Theory of Quantum Matter Unit
Associate Professor Nic Shannon
Abstract
The Theory of Quantum Matter (TQM) unit grew considerably in size during the 2013 Financial Year. First to arrive was Dr Hiroaki Ueda in April 2013, from Tokyo Metropolitan University. Dr Ueda was followed by Dr Karim Essafi (Orsay, Paris) and Dr Matthieu Taillefumier (University of Frankfurt). Meanwhile, Mr Rico Pohle returned to the unit as a candidate PhD student. In August 2013, Dr Andrew Smerald, a founding member of the unit, moved on to a postdoc position in the group of Prof. Frederic Mila at EPFL, Switzerland.
The unit continued to work on a range of problems in frustrated magnetism, correlated electrons and statistical mechanics. Scientific highlights included the discovery of a new form of spin liquid on the pyrochlore lattice, and further progress in understanding the ground state of quantum spin ice. Significant publications included works on the field-theoretical description of excitatons in spin-nematics, the crytsalization of magnetic monopoles in spin ice, and a paper on the magnetism of thin films of 3He which was singled out as both an Editor's Suggestion in Physical Review Letters, and a Research Highlight in Nature Materials.
Members of the TQM unit were also active in presenting their findings to the scientific community, giving over 30 presentations at international conferences and research institutions, including more than 20 invited talks. Unit members participated in 8 international workshops and conferences, including the March Meeting of the American Physical Society, and both the Spring and Autumn Meetings of the Japanese Physical Society.
The TQM unit also brought a large number of visitors to OIST in FY 2013, hosting two Research Interns, Mr. Sameer Wagh (IIT Madras) and Mr. Valentin Raban (ENS Lyon), and 16 visitoring reserchers, including Prof. Sir Michael Berry, FRS (Bristol). Each of the visiting researchers gave a research seminar. In addition, Prof Berry gave an extremely well-recieved public lecture in OIST's 500-seat auditorium.
1. Staff
- Prof. Nic Shannon, Associate Professor
- Dr. Ludovic Jaubert, Group Leader
- Dr. Karim Essafi, Researcher
- Dr. Andrew Smerald, Researcher
- Dr. Mathieu Taillefumier, Researcher
- Dr. Hiroaki Ueda, Researcher
- Mr. John Owen Benton, Special Student
- Mr. Rico Pohle, PhD Student
- Mr. Han Yan, PhD Student (Supervised by Prof. Hirotaka Sugawara)
- Mr. Sameer Narahari, Resarch Intern (Summer 2013)
- Mr. Valentin Raban, Research Intern (Summer 2013)
- Ms. Shiho Saito, Research Administrator
2. Collaborations
- Theme: Quantum and classical spin liquids on the pyrochlore lattice
- Type of collaboration: Joint research
- Researchers:
- Mr. Owen Benton, University of Bristol, UK and OIST
- Dr. Ludovic Jaubert, OIST
- Dr. Paul McClarty, MPI-PKS Dresden, Germany and ISIS Oxfordshire, UK
- Prof. Roderich Moessner, MPI-PKS Dresden, Germany
- Prof. Karlo Penc, Research Institute for Solid State Physics, Hungary
- Dr. Frank Pollmann, MPI-PKS Dresden, Germany
- Prof. Nic Shannon, OIST
- Dr. Olga Sikora, National Taiwan University, Taiwan
- Dr. Mathieu Taillefumier, OIST
- Mr. Han Yan, OIST
- Theme: Quantum Spin Nematics
- Type of collaboration: Joint research
- Researchