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Oral Abstract

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Mo-1-1: Doping evolution of charge orders in cuprates revealed by scanning tunneling microscopy Yayu Wang Department of Physics, Tsinghua University, Beijing, China 100084 One of the key issues in cuprates is the nature of the ubiquitous charge order and its relationship to superconductivity. Scanning tunneling microscopy (STM) is a powerful tool for answering these questions because it can detect the atomic scale charge order and electronic structure simultaneously. In this talk we report STM studies on Bi-based cuprates over a large doping range across the phase diagram. In lightly doped insulating Bi-2201, we find that a sharp energy gap reminiscent of the pseudogap starts to form near the Fermi level, and is accompanied by the emergence of a checkerboard-like charge order with 4 a0 periodicity. In the heavily overdoped non-superconducting Bi-2201, spectroscopic imaging reveals the van Hove singularity features near the Fermi level, as well as nanoscale patches of static charge orders

with √2 × √2 periodicity. We further show how the checkerboard charge order gradually

evolves into the √2 × √2 charge order with increasing doping, and will discuss the implications to superconductivity.

Mo-1-2: Evolution of pairing orders between superconducting and pseudogap phases of copper oxide superconductors Y. H. Liu1,2, W. L. Tu1, and T. K. Lee1 1Institute of Physics, Academia Sinica, Taipei, 11529, Taiwan 2Department of Physics, National Tsinghua University, Hsinchu 30013, Taiwan Recently there are many evidences to support the presence of charge density and/or spin density waves in the superconducting and pseudogap phases. By making a postulate of the presence of pair density waves, or Cooper pairs with a finite momentum, intertwined with these other density waves, we will be able to provide a consistent and simple way to understand the relationship between the superconducting and pseudogap phases. Specifically we will present self-consistent inhomogeneous solutions of the t-J model by using the renormalized mean-field theory. These solutions although quite complex, it is in fact dominated by the large energy scale defined by this finite momentum pairing order, or the FFLO order. Properties of these solutions are shown to include all the important features reported by Scanning tunneling spectra and angle-resolved photoemission spectra for copper oxide superconductors. The temperature and doping dependence of these spectra can be also accounted for.

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Mo-1-3: Visualizing the electronic structure of monolayer flakes of cuprate superconductors Yuanbo Zhang1 1 Department of Physics, Fudan University, Shanghai 200433, China

The role of dimensionality in high Tc superconductivity is an interesting issue: many of the high Tc superconductor have layered atomic structures, and yet the link between the high Tc superconductivity and the two-dimensional nature of the crystal structure remains elusive. We

fabricated atomically thin Bi2Sr2CaCu2O8+d (Bi-2212) and Bi2Sr2CuO6+d (Bi2201) flakes, and used scanning tunneling microscopy/spectroscopy (STM/STS) to investigate their electronic structure. In this talk, I will discuss our recent results on the superconducting gap, pseudogap and charge order in Bi-2212 and Bi-2201 in the ultimate 2D limit.

Mo-1-4: Uniaxial pressure control of competing orders in the cuprates H.-H. Kim1, S. M. Souliou2, M.E. Barber3, E. Lefrancois1,2, M. Minola1, R. Heid4, A. Bosak2, A. P. Mackenzie3, B. Keimer1, C. W. Hicks3, M. Le Tacon4

1Max Planck Institute for Solid State Research, Stuttgart, Germany. 2European Synchrotron Radiation Facility (ESRF), Grenoble, France. 3Max Planck Institute for Chemical Physics of Solids, Dresden, Germany. 4Institute for Solid State Physics, Karlsruhe Institute of Technology, Karlsruhe, Germany.

External control of electronic phases in correlated-electron materials is a long-standing challenge of condensed-matter research. Layered cuprates exhibit antiferromagnetic, charge-density-wave (CDW), and high-temperature superconducting ground states which can be tuned by doping and external magnetic fields. However, disorder generated by lattice defects and randomly pinned magnetic vortices greatly complicates the interpretation of these experiments. Here, we report a high-resolution inelastic x-ray scattering study of the high-temperature superconductor YBa2Cu3O6.67 under uniaxial stress, and show that a three-dimensional long-range-ordered CDW state can be induced by pressure along the a-axis, in the absence of magnetic fields. The amplitude of the CDW is strongly suppressed below the superconducting transition temperature, indicating strong thermodynamic competition with superconductivity. We also show that the transition is driven by the complete softening of an optical phonon mode. The results provide new insights into the anomalous normal-state properties of high-temperature superconductors an illustrate the potential of uniaxial-pressure control of competing orders in quantum materials [1]. [1] H.-H. Kim, S. M. Souliou et al., Science 362, 1040 (2018).

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Mo-1-5: ARPES study on cuprates across critical dopings M. Hashimoto1 1SSRL, SLAC National Accelerator Laboratory, Menlo Park, CA, 94025, USA. In the hole-doped cuprate superconductors, the special doping p = 0.19 with various anomalies has attracted considerable research interest, with close connection to the pseudogap and strange metal [1]. In this talk, we present systematic ARPES studies across p = 0.19 in Bi2212. The results provide important insights about the nature of this special doping and the phenomenology of the cuprates [2, 3]. Further, we identify and discuss significant superconducting fluctuations on a single coherent, hole-like Fermi surface in heavily overdoped regime. We also show ARPES evidence for FS reconstruction in electron-doped cuprate NCCO in optimal- and over-doped regime (x=0.15-0.17) [4], where an unexpected FS reconstruction by quantum oscillation measurements (QOM) has been reported. [1] M. Hashimoto, et al., Nature Phys. 10, 483–495 (2014). [2] Y. He*, M. Hashimoto*, et al., Science 362, 62 (2018). [3] S. Chen*, M. Hashimoto*, et al., submitted. [4] J. He, et al., PNAS 116, 3449 (2019).

Mo-2-1: Evidence for electronic nematicity in the pseudogap state in cuprates

�� Shibauchi1

1Department of Advanced Materials Science, University of Tokyo, Kashiwa 277-8561, Japan In the quest to understand the superconductivity in cuprates, a key issue is the nature of the enigmatic pseudogap region of the phase diagram. An especially important question is whether the pseudogap state is a distinct thermodynamic phase characterized by broken symmetries

below the onset temperature T . We use torque-magnetometry and elastoresistance

measurements to test the rotational symmetry breaking in the pseudogap state in single crystals

of YBa2Cu3Oy (YBCO) and Bi2Sr2CaCu2O8+d (Bi2212). In YBCO, anisotropic susceptibility within the ab planes was measured by the torque magnetometry with exceptionally high precision [1]. The in-plane anisotropy displays a significant increase with a distinct kink at the

pseudogap onset temperature T , showing a remarkable scaling behavior with respect to T/T

in a wide doping range. Our systematic analysis reveals that the rotational symmetry breaking

sets in at T in the limit where the effect of orthorhombicity is eliminated. In Bi2212, the

nematic susceptibility that describes the electronic anisotropy response to a uniaxial strain was

measured by elastoresistance above T [2]. We find that the nematic susceptibility increases

upon cooling with a kink anomaly at T , evidencing a second-order transition with neamtic

fluctuations. These results provide thermodynamic evidence that the pseudogap in cuprates accompanies an electronic nematic order, which differs from the recently reported charge-density-wave transition that accompanies translational symmetry breaking. [1] Y. Sato et al., Nat. Phys. 13, 1074 (2017). [2] K. Ishida et al., preprint.

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Mo-2-2: Nematic Quantum Criticality in the Cuprate Superconductor Bi2Sr2CaCu2O8+δ N. Auvray1, B. Loret1, S. Benhabib1, M. Cazayous1, R.D. Zhong2, J. Schneeloch2, G. D. Gu2, A. Forget3, D. Colson3, I. Paul1, A. Sacuto1, Y. Gallais1

1Université Paris Diderot and CNRS, France, 2Brookhaven National Lab, USA, 3CEA Saclay, France The presence and nature of a quantum critical point in their phase diagram is a central enigma of the high-temperature superconducting cuprates. It could explain their pseudogap and strange metal phases, and ultimately their high superconducting temperatures [4]. Yet, while solid evidences exist in several unconventional superconductors of ubiquitous critical fluctuations associated to a quantum critical point, their presence has remained elusive in the cuprates, thus questioning the validity of the quantum critical point scenario in their case. Here using symmetry-resolved electronic Raman scattering, we report the observation of critical electronic nematic fluctuations near the endpoint of the pseudogap phase of the cuprate Bi2Sr2CaCu2O8+δ. Their doping and temperature dependences demonstrate the presence of an incipient nematic quantum critical point under the superconducting dome. We further show that the nematic instability weakens upon entering the pseudogap regime, suggesting a non-trivial link between the pseudogap phenomenon and quantum criticality. Our result establishes nematic quantum criticality as a key ingredient to understand the phase diagram of cuprates. [1] N. Auvray et al. arXiv1932 :03508 (2019)

Mo-2-3: Superconductivity, nematicity and BCS-BEC crossover in FeSe1-xSx T. Hanaguri1 1RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan We review spectroscopic-imaging STM studies on bulk FeSe1-xSx in which superconductivity and nematicity coexist. The electronic nematicity is suppressed with increasing sulfur content x and disappears above the nematic end point at x ~ 0.17. By analyzing the evolution of the quasiparticle interference patterns and superconducting gap spectrum, we show that there are two distinct superconducting pairing states depending on the presence or absence of nematicity [1]. Besides nematicity, FeSe is known as a superconductor with very small Fermi energy that is only a several times larger than the superconducting gap amplitude. This places FeSe as the prime candidate of the BCS-BEC crossover superconductor where well separated discrete bound states in the vortex core and a pseudogap above Tc are expected. Spectroscopic-imaging STM data indicate the discrete vortex bound states but the pseudogap was not detected. These conflicting observations can be resolved if multi-band nature is taken into account [2]. This work has been done in collaboration with Y. Matsuda’s group (Kyoto), T. Shibauchi’s group (Tokyo), and I. Eremin’s group (Bochum).

[1] T. Hanaguri et al., Science Adv. 4, eaar6419 (2018). [2] T. Hanaguri et al., Phys. Rev. Lett. 122, 077001 (2019).

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Mo-2-4: Optical spectroscopy of FeSe thin films on different substrates M. Nakajima1, K. Yanase1, Y. Senoo1, M. Kawai2, T. Ishikawa2, N. Shikama2, F. Nabeshima2, A. Maeda2, and S. Tajima1 1Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan 2Department of Basic Science, The University of Tokyo, Meguro, Tokyo 153-8902, Japan In iron-based superconductors, the electronic structure is strongly affected by in-plane strain. In particular, the band structure of FeSe thin films differs from that of bulk FeSe due to epitaxial strain [1]. To investigate the effect of in-plane strain on charge dynamics, we performed optical spectroscopy on thin films of FeSe with different substrates. The spectral weight of the coherent Drude component, which is responsible for the coherent conduction channel, gradually decreases with decreasing temperature in the low-temperature orthorhombic phase [2]. With applying tensile strain, the coherent Drude weight decreases, which is likely related to the suppression of Tc. For the film on LaAlO3 substrate with a tensile strain, we observed a transfer of the spectral weight below ~ 500 cm-1 to a higher energy region. This behavior, distinct from the charge dynamics of FeSe for CaF2 substrate with compressive strain, can be explained by the change in the band structure. [1] G. N. Phan et al., Phys. Rev. B 95, 224507 (2017). [2] M. Nakajima et al., Phys. Rev. B 95, 184502 (2017).

