Cafe Scientifique

This is our weekly Monday morning coffee which aims to foster scientific discussions between our group and our colleagues in different departments to promote collaboration and enrich our research.

To register your contribution please use this page of Cafe Scientifique Talks.

Cafe Scientifique

Mondays 10.30am-11.30am
Clarendon Laboratory,
Audrey Wood Room
Coffee from 10.15am

Cafe Scientifique 2016

24 October 2016

Visualizing Quantum Matter

Professor J.C. Séamus Davis
Tyndall National Institute, UCC, Cork, Ireland.
Physics Dept., Cornell University, NY, USA
CMPMS Dept., Brookhaven Nat. Lab, NY, USA

Everything around us, everything each of us has ever experienced, and virtually everything underpinning our technological society and economy is governed by quantum mechanics. Yet this most fundamental physical theory of nature often feels as if it is a set of somewhat eerie and counterintuitive ideas of no direct relevance to our lives. Why is this? One reason is that we cannot perceive the strangeness (and astonishing beauty) of the quantum mechanical phenomena all around us by using our own senses. I will describe the recent development of techniques that allow us to image electronic quantum phenomena directly at the atomic scale. As examples, we will visually explore the previously unseen and very beautiful forms of quantum matter making up electronic liquid crystals and high temperature superconductors and find that they are closely related. I will discuss the implications for fundamental physics research and also for advanced materials and new technologies, arising from quantum matter visualization.

20 June 2016

What we know and don’t about multiferroic BiFeO3

Professor Je-Geun Park
*Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul 151-747, Korea
*Department of Physics & Astronomy, Seoul National University, Seoul 151-747, Korea

Multiferroic materials that have the coexistence of both magnetic and ferroelectric ground states have drawn significant attention in materials science over the past ten years or so. The underlying origin of this unusual behavior in either naturally occurring materials or artificially synthesized thin films has been a strong enough motivation for material scientists to discover or rediscover new multiferroic materials. Among a long list of multiferroic materials, BiFeO3 is arguably one of the most interesting multiferroic materials. BiFeO3 has been extensively investigated for potential applications by virtue of its magnetic and ferroelectric transitions occurring above room temperature: TN=650 K and TC=1050 K. Moreover, it has a very interesting incommensurate magnetic phase transition with an extremely long period of 650 Å. Over the past few years or so, there have been a multitude of studies done on BiFeO3. Largely thanks to these extensive studies, we have now come to know extremely details about the physical properties of this material. In this talk, I will present our results [1-4] obtained from high resolution neutron scattering experiments on this fascinating material.

[1] Jaehong Jeong, et al., Phys. Rev. Lett. 108, 077202 (2012)
[2] Sanghyun Lee et al., Phys. Rev. B Rapid Comm. 88, 060103R (2013)
[3] Jaehong Jeong, et al., Phys. Rev. Lett. 113, 107202 (2014)
[4] [Review] Je-Geun Park, et al., J. Phys.: Condens. Matter 26, 433202 (2014)

13 June 2016

Dr Daniel Woodruff (Oxford Inorganic Chemistry)

Exploring the Chemistry of Iron Selenide Based Superconductors

In order to explore the various physical and magnetic properties of superconducting β-FeSe, a number of groups have attempted to dope various species either onto the Fe site within the FeSe layers or in the void space between FeSe layers. Different approaches have yielded varying levels of success, with a large number of materials exhibiting enhanced properties whilst still being far from structurally well characterised. Our group has isolated and explored a number of systems via the use of “soft chemistry” synthetic approach. These have included Li1 xFex(OH)Fe1 ySe materials in which superconductivity can be “switched on” by a post synthetic treatment, as well as metal ammonia intercalated systems. This talk will focus on work previously performed within the group on a variety of iron chalcogenide superconducting systems as well as current and future work on more chemically challenging and complex systems.

1. H. Hosono and K. Kuroki, Physica C, 2015, 514, 399
2. A. Krzton-Maziopa, V. Svitlyk, E. Pomjakushina, R. Puzniak and K. Conder, J. Phys.: Condens. Matter, 2016, 28, 293002
3. H. Sun, D. N. Woodruff, S. J. Cassidy, G. M. Allcroft, S. J. Sedlmaier, A. L. Thompson, P. A. Bingham, S. D. Forder, S. Cartenet, N. Mary, S. Ramos, F. R. Foronda, B. H. Williams, X. Li, S. J. Blundell and S. J. Clarke, Inorg. Chem., 2015, 54, 1958
4. M. Burrard-Lucas, D. G. Free, S. J. Sedlmaier, J. D. Wright, S. J. Cassidy, Y. Hara, A. J. Corkett, T. Lancaster, P. J. Baker, S. J. Blundell and S. J. Clarke, Nature Mat., 2013, 12, 15

