Oxford Symposium on Quantum Materials 2011 · Oxford Symposium on Quantum Materials 2011 Spin fluctuations in iron-based superconductors observed by neutron scattering Alice E. Taylor

Oxford Symposium on Quantum Materials 2011

Wolfson College, 6 May 2011

Abstracts

POSTERS


Monopole Dynamics in Spin Ice

Dr Claudio Castelnovo SEPnet and Hubbard Theory Consortium,

Royal Holloway University of London

The last couple of years have witnessed intense interest in spin ice materials due to the unique nature of its low energy excitations, which take the form of emergent magnetic monopoles. Through combined theoretical and experimental work, it has become increasingly apparent that an effective description of these excitations in terms of free, Coulomb interacting point-like quasiparticles is essential to develop an understanding of the thermodynamic properties of these materials beyond numerical simulations. On the other hand, we are only just beginning to unravel the repercussions of such exotic excitations on the dynamics of spin ice, in relation for instance to how the system relaxes when driven out of equilibrium, or in relation to thermal transport experiments. In this talk we review some of the latest theoretical and experimental results on the out of equilibrium properties of spin ice materials, ranging from thermal and field quenches [Castelnovo, Moessner, & Sondhi, PRL 104, 107201 (2010) and ongoing work] to thermal runaways in response to a varying magnetic field [Slobinsky et al., arXiv:1010.4143v1]. In particular, we discuss how these phenomena can be understood as consequences of the specific nature of the low energy excitations.

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Measurements on Correlated Electron Systems at the Nicholas

Kurti Magnetic Field Laboratory

Mr Saman Ghannadzadeh Clarendon Laboratory, Oxford Physics

I will introduce the Nicholas Kurti Magnetic Field Laboratory located in the Clarendon Laboratory, Oxford. This is a pulsed magnetic field laboratory capable of fields up to 60T, the highest magnetic field available in the UK. The laboratory is well suited to high field experiments at low temperatures in Condensed Matter Physics, such as exploration of superconductivity and low-dimensional magnetism. As well as presenting recent results taken on such materials, I will also describe the various available techniques - such as the contactless transport and absolute magnetisation.

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Quantum oscillations in novel electronic materials

Amalia I Coldea

Clarendon Laboratory, Oxford Physics

Quantum oscillations is a bulk probe that allows us to map out the full Fermi surface of a superconducting system in its normal metallic state. These oscillations are determined by the Landau quantization in high magnetic fields and are usually observed at very low temperatures and in very clean samples. By knowing the exact nature of the quasi-particles in the normal state and the degree of electronic correlations, one can simplify and restrict theoretical models required to understand the pairing mechanism in novel superconductors. Here I present examples of quantum oscillations experiments [1-4] and the comparison with band structure calculations for LiFePO (Tc=6K) and the effect of mass enhancement as approaching maximum Tc in BaFe2(As1-xPx)2. [1] A. I. Coldea, J. D. Fletcher, A. Carrington et al., PRL. 101, 216402 (2008); [2] A. I. Coldea, C. M. J. Andrew, J. G. Analytis, et al., PRL 103, 026404 (2009); [3] J. G. Analytis, C. M. J. Andrew, A. I. Coldea et al., PRL.103 076401 (2009); [4] H. Shishido, A. F. Bangura, A. I. Coldea et al., PRL.104, 057008 (2010);

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http://link.aps.org/doi/10.1103/PhysRevLett.101.216402





Magnetic Structure Solution of Frustrated Spin Systems

Joe Paddison

Inorganic Chemistry Oxford

Frustrated magnets exhibit a plethora of interesting quantum phenomena. However, devising models of their local magnetic structure is difficult, since the magnetic component of neutron scattering shows no Bragg peaks. The reverse Monte Carlo method is able to provide valuable insight into this problem, accurately predicting the single-crystal diffuse magnetic scattering and short-range spin correlations from powder neutron data alone. This approach is discussed and applied to a variety of topical frustrated systems.

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Control of structure, superconductivity and magnetism in iron-

based superconductors

Simon J Clarke Inorganic Chemistry Oxford

The synthesis, detailed structural characterisation and the sensitivity of superconductivity and magnetism to composition in the derivatives of LiFeAs and NaFeAs willbe described. Contrast will be made with the other members of the iron-based superconductor family.