Mo-2-5: Hints of orbital-selectivity and charge-order in AFe2As2 (A = Cs, Rb) iron-based superconductors by means of 75As nuclear quadrupole resonance G. Prando1, M. Moroni1, S. Aswartham2, I. Morozov2,3, Z. Bukowski4, B. Büchner2, H.-J. Grafe2, and P. Carretta1 1Department of Physics, University of Pavia, 27100 Pavia, Italy 2Leibniz-IFW Dresden, 01069 Dresden, Germany 3Lomonosov Moscow State University, 119991 Moscow, Russia 4Institute of Low Temperature and Structure Research, 50422 Wroclaw, Poland We discuss the results of 75As nuclear quadrupole resonance in AFe2As2 (A = Cs, Rb) single crystals. We demonstrate a crossover in the nuclear spin-lattice relaxation rate associated with the onset of an inhomogeneous local charge distribution causing the broadening or even the splitting of the spectra. We argue that this crossover, occurring at temperatures well above the phase transition to the nematic long-range order, is associated with a charge disproportionation at the Fe sites induced by competing Hund’s and Coulomb couplings. These different trends are discussed in light of an orbital-selective behavior expected for the electronic correlations. Moreover, in RbFe2As2 we observe a peak in the spin-lattice relaxation rate which is possibly associated with the critical slowing down of electronic nematic fluctuations on approaching the transition to the nematic long-range order. [1] M. Moroni, G. Prando et al., arXiv:1903.05029 (2019).

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Mo-3-1: Nonreciprocal transport in noncentrosymmetric 2D superconductors Y. Iwasa1,2 1 Department of Physics, University of Tokyo, Hongo, Tokyo 113-0033, Japan 2 RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan The common features of recently emerging 2D superconductors fabricated either by MBE, exfoliation, interfaces, and field effect, are the high crystallinity. This allows us to approach the intrinsic nature of 2D superconductors [1], in sharp contrast to the conventional 2D superconductors which are amorphous or granular materials, and thus are predominantly controlled by quenched disorder. In this presentation, we discuss on the nature of the recent highly crystalline superconductors, with particular focus on the vortex dynamics. To probe the vortex states in the 2D superconductors we utilized nonreciprocal transport, the inequivalent resistance for the leftward and rightward current flow under the broken inversion and time reversal symmetry [2]. With this approach, we discuss on the vortex states which have been left to be addressed in the highly crystalline 2D superconductors [3]. [1] Y. Saito, T. Nojima, and Y. Iwasa, Nature Reviews Materials 2, 16094 (2016).. [2] R. Wakatsuki, Y. Saito, S. Hoshino, Y. M. Itahashi, T. Ideue, M. Ezawa, Y. Iwasa, N. Nagaosa, Science Advances, 3, e1602390 (2017). [3] Y. M. Itahashi, Y. Saito, T. Ideue, T. Nojima, and Y. Iwasa, arXiv 1904.00611

Mo-3-2: Non-equilibrium superconductivity on demand

D. Kennes1 1Freie Universitaet Berlin, Arnimallee 14, 14195 Berlin, Germany Obtaining time-dependent quantum control in solids on ultra-fast time scales in a flexible and reversible manner is a crucial step along the long road of realizing quantum technologies of potential high-impact. Here, I will present three mechanisms of control using non-equilibrium means, with a particular focus on how superconductivity can be induced: (i) First, I will illustrate how the dominant symmetry-allowed non-linear coupling between electrons and dipole active phonon modes implies an electron density-dependent squeezing of the phonon state which provides an attractive contribution to the electron-electron interaction [1]. (ii) Then, I will turn to chiral superconductors and identify a sequence of combined linear and circular polarized light that can be used to switch the chirality of the superconducting condensate [2]. (iii) Finally, I will describe an example of Floquet engineering --- the idea to steer system properties by periodic drive. I will consider a two-dimensional Hubbard model of interacting electrons for which exotic d-wave superconductivity can be induced. [1] Nature Physics 13, 479-483 (2017) [2] arXiv:1810.06536 (accepted in Nature Physics) [3] arXiv:1808.04655 (under review in PRL)

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Mo-3-3: Correlation in twisted bilayer graphene: Many-body physics around Van Hove singularities Hiroki Isobe1 1Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Superconductivity and correlated insulating states were observed in bilayer graphene at a certain “magic” twist angle. Since the discovery, there has been a growing interest in correlated physics in superstructures of van der Waals materials. Graphene by itself is a good conductor, but the bandwidth is drastically reduced in a superlattice structure of twisted bilayers, which enhances electron correlation effects. We theoretically study the origin of these insulating and superconducting states [1]. Our theory is motivated by the Van Hove singularity (VHS) and the Fermi surface nesting. The pronounced peak in the density of states near the VHS enhances correlation effects and the Fermi surface nesting drives an electronic instability. From a renormalization group analysis, we find d/p-wave superconductivity and charge/spin density wave as leading instabilities driven by Coulomb interaction. Then, we introduce a high-order VHS as a possible account for the “magic” angle, where the density of states shows a power-law divergence [2]. We also discuss correlation effects due to the high-order VHS. [1] H. Isobe, N. F. Q. Yuan, and L. Fu, Phys. Rev. X 8, 041041 (2018). [2] N. F. Q. Yuan, H. Isobe, and L. Fu, arXiv:1901.05432.

Mo-3-4: Extremely flat band in bilayer graphene D. Marchenko1, D. Evtushinsky1, E. Golias1, A. Varykhalov1, Th. Seyller2, O. Rader1 1Helmholtz-Zentrum Berlin für Materialien und Energie, 12489 Berlin, Germany 2Institut für Physik, Technische Universität Chemnitz, 09126 Chemnitz, Germany Frequent reports of superconductivity in graphite at elevated temperatures even above 300 K demand an explanation. It has been suggested that this is an effect of flat band formation at graphite surfaces and/or interfaces. In the present work we discover by angle-resolved photoemission that bilayer graphene on SiC shows an extremely flat band (< 2 meV dispersion). We demonstrate that the band extends two-dimensionally around the K point and present a general model for flat band formation in bilayer graphene and related systems. This model is based on the relative biasing of only one sublattice against other sublattices in a honeycomb lattice bilayer and allows modification of the band dispersion from parabolic to “Mexican hat”–like through the formation of a flat band. The mechanism is applicable for doped and undoped bilayer graphene. We argue that our model and the experimental results have the potential to contribute to achieving superconductivity of graphene- or graphite-based systems at elevated temperatures. [1] D. Marchenko, D. Evtushinsky, E. Golias, A. Varykhalov, Th. Seyller, O. Rader, Sci. Adv. 4, eaau0059 (2018).

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Mo-3-5: Chiral spin density wave and d + id superconductivity in the magic-angle-twisted bilayer graphene Fan Yang Beijing Institute of Technology, China We model the newly synthesized magic-angle-twisted bilayer graphene superconductor with two px,y like Wannier orbitals on the superstructure honeycomb lattice, where the hopping integrals are constructed via the Slater-Koster formulism by symmetry analysis. The characteristics exhibited in this simple model are well consistent with both the rigorous calculations and experiment observations. A van Hove singularity and Fermi-surface (FS) nesting are found in the doping levels relevant to the correlated insulator and unconventional superconductivity revealed experimentally, based on which we identify the two phases as weak-coupling FS instabilities. Then, with repulsive Hubbard interactions turned on, we performed random-phase-approximation based calculations to identify the electron instabilities. As a result, we find chiral d+id topological superconductivity bordering the correlated insulating state near half-filling, identified as noncoplanar chiral spin-density wave ordered state, featuring the quantum anomalous Hall effect. The phase diagram obtained in our approach is qualitatively consistent with experiments.

Reference: C.-C. Liu, L.-D. Zhang, W.-Q. Chen and F. Yang, Phys. Rev. Lett. 121,217001 (2018).

Mo-4-1: Unbiased T=0 and finite T simulations of the Hubbard model (and variants) Thomas P. Devereaux Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA Geballe Laboratory for Advanced Materials, Stanford University, Stanford, CA 94305, USA It is now well accepted that there are many competing and intertwined phases as "solutions" of the Hubbard model that are separated by very small energies, typically 1/100th of the hopping strength. Therefore it is important to benchmark using unbiased methods which are the stable phases, and how they can be manipulated. In this talk I will present the results of quantum Monte Carlo and DMRG simulations that explore the relationship between stripes and superconductivity. We can show a strong interplay between CDW and long-range superconductivity that can be controlled by varying the next nearest neighbor hopping t'.

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Mo-4-2 : Superconductivity and nematicity in FeSe Andrey V Chubukov University of Minnesota Bulk FeSe is a special iron-based material in which superconductivity emerges inside a well-developed nematic phase. I will argue that the nematicity (spontaneous breaking of C4 lattice rotational symmetry) is the consequence of d-wave orbital order, which distinguishes between dxz and dyz orbitals. I argue that the sign of the orbital order must be different between hole and electron pockets. I compare the results with ARPES data and present the explanation of the Raman data in the nematic phase. I next move to the superconducting state and analyze the gap anisotropy. I present the model for the nematic superconducting state, which takes into account the mixing between s-wave and d-wave pairing channels and the changes in the orbital spectral weight promoted by the sign-changing nematic order parameter. I show that

nematicity gives rise to a cos2q variation of the pairing gap on the hole pocket, consistent with ARPES and STM data for experimentally-extracted Fermi surface parameters. I further argue that, dxz and dyz orbitals give nearly equal contributions to the pairing glue, i.e. nematic order alone accounts for the gap anisotropy, but has little effect on Tc. I compare our results with the concept of orbital-selective pairing.

Mo-4-3: Towards a first-principles description of stronger correlations: Novel superconductors to topological materials Arun Bansil Physics Department, Northeastern University, Boston, Massachusetts USA I will discuss how advanced density functionals are enabling new insights into the electronic structure, phase diagrams and magnetism of a wide variety of materials that have until now been considered to be so strongly correlated as to lie outside the scope of first-principles treatment. A spectacular example is provided by the cuprate high-Tc superconductors in which the density functional theory fails to correctly predict the half-filled parent compounds to be insulators. In sharp contrast, however, the recently constructed SCAN functional [1] not only reproduces the insulating character and magnetism of the half-filled cuprates, but also captures the transition to the metallic state with doping without invoking any free parameters such as the Hubbard U. [2-4] I will also comment on the opportunities for a new generation of predictive modeling in correlated materials more generally, including the topological phases of quantum matter, which are drawing intense current interest [5]. Work supported by the U.S. Department of Energy. [1] J. Sun, A. Ruzsinszky, J. P. Perdew, Phys. Rev. Lett. 115, 036402 (2015). [2] J. W. Furness et al., Nature Communications Physics 1, 11 (2018). [3] C. Lane et al., Phys. Rev. B. 98, 125140 (2018). [4] Y. Zhang et al., arXiv:1809.08457 (2018). [5] A. Bansil, H. Lin and T. Das, Reviews of Modern Physics 88, 021004 (2016).

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Mo-4-4: Nematicity, pairing mechanism and spin resonance in Fe-based SCs H. Kontani1, Y. Yamakawa1, and S. Onari1 1Department of Physics, Nagoya University, Nagoya, Aichi 464-8602, Japan

Unexpected diversity of nematic states and SC states discovered in various Fe-based SCs attracts increasing attention. To understand this key characteristic of Fe-based SCs, we focus on the higher-order many-body effects due to the vertex corrections (VCs) and self-energy effects. We find that the Aslamazov-Larkin (AL) VC, which describes the interference between charge- and spin-fluctuations, induces various types of nematicity. We predict that the “B2g symmetry bond order” emerges in heavy electron-doped compound AFe2As2 (A=Cs,Rb) [1], and the “antiferro nematic order” appears in BaFe2As2 prior to the structure transition temperature TS. The development of these orbital and bond fluctuations induces the nodal s-wave state in BaFe2(As,P)2 [2] and s++ wave state in LiFeAs. The same mechanism will be responsible for the s++ wave SC in heavy fermion CeCu2Si2 [3].

