Oxford Symposium on Quantum Materials 2011 · Web view2013/04/27 · Oxford Symposium on Quantum Materials 2013 Somerville College, 10 May 2013 Abstracts Oxford Symposium on Quantum

Oxford Symposium on Quantum Materials 2013

Somerville College, 10 May 2013

Abstracts


Oxford Symposium on Quantum Materials 2013 – Welcome

Paolo RadaelliOxford Physics

Talk: 9.00-09.15 am QM@oxford


Forging a Theory of Iron-Based Superconductors

Peter HirschfeldFlorida State University, USA

In contrast to cuprate superconductors, which manifest low-temperature

properties which correspond to a gap with a common (dx2-y2) form, the gap structure in Fe-based superconductors appears to be far from universal [1]. I argue that, while aesthetically unappealing from a traditional theoretical perspective, such a situation affords a novel opportunity to develop a quantitative theory of unconventional superconductivity. I will review approaches to this problem that focus on pairing from spin and orbital fluctuations in the Fe d states, and discuss how well they allow us to understand current experiments on the superconducting state of various Fe-based systems. I will review experimental evidence for the sign-changing s-wave pair state, and discuss recent electron irradiation experiments which appear capable of resolving the debate on the role of disorder in these materials.

[1] "Gap symmetry and structure of Fe-based superconductors", P.J. Hirschfeld, M.M.Korshunov, and I.I. Mazin, Rep. Prog. Phys. 74, 124508 (2011).



Chemistry of Layered Chalcogenides and Oxide Chalcogenides

Simon ClarkeOxford Chemistry

The chemistry of layered chalcogenides and oxide chalcogenides in which intercalation or deintercalation reactions are used to tune the formal oxidation states of mid transition metals will be described. The systems will include mixed-valent manganites and high temperature superconductors derived from iron selenide.



Field-Induced Magnetic Transition in an Iron Pnictide Superconductor, Ca10(Pt3As8)((Fe/Pt)2As2)5

Matthew WatsonOxford Physics

I will present a high magnetic field study (up to 55 T) of a10(Pt3As8)(Fe1−xPtxAs2)5 (x = 0.097) with a TC ˜ 10 K using magnetic torque, tunnel diode oscillator technique and transport measurements. The superconducting phase diagram is determined, revealing an anisotropy of the irreversibility field up to a factor of 10 near TC. Unexpectedly, beyond the superconducting state we find a field-induced magnetic transition at ˜22 T in the magnetic torque data when the magnetic field is applied perpendicular to the conducting layers. This transition becomes significantly sharper by suppressing the thermal fluctuations while lowering the temperature to 0.3 K. The origin of the anomaly is likely to be the stabilization of a collinear magnetic ordering of the Fe or Pt sublattices by the magnetic field.

Talk:10.50-11.05 am QM@oxford


High Resolution Microstructural Analysis of Phase Separation Phenomena in Fe-Based Superconductors

Susie SpellerOxford Materials

The co-existence of magnetic order and superconductivity is a common feature of the iron-based superconductors, raising the question of whether these phases are spatially distinct or whether the same electrons are responsible for both phenomena. To investigate the nature of the phase separation in Fe-based single crystals, we have employed a combination of microanalytic techniques to assess the chemical and structural uniformity of single crystalline samples used for fundamental property measurements. In particular we have used the high resolution Electron Backscatter Diffraction (EBSD) technique, pioneered for strain mapping in aerospace alloys [1], to map local variations in lattice parameter with exceptional precision and sub-micron spatial resolution for a range of different iron-based compounds including Fe(Se,Te) [2] and AFe2Se2 (A=Cs,Rb) [3]. In AFe2Se2, the intrinsic phase separation is very extreme, with significant chemical and structural differences associated with the spatially distinct electronic phases. Here we will present recent results on the microstructural development in AFe2Se2 and CaFe2As2 single crystals.

Talk:11.05 -11.25 am QM@oxford


The Upper Critical Field of NaFe1-x CoxAs Superconductors

Saman GhannadzadehOxford Physics

The recent discovery of iron-pnictide superconductors has generated great interest, leading to an increased worldwide effort towards understanding the superconducting mechanism. A crucial part of this effort is the measurement of the upper critical field, which is a fundamental property of type-II superconductors. The critical fields give the coherence and penetration depths, and provide information on the pair-breaking mechanism. Furthermore, the upper critical field anisotropy sheds light on the multi-band nature of the superconductivity, and is sensitive to the dimensionality and the underlying electronic structure of the system.

Here, we will present and discuss the results of our recent study on the upper critical fields of the NaFe1-xCoxAs superconductor family, which was carried out at the Nicholas Kurti High Magnetic Field facility in fields of up to 45 T.