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Magnetic properties of complex inorganic materials

Peter Battle


A number of selected compounds will be described, with an emphasis on the way in which the magnetic properties change with chemical composition.

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New solids containing extended sheets of square-planar

transition metal centres

Michael Hayward Inorganic Chemistry Oxford

The low temperature topotactic reduction of complex solid phases allows the preparation of novel materials containing transition metal centres in unusual oxidation states and coordination geometries. Phases prepared in this manner tend to adopt structures with well ordered arrays of anion vacancies and the reduction reactions used to prepare them often display striking structural selectivity. Recently we have investigated the reduction chemistry of a series of complex transition metal oxychlorides. Reduction of these phases yields materials which contain apex-linked sheets of square-planar MO4 transition metal centres, analogous to the CuO2 sheets in high Tc cuprate superconductors. The structural and magnetic properties of these phases will be discussed.

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Insulator-to-metal transition in BaBiO3 under pressure from first

principles DFT+U

Dr Davide Ceresoli Materials Modelling Laboratory, Oxford Materials

At zero pressure and temperature BaBiO$_3$ is an insulator with a structural dimerization, equivalent to a static valence disproportionation of the two Bi ions per cell from 4+ to 3+/5+. Under pressure one would expect an insulator-metal transition and the eventual disappearance of the dimerization. Moreover, the metallic phase should be superconducting, in analogy the metal doped Ba$_{1-x}$K$_x$BiO$_3$ compounds. To date, there are no accurate ab initio predictions under pressure, essentially because LDA or GGA fail to stabilize an insulating phase with the correct distortion and electronic gap. We carried out first principles DFT+U calculations by determining the effective Hubbard U self-consistently at every pressure, and found that the presence of U is mandatory for a correct description of the zero-pressure state. Upon increasing pressure, we found an insulator to metal transition at ~20 GPa. By further increasing the pressure, we predict the appearance of a superconducting phase, characterized by quantum melting of the weakly dimerized CDW lattice. The dimerization tendency and superconductivity are expected to weaken only at much higher pressures, presently under investigation.

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A Curious Case of Hexahydroxytriphenylene

Jasper Adamson


The D3h symmetry molecule 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) has been an attractive building block for covalent organic frameworks (COFs) [1] and has found use as a mesogen unit in discotic liquid crystals [2]. In the current study the crystal structure of HHTP*4H2O has been investigated. Various single crystal x-ray diffraction (SCXRD) measurements performed at different temperatures yielded vital information about the structure and structural changes of the crystals. The as synthesised crystals illustrate the π-stacking motif common to triphenylene systems: the HHTP triphenylene cores assemble in columnar stacks, arranged in a distorted hexagonal array. The solvent water molecules reside in the pores between the stacks and hence an extensive three-dimensional hydrogen bonding network is created. Our hypothesis is that upon heating and over time the crystals lose the solvate water molecules, resulting in a mixture of phases. Correlating powder x-ray diffraction (PXRD) data with SCXRD data, we can confirm that structural changes do indeed take place over time. It is noteworthy that the as synthesised red and transparent crystals of HHTP*4H2O turn black and opaque upon heating. SQUID measurements indicate slight diamagnetism for the red crystals and paramagnetism for the black crystals of HHTP. Therefore we believe that the HHTP stacks are subject to oxidation, which results in unpaired electrons in the system. The system is unique in its potential of being a single organic molecule based conductor, because of the unpaired electrons present. Further work on the system will involve conductivity measurements and setting up computer models in the hope to replicate experimental data. 1. O. M. Yaghy et al. (2007). Science, 316, 5822, 268-272. 2. K. Bechgaard et al. (2000). Chem. Mater.,12, 2428-2433.

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Utilzing boron nitride sheets as thin supports for high resolution

imaging of nanocrystals

Yimin Wu Oxford Materials

We demonstrate the use of thin BN sheets as supports for imaging nanocrystals using low voltage (80 kV) aberration-corrected high resolution transmission electron microscopy. This provides an alternative to the previously utilized 2D crystal supports of graphene and grapheme oxide. A simple chemical exfoliation method is applied to get few layer boron nitride (BN) sheets with micrometer-sized dimensions. This generic approach of using BN sheets as supports is shown by depositing Mn doped ZnSe nanocrystals directly onto the BN sheets and resolving the atomic structure from both the ZnSe nanocrystals and the BN support. Phase contrast images reveal moir´e patterns of interference between the beams diffracted by the nanocrystals and the BN substrate that are used to determine the relative orientation of the nanocrystals with respect to the BN sheets and interference lattice planes. Double diffraction is observed and has been analyzed.

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Spin fluctuations in iron-based superconductors observed by

neutron scattering

Alice E. Taylor Oxford Physics

Results of inelastic neutron scattering measurements on the superconductor LiFeAs are presented, within the context of results on other families of iron-based superconductors. The observation of spin fluctuations in these systems informs us on the symmetry of the superconducting pairing mechanism. Our results do not support a recent theoretical prediction of novel pairing in LiFeAs, resulting in spin-triplet p-wave superconductivity. Instead our results suggest that the mechanism of superconductivity is similar to that in other iron-based superconductors.

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Crystal structure, magnetic order and dynamics in honeycomb

iridate Na2IrO3

Sungkyun Choi Oxford Physics

We explore the crystal structure and magnetism in the layered insulator Na2IrO3, proposed to realize a frustrated honeycomb lattice of exchange-coupled Ir4+ ions with J=1/2 moments, possibly stabilized by strong spin-orbit coupling. Single crystal X-ray diffraction sees rods of diffuse scattering as well as Bragg peaks. Based on a speical periodicity of diffuse scattering, we have simulated microscopic model of stacking faults. Turning to magnetism, magnetic anomaly at low temperature observed by magnetic susceptibility measurment turns out to be the signature of a long-range magnetic ordering by muon-spin rotation experiments. Also, inelastic neutron scattering experiment shows a dispersive low energy mode, which is examined in the context of two exchange models for the honeycomb lattice using linear spin-wave theory.

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Time-resolved spectroscopy of multiferroic materials

Katie Doig, Samuel Jones, James Lloyd-Hughes*

Oxford Physics

Multiferroic materials – in particular magnetoelectric multiferroics – show great potential in future data storage technologies in which data can be written electrically and read magnetically. We are studying the equilibrium and non-equilibrium optical properties of multiferroics via time-resolved spectroscopy in the visible and terahertz frequency ranges. The aim of this work is to obtain a better understanding of the light-matter interaction in multiferroics, by investigating low-energy excitations such as magnons and electromagnons. * [email protected]

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Determining the Electronic Ground State in a Bilayer Manganite

Graeme Johnstone

Oxford Physics

There has been a debate in recent years as to the true electronic ground state in the CE-type antiferromagnetic phase in materials related to the colossal magnetoresistive manganites. Two possible models have been suggested, the first is the truly charged ordered CE-phase model with a checkerboard lattice of Mn3+ and Mn4+ ions, based on the Goodenough-Kanamori rules. The second proposed model is known as the Zener polaron model, where two neighbouring manganese ions are tied together ferromagnetically by the Zener double exchange interaction and the lattice is made up of these Zener polarons. I will report inelastic neutron scattering measurements on Pr(Sr0.1Ca0.9)2Mn2O7 that the measured dispersion agrees with the predicted for the CE-phase model, but also reveals some interesting competing interactions within the antiferromagnetically ordered layers.

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Signature of macroscopic phase separation in Ce-based pnictides

from high resolution neutron powder

Daoud-Aladine Aziz ISIS, Rutherford Appleton Laboratory

The phase diagram of those Ce-based pnictides as function of x has been derived assuming a single phase at all temperatures and compositions from medium resolution neutron powder diffraction data of Ref[1]. We have collected high resolution neutron powder data in order to compare the parent CeFeAsO compound with the optimally doped superconducting CeFeAsO_{1-x}F_x (x=0.16). Using the higher resolution available using Time of Flight diffraction (HRPD at ISIS), we observe a striking difference between the two compostions : a clearly visible asymetry of the profile of the (00l) reflections occurs in the optimally doped SC sample, which is not present in the parent material. As a result, single phase refinements holds in the whole temperature range for the parent compound and describe well the tetragonal-orthorhombic transformation that it presents. However, the asymetry observed in the SC sample is much improved considering a mixture of two similar tetragonal phases with distinct c parameters whose phases fractions are constant in the whole temperature range. This analysis leads us to speculate the existence of a macroscopic phase separation in the SC sample, where only the minority phase would play a role in the SC property. Unlike the majority phase, the miniority phase has a phase fraction apparently well related to the composition (~2x), and its cell parameter shows an anomaly jsut at the SC transition [1] Jun Zhao et al., Nature Materials 7, 953 - 959 (2008)

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A new triclinic multiferroic: Cu3Nb2O8

Roger Johnson

Clarendon Laboratory, Oxford Physics

In this poster we present physical property measurements that have lead to the discovery of a new, triclinic multiferroic compound, Cu3Nb2O8. In addition, through both neutron and x-ray diffraction experiments, we have solved the previously unknown crystal and magnetic structure. By comprehensive analysis of the solved structures and bulk properties, we will uncover the microscopic mechanism that results in the magneto-electric properties of this novel multiferroic compound.

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Polaronic Conductivity in the Photoinduced Phase of 1T-TaS2

Nicky Dean

Oxford Physics

1T-TaS2 is a quasi-two-dimensional transition metal dichalcogenide in which charge density wave (CDW) behaviour coexists with strong electron-electron correlations. Below 180 K, a periodic lattice distortion causes the CDW to lock-in to the lattice, becoming fully commensurate. This reduces the bandwidth below a critical value, leading to the opening of a 100-meV Mott gap. After photoexcitation, time-resolved ARPES [1] and electron diffraction [2] reveal prompt collapse of the insulating gap but incomplete relaxation of the structural distortion. We investigate the transport properties of this photoinduced phase through measurements of the transient optical conductivity. By combining THz and infrared time-resolved spectroscopies, this is achieved over a three-order-of-magnitude frequency range [3]. Prompt collapse and recovery of the gap is observed, in line with previous measurements [1]. However, important differences arise between this phase and the thermally-induced one. Suppressed low-frequency conductivity, transient Fano effects, and the appearance of a mid-infrared absorption band point to polaronic transport in the transient phase. This suggests a unique phase in which the Mott gap is melted but the lattice retains its low-temperature symmetry, a regime only accessible by photo-doping. [1] L. Perfetti et al., Phys. Rev. Lett. 97, 067402 (2006). [2] M. Eichberger et al., Nature 468, 799 (2010). [3] N. Dean et al., Phys. Rev. Lett. 106, 016401 (2011).

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Dissecting charge ordering, lattice ordering, and localisation on the ultrafast timescale in 2D CDW materials

Jesse Petersen Oxford Physics

Charge density waves (CDWs) underpin the electronic properties of many complex materials [1]. When electrons are uncorrelated, electron-phonon interactions create CDW order through the Peierls mechanism. In layered dichalcogenides, the Peierls instability coexists with strong electron-electron interactions, and groundbreaking time-resolved experiments [2] hint at a role for electronic correlations in the CDW ordering process. In 1T–TaS2, a nested Fermi-surface geometry coexists with Mott physics. A commensurate CDW periodically distorts the crystal structure, reduces the Brillouin zone (BZ) and splits off several Umklapp or `shadow' bands [3]. A narrow band left at the Fermi level is then further split by intra-cluster Coulomb repulsion into Hubbard bands separated by a Mott gap [4], creating an insulating ground state. We have used time-resolved ARPES to map out changes in momentum-dependent electronic structure during an ultrafast insulator–metal transition in this material. By clocking the evolution of various changes, we assign specific features of the electronic structure to individual aspects of the ordered ground state. Gaps in the zone-boundary spectral function melt on sub-vibrational time scales, indicating an electronic origin for these features. Subsequently, spectral intensity migrates from shadow bands in the first zone back out to higher momentum, revealing the unfolding of the BZ that accompanies incipient lattice relaxation. This hierarchy of time scales indicates that charge and lattice order decouple at early times, so that it will be necessary to go beyond the phase- and amplitude-mode picture of CDW dynamics [5] to understand this system. An electronic origin for the Umklapp gaps also casts into doubt the dominant conception of the thermal phase transition, as being driven by nesting with Mott physics following. Instead, electronic correlations seem to play a central part in the CDW ordering itself. [1] F. Clerc et al., J. Phys.: Condens. Matter 19, 55002 (2007); M. Grioni, L. Perfetti, and H. Berger, J. of Electron Spectroscopy and Related Phenomena 137-140, 417 (2004); G. Gweon et al., ibid. 117-118, 481 (2001); D. Jérome and H. J. Schulz, Adv. Phys. 31, 299 (1982).[2] T. Rohwer et al., Nature 471, 490 (2011); M. Eichberger et al., ibid. 468, 799 (2010). [3] F. Clerc et al., Phys. Rev. B 74, 155114 (2006); K. Rossnagel and N.V. Smith, ibid. 73, 073106 (2006).[4] L Perfetti et al., Phys. Rev. Lett. 97 067402 (2006). [5] G. Grüner, Density Waves in Solids (Perseus, 2004).