18 January 2016

Andrew Princep (Oxford Physics),
Tackling the complicated single ion properties of Osmium

Cafe Scientifique 2015

9 November 2015

Fabio Caruso (Oxford Materials),
Ubiquitous electron-plasmon coupling in semiconductors

James Beilsten-Edmands (Oxford Physics),
Anomalous electronic behaviour of halide perovskites

Cafe Scientifique 2014

Special Quantum Materials Seminars

Professor Roser Valenti
Goethe-Universität Frankfurt am Main

Wednesday 3 December 2014 10:00 am - 11:15, Audrey Wood

Hexagonal iridates: A trip between itinerant quasi-molecular orbitals and Heisenberg-Kitaev physics

In this talk I will present an overview on the electronic and magnetic properties of hexagonal iridates and discuss our present understanding of the behavior of these materials in terms of two alternative descriptions; the Heisenberg-Kitaev model and a new type of quasi-molecular orbitals.

Thursday 4 December 2014, 11.00 - 12.30, Audrey Wood

Unified picture of the doping dependence of superconducting transition temperatures
in alkali/meal/ammonia intercalated FeSe

In the recently synthesized Li_x(NH_2)_y(NH_3)_zFe_2Se_2 family of iron chalcogenides
a molecular spacer consisting of lithium ions, lithium amide and ammonia separates
layers of FeSe. It was shown [1,2] that upon variation of the chemical composition
of the spacer layer, superconducting transition temperatures can reach Tc=44 K,
but the relative importance of the layer separation and effective doping to the Tc
enhancement is currently under debate. Using state-of-the-art band structure unfolding techniques
as well as an RPA spin-fluctuation approach we will show [3] that the electron doping enhances the superconducting pairing and explain the experimentally observed limit to Tc in the molecular-spacer-intercalated FeSe class of materials.

[1] S. J. Sedlmaier et al. J. Am. Chem. Soc. 136, 630 (2014)
[2] Burrad-Lucas et al., Nat. Mater. 12, 15 (2013)
[3] Guterding et al., arXiv:1410.7565

8 December 2014, Quantum Kapitza Pendulums: Fluctuation-driven ordering phenomena in metallic ferromagnets

Frank Kruger,
London Centre for Nanotechnology, University College London (UCL)
ISIS, Rutherford Appleton Laboratory

]Quantum Kapitza Pendulums: Fluctuation-driven ordering phenomena in metallic ferromagnets

The coupling between the magnetic order parameter and soft electronic particle-hole fluctuations can have profound effects. In itinerants ferromagnets, it generically leads to fluctuation-induced first-order behaviour at low temperatures. In this talk, I will show that the interplay between magnetic and electronic low-energy fluctuations causes a range of exotic ordering phenomena that are a consequence of a fermionic quantum order-by-disorder mechanism. Examples include (i) the formation of incommensurate spiral order on the border of ferromagnetism and (ii) a fluctuation-driven switching of moments from an easy direction to a magnetic hard axis. Experimental realisations of these phenomena will be discussed.

3 November 2014, A Glance at the Exotic Quantum Liquids and Solids of the Two-Dimensional Electron Gas

Gábor A. Csáthy, Associate Professor
Purdue University, Department of Physics

A Glance at the Exotic Quantum Liquids and Solids of the Two-Dimensional Electron Gas

The two-dimensional electron gas is a fascinating model system with an astonishingly large number of ground states. The study of this system has enriched quantum many-body physics with novel concepts such as composite fermions, topological invariants, and quasiparticles with fractional charge. The description of a small number of recently observed fractional quantum Hall states, however, may require an even more sophisticated set of concepts such as Pfaffian electronic correlations and non-Abelian statistics.

Cooling of the electrons to ultra-low temperatures has led to the discovery of new fractional states and the chance to study others in the new regime of very low densities. In this talk I will discuss how these results have shaped our understanding of both the even- and odd-denominator fractional quantum Hall states developing in the area of phase space called the second Landau level. I will also highlight new results on the exotic electronic solids forming in this region

Figure. Magnetoresistance in the region of the second Landau level, exhibiting a rich variety of ground states. The fractional quantum Hall states labeled by their quantum number are believed to have exotic quantum correlations

27 October 2014, Fractional quantum Hall effect

Woowon Kang
University of Chicago

Fractional quantum Hall effect

16 June 2014, Local magnetism and spin correlations in the geometrically frustrated cluster magnet LiZn2Mo3O8

Stephen Blundell

Local magnetism and spin correlations in the geometrically frustrated cluster magnet LiZn2Mo3O8