Talk: 11.25 – 11.40 am QM@oxford


Next Generation of High Field Superconducting Magnets

Ziad MelhemOxford Instruments

Users need for superconducting magnet greater than 23T for physical sciences, NMR or ICR applications will require new innovation in superconducting magnet engineering. The talk presents an overview of high fields for research and industry and the dependence of the future systems on the use of advanced superconducting materials. The production of high magnetic fields using low temperature superconductors (LTS) has become common place. However, the large magnet sizes and their associated high cooling costs have often precluded the full utilization of these research capabilities. Recent advances in internal Sn superconductors and Cryofree® technology together with advances in high temperature superconductors (HTS) have opened up a new era in superconducting magnet technology. A new generation of superconducting magnets is undergoing development and exploits the current performance of HTS and LTS materials together with innovative solutions in engineering the integration of coils from different materials.

Talk: 11.40 am -12.00 pm QM@oxford


Probing Frustration: Muons and Spin Ice

Stephen BlundellOxford Physics

The detection of a current of monopoles in spin ice has proved controversial. An analysis is presented of the behavior of muons in the low-temperature state in spin ice [1]. It is shown in detail how the behavior observed in some previous muon experiments on spin ice in a weak transverse field may result from the macroscopic stray field of magnetized spin ice. A model is presented which allows these macroscopic field effects to be simulated and the results agree with experiment. The persistent spin dynamics at low temperature originate from the

sample and could be a muon-induced implantation effect that is operative in out-of-equilibrium systems with long relaxation times. New data on various other pyrochlore systems will be presented to help assess the role of muon diffusion (which has been proposed [2] to account for some of the behaviour observed) and also quantum spin ice behaviour (which can be realised in some Pr-containing pyrochlores).

[1] S. J. Blundell, Phys. Rev. Lett. 108, 147601 (2012).

[2] P. Quémerais, P. McClarty, and R. Moessner, Phys. Rev. Lett. 109, 127601

Talk: 14.00-14.20 pm M@oxford


Crystal Field States in Candidate Quantum Spin Ice Pr2Sn2O7

Andrew PrincepOxford Physics

Magnetic pyrochlores A2B2O7 have very strong geometric frustration, leading to exotic phenomena such as classical spin liquids and magnetic monopole excitations [1]. Recently the idea of a “quantum” spin ice, where the quantum fluctuations preventing spin freezing, has attracted considerable interest. This state should realise an analogue of quantum electromagnetism with linearly dispersing magnetic excitations equivalent to photons [2]. It has been proposed that Pr2Sn2O7 is one of the most promising candidates for quantum spin ice as Neutron diffraction measurements have shown a strong quasielastic peak, but no evidence for long-range magnetic order down to 200 mK [3]. Specific heat measurements also indicate a greater residual entropy than other spin ice compounds. All these results point towards a ground state which is dynamic at T = 0.

The specific admixture of angular momentum states in the ground doublet, together with the strengths of the exchange and dipolar interactions, control

whether or not a material can exhibit quantum ice behaviour. When the ground state is not of the Kramers type, the mapping to a quantum ice model is non-trivial and the details of the wavefunction are important for the quantitative calculations needed to confirm whether or not the system falls within the realm of spin-ice models [4]. By performing inelastic neutron scattering on the MERLIN instrument at ISIS, we have determined the single-ion ground state wavefunction of the Pr3+ ions, and lowest lying level is indeed a non-kramers doublet. We also observe and reproduce intermultiplet transitions at approximately 280 meV, and show that an excitation around 38 meV, previously assigned to a crystal field transition, is actually a phonon.

[1] J. S. Gardner et. al., Rev. Mod. Phys., 82, (2010), 53[2] O. Benton, O. Sikora, N, Shannon, Phys. Rev. B, 86, (2012), 075154[3] H. D. Zhou. et. al., Phys Rev. Lett., 101, (2008). 227204[4] S. Lee, S. Onoda, and L. Balents, Phys. Rev. B, 86, (2012), 104412

Talk: 14.20-14.35 pm QM@oxford


Dynamical Spin Waves in the Pyrochlore Magnet

Eliot KapitOxford Physics

We demonstrate a numerical method for studying the dynamical behavior of the spin excitations of a pyrochlore antiferromagnet in the large-S limit. The classical pyrochlore magnet is a highly frustrated compound, with a massive ground state degeneracy of disordered states, and therefore provides an ideal system to study the effects of frustration on 3D quantum magnetism. While many previous studies have treated the static and finite temperature spin structure of these compounds, the dynamical behavior of the system's finite energy excitations is much less well understood. We show that by repeatedly generating ground states using classical Monte Carlo methods and diagonalizing the full linear spin wave Hamiltonian which results from each state, we can calculate the time-dependent spin structure factor S[Q,w] for systems of up to 8000 spins. In addition to the familiar massive degeneracy of zero energy excitations, we find a coherent, linearly dispersing spin wave mode even in the disordered ground states. Preliminary comparison between our numerical simulations and energy-resolved neutron scattering data in MgCr2O4 shows strong qualitative agreement.



Dispersion Relations near Quantum Criticality in the Quasi 1D Ising Chain CoNb2O6 in Transverse Magnetic

Field

Ivelisse CabreraOxford Physics

The Ising chain in a transverse magnetic field is one of the canonical examples of a quantum phase transition. We have recently realized this model experimentally in the quasi-one-dimensional (1D) Ising-like ferromagnet CoNb2O6 [1]. Here, we present single-crystal inelastic neutron scattering measurements of the magnetic dispersion relations in the full three-dimensional (3D) Brillouin zone for magnetic fields near the critical point and in the high field paramagnetic phase. We explorethe gap dependence as a function of field and quantify the cross-over to 3D physics at the lowest energies due to the finite interchain couplings. We parametrize the dispersion relations in the high-field paramagnetic phase to a spin wave model to quantify the sub-leading terms in the spin Hamiltonian beyond the dominant 1D Ising exchange.

1. [1] R. Coldea, D.A. Tennant, E.M. Wheeler et al, Science 327 177-180 (2010).



Magneto-Orbital Helices: A Novel Route to Coupling Magnetism and Ferroelectricity in Multiferroic

CaMn7O12

Natasha PerksOxford Physics

Multiferroic materials, specifically those possessing a magnetically induced improper ferroelectric polarisations attract considerable interest, due to both their novel physics, and their potential technological application, exploiting cross coupling of electrical properties. Work presented here forms part of a wider effort to develop strongly coupled, high temperature multiferroics with enhanced

functionality.Orbital physics drives a rich phenomenology in transition metals, providing microscopic underpinning for effects such as colossal magnetoresistance. Magnetic and lattice degrees of freedom are coupled through orbital ordering, and it has long been hoped that this coupling could be exploited to create new multiferroic materials. Here, we report an unprecedented magneto-orbital texture in multiferroic CaMn7O12, giving rise to the largest magnetically induced polarisation measured to date. Neutron and x-ray diffraction has characterised magnetic and structural modulations, which are intertwined to form an ‘incommensurate magneto-orbital helix’. Analysis of magnetic exchange shows that orbital order is crucial in stabilising the chiral magnetic structure, subsequently allowing for electric polarisation. The presence of a global structural rotation promotes the coupling between this polarisation and magnetic helicity required for multiferroicity. These observations open up the possibility of finding a new class of strongly coupled multiferroic materials, underpinning their technological development.



Terahertz Frequency Electromagnon and Magnon Modes in Multiferroic Cupric Oxide

Sam JonesOxford Physics

We examined the terahertz magnetoelectric responseof cupric oxide, a high-temperature multiferroic with a cycloidalspin structure that induces an electrical polarisation. Terahertztime-domain spectroscopy at temperatures from 10-300K and magneticfields 0-7T uncovered electromagnon and magnon modes, providinginsights into the spin Hamiltonian.

Talk: 15.20 – 15.35 pm QM@oxford


Antiferromagnetic Ordering in Quasi-Low Dimensional Polymeric Magnets: a CW-ESR Study

Danielle KaminskiOxford Physics

In both 1D and 2D systems, the formation of long-range magnetic order is impossible at temperature T>0K. However, attempts experimentally to realise such systems are imperfect, resulting in 'quasi' 1D (Q-1D) and 2D (Q-2D) systems. These have a small but finite coupling across the remaining dimensions, meaning magnetic ordering at T>0K is a possibility.

Polymeric magnets are quasi-low dimensional and consist of magnetic ions linked by organic groups, forming chains and layers. With the ability we have to fine-tune their magnetic parameters, they provide a fertile test ground for aspects of the quantum theory of magnetism [1,2].

We here explore, via continuous wave electron spin resonance (CW-ESR) at variable temperature, a group of polymeric magnets which form an antiferromagnetic state at T<TN. Through careful extraction of linewidth and principle g-tensor components, we are successful in probing the temperature region above TN. We therefore propose use of this method across a wider range of compounds to gain insight into the ordering process itself.

[1] P.A. Goddard et al., New J. Phys. 2008 (10) 083025.[2] P.A. Goddard et al., Phys. Rev. Lett. 2012 (108) 0077208.