In the SC state, it is widely believed that the spin resonance is a hallmark of the sign reversal in the gap. To verify this, we perform the fluctuation-exchange approximation below Tc at very low T (~1meV) very precisely, by newly developing the multistage algorithm [4]. It is clarified that the spin resonance peak appears even in the s++ wave state, due to the strong energy-dependence of the self-energy below Tc. [1] S. Onari and H. Kontani, arXiv:1809.08017 [2] H. Nakaoka, Y. Yamakawa and H. Kontani, Phys. Rev. B 98, 125107 (2018) [3] R. Tazai and H. Kontani, Phys. Rev. B 98, 205107 (2018) [4] L. Takeuchi, Y. Yamakawa, and H. Kontani, Phys. Rev. B 98, 165143 (2018)

Tu-1-1: Collaborations between Electron Phonon Interactions and Electron Correlations in the Superconductivity of FeSe/STO and (Ba,K)BiO3 D. L. Feng1,2, H. C. Xu1, and R. Peng1 1Dept. of Physics, State Key Laboratory of Surface Physics, and Adv. Materials Laboratory, Fudan University, Shanghai, 200438, China 2Hefei National Laboratory for Physical Sciences at the Microscale, and Department of Physics, Univ. of Sci. & Tech. of China, Hefei, 230026, China Electron-phonon interactions (EPI) are the pairing glue for conventional BCS superconductors, while electron correlations are believed to be responsible for unconventional superconductors. Whether they could collaborate remain elusive, despite extensive studies on cuprates among others. Here we show that 1. forward-scattering type of EPI and short-range electron correlations collaborate to generate the high temperature interfacial superconductivity in FeSe/STO [1,2]; 2. long-range electron correlations strongly enhance EPI, which subsequently gives the high superconducting temperature in (Ba,K)BiO3 [3]. [1] W. H. Zhang et al. Nano Lett. 16, 1969 (2016). [2] Q. Song et al. Nature Commun. 10:758 (2019). [3] H. P. Wen-Chen. et al. Phys. Rev. Lett. 121, 117002 (2018).

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Tu-1-2: Ultrafast nematic orbital excitation in FeSe superconductor K. Ishizaka Quantum-Phase Electronics Center & Department of Applied Physics, University of Tokyo, Hongo, Tokyo 113-8656 Japan This Electronic nematicity is a mysterious state of matter that spontaneously breaks the rotational symmetry of electrons. In iron-pnictides/chalcogenides and cuprates, the nematic order and fluctuations are suggested to play as-yet-unknown important roles on superconductivity. Here we use femtosecond optical pulse to perturb the electronic nematic order in FeSe. Through the time-, energy-, momentum- and orbital-resolved photo-emission spectroscopy, we detect the ultrafast dynamics of the electronic nematicity. In the strong-excitation regime, we find a quick disappearance of the nematicity followed by a heavily-damped oscillation, through the observation of Fermi surface anisotropy. This short-life nematicity oscillation is seemingly related to the unbalance of Fe 3dxz and dyz orbitals. These phenomena show a critical behavior as a function of pump fluence, resembling the nematic fluctuation reported by electronic Raman scattering. Our real-time observations reveal the nature of the electronic nematic excitation decoupled from the underlying lattice.

[1] T. Shimojima, K. Ishizaka et al., Nat. Comm. (in press).

Tu-1-3: Largely system dependent orbital occupation imbalance in iron based superconductor Y. K. Kim1,2 1Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea 2Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea Establishing the microscopic understanding of the nematic phase in iron based superconductor is of most important task in iron based superconductivity research. It is complicated task as all the degrees of freedom-lattice, orbital and spin are active and even closely intertwined especially for spin and orbital case. We focused on the orbital side, the occupation imbalance between dxz and dyz orbital that has been regarded as an important parameter of the nematic phase. We examined the examination of the occupation imbalance for various iron based superconductor systems with X-ray absorption spectroscopy. Our result reveal that the sign and the degree of occupation imbalance largely alter depending on the system. Accounting the almost system independent band splitting between dxz and dyz bands that only depends on the transition temperature, our result implies that occupation imbalance is likely the by-product of the nematic order, not a driving instability.

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Tu-1-4: Unprecedented Optical Anisotropy in Optimally Doped Iron-Based Superconductor L. Degiorgi1 1Laboratorium für Festkörperphysik, ETH - Zürich, 8093 Zürich, Switzerland The divergent nematic susceptibility, obeying a simple Curie-Weiss power law over a large temperature interval, is empirically found to be a ubiquitous signature in several iron-based materials across their doping-temperature phase diagram. The composition at which the associated Weiss temperature extrapolates to zero is found to be close to optimal doping, boosting the debate to what extent nematic fluctuations contribute to the pairing-mechanism and generally affect the electronic structure of iron-based superconductors. Here, we offer a comprehensive optical investigation of the optimally hole-doped Ba0.6K0.4Fe2As2 over a broad spectral range, as a function of temperature and of tunable applied stress, which acts as an external symmetry breaking field. We show that the stress-induced optical anisotropy in the infrared spectral range is reversible upon sweeping the applied stress and occurs only below the superconducting transition temperature. These findings demonstrate that there is a large electronic nematicity at optimal doping which extends right under the superconducting dome [1].

[1] A. Pal et al., npj Quantum Materials 4, 3 (2019).

Tu-1-5: Nature of zero-energy vortex bound state in superconducting topological surface state of Fe(Se,Te) T. Machida1, Y. Sun2, S. Pyon3, S. Takeda4, Y. Kohsaka1, T. Hanaguri1, T. Sasagawa4 and T. Tamegai3 1RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan 2Department of Physics and Mathematics, Aoyama Gakuin University Sagamihara, Kanagawa 252-5258, Japan 3Department of Applied Physics, University of Tokyo, Hongo, Tokyo 113-8656, Japan 4Laboratory for Materials and Structures, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan At the surface of FeTe0.6Se0.4, where topological superconductivity emerges, vortex cores are

expected to host Majorana Fermions. Using a dilution-refrigerator STM [1], we have systematically examined a large number of vortices in FeTe0.6Se0.4. We found that some

vortices possess the zero-energy vortex bound state (ZVBS) below 20 µeV, which suggests its Majorana bound state origin. However, we also found vortices without the ZVBS. Interestingly, emergence of the ZVBS is not related to the preexisting quenched disorders, and the fraction of vortices with the ZVBS decreases with increasing magnetic field [2], suggesting the importance of the inter-vortex interaction for the ZVBS formation. [1] T. Machida et al., Rev. Sci. Instrum. 89, 093707 (2018) [2] T. Machida et al., arXiv.1812.08995 (2018)

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Tu-2-1: Exotic superconductivity in multipole systems Y. Yanase1, J. Ishizuka1, S. Sumita1, and T. Nomoto2 1Department of Physics, Kyoto University, Sakyo, Kyoto 606-8502, Japan 2Department of Applied Physics, University of Tokyo, Hongo, Tokyo 113-8656, Japan Using a microscopic calculation and symmetry argument we reveal superconductivity in the vicinity of parity-violating magnetic order [1]. We show unconventional superconductivity induced by an odd-parity magnetic multipole fluctuation in a locally-noncentrosymmetric two-sublattice Hubbard model. We find that even-parity superconductivity is more significantly suppressed by spin-orbit coupling than that in a globally noncentrosymmetric system. Consequently, two odd-parity superconducting states are stabilized by magnetic multipole fluctuations in a large spin-orbit coupling region. Both of them are identified as time-reversal invariant Z2 topological superconducting states. Our finding implies a new family of odd-parity topological superconductors. Candidate materials are proposed. We also study coexistent multipole superconducting state [2]. Combining the group

theoretical analysis and numerical calculations for an effective Jeff=1/2 model of Sr2IrO4, we show unusual superconducting gap structure protected by nonsymmorphic magnetic space group symmetry and the Fulde-Ferrell-Larkin-Ovchinnikov superconducting state stabilized at zero magnetic field. [1] Jun Ishizuka and Youichi Yanase, Phys. Rev. B 98, 224510 (2018). [2] S. Sumita, T. Nomoto, and Y. Yanase, Phys. Rev. Lett. 119, 027001 (2017).

Tu-2-2: Spectroscopic properties of an SYK-model for non-quasiparticle superconductivity Jörg Schmalian1 and Ilya Esterlis2 1Institute for Theory of Condensed Matter and Institute for Solid State Physics, Karlsruhe Institute of Technology, Karlsruhe, 76131 Germany 2Department of Physics, Stanford University, Stanford, California 94305, We introduce and solve a model for strong electron-boson interactions that is a natural generalization of the SYK-model and that gives rise to both, non-Fermi liquid behavior and superconductivity. We present the solution of this model as function of the electron-boson coupling strength and analyze the key spectroscopic features in the normal and superconducting state. At weak coupling superconductivity sets in at the crossover to the non-Fermi liquid state, while in the strong coupling limit we find genuine non-quasiparticle superconductivity with small coherent peaks and large incoherent background. This analysis offers a rigorously solvable toy model for superconductivity in strongly-coupled quantum critical systems, such as the cuprate and possibly the iron-based superconductors.

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Tu-2-3: Majorana fermion energy oscillations, localization, and disorder robustness in magnetic impurity chains O. Awoga1, A. Theiler1, K. Björnson1,2, and A. Black-Schaffer1 1Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden 2Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark Magnetic impurity chains on the surface of spin-orbit coupled but otherwise conventional superconductors offer the possibility to create topological phases with Majorana fermions at the chain end points without having to apply an external magnetic field. In this talk I will present some of our resent results modeling such magnetic impurity chains, with an accurate treatment of the superconducting state through a self-consistent approach. In particular, I will show how the Majorana fermions strongly hybridize with in-gap Yu-Shiba-Rusinov (YSR) states, causing large oscillations in the Majorana fermion energies. Still, by treating the Majorana fermion as a topological boundary mode, dependent only on the effective mass gap, we arrive at a fully parameter-free fitting of the Majorana localization length, which stays very short [1]. I will also show how the Majorana fermions are very robust against disorder, despite individual YSR states being extremely sensitive to disorder [2].

[1] A. Theiler, K. Björnson, and A. M. Black-Schaffer, arXiv:1808.10061.

[2] O. A. Awoga, K. Björnson, and A. M. Black-Schaffer, Phys. Rev. B 95, 184511 (2017).

Tu-2-4: Higgs Spectroscopy of Nematic and Chiral Superconductors T. Mizushima1, H. Uematsu1, A. Tsuruta1, S. Fujimoto1, and J. A. Sauls2 1Depatment of Materials Engineering Science, Osaka University, Toyonaka 560-8531, Japan 2Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA Nematic/chiral superconductivity accompanying the spontaneously broken rotational/time-reversal symmetry has been reported in doped topological insulators, MxBi2Se3 (M=Cu, Sr, Nb) and Uranium compounds. Here we propose that transverse electromagnetic wave response of long-lived massive bosonic (“Higgs”) excitations provide fingerprint spectroscopy of nematic/chiral superconductivity. Using quasiclassical Keldysh theory, we show the existence of characteristic bosonic modes in nematic/chiral superconductors [1,2]. For instance, there exist nematicity and chirality vibration modes in nematic superconductors. The former is the pseudo Nambu-Goldstone boson associated with the broken rotation symmetry. The latter can be identified as the fluctuation of the orbital angular momentum of Cooper pairs. We find that in MxBi2Se3, the mass gap of the chirality mode closes at the critical doping rate, signaling the dynamical instability of the nematic state towards the chiral state with broken time reversal symmetry [2]. We demonstrate that the spectral evolution of bosonic excitations can be clearly captured by the power absorption spectrum of electromagnetic waves. [1] J. A. Sauls, H. Wu, and S.-B. Chung, Front. Phys. 3, 36 (2015). [2] H. Uematsu, T. Mizushima, A. Tsuruta, S. Fujimoto, and J. A. Sauls, arXiv:1809.06989.