LiZn2Mo3O8 has been proposed to contain S = 1/2 Mo3O13 magnetic clusters arranged on a triangular lattice with antiferromagnetic nearest-neighbour interactions. By using a combination of microwave and terahertz electron spin resonance, 7Li nuclear magnetic resonance, and muon-spin rotation spectroscopies we have characterised the local magnetic properties of LiZn2Mo3O8. These results show the magnetism in LiZn2Mo3O8 arises from a single isotropic S = 1/2 electron per cluster and that there is no static long-range magnetic ordering down to T = 0.07 K. Further, there is evidence of gapless spin excitations with spin fluctuations slowing down as the temperature is lowered. These data indicate strong spin correlations, which, together with previous data, suggest a low-temperature resonating valence-bond state in LiZn2Mo3O8.

Reference: J.P. Sheckleton et al. Phys. Rev. B 89, 064407 (2014).

23 June 2014, Depletion of Spectral Weight in One-Dimensional Chain Compounds M2Mo6Se6

Moritz Hoesch
Diamond Light Source

Depletion of Spectral Weight in One-Dimensional Chain Compounds M2Mo6Se6

The family of M2Mo6Se6, where M is and alkali metal or Tl or In, forms a quasi- one-dimensional crystal structure with a host lattice of (Mo3Se3) chains that are weakly coupled by the M ions. Within the chain a competition between the formation of Charge Density Wave (CDW) order and a superconducting ground state leads to a metal-to-insulator transition at e.g. Tcrit~150 K for rubidium guests and superconductivity with Tc = 4.2 K and 2.85 K for thallium and indium guests, respectively. We have measured the band structure and spectral weight near the Fermi level by angle-resolved photoemission spectroscopy (ARPES) at various temperatures between 20 K and room temperature. An intriguing depletion of spectral weight is observed in all samples, both with insulating and with metallic ground states. The spectra can be analysed in terms of gaps, in which case a trend to closing the gaps at higher temperatures is found. Alternatively, the power-law dependence of the spectral intensity as a function of binding energy is reminiscent of a Luttinger-liquid behaviour, although no spin-charge separation can be distinguished. In the talk the electronic structure of this most one-dimensional exotic superconductor and metal-to-insulator transition system will be discussed based on calculations and the ARPES results.

20 January 2014, Superconductivity in δ´-FeSe1-x

Amir-Abbas Haghighirad,
Superconductivity in δ´-FeSe1-x

The iron chalcogenides FeM (M = Se , Te) are the parent compounds of the recent discovered high-Tc superconducting iron-based layered oxypnictides RFeAsO1-xFx (R = La, Nd, Sm, etc.). The β polymorph of the FeSe phase exhibit superconductivity at ambient pressure with Tc ~ 8-12 K. This compound possesses a layered anti-PbO tetragonal structure at ambient temperature that becomes orthorhombic below 90 K. One of the interesting aspects of superconductivity in FeSe is the enormous effect of Tc when pressure is applied. The latter indicates an extraordinarily high sensitivity response of the superconducting state to structural deformations. However, the accurate structure and the terminology for describing the FeSe phase that exhibits superconductivity is still not well understood. Several groups as well as ours have undertaken the effort to refine the synthesis steps to produce better quality single crystalline samples of FeSe which crucial for resolving the physical properties, such as „anomalous“ resistivity, the role of the structural phase transition as well as the role of magnetism in enabling superconductivity. We have improved our methods of synthesis using ambient- and high-pressure techniques. The effect of stoichiometry on the phase purity of FeSe and its superconducting properties were investigated.

17 February 2014, Neutron scattering studies in CoNb2O6 and layered honeycomb iridates

17 February 2014
Ivelisse M Cabrera, Observation of a gapless linear dispersion at quantum criticality in the Ising chain ferromagnet CoNb2O6 in transverse field

The Ising chain in transverse field is one of the canonical paradigms for a continuous field-driven quantum phase transition between spontaneous magnetic order and a quantum paramagnet. The mechanism driving this phase transition has long been predicted to involve the closing of the spin gap, or minimum excitation energy, at the quantum critical point, where a characteristic linear dispersion is expected at low energies. We report single-crystal neutron diffraction and inelastic neutron scattering measurements that unveil how the magnetic order and excitations evolve in the very close proximity of the quantum critical point in the quasi-1D Ising chain ferromagnet CoNb2O6. Near criticality, we observe an essentially gapless spectrum with an almost perfectly-linear dispersion along the chain direction. Below the critical field, the frustrated interchain couplings stabilize 3D incommensurate spin-density-wave order, as observed through diffraction measurements. To our knowledge, this is the first time that essentially-gapless, linearly dispersive excitations have been observed in the very close proximity of a transverse field-tuned quantum critical point.