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IR MICROSPECTROSCOPY BEAMLINE AT DIAMOND

Gianfelice Cinque

Diamond Light Source

Infrared (IR) Micro-Spectroscopy is a quantitative analytical and non-destructive technique which has undergone a renaissance since synchrotrons have been used as a highly brilliant source. Synchrotron IR radiation spans a larger spectral range - extending from the near-IR towards the far-IR/THz region - and can be 100-1000 times brighter in the mid- and far-IR than any other broadband IR source. Therefore SR IR is among the best IR microprobe to perform absorption spectroscopy and reveal small features at the highest spatial resolution in a broad spectral range.

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Coulour centres in diamond for quantum spintronics and

photonics

Jason Smith Oxford Materials

In the past few years colour centres in diamond have emerged as one of the most attractive candidates for solid state quantum information technologies. Initialisation, manipulation, and readout of single spins is now routine in several labs, combined with some of the longest coherence times ever seen in the solid state. Experiments towards achieving similar levels of control over the optical transitions are underway, to enable scalable quantum entanglement between remote qubits that can facilitate the development of a universal quantum processor. In this presentation I will give an overview of the current state of the art in this rapidly moving field, the progress we are making in Oxford towards these goals, and the challenges that remain to be addressed.

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Quantum Effects in Photosynthesis

Animesh Datta Oxford Physics

There have been recent experiments studying efficient energy transfer across light-harvesting complexes [PNAS, 107, 12766 2010]. These experiments show coherences spread across macroscopic length scales in the complexes. There coherences are believed to involve the electronic states, and not possible classically. These have taken as an evidence of quantum features in physiological systems. We will discuss the role quantum coherence and entanglement in efficient energy transfer in light harvesting complexes.

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Violation of a Leggett-Garg inequality with a solid state ensemble

Simon Benjamin Oxford Materials

An inequality devised by Leggett and Garg can be used to experimentally test whether an object exhibits true quantum behaviour. Building upon Leggett's concept of an 'ideal negative result' measurement, we develop a general protocol for Leggett-Garg tests on ensembles of two level systems (macroscopic or otherwise) by equipping a highly polarised ancillary structure to each unit of the ensemble to act as a local measuring device. We demonstrate this protocol using an ensemble of spin-bearing phosphorous impurities in silicon at 3.36 Tesla and 2.7 Kelvin, thus ruling out non-quantum theories for this type of system.

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Controllable length N@C60 dimers for use in quantum

information processing

Benjamin Farrington Oxford Materials

Quantum information processing (QIP) systems which force the nuclear spin states of the atoms of a molecule to behave as Quantum-bits (qubit) have shown great promise, producing the first physical implementation of Shor’s factorising algorithm in 20011 and the largest entangled array of qubits reported2 to date. Current research at Oxford Materials focuses on assessing the potential for using the electron spin states of the molecular species N@C60 as a qubit in a scalable quantum computer. The presentation herein centres on optimising the magnitude of the coupling between two nitrogen centres in an N@C60 dimer. The aim is to enhance the distinguishability of the energy transitions in a dimer from the hyperfine splitting of pristine N@C60 when measured as disordered powder using electron spin resonance (ESR). Our current approach modifies the dipolar coupling strength by changing the separation of the N@C60 cages in the dimer. The identification of suitable bridging units that can be attached region-specifically using conditions which are sympathetic to the thermal and photosensitivity of N@C60 is by no means trivial. The most successful bridging system to date uses a Prato cyclo-addition reaction to affix a C60 cage to either end of bis-aldehyde terminated molecular wires Reaction times of 2-10mins have produced significant dimer yields.

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Documents

Oxford Symposium on Quantum Materials 2011 · Oxford Symposium on Quantum Materials 2011 Spin fluctuations in iron-based superconductors observed by neutron scattering Alice E. Taylor