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Tu-2-5: Quasicrystalline superconductors under magnetic field S. Sakai RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan We theoretically study superconducting states in quasicrystals under magnetic field, partly motivated by the recent discovery of superconducting quasicrystal [1]. We first introduce an attractive Hubbard model on Penrose-tiling structure as a simple model to discuss the issue [2]. We then solve the model within the Bogoliubov-de Gennes theory, and find an exotic superconducting state at high magnetic field just below the critical field [3]. In that state, the superconducting order parameter changes its sign in real space, with keeping the five-fold rotational symmetry inherent to the Penrose structure. The exotic superconducting state is in part analogous to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in periodic systems. However, considering that the FFLO state is unstable against impurities, it is remarkable that such an alternating-sign superconducting state appears under the quasiperiodic potential, which often acts as a random potential: Electron system finds a way to keep the coherence under magnetic field by organizing itself in a spatially sign-changing pattern conforming to the underlying quasiperiodic structure. [1] K. Kamiya et al., Nature Commun. 9, 154 (2018). [2] S. S, N. Takemori, A. Koga, and R. Arita, Phys. Rev. B 95, 024509 (2017). [3] S. S and R. Arita, in preparation. Tu-3-1: Advances Photo-induced superconductivity Andrea Cavalleri Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany Department of Physics, University of Oxford

I will discuss how coherent electromagnetic radiation at infrared and TeraHertz frequencies can be used to induce superconducting coherence above the transition temperature Tc. In this talk, I will report the most recent advances, which cover the creation of metastable high-temperature superconductivity, the direct detection of oscillatory supercurrents in cuprates and the discovery of the same phenomenon in new materials.

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Tu-3-2: Photo-induced nonequilibrium response in heavily underdoped YBa�Cu�O���� probed by time-resolved terahertz spectroscopy

S. J. Zhang�, Z. X. Wang

�, X. Yao

�, L. Y. Shi

�, T. Lin

�, D. Wu

�, T. Dong

�, and N. L. Wang1

1International Center for Quantum Materials, Peking University, Beijing, China 2School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, China We present a near and midinfrared pump, c-axis terahertz probe measurement on an

underdoped YBa�Cu�O��� superconductor with T=35 K. We observe strong pump-induced

spectral change in the terahertz frequency range. In the superconducting state, the Josephson edge is removed upon initial photo excitations, yielding evidence for a collapse of superconducting condensate. However, a Josephson plasma edge reappears at frequency lower than the static plasma edge within a short time delay, whose feature becomes more pronounced and shifts slightly to higher energy scale with increasing time delay. Meanwhile, incoherent quasiparticle excitations build up at energy scale above the static plasma edge. Above Tc, much smaller pump induced effect is detected. A weak edge-like shape develops in the reflectance spectrum and sustains even above room temperature. We elaborate that weak edge structure above Tc is unlikely to be explained by the photo-induced transient superconductivity. In addition, there is no substantial difference between the near and midinfrared pump cases both in superconducting phase and normal phase, which indicates that phonon pumping as a scenario for the photo excitation effect can be excluded.

Tu-3-3: Higgs spectroscopy of high Tc cuprates in equilibrium and nonequilibrium R. Shimano1,2 1Cryogenic Research Centre, University of Tokyo, Yayoi, Tokyo 113-0032, Japan 2Department of Physics, University of Tokyo, Hongo, Tokyo 113-0033, Japan In the last few years, it has been elucidated that there exists a nonlinear coupling between light and the Higgs mode in superconductors, which makes the Higgs mode visible in various nonlinear optical responses in a time-resolved manner [1,2,3]. Since the Higgs mode is an amplitude mode of superconducting order parameter, it can be used as an optical probe for the order parameter both in equilibrium and nonequilibirum. In this talk, firstly I will report on our recent experiments on the observation of Higgs mode in various superconductors, including high Tc cuprates [4,5]. Secondly I will report on the potential application of Higgs spectroscopy to the study of photoexcited dynamics in high Tc cuprates, in particular in view of light-induced superconductivity. [1] R. Matsunaga et al., Phys. Rev. Lett. 111, 057002 (2013). [2] R. Matsunaga et al., Science 345, 1145 (2014). [3] Y. Murotani and R. Shimano, arXiv:1902.01104. [4] K. Katsumi et al., Phys. Rev. Lett. 120, 117001 (2018). [5] H. Chu et al., arXiv:1901.06675.

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Tu-3-4: New “Rabi-Higgs” mode in a driven superconductor H. P. Ojeda Collado1,, J. Lorenzana2, G. Usaj1,2, and C. A. Balseiro1

1CONICET, Centro Atómico Bariloche and Instituto Balseiro CNEA, Bariloche, Argentina 2ISC-CNR and Department of Physics, Sapienza, Piazzale Aldo Moro 2 Rome, Italy

We consider a superconductor under a periodic drive solving for the non-linear dynamics to all orders in the drive. We show that a subset of quasiparticle excitations enter into resonance and perform synchronous Rabi oscillations leading to cyclic population inversion. As a consequence a new highly non-linear “Rabi-Higgs” mode emerges[1]. Turning off the drive at different times and modulating the strength allows access to all known dynamical phases of the order parameter: persistent oscillations, oscillations with damping and overdamped dynamics. We discuss physical realizations of the drive and methods to detect the dynamics accessible with existing experimental setups[2-4]. [1] H. P. Ojeda Collado, J. Lorenzana, G. Usaj, and C. A. Balseiro, Phys. Rev. B 98, 214519 (2018). [2] B. Mansart, J. Lorenzana, A. Mann, A. Odeh, M. Scarongella, M. Chergui, and F. Carbone, PNAS 110, 4539 (2013). [3]R. Matsunaga, Y. I. Hamada, K. Makise, Y. Uzawa, H. Terai, Z. Wang, and R. Shimano, Phys. Rev. Lett. 111, 057002 (2013). [4]R. Matsunaga, N. Tsuji, H. Fujita, A. Sugioka, K. Makise, Y. Uzawa, H. Terai, Z. Wang, H. Aoki, and R. Shimano, Science 345, 1145 (2014).

Tu-3-5: Pressure induced visibility of the superconducting Higgs mode in 2H-TaS2 R. Grasset1, Y. Gallais1, A.Sacuto1, M. Cazayous1, S. Mañas-Valero2, E. Coronado2 and M.-A. Méasson1,3

1 Laboratoire Matériaux et Phénomènes Quantiques, Paris, France 2 Universidad de Valencia (ICMol), Paterna, Spain 3 Institut NEEL CNRS/UGA, Grenoble, France We explore the mechanism of observability that makes the Higgs mode of the superconducting state detectable by Raman spectroscopy based on the interplay between the charge-density-wave and superconducting states. By application of high pressure on the transition metal dichalcogenides 2H-TaS2, reaching a region of coexistence of both states, we unravel an intense and narrow sub-gap superconducting mode, attributed to a Higgs mode, coexisting with the expected incoherent Cooper-pair breaking signature [1]. Only the latter remains in the pure superconducting state reached above 8.5 GPa. Our report constitutes a new observation of such Raman active superconducting mode, attributed to a signature of the Higgs mode, since the longstanding unique case 2H-NbSe2. [1] ‘Pressure induced collapse of the charge density wave and Higgs mode visibility in 2H-TaS2’, R. Grasset, Y. Gallais, A. Sacuto, M. Cazayous, S. Mañas-Valero, E. Coronado, and M.-A. Méasson, Phys. Rev. Lett. 122, 127001 (2019)

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Tu-4-1: Implications of the charged phonon effect for the wo-phonon pairing mechanism in doped SrTiO3 D. van der Marel1, F. Barantani1 and C.W. Rischau1 1Department of Physics, University of Geneva, Quai Ernest-Ansermet 24, CH-1211 Geneva, Switzerland We addressed the 50 year old puzzle of the superconducting pairing interacting in doped SrTiO3. In recent publications Edge, Balatsky, Spaldin, and Kedem proposed that pairing is mediated by soft ferroelectric modes [1]. This was challenged in a recent comment Jonathan Ruhmann and Patrick Lee, who pointed out that these processes are symmetry forbidden [2]. Here we demonstrate that pairing mediated by soft ferroelectric modes is in fact possible, but involves two-phonon exchange processes as proposed by Ngai [3], which are allowed by symmetry. The coupling parameters are derived from the observed charged phonon effect in SrTiO3 [4], and are found to have the correct magnitude. [1] J. M. Edge, Y. Kedem, U. Aschauer, N. A. Spaldin, and A. V. Balatsky, Phys. Rev. Lett. 115, 247002 (2015). [2] J. Ruhman, and P. A. Lee, Phys. Rev. B 94, 224515 (2016). [3] K. L. Ngai, Phys. Rev. Lett. 32, 215 (1974). [4] D. van der Marel, F. Barantani and C.W. Rischau, arXiv:1903.08394 (2018).

Tu-4-2: High temperature superconductivity in FeSe-based materials Dung-Hai Lee1,2 1Department of Physics, University of California, Berkeley, CA 94720, USA. 2Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Raising the superconducting transition temperature to a point where applications are practical is one of the most important challenges in science. In this talk I aim at discussing insights gained on the Tc controlling factors for a particular high temperature superconductor family -- the FeSe-based superconductors. [1] Dung-Hai Lee, Annual Review of Condensed Matter Physics 9, 261-282 (2018).

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Tu-4-3: TBA Suchitra Sebastian Cambridge University, UK

Tu-4-4: Impact of magnetic fluctuations on optical response and superconductivity in 2D strongly correlated systems A. Kauch1, P. Pudleiner1, K. Astleithner1, T. Ribic1, F. Hörbinger1, G. Li2, and K. Held1 1Institute of Solid State Physics, TU Wien, Vienna, Austria 2School of Physical Science and Technology, ShanghaiTech University, Shanghai, China Thanks to new computational advances in parquet equations methods [1] we can unbiasedly study the interplay between fluctuations in the magnetic, density and pairing channels in 2D lattice models. The antiferromagnetic or chanrge density wave fluctuations constitute the dominant contribution to optical conductivity beyond the bubble part, forming new polaritons, which we called p-tons [2]. In the pairing channel the results confirm that strong antiferromagnetic fluctuations lead to d-wave pairing in the optimally doped regime of the Hubbard model on square lattice and point to a possible triplet s-wave pairing symmetry in the overdoped regime, where magnetic fluctuations become incommensurate [3]. The applied methods: parquet dynamical vertex (DGA) and parquet approximations allow for diagrammatic decomposition of the results and physical analysis of the contributions. [1] G. Li, A. Kauch, P. Pudleiner, and K. Held, arXiv:1708.07457 (Comp. Phys. Com. 2019). [2] A. Kauch, P. Pudleiner, K. Astleithner, T. Ribic, and K. Held, arXiv:1902.09342 (2019). [3] A. Kauch, F. Hörbinger, G. Li, and K. Held, arXiv:1901.09743 (2019).

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Wed-1-1: Density fluctuations in strange metals Peter Abbamonte University of Illinois, Urbana-Champaign Metals exhibit plasmon excitations, which are collective modes one can think of as sound waves in the electron density (as opposed to the atomic density). The so-called "strange metals" are bizarre phases of matter that fail to exhibit well-defined quasiparticles but somehow are still good conductors, leading one to wonder what degree of freedom is actually carrying the charge. A sensible question to ask is, Do strange metals exhibit plasmons? In this talk I will describe momentum-resolved EELS (M-EELS) measurements of several strange metals, notably Bi2Sr2CaCu2O8+x which is also a high-temperature superconductor. I will show that plasmon excitations are barely defined in these materials, which instead exhibit an incoherent continuum of charge fluctuations with no particular length or time scale. These fluctuations obey a simple, power law form, suggesting some kind of scale-invariant phase is present, though the data are not consistent with a (textbook) quantum critical point. I will discuss efforts by theorists to explain this phenomenon using holographic approaches based on the AdS-CFT correspondence. [1] M. Mitrano, et al., PNAS 115, 5392 (2018) [2] A. A. Husain, et al., arXiv:1903.04038

Wed-1-2 : Spin and charge correlations at cuprate interfaces B. Keimer Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany We report resonant elastic and inelastic x-ray scattering experiments on cuprate thin films and heterostructures grown by oxide molecular beam epitaxy on different substrates. In particular, we will discuss the influence of epitaxial strain [1] and interfacial charge transfer [2,3] on charge ordering in YBa2Cu3O6+x. We will also outline the results of a recent study of spin excitations in ultrathin La2-xSrxCuO4 films down to the monolayer limit, which elucidate the influence of the interfacial oxygen positions on the superexchange interactions. Interface effects thus offer fresh perspectives for the controlled manipulation of the interplay between magnetism, charge order, and superconductivity. [1] M. Bluschke, A. Frano, E. Schierle, D. Putzky, F. Ghorbani, R. Ortiz, H. Suzuki, G. Christiani, G. Logvenov, E. Weschke, R.J. Birgeneau, E.H. da Silva Neto, M. Minola, S. Blanco-Canosa, and B. Keimer, Nature Comm. 9, 2978 (2018). [2] D. Putzky et al., to be published. [3] A. Frano, S. Blanco-Canosa, E. Schierle, Y. Lu, M.Wu, M. Bluschke, M. Minola, G. Christiani, H. U. Habermeier, G. Logvenov, Y.Wang, P. A. van Aken, E. Benckiser, E.Weschke, M. Le Tacon, and B. Keimer, Nature Mater. 15, 831 (2016). [4] M. Minola et al., to be published.