Sungkyun Choi,
Spin dynamics in layered honeycomb iridates: implications for Kitaev physics

We explore the spin dynamics in the frustrated honeycomb magnets Na2IrO3 [1] and Li2IrO3, candidates to display novel magnetic states stabilized by the strong spin-orbit coupling at the 5d Ir ions. Theory predicts composite spin-orbital J=1/2 moments at the Ir ions coupled by strongly-anisotropic and bond-directional exchanges, the so-called Kitaev honeycomb model, which has in its phase diagram novel magnetically-ordered ordered phases and a quantum spin liquid with exotic excitations. To search for such physics the experimental technique of choice is inelastic neutron scattering to probe the spin dynamics, however this is technically very challenging due to the large absorption cross-section of neutrons by the Ir nuclei. Using an optimised setup to minimise neutron absorption we have been successful in observing strongly dispersive spin-wave excitations of the Ir moments in both compounds and results are compared with predictions for a Kitaev-Heisenberg model as well as a Heisenberg model with further neighbour

24 February 2014, High magnetic field studies of IrTe2 and Ca10(Pt3As8)((Fe1−xPtx)2As2)5

24 February 2014

Samuel Blake,
The low temperature Fermi surface of IrTe2 probed by quantum oscillations
The transition metal dichalcogenide IrTe2 undergoes a structural transition at 280K [1]; doping on the Ir site suppresses this transition and induces superconductivity with Tc of about 3K [2]. The nature of the structural transition is possibly driven by charge disproportionation and the effect this has on the electronic structure of the superconducting state is not fully understood. We report a low temperature investigation of the Fermi surface of IrTe2 from quantum oscillations, using torque measurements performed in magnetic fields up to 33T and temperatures down to 0.3K. The observed extremal areas of the Fermi surface likely correspond to frequencies of a reconstructed Fermi surface, with light effective masses below 0.8me. The angular dependence of these frequencies across multiple crystals of IrTe2 suggests these materials are prone to domain formation upon cooling. We compare our measured Fermi surface with those predicted by electronic structure calculations, based upon the existing structural models, for both above and below the structural transition.

Matthew Watson,
Field-induced nematic-like magnetic transition in an iron pnictide superconductor, Ca10(Pt3As8)((Fe1−xPtx)2As2)5

We report a high magnetic field study up to 55~T of the nearly optimally doped iron-pnictide superconductor Ca10(Pt3As8)((Fe1−xPtx)2As2)5 with a Tc≈ 10 K using magnetic torque, tunnel diode oscillator technique and transport measurements. We determine the superconducting phase diagram, revealing an anisotropy of the irreversibility field up to a factor of 10 near Tc and signatures of multiband superconductivity. Unexpectedly, we find a spin-flop like anomaly in magnetic torque at 22~T, when the magnetic field is applied perpendicular to the {\it ab} planes, which becomes significantly more pronounced as the temperature is lowered 0.33~K. As our superconducting sample lies well outside the antiferromagnetic region of the phase diagram, the observed field-induced transition in torque indicates a spin-flop transition \textit{not of long-range ordered moments}, but of nematic antiferromagnetic fluctuations.

10 March 2014, Persistent Spin Dynamics in Magnets at Low Temperature

10 March 2014
Paul McClarty
Persistent Spin Dynamics in Magnets at Low Temperature

Many disparate magnetic materials exhibit a temperature independent relaxation rate at low temperatures. In this talk, I will discuss possible mechanisms for this relaxation.

Cafe Scientifique 2013

2 December 2013, Neutron scattering and Excitations of CoO

2 December 2013
Roger Cowley,
Neutron scattering and Excitations of CoO

CoO is probably the simplest material for which the magnetic structure has not been solved. It has the NaCl cubic structure above 290 K and the phonon spectra was successfully measured in 1967. Below 290 K it is an antiferromagnet but the magnetic structure and excitations have not been successfully described so far. There are several reasons for this. The magnetic moments have spin and orbital components. The magnetic structure may have non-collinear components. The domain structure has each magnetic excitation having possibly 12 different components. The spin-orbit interaction, the magnetic exchange interactions and the phonon structure all have the same order of magnitude.