Talk: 15.35 – 15.50 pm QM@oxford


New Suprises from a Famous Organic Superconductor

Francis PrattRutherford Appleton Laboratory

The low field phase of the first organic superconductor (TMTSF)2ClO4 has been studied by muon spin rotation to get information about the vortex state and an accurate value for the penetration depth. The muon spin relaxation shows no indication of gap nodes on the Fermi surface nor of any spontaneous fields due to time-reversal-symmetry breaking. The overall evidence does however suggest that the symmetry of this low field phase is odd-frequency p-wave singlet; a novel example of odd-frequency pairing in a bulk superconductor.

16.20 – 16.40 pm QM@oxfordOxford Symposium on Quantum Materials 2013

Unconventional Superconductivity in Strontium Ruthenate

Jesper RomersOxford Physics

In 1994 superconductivity was discovered in the material Sr2RuO4, strontium ruthenate (SRO). Closely related materials show ferromagnetism in the metallic state and by analogy it was argued that SRO might exhibit spin triplet pairing in the superconducting state.

The spin triplet pairing is well-established experimentally by now, but the form of the orbital part of the pairing wave function is still the subject of debate. There is hope SRO is an electronic analogue of the A phase of He-3, a chiral p-wave superconductor. This state of matter carries topologically protected edge modes and supports half quantum vortices with Majorana zero modes.

However, many more types of p-wave pairing are in principle possible and to date experiments are inconclusive and sometimes at odds with one another. From a theoretical point of view, the problem is very rich given the fact that SRO is a layered two-dimensional system with three bands. Most theories assume the dominant contribution to the superconductivity comes from the 2D band - recent proposals however involve the other two quasi-1D bands.

I will give an overview of the experimental status of the field and pinpoint where the difficulties are in reconciling the experimental data with the theoretical models. Furthermore I will present work we are doing right now which combines taking into account all three energy bands as well as strong spin-orbit coupling, which is important in SRO.

16.40 – 17.00 pm QM@oxfordOxford Symposium on Quantum Materials 2013

Anharmonicity due to Electron-Phonon Coupling in Magnetite

Moritz HoeschDiamond Light Source

We present the results of inelastic x-ray scattering for magnetite and analyze the energies and widths of the phonon modes with different symmetries in a broad range of temperature 125 < T < 293 K. The phonon modes with X4 and 5 symmetries broaden in a nonlinear way with decreasing T when the Verwey transition is approached. It is found that the maxima of phonon widths occur away from high-symmetry points which points at

the incommensurate character of critical fluctuations.Strong phonon anharmonicity induced by electron-phonon coupling is discovered by a combination of these experimental results with ab initio calculations which take into account local Coulomb interactions at Fe ions. It (i) explains observed anomalous phonon broadening, and (ii) demonstrates that the Verwey transition is a cooperative phenomenon which involves a wide spectrum of phonons coupled to the electron charge fluctuations condensing in the low-symmetry phase.

17.00 – 17.20 pm QM@oxford


Quantum Mechanics in Superconducting CircuitsPeter Leek

Oxford Chemistry

Microwave frequency electrical circuits built from superconducting components and cooled to millikelvin temperatures can display quantum mechanical behaviour. For example, an electrical resonator cooled to its ground state, and coupled to one of a variety of solid state quantum devices, realises an electrical circuit analogue of cavity quantum electrodynamics. In this talk I will give a basic introduction to this field, and an overview of the research within it that we are pursuing at Oxford.

17.20 – 17.35 pm QM@oxford


Do Birds Use Entangled Spins in Their Magnetic Compass Sensor?

Peter HoreOxford Chemistry

Most physical scientists would probably treat with scepticism the suggestion that a chemical reaction could respond to a magnetic field as weak as the Earth’s. After all, the energy of interaction of a molecule with a 50 microtesla magnetic field is more than a million times smaller than the thermal energy per molecule at room temperature. Nevertheless, the kinetics of certain chemical reactions are magnetically sensitive. The key molecular species are pairs of transient free radicals whose electron-nuclear spin systems evolve coherently under the influence of internal and external magnetic interactions.

In this short talk, I will discuss aspects of the proposal that the coherent quantum spin-dynamics of photo-induced radical pairs in cryptochromes (photo-active proteins) could be the mechanism of the light-dependent magnetic compass sense of migratory birds and other animals.

17.35 – 17.55 pm QM@oxford

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Oxford Symposium on Quantum Materials 2011 · Web view2013/04/27 · Oxford Symposium on Quantum Materials 2013 Somerville College, 10 May 2013 Abstracts Oxford Symposium on Quantum