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Wed-1-3: Charge fluctuations and electron phonon coupling in HTcS cuprates G. Ghiringhelli1,2 1 Dipartimento di Fisica, Politecnico di Milano, Italy 2 CNR/SPIN, Italy In the last 10 years, resonant inelastic soft x-ray scattering (RIXS) has decisively contributed to the assessment of the magnetic proper-ties [1,2,3,4] and to the discovery of charge density waves of high Tc superconducting cuprates [5,6]. I will review some of the most recent results of our group. Especially significant is the discovery of dynamic charge density fluctuations in underdoped and optimally doped Nd123 and Y123, that are directly related to the marginal Fermi liquid behavior of cuprates in the normal state [7]. And I will present a series of experiments aimed at extracting the momentum dependent electron-phonon coupling in cuprates directly from the RIXS spectra [8]. [1] L. Braicovich, et al, Phys. Rev. Lett. 104, 077002 (2010) [2] M. Le Tacon, et al, Nature Physics 7, 725 (2011) [3] YY Peng, et al, Nature Physics 13, 1201 (2017) [4] YY Peng, et al, Phys. Rev. B 14, 144507 (2018) [5] G Ghiringhelli, et al, Science 337, 821 (2012) [6] YY Peng, et al, Nature Materials 17, 697 (2018) [7] R Arpaia, et al, arXiv:1809.04949 [8] Matteo Rossi, et al, arXiv:1902.09163

Wed-1-4: Strain-Engineering Mott-Insulating La2CuO4 O. Ivashko1, M. Horio1, W. Wan2, N. B. Christensen2, D. E. McNally3, E. Paris3, Y. Tseng3,

N. E. Shaik4, H. M. Rønnow4, H. I. Wei5, C. Adamo6, C. Lichtensteiger7, M. Gibert1, M. R.

Beasley6, K. M. Shen5, J. M. Tomczak8, T. Schmitt3, and J. Chang1

1Physik-Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland 2Department of Physics, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark 3Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 4Institute of Physics, Ecole Polytechnique Fedérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland 5Department of Physics, Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York 14853, USA 6Department of Applied Physics, Stanford University, Stanford, CA 94305, USA 7Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest Ansermet, 1211 Geneva, Switzerland 8Institute of Solid State Physics, Vienna University of Technology, A-1040 Vienna, Austria

An outstanding challenge is to understand the optimal conditions for superconductivity in thin films? Here we demonstrate, by a combination of x-ray absorption and resonant inelastic x-ray scattering spectroscopy, how the Coulomb and magnetic-exchange interaction of La2CuO4 thin films can be enhanced by compressive strain. Our experiments and theoretical calculations establish that the substrate producing the largest Tc also generates the largest nearest neighbour hopping integral, Coulomb and magnetic-exchange interaction [1].

[1] O. Ivashko et al., Nature Communications 10, 786 (2019)

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Wed-1-5: Acoustic-like plasmons in high-temperature cuprate superconductors H. Yamase1, A. Greco2, and M. Bejas2 1National Institute for Materials Science, Tsukuba 305-0047, Japan 2Facultad de Ciencias Exactas, Ingeniería y Agrimensura and Instituto de Física Rosario (UNR-CONICET), Av. Pellegrini 250, 2000 Rosario, Argentina Employing the layered t-J model with the long-range Coulomb interaction, we show that charge excitation spectrum around the in-plane momentum transfer q equal to (0,0) is dominated by plasmon excitations [1,2]. When the out-of-plane momentum transfer qz is zero, we obtain the well-known optical plasmons observed by electron energy-loss spectroscopy (EELS). Once qz becomes finite, the optical plasmon energy drops around q=(0,0), leading to a V-shaped dispersion with a gap proportional to the interlayer hopping integral tz. This acoustic-like plasmon well describes the charge excitations around q=(0,0) recently observed by resonant inelastic x-ray scattering (RIXS) experiments in both electron- [3,4,5] and hole-doped cuprates [6]. [1] A. Greco, H. Yamase, and M. Bejas, Phys. Rev. B 94, 075139 (2016). [2] A. Greco, H. Yamase, and M. Bejas, Commun. Phys. 2, 3 (2019). [3] W. S. Lee et al., Nat. Phys. 10, 883 (2014). [4] K. Ishii et al., Nat Commun. 5, 3714 (2014). [5] M. Hepting et al., Nature 563, 374 (2018). [6] K. Ishii et al., Phys. Rev. B 96, 115148 (2017).

Wed-2-1: Continuous doping of cuprate surface: new insights from in-situ ARPES Hong Ding1, Yigui Zhong1 1Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Recently we developed a new technique of continuously doped surface of Bi2Sr2CaCu2O8+x through ozone/vacuum annealing and a systematic measurement over the nearly whole superconducting dome on a same sample surface by in-situ angle-resolved photoemission spectroscopy. We will report our comprehensive results on the doping evolution of Fermi surface [1], superconducting gap, pseudogap, quasiparticle weight [2], and dispersion kinks, which offer new insights to the physics of cuprate superconductivity. [1] Y. G. Zhong et al., Sci. China-Phys. Mech. Astron. 61, 127403 (2018). [2] Y. G. Zhong, et al., Physical Review B 98, 140507(R) (2018).

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Wed-2-2: Photoemission study on the phase diagram and low/high-energy electronic structure of electron-doped cuprates M. Horio1,2, C. Lin1, K. P. Kramer2, C. E. Matt2, J. Chang2, and A. Fujimori1 1Department of Physics, University of Tokyo, Hongo, Tokyo 113-0033, Japan 2Physik-Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland In recent years, the phase diagram of cuprates especially on the electron-doped side has attracted renewed interest since superconductivity is realized even in nominally undoped thin films. To establish the phase diagram of these new types of electron-doped cuprates, we have performed (angle-resolved) photoemission spectroscopy [(AR)PES] measurements on single crystals and thin films. From Fermi surface area and chemical-potential shifts, we found that the reduction annealing, which is indispensable to induce superconductivity, can considerably vary the electron concentration. The resultant phase diagram thus differs from the conventionally accepted one with a small superconducting dome, but rather resembles that of the hole-doped counterpart. Further detailed ARPES studies on the electron-doped cuprates suggest the common origin of the pseudogap as well as distinct high-energy (2-3 eV) band structure between the hole- and electron-doped cuprates. The present results [1,2] thus shed new light on the common/specific aspects of the cuprate superconductors.

[1] M. Horio and A. Fujimori, J. Phys.: Condens. Matter 30, 503001 (2018). [2] K. P. Kramer et al., arXiv:1903.00301.

Wed-2-3: Hidden self-energy structures of high-temperature cuprate superconductors Y. Yamaji1 1Department of Applied Physics, the University of Tokyo, Hongo, Tokyo 113-8656, Japan How electrons are mutually interacting is the key to identify the origin of superconductivity. We are, however, able to observe motion of electrons only after projection onto experimentally accessible degrees of freedom. In this study, the Boltzmann machine learning is examined to extract physical quantities hidden in experimental data. The method is applied to a small set of the angle-resolved photoemission spectroscopy (ARPES) spectra of cuprate superconductors to extract the normal and anomalous contributions in the self-energy separately. We reveal that prominent peak structures emerge both in the normal and anomalous self-energies, which are canceled in the total self-energy and hence invisible in experiments including ARPES. The revealed peaks make dominant contribution to the superconducting gap and provide the decisive testimony for the origin of the superconductivity [1]. [1] Y. Yamaji, T. Yoshida, A. Fujimori, and M. Imada, arXiv:1903.08060.

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Wed-2-4: Unveiling the relation between short-range spin-correlations and the pseudogap in electron-doped cuprates F. Boschini1, M. Zonno1, E. Razzoli1, R. P. Day1, M. Michiardi1, E. H. Da Silva Neto2, S. Zhdanovich1, A. K. Mills1, G. Levy1, A. Erb3, C. Giannetti4, D. J. Jones1, and A. Damascelli1 1Quantum Matter Institute, University of British Columbia, Vancouver, Canada 2Department of Physics, University of California, Davis, USA 3Walther-Meißner-Institute for Low Temperature Research, Garching, Germany 4Dipartimento di Matematica e Fisica, Universita Cattolica del Sacro Cuore, Brescia, Italy The phase diagram of copper-oxides hosts intertwined phases as disparate as high-temperature superconductivity, charge order and the pseudogap. In particular, the pseudogap is a puzzling phenomenon of correlated matter where the interplay between different orders is believed to assume a critical role. Here we apply ultrafast angle-resolved photoemission spectroscopy (ARPES) [1] to study the temperature evolution of the low-energy density of states in the electron-doped cuprate Nd2-xCexCuO4. We reveal the direct relation between the momentum-resolved pseudogap and the spin-correlation length, establishing the pseudogap in electron-doped cuprates as a precursor to the incipient antiferromagnetic order even when long-range antiferromagnetic correlations are not settled, as in the case of optimal doping [2]. [1] Boschini et al., Nature Materials 17, 416-420 (2018) [2] Boschini*, Zonno* et al., arXiv:1812.07583 (2018)

Wed-2-5: Three observations on the ARPES data of the cuprate superconductors - what the AF spin fluctuation can do and what it cannot Tao Li1 1Department of physics, Renmin University of China, Beijing, China We discuss the origin of the pseudogap phenomena in the cuprate superconductors from the perspective of the spin-Fermion model. We find that the vanishing of the pseudogap around

($,0) as observed recently in Pr1.3−xLa0.7CexCuO4 is consistent with the AF band folding picture of the pseudogap in the electron-doped cuprates, if we assume a strongly momentum dependent quasi-particle scattering rate on the Fermi surface[1,4]. However, we find that the pseudogap in the hole-doped cuprates is unlikely an AF band folding gap. In particular, we show that electron pairing is indispensable to eliminate the Fermi level crossing along ($,0)-($,$) in a way consistent with the ARPES observation on the underdoped Bi-2201 system around T*[2,4]. Nevertheless, we find that the AF spin fluctuation in the hole-doped cuprates is responsible for the high energy hump structure, the mismatch between the hump back-bending momentum and the bare Fermi momentum, and in particular, the extremely flatness of the anti-nodal quasi-particle dispersion in the superconducting state [3,4]. [1] Hario et al, arXiv: 1801.04147. [2] Hashimoto et al, Nat. Phys. 6, 414 (2010). [3] R.H. He et al, Science 331, 1579 (2011). [4] Tao Li and Da-Wei Yao, arXiv: 1803.08226(EPL,124,47001,2018); arXiv:1805.05530; arXiv:1805.04883.