9 December 2013, Hour-glass magnetic spectrum in La1.75Sr0.25CoO4

9 December 2013
Stephen Gaw,
Hour-glass magnetic spectrum in La1.75Sr0.25CoO4

An hour-glass magnetic spectrum has been measured in a variety of the high-Tc superconducting cuprates as well as in the insulting cobaltates and manganates. While the relation between the hour-glass and superconductivity within the cuprates is still disputed, the range of electronic properties exhibited by these systems has led many to believe that the hour-glass arises due to local moment physics. Previous studies have shown that an hour-glass spectrum can arise due to two factors: quasi-one dimensional magnetic correlations and disorder. The condition for quasi-1D correlations is satisfied in the charge and magnetic stripe ordering seen in all of these doped systems. In this talk, I will present our recent work investigating the magnetism of La1.75Sr0.25CoO4 using inelastic neutron scattering and bulk magnetization measurements. I will show how the magnetic spectrum changes with doping across the cobaltate’s phase diagram and look at a new model for the magnetic ground state in this system which sheds more light onto the nature of the disorder present.

Cafe Scientifiques May- June 2013

20 May 2013
Simon Benjamin, A new model for magnetoreception

3 June 2013
Amy Webber, Tuning Molecular Magnets for Quantum Information Processing
Junjie Liu, High-Field Electron Paramagnetic Resonance Studies of Anisotropic Molecular Magnets

10 June 2013
Gábor Halász, Mobile defects in the Kitaev honeycomb model

17 June 2013
Andrew Boothroyd, A ferroelectric-like transition in a metal
Imke Schneider, Low-energy spectral weight of the 1D Hubbard chain

24 June 2013
Roger Johnson, MnSb2O6: A polar magnet with a chiral crystal structure

Cafe Scientifiques 2012

Microwave: from classical to quantum applications

Monday 14 March 2012
Microwave: from classical to quantum applications

This session is dedicated to microwaves on how to detect them and probe classical and quantum
phenomena. Demonstration will be presented by Peter and Alexy.

Designing new materials: complex magnetic oxides and superconductors

Monday 5 March 2012
Designing new materials: complex magnetic oxides and superconductors

This session is dedicated on how to syntesize complex oxides and grow single
crystals of superconducting materials.

Molecular magnets: complex and tunable magnetic architectures probed by experiments

Monday 20 February 2012
Molecular magnets: complex and tunable magnetic architectures probed by experiments

Background reading:

Organic and molecular magnets

Band structure calculations: predictions and comparison with experiments

Monday 6 February 2012
Band structure calculations: predictions and comparison with experiments

Modelling with Quantum Expresso

DFT calculations with Wien2K

Topological insulators: from theoretical concepts to experimental results

Monday 23 January 2012

Topological insulators: from theoretical concepts to experimental results

CuO - a simple material with complex behaviour: orbital currents and multiferroicity

Monday 16 January 2012

CuO - a simple material with complex behaviour: orbital currents and multiferroicity

Orbital currents in CuO

Multiferroics: mechanism, experiments and new materials

Monday 28 November 2011

Multiferroics: mechanism, experiments and new materials

Useful reading:

Helical scattering signatures of strain and electronic textures in YbFe2O4

Multiferroics: a magnetic twist for ferroelectricity

Iron Superconductivity

Monday 7 November 2011

Iron Superconductivity: mechanism, experiments and new materials.

Useful reading:

Iron Superconductivity Weathers Another Storm

Spin ice and monopole

Monday 24 October 2011

Quantum Ice and Monopoles: Basic concepts, experiments and new materials

Background reading:

Graduate Lectures:

Graduate Lectures:

Graduate Lectures 2013: Superconductivity

Lecturer: Prof Peter Hirschfeld, University of Florida
Full Title: Mechanism of superconductivity
Course: Condensed Matter Graduates
Paper: Graduate
Place: Audrey Wood room, Clarendon Laboratory
Time: Wednesday 1 May 2013 at 11 am

Lecturer: Prof Peter Hirschfeld, University of Florida
Full Title: Disorder and superconductivity
Course: Condensed Matter Graduates
Paper: Graduate
Place: Audrey Wood room, Clarendon Laboratory
Time: Tuesday 7 May 2013 from 10 am

Graduate Lectures 2012: Design and Growth of Novel Materials

Lecturer: Prof Paul C Canfield
Full Title: Design and Growth of Novel Materials
Course: Condensed Matter Graduates
Paper: Graduate

Audrey Wood, Clarendon Laboratory

Tuesday 24/04 10.00 - 11.30 am
Thursday 26/04 12.00 - 13.30 pm

Lecturer: Prof Paul C Canfield
Full Title: Characterisation of Novel Materials
Course: Condensed Matter Graduates
Paper: Graduate

Audrey Wood, Clarendon Laboratory

Tuesday 01/05 10.00 - 11.30 am
Thursday 03/05 10.00 - 11.30 am