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Th-1-1: High-temperature superconductivity in a new type of copper oxide S. Uchida1,2 1Institute of Physics, Chinese Academy of Sciences, Beijing, China 2Department of Physics, University of Tokyo, Japan A new high-Tc cuprate with a simple chemical formula Ba2CuO4-y has been synthesized at high pressure (~ 18 GPa). In this new cuprate superconductivity (Tc = 73 K) occurs in the Cu-O plane with enormous oxygen vacancies (nominal composition Ba2CuO3.2) and with very high doped hole density (p ~ 0.4) , and the apical-O is located very close to the plane. These characteristics had been thought to be detrimental to high-Tc. The discovery of Ba2CuO4-y opens a new road toward higher Tc. A possible structure of the oxygen-deficient Cu-O plane and a way to further enhance Tc will be discussed. The present work has been performed in collaboration with C.Q Jin, W.M. Li, L.P. Cao, J.F. Zhao, Y. Liu, Q.Q. Liu, (IOP, Beijing), Z. Hu (MPI, Dreseden), Q.Z. Huang, H. Wu (NIST, Gaithersburg), H.J. Lin, C.T. Chen (NSRRC, Hsinchu), Z. Li (Nanjing University of Science and Technology), Z.Z. Gong, Z. Guguchia, Y.J. Uemura (Columbia University), J.S. Kim, G. Stewart (University of Florida), S.D. Conradson, and T.H. Geballe (Stanford University).

Th-1-2: The magnon-mediated attraction between two holes doped in a CuO2 layer M.M. Moller1,2, G.A. Sawatzky1,2, and M. Berciu1,2 1Stewart Blusson Quantum Matter Institute, Vancouver BC V6T 1Z4, Canada 2Department of Physics, University of British Columbia, Vancouver BC V6T 1Z1, Canada We study a realistic three-band model for two holes doped into a CuO2 layer, using a variational method that allows us to turn on/off the exchange of magnons between the holes. This enables us to verify that the magnon-mediated effective hole-hole interaction is attractive and could therefore indeed be (part of) the superconducting glue. We derive its analytical expression and show that it consists of a novel kind of pair-hopping+spin-exchange terms. The coupling constant is fitted to numerical results obtained with the variational exact diagonalization. For realistic parameter values, this effective interaction is borderline strong enough to bind the holes into a pre-formed pair.

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Th-1-3: The Superconductivity of YBa2Cu3O6.67 Under Uniaxial Stress M. E. Barber1, M. Konczykowski2, T. Loew3, J. Porras3, B. Keimer3, M. Le Tacon4, A. P. Mackenzie1, C. W. Hicks1 1Max Planck Institute for Chemical Physics of Solids, Dresden 01187, Germany 2Laboratoire des Solides Irradiés, Ecole Polytechnique, 91228 Palaiseau cedex, France 3Max Planck Institute for Solid State Research, D-79569 Stuttgart, Germany 4Karlsruhe Institute of Technology, Institute for Solid State Physics, D-76021 Karlsruhe, Germany

The superconductor YBa2Cu3O6.67 has Tc = 65 K, and also has a strong tendency towards competing charge density wave order. Under strong uniaxial compression along the a axis, which increases the orthorhombicity of the CuO2 planes, Tc is reduced and quasi-long-range, three-dimensional CDW order is stabilised [1]. Here, we present the uniaxial stress dependence

of Tc of YBa2Cu3O6.67 for uniaxial stresses sa of up to ~2 GPa, using a Hall probe susceptometer to enable high-precision measurements on a short length scale. Tc is suppressed

to ~40 K, and the form of the Tc(sa) curve is consistent with strong competition with CDW order. [1] H.-H. Kim et al, Science 362, 1040 (2018).

Th-1-4: High Magnetic Field Investigation of High-Tc Superconductors C. M. Moir1, Scott C. Riggs2, J.A. Galvis2, X. Lian2,3, P. Giraldo-Gallo2, Jiun-Haw Chu4, P. Walmsley5, Ian R. Fisher5,6, Arkady Shekhter2, and G.S. Boebinger2

1Central Research Institute for Energy Power Industry, Yokosuka, Japan 2 National High Magnetic Field Laboratory, Florida State University,USA 3Department of Physics, Florida State University, USA 4 University of Washington, Seattle, USA 5 Stanford University, Stanford, USA 6 SLAC National Accelerator Laboratory, USA By applying magnetic fields, up to 35 T, we are able to suppress superconductivity and reveal the normal state of the iron-based high-temperature superconductor, BaFe2(As1-xPx)2. We measure the specific heat and resistivity of BaFe2(As1-xPx)2 and observe √H behavior indicating a nodal superconducting gap, saturation of the heat capacity at a magnetic field corresponding to the onset of the normal state, and enhancement of the quasiparticle mass sum as calculated from electronic specific heat coefficient as optimal doping is approached [1]. Together, our observations form a consistent treatment of specific heat measurements in high-temperature superconductors which can be applied to materials such as La2-xSrxCuO4.

[1] C. M. Moir et. al. npj Quantum Mat. 4, 8 (2019).

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Th-1-5: Anisotropic superconductivity in organic conductor λ-(BETS)2GaCl4 H. Aizawa1, T. Koretsune2, K. Kuroki3, and H. Seo4,5

1Institute of Physics, Kanagawa University, Yokohama, Kanagawa 221-8686, Japan 2Department of Physics, Tohoku University, Sendai 980-8578, Japan 3Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan 4Condensed Matter Theory Laboratory, RIKEN, Wako, Saitama 351-0198, Japan 5Center for Emergent Matter Science (CEMS), RIKEN, Wako, Saitama 351-0198, Japan Quasi-two-dimensional molecular conductor λ-(BETS)2GaCl4 shows superconductivity (SC) below 5.5 K, neighboring the dimer-type Mott insulating phase. Recently experiments have been carried out to investigate its SC gap function, but the results are controversial [1]. Motivated by this situation, we performed first-principles band calculation to derive a four-band tight-binding model, and treated the Hubbard model analyzed within the random phase approximation [2]. The results show that the SC gap changes its sign four times along the Fermi surface in the unfolded Brillouin zone, suggestive of a d-wave-like SC gap, but which only has the two-fold symmetry because of the low symmetry of the crystal structure. Implications to the experiments will be discussed. [1] S. Imajo et al., J. Phys. Soc. Jpn. 85, 043705 (2016); D. P. Sari, Ph. D. thesis, Osaka University (2018). [2] H. Aizawa, T. Koretsune, K. Kuroki, and H. Seo, J. Phys. Soc. Jpn. 87, 093701 (2018).

Th-2-1: What Makes Cuprate Superconductors so Exceptional? Ivan Božović1,2 1Brookhaven National Laboratory, Upton NY 11973, USA 2Applied Physics Department, Yale University, New Haven, CT 06520, USA An account will be given of a comprehensive study of over 2,000 single-crystal LSCO films grown by ALL-MBE, over the course of 13 years. The key parameters of the normal and

superconducting states — r, RH, magnetoresistance, Tc, l, x — were measured precisely as a function of temperature T (down to 300 mK), magnetic field B (up to 90 T), doping, and in-

plane azimuth angle f. [1-3] The key findings are as follows. (i) The superconducting phase stiffness is extremely low, comparable to Tc. (ii) The superfluid density Ns(T) decreases linearly with T, up to Tc. (iii) Tc scales linearly with Ns0 except near the dome edges where it scales as √Ns0. (iv) The superconducting state develops from an electronic nematic state that breaks the C4 symmetry of the lattice. (v) The electron fluid behaves as if it were comprised of two

components, one FL-like and the other showing r linear in T and B; the later component diminishes and disappears with increased doping, together with the nematicity, Ns0, and Tc. (vi) Pairs are detected directly way outside the superconducting gap region. [1] I. Božović, X. He, J. Wu and A. T. Bollinger, Nature 536, 309 (2016). [2] J. Wu, A. T. Bollinger, X. He and I. Božović, Nature 547, 432 (2017). [3] P. Girardo-Gallo et al., Science 361, 479 (2018).

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Th-2-2: Giant thermal Hall conductivity in the pseudogap phase of cuprates G. Grissonnanche1, A. Legros1, S. Badoux1, E. Lefrancois1, V. Zatko1, M. Lizaire1, F. Laliberté1, A. Gourgout1, J.-S. Zhou3, S. Pyon4, T. Takayama4, H. Takagi4, S. Ono5, N. Doiron-Leyraud1, L. Taillefer1

1 Université de Sherbrooke, Sherbrooke, QC, Canada 3 University of Texas - Austin, Austin, TX, USA 4 University of Tokyo, Tokyo, Japan 5 Central Research Institute of Electric Power Industry, Tokyo, Japan The nature of the pseudogap phase of cuprates remains a major puzzle. Although there are indications that this phase breaks various symmetries, there is no consensus on its fundamental nature [1]. Here we report measurements of the thermal Hall conductivity κxy in the normal state of four different cuprates and show that a large negative κxy signal is a property of the pseudogap phase, appearing with the onset of that phase at the critical doping p* [2]. Since it is not due to charge carriers – as it persists when the material becomes an insulator, at low doping – or magnons – as it exists in the absence of magnetic order – or phonons – since skew scattering is very weak, we attribute this κxy signal to exotic excitations, presumably with spin chirality. The thermal Hall conductivity in the pseudogap phase of cuprates is reminiscent of that found in insulators with spin-liquid states [3]. [1] Keimer, B. et al. Nature 518, 179-186 (2015). [2] G. Grissonnanche et al. arXiv:1901.03104 (2019). [3] Y. Kasahara et al. Nature 559, 227 (2018).

Th-2-3: Loop currents in superconducting cuprates and iridates Philippe Bourges1 1 Laboratoire Léon Brillouin-Orphée, CEA Saclay, France Polarized neutron diffraction has revealed the existence of an ordered magnetic phase, hidden inside the pseudo-gap state of underdoped copper oxides [1], which matches the pseudo-gap phase. The magnetic order can be described as an Intra-Unit-cell (IUC) magnetic order, breaking time-reversal and C4 rotational symmetries. It corresponds to the broken symmetry predicted in the loop current theory where staggered current loops give rise to orbital-like magnetic moments within the CuO2 unit cell. Recently, polarized neutron investigation in spin ladder compound reveal short range magnetism with the same symmetry as the IUC order [2]. We also reported similar magnetic signal in the antiferromagnetic iridates system Sr2(Ir,Rh)O4 [3] below a temperature different from TN suggesting the existence of loop-current electronic states in different oxides. These observations will be discussed in light of loop current as well as magneto-electric quadrupoles models. [1] Ph. Bourges et al, Phys. Rev. B 98, 016501 (2018); Yang Tang et al, Phys. Rev. B 98, 214418 (2018). [2] D. Bounoua et al, to be published (2019). [3] Jaehong Jeong et al, Nat. Comms., 8, 15119 (2017).

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Th-2-4: Development of ferromagnetic fluctuations in heavily overdoped Bi-2201 cuprates T. Adachi1, K. Kurashima2, S. Onishi1, M. Harada1, K. Kawabata1, H. Kuwahara1, H. Kuroe1, K. M. Suzuki3, Y. Fukunaga2, T. Kawamata2, T. Noji2, Y. Koike2, H. Miyasaka3, I. Watanabe4, M. Miyazaki5, A. Koda6, and R. Kadono6 1Dept. of Eng. and Appl. Sci., Sophia University, Kioi-cho, Tokyo 102-8554, Japan 2Dept. of Appl. Phys., Tohoku University, Aoba Aramaki, Sendai 980-8579, Japan 3Institute for Materials Research, Tohoku University, Katahira, Sendai 980-8577, Japan 4Nishina Center for Accelerator-Based Science, RIKEN, Hirosawa, Wako 351-0198, Japan 5Grad. Sch. of Eng., Muroran Institute of Technology, Mizumoto, Muroran 050-8585, Japan 6Institute of Materials Structure Science, KEK, Oho, Tsukuba 305-0801, Japan In order to investigate the details of ferromagnetic fluctuations suggested theoretically [1] and experimentally [2] in the heavily overdoped cuprates, we have performed measurements of electrical resistivity, magnetization, specific heat, muon spin relaxation using heavily overdoped single crystals of Bi-2201 cuprates [3]. It has been concluded that the two-dimensional ferromagnetic fluctuations exist in the heavily overdoped regime of Bi-2201, suggesting the universality of ferromagnetic fluctuations in heavily overdoped cuprates. [1] A. Kopp et al., Proc. Natl. Acad. Sci. USA 104, 6123 (2007). [2] J. E. Sonier et al., Proc. Natl. Acad. Sci. USA 107, 17131 (2010). [3] K. Kurashima et al., Phys. Rev. Lett. 121, 057002 (2018).

Th-2-5: Charge trapping and super-Poissonian noise centers in a cuprate superconductor KM Bastiaans1, D Cho1, T Benschop1, I Battisti1, D Chatzopoulos1, Y Huang2, MS Golden2, Q Dong3, Y Jin3, J Zaanen1, MP Allan1 1 Leiden Institute of Physics, Leiden University, 2333 CA Leiden, The Netherlands 2 Institute of Physics, University of Amsterdam, 1098 XH Amsterdam, The Netherland 3 CNRS, Univ. Paris-Sud, Univ. Paris-Saclay, C2N – Marcoussis, 91460 Marcoussis France I will present evidence for highly polarizable insulator properties in a cuprate high-temperature superconductor through novel, atomic-scale noise spectroscopy. Our results indicate that charge can be trapped on macroscopic timescales in the cuprates and provide new insight into the mystery of their highly anisotropic transport characteristics. We employ our newly developed scanning noise spectroscopy technique [1] and discover surprising deviations from the expected Poissonian noise of uncorrelated electrons. Such behavior can only happen in highly polarizable insulators and represents strong evidence for trapping of charge in the charge reservoir layers of the cuprates [2]. I will discuss how this connects to the physics of the cuprates including the c-axis transport characteristics. [1] KM Bastiaans et al. Rev. Sci. Instrum. 89, 093709 (2018), [2] KM Bastiaans, D Cho et al. Nature Physics 14, 1183 (2018)

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Th-3-1: Femtosecond electron-phonon lock-in via photoemission and x-ray free-electron laser Zhi-Xun Shen1,2 1Departments of Physics and Applied Physics, Stanford University 2Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory High-temperature superconductivity in cupper and iron based materials, with critical temperature well above what was anticipated by the conventional theory, remains a major unsolved physics problem today. The challenge of this problem is symbolized by a complex phase diagram consists of intertwined states with unusual properties in addition to unconventional superconductivity. These states are different manifestations of the same underlying physical system consists of multiple degrees of freedom, making an integrated understanding a necessity. In this talk, we present experimental evidence for electron-spin and electron-phonon interactions in these superconductors. We then discuss a general theme of cooperative enhancement and positive feedback loop of different interactions exemplified by electron-electron and electron-phonon interactions [1-2]. In particular, we focus on the capabilities from newly developed ultrafast spectroscopy and diffraction capabilities from x-ray free electron laser (XFEL). In such multimodal experiments, the precision measurements of electron’s energy, momentum and time dynamics provide evidence for cooperative interplay of different degrees of freedom as a route to increase the effective interactions, and the resulting superconducting properties. We will also provide some general outlook for the future application of ultrafast technique for condensed matter and materials physics research. [1] J.J. Lee et al., Nature 515 (2014) [2] S. Gerber et al., Science 357 (2017)

Th-3-2: Laser ARPES studies of the nematic superconductor FeSe Kozo Okazaki Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan The iron-based superconductor FeSe has been intensively studied because of its nematic

electronic state without any long-range magnetic order. From the aspect of the mechanism of its superconductivity, several controversial results had been reported whether FeSe has line nodes in the superconducting (SC) gap or not. Determination of the positions of the SC gap nodes is important for clarifying the SC mechanism. In this talk, I will show the results for the SC gap measurements of the zone-centered hole FS based on the laser-excited ARPES measurements of single-crystal FeSe [1]. We find that while the SC gap node is not observed for multi-domain samples, it exists at the vertices of the major axis of the elliptical FS for single-domain sample. This can be attributed to breaking of time-reversal symmetry at the twin boundaries and our results reveal the effects of time-reversal symmetry breaking on the nodal SC gap anisotropy. Also, FeSe is known as significant increase of Tc can been achieved with various external stimuli. In order to investigate the electronic response of FeSe to photo-excitation, we have performed time-resolved ARPES using extreme ultraviolet laser [2]. I will also talk about the result for the time-resolved measurements. [1] T. Hashimoto, K.Okazaki et al., Nat. Commun. 9, 282 (2018). [2] T. Suzuki, K.Okazaki et al., submitted.

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Th-3-3: To unravel the gap sign in unconventional superconductors by phase-referenced quasiparticle interference Hai-Hu Wen Department of Physics, Nanjing University, China Quasiparticle interference (QPI) technique has been widely used in determining the Fermi

surface contour in superconductors. The Fourier transformed QPI intensity r(q,E) contains

both the message of magnitude and phase at each energy and vector, namely r(q,E) = |r0(q,E)|

exp(-ij). The referenced phase difference at positive and negative energies reflects the gap

structure. STS measurements and QPI analysis in (Li1-xFex)OHFeSe (Tc»40 K) without hole pocket indicates the presence of two sets of Fermi surfaces and double gaps [1], and the gap sign reversal [2,3]. Practice on Fe(Te,Se) with well defined hole and electron pockets[4] also receives a positive echo. In Bi-2212, our results illustrate nice consistency between the expectation of a d-wave gap[5]. [1] Zengyi Du, Huan Yang, Hai-Hu Wen et al., Nature Commun. 7, 10565(2016). [2] Zengyi Du, Hai-Hu Wen et al., Nature Physics 14, 134 (2018). [3] Qiangqiang Gu, Huan Yang, Hai-Hu Wen et al., Phys. Rev. B 98, 134503 (2018). [4] Mingyang Chen, Huan Yang, Hai-Hu Wen et al., Phys. Rev. B 99, 014507 (2019). [5] Qiangqiang Gu, Huan Yang, Hai-Hu Wen et al., Nature Commun. 10, 1603(2019).

Th-3-4: Fermi surface with Dirac fermions in an iron-based superconductor parent compound CaFeAsF via quantum oscillation measurements Taichi Terashima,1 Hishiro T. Hirose,1 David Graf,2 Yonghui Ma,3 Gang Mu,3 Tao Hu,3 Katsuhiro Suzuki,4 Shinya Uji,1 and Hiroaki Ikeda5 1National Institute for Materials Science, Tsukuba 305-0003, Japan 2National High Magnetic Field Laboratory, Florida State University, Tallahassee 32310, USA 3State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, and CAS Center for Excellence in Superconducting Electronics (CENSE), Shanghai 200050, China 4Research Organization of Science and Technology, Ritsumeikan University, Kusatsu 525-8577, Japan 5Department of Physics, Ritsumeikan University, Kusatsu 525-8577, Japan We have performed comprehensive quantum-oscillation studies of a 1111-type iron arsenide parent compound CaFeAsF [1, 2] in the antiferromagnetic phase. We show that the Fermi surface consists of one hole and a symmetry-related pair of electron cylinders, and that the carrier density is extremely small, of the order of 10-3 per Fe. From analyses of the oscillation phase we further show that the electron cylinders carry a nontrivial Berry phase: that is, the electrons are Dirac fermions. [1] Y. Ma, et al., Supercond. Sci. Technol. 28, 085008 (2015). [1] T. Terashima et al., Phys. Rev. X 8, 011014 (2018).

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Th-3-5: Novel spin fluctuations with gap at low energies in heavily electron-doped Fe-pnictides K. Suzuki, M. Yashima1, T. Shiota1, H. Mukuda1, M. Uekubo2, H. Tsuji2, S. Miyasaka2, and S. Tajima2 1 Graduate school of engineering science, Osaka university, Osaka 560-8531, Japan 2 Graduate school of science, Osaka university, Osaka 560-0043, Japan High temperature superconductivity (SC) in Fe-based compounds appears frequently on the large electron Fermi surfaces [1,2]. We have performed systematic 31P-NMR measurements on heavily electron-doped state of LaFe(As1-xPx)(O1-yHy), which provides a good opportunity to unravel an important factor for superconductivity in Fe-pnictides since the SC transition temperature (Tc) exhibits a nonmonotonic variation with x and y [2-4]. As a result, we observed gap-like behavior at low energies in antiferromagnetic spin fluctuations (AFMSFs), which becomes small from x=1 (non-SC) to x=0 (SC). It may be related to that the incipient hole band derived from dxy oribital approaches Fermi level toward x=0. This is in contrast with the previous results on lightly-electron-doped LaFe(As1-xPx)(O1-yFy), where the AFMSFs are significant at low energies in association with the enhancement of Tc[4]. [1] G. –F. Ge et al., Nature Mater. 14, 285 (2015). [2] S. Iimura et al., Nat. Commun. 3, 943 (2012). [3] S. Miyasaka et al., Phys. Rev. B 95, 214515 (2017). [4] T. Shiota et al., J. Phys. Soc. Jpn. 85, 053706 (2016).

Th-4-1: Topological Superconductivity and associated Time Reversal Symmetry Breaking in the FeTe1-xSex family Peter D. Johnson1, N. Zaki,1 J.D. Rameau,1 G.D. Gu1 and M. Weinert2

1Condensed Matter Physics and Materials Science Division, Brookhaven National Laboratory, Upton, NY 11973 USA

2Department of Physics, University of Wisconsin-Milwaukee, WI, USA

Laser-based ARPES with variable light polarization offers a powerful probe of the electronic structure near the center of the Brillouin zone. Here the technique is used to examine the Fe-based superconductor family, FeTe1-xSex. At the zone center we observe the presence of Dirac cones with helical spin structure as expected for topological surface states and as previously reported in the related FeTe0.55Se0.45.[1] These studies are compared with theoretical studies that take account of the disordered local magnetic moments related to the paramagnetism observed in this system. Indeed including the magnetic contributions in the theoretical description is necessary to bring the chemical potential of the calculated electronic band structure into alignment with experimental observation. In the bulk superconducting state for FeTe0.7Se0.3 the system appears to reflect the presence of some level of orbital selectivity in the pairing even though no structural transition occurs at the transition temperature Tc. At the same time the topological state appears to acquire mass below Tc.

[1] P. Zhang et al., Science 360, 182 (2018)

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Th-4-2: Electronic structure of topological-superconductor candidates studied by ARPES Takafumi Sato1,2,3

1 WPI Research Center, AIMR, Tohoku University, Sendai 980-8577, Japan 2Department of Physics, Tohoku University, Sendai 980-8578, Japan 3 Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan Topological superconductors manifest a novel quantum state of matter where the nontrivial topology of the bulk leads to the emergence of Majorana femrions. Although various heterostructures consisting of BCS superconductors as well as doped topological insulators were intensively investigated, Majorana fermions are still elusive. We present possible realization of topological superconductivity at high temperatures in a hybrid of Bi(110) ultrathin film and cuprate superconductor Bi2Sr2CaCu2O8 (Bi2212) [1]. Using ARPES and STM, we fond that this hybrid exhibits a proximity-effect-induced s-wave energy gap as large as 7.5 meV which persists up to 85 K. We suggest that this system is a suitable platform for exploring Majorana fermions at high temperatures.

This work has been done in collaboration with N. Shimamura, K. Sugawara, S. Souma, K. Nakayama, C. X. Trang, T. Noji, Y. Koike, T. Takahashi (Tohoku Univ.), K. Yamauchi, T.Oguchi (Osaka Univ.), K. Kudo (Okayama Univ.), S. Sucharitakul, K. Iwaya, and T. Hanaguri (RIKEN). [1] N. Shimamura et al., ACS Nano 12, 10977 (2018).

Th-4-3: A New Pairing Mechanism in the Market Tanmoy Das1, Priyo Adhikary1 1Department of Physics, Indian Institute of Science, Bangalore 560012, India One of the fascinating facts about superconductivity is that despite more than 100 years of extensive research and 7 Nobel prizes, we are still away from our main goals. The race continues to both achieving room temperature superconductivity as well as to obtain a novel theory of attractive potential between electrons. By now we know two mechanisms for attractive interaction between the same-charge fermions: Meson mediates an attraction between protons. Phonon mediates attractive potential between electrons with superconductivity. In this talk, I will present a new mechanism of attractive potential between electrons, forming superconductivity. The basic principle is as follows. The flat bands or f-orbitals have strong Coulomb interaction, which prohibit double occupancy. However, the unoccupied f-site can be occupied by a conduction electron since the presence of valence fluctuation channel allows mutation between the f- and conduction electrons. We show that the doubly occupied state with f- and conduction electrons condensates like a Cooper pair. I will present this theory along with detailed comparison with recent experimental surprises of conventional superconductivity in heavy-fermion materials where decades old studies predicted unconventional superconductivity.

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Th-4-4: d-vector direction and transition of gap symmetry in topological superconductors CuxBi2Se3 Guo-qing Zheng1 1Department of Physics, Okayama University, Okayama 700-8530, Japan

The observation of spontaneous spin-rotational symmetry breaking in the doped topological insulator CuxBi2Se3 established spin-triplet, odd-parity superconducting state of this material [1]. However, the pinning mechanism of the vector order parameter d and the gap symmetry are still unclear. We have grown new single crystals by electrochemical doping, and measured the 77Se nuclear magnetic resonance spectra and the angle-resolved upper critical field (Hc2) in the hexagonal plane. For samples with x = 0.28, 0.36 and 0.37, Hc2 shows a large in-plane anisotropy that has a two-fold symmetry. However, the angle at which the Hc2 becomes minimal is different by 90◦ among the samples. The results indicate that the d-vector direction is different for each sample due to a different local structure. The Knight shift for x = 0.46 and 0.54 increases substantially compared to x < 0.37, indicating more carriers in these two samples. We present evidence for the gap symmetry transition as carrier increases. Work done in collaboration with T. Kawai, Y. Kandori, Y. Honoki, K. Matano, T. Kambe. [1] K. Matano, M. Kriener, K. Segawa, Y. Ando, and G.-q. Zheng, Nat. Phys. 12, 852 (2016).

Fr-1-1: Starfish-shaped Cooper pairs with ultrashort antinodal length scales across all doping levels in cuprate superconductors Dan Dessau, Department of Physics, University of Colorado, USA

We access the fully causal electronic self-energy utilizing a brand new 2-dimensional method of ARPES analysis [1], which removes the critical limitations of the previous one-dimensional MDC (Momentum Distribution Curve) and EDC (Energy Distribution Curve) methods. This new method, which utilizes orders-of-magnitude fewer parameters than the MDC and EDC methods, brings in the energy, momentum, and temperature -dependence of the self-energies. The full set of parameters we access allows us to make the first direct measurements of the shape and size of the pairs of a cuprate superconductor (Bi2Sr2CaCu2O8), which we find to be starfish-shaped with long arms along the node and a very short body along the antinodes [2]. We find the center of mass antinodal length scale (pair size) to be ~ 4.5Å for all doping levels – a surprising finding considering how dramatically the other main parameters vary with doping.

1. Haoxiang Li, Xiaoqing Zhou, Stephen Parham, Theodore J. Reber, Helmuth Berger, Gerald Arnold, Daniel S. Desssau, Nature Communications 9, 26 (2018)

2. Haoxiang Li, Xiaoqing Zhou, S. Parham, Kyle N. Gordon, R. D. Zhong, J. Schneeloch, G. D. Gu, Y. Huang, H. Berger, G. B. Arnold, D. S. Dessau, arXiv:1809.02194

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Fr-1-2: Non-Fermi Liquid Behaviors, Nodal Gap and Insulating Parent Phase in Iron-Based Superconductors Xingjiang ZHOU Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China We have studied electronic structure of iron-based superconductors by angle-resolved photoemission spectroscopy. In this talk, I will introduce three of our recent work. (1). Emergence of superconductivity from fully incoherent normal state in optimally-doped

(Ba0.6K0.4)Fe2As2 superconductor (Tc~38 K)[�]. We find that, while sharp superconducting

coherence peaks emerge in the superconducting state on the hole-like Fermi surface sheets around the zone center, no quasiparticle peak is present in the normal state. Its electronic behaviors deviate strongly from a Fermi liquid system; (2). Orbital origin of robust nodal superconducting gap in the nematic state of bulk FeSe superconductor (Tc=8~9K) [2]. We reveal highly anisotropic Fermi surface and extremely anisotropic superconducting gap in the nematic

state of the FeSe superconductor that are robust against nematic domains and disorder�(3). We

find that the parent phase of the single-layer FeSe/SrTiO3 films is insulating, and its doping evolution is very similar to doping a Mott insulator in cuprate superconductors.

[1]. Jianwei Huang, X. J. Zhou et al., Science Bulletin 64, 11 (2019);

[2]. Defa Liu, X. J. Zhou et al., Physical Review X 8, 031033 (2018);

Fr-1-3: Electronic structure of multilayered high-Tc copper oxide superconductors

Takeshi Kondo ISSP, the University of Tokyo, Kashiwa, Chiba 277-0882, Japan

The Tc value in cuprates is sensitive to the number of CuO2 layers per unit cell, and it is

maximized in triple-layer systems. Significantly, the cuprates are categorized to two kinds according to the chemical situation of CuO2 layers in crystal. One is single- and double-layer systems, where the CuO2 plane is adjacent to the dopant layer, which possesses random atomic vacancies, thus generally causes spatially inhomogeneous state in the underling conduction sheet. The situation is changed in the triple and more layered systems (the second category), where inner CuO2 planes are sandwiched by outer CuO2 planes, thus protected from the outermost dopant layers. The cleanness in CuO2 plane seems to get improved with increasing the number of CuO2 plane per unit cell [1]. In this study, we have particularly selected a five-layered system with lightly doped inner CuO2 planes, which are ideally flat and homogeneously hole doped, thus provide an excellent platform to unveil inhere properties of the lightly doped electronic state in cuprates. The investigation of this compound is especially significant since the electronic properties would share those of triple-layer systems, which commonly have the highest Tc in homologous series of cuprate families. I will present recent results of multilayered cuprates investigated by laer-based angle-resolved photoemission spectroscopy (laser-ARPES).

[1] H. Mukuda et al., J. Phys. Soc. Jpn. 81 (2012) 011008.

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Fr-1-4: Multiple topological states and topological superconductivity in iron-based superconductors Peng Zhang 1 1Institute for Solid State Physics, The University of Tokyo, Japan

Topological insulators/semimetals and unconventional iron-based superconductors have attracted major recent attentions in condensed matter physics. The combination of topological bands and superconducting states will produce more exotic topologically superconducting states and Majorana states. With the progress in laser-based high-resolution photoemission spectroscopy, we resolved the topological insulator (TI) bands and topological Dirac semimetal (TDS) bands near Fermi level in the iron-based superconductors Fe(Te,Se) and Li(Fe,Co)As. In the superconducting state, these multiple topological states will further lead to different topologically superconducting states, which can be separately tuned to Fermi level by carrier doping, allowing a detailed study of these states. Our results provide a potential high-Tc platform for the study of topological superconductivity and Majorana states.

[1] P Zhang et al, APL 105, 172601 (2014) [2] Z Wang, P Zhang et al, PRB 92, 115119 (2015) [3] P Zhang et al, Science 360, 182 (2018) [4] P Zhang et al, Nature Physics 15, 41 (2019)

Fr-1-5: Fermi arcs, nodal and antinodal gaps : the ‘pairon’ model approach W. Sacks1, A. Mauger1, and Y. Noat2 1Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, 2Institut des Nanosciences de Paris Sorbonne Université, Faculté des Sciences et Ingénierie, 75005 Paris, France

Angle-resolved photoemission, in addition to tunneling, has provided key information on the cuprate pairing on the microscopic scale. However in the underdoped regime, enigmatic and still unexplained phenomena have been clearly observed. In particular, the angular dependence

of the gap function Δ(q) deviates from a pure d-wave form such that the antinodal gap ΔAN and the nodal gap ΔN values diverge. Moreover, some disparity exists in the reported data and in their interpretation. On another front, ARPES has firmly established that the enigmatic Fermi arcs, i.e. normal electron excitations around the nodes, exist even below Tc. In this work, we will interpret these experiments based on the ‘pairon’ model [1] in which the fundamental object is a hole pair bound by its local antiferromagnetic (AF) environment on the

scale of the AF coherence length xAF. The pairon model agrees quantitatively with both the gap

function Δ(q) and the Fermi arcs seen at finite temperature. [1] W. Sacks, A. Mauger, and Y. Noat, Europhysics Lett. 119, 17001 (2017).

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Fr-2-1: Locating the missing superconducting electrons in overdoped cuprates Fahad Mahmood1, Xi He2, Ivan Bozovic2, N. P. Armitage1 1Johns Hopkins University, Baltimore, MD, USA 2Brookhaven National Laboratory, Upton, NY, USA Overdoped high-temperature cuprate superconductors have been widely believed to be described by the physics of d-wave BCS-like superconductivity. However, recent measurements indicate that as the doping is increased, the superfluid density decreases smoothly to zero rather than increasing as expected by BCS theory in the absence of disorder. Here, we combine time-domain THz spectroscopy with kHz range mutual inductance measurements on the same overdoped La2-xSrxCuO4 films to determine both the superfluid and the uncondensed carrier density as a function of doping. A significant fraction of the carriers remains uncondensed in a wide Drude-like peak even as T→0, which, when taken with the linear-in-temperature superfluid density, is inconsistent with existing theories for the role of disorder in suppressing the superfluid density in a d-wave superconductor. Our large measurement frequency range gives us a unique look at the low frequency spectral weight distribution, which may suggest the presence of quantum phase fluctuations as the critical doping is approached.

Fr-2-2: Charge-density-wave order in single-layered Bi2Sr2-xLaxCuO6+δ superconductor S. Kawasaki1, D. Kamijima1, M. Kitahashi1, Z. Li2, C. T. Lin3, P. L. Kuhns4, A. P. Reyes4, and Guo-qing Zheng1,2 1Department of Physics, Okayama University, Okayama 700-8530, Japan 2IOP, CAS, and Beijing National Lab. for Condensed Matter Physics, Beijing 100190, China 3MPI fur Festkorperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany 4National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA

Recently, a charge density wave (CDW) order was found below the superconducting dome in

bi-layer YBa2Cu3Oy when a high magnetic field is applied perpendicular to the CuO2 plane, which was suggested to arise from incipient CDW in the vortex cores that becomes overlapped [1]. In this presentation, we report 63Cu-nuclear magnetic resonance (NMR) study that discovered a long-range CDW order in single-layered Bi2Sr2-xLaxCuO6+δ (Bi2201) [2]. In contrast to YBCO, the CDW in Bi2201 sets in above the superconducting dome, under in-plane

(H c) fields H >10 T. We will also report recent results under out-of-plane (H//c) fields. Based

on systematic NMR studies, we will discuss the relationship between the CDW and superconductivity, pseudogap, and antiferromagnetic order in Bi2201.

[1] T. Wu et al., Nature 477, 191 (2011)., T. Wu et al., Nat. Commun. 4, 2113 (2013). [2] S. Kawasaki et al., Nat. Commun. 8, 1267 (2017).

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Fr-2-3: Field-dependent specific heat in overdoped cuprates – electronic phase separation J. L. Tallon1 1Robinson Research Institute, Victoria University of Wellington, PO Box 33436, Lower Hutt 5046, New Zealand The electronic specific heat to 300 K captures the full spectrum of low-lying spin and charge excitations up to about 100 meV. In the case of the cuprates measurements of the electronic specific heat have provided deep insights into the various correlated states that compete with superconductivity, especially the still-enigmatic pseudogap. We present additional measurements of the field-dependent electronic specific heat in Tl2Ba2CuO6, Y0.8Ca0.2Ba2Cu3Oy and YBa2Cu4O8 with a view to addressing the current interest in overdoped cuprate physics. We show (i) the reported coincidence of the pseudogap with a Lifshitz transition is not supported, and neither is the associated re-entrant superconductivity; and (ii) in the overdoped state scaling behaviour reveals a progressive electronic phase separation, causing the superfluid density to fall along with Tc and critical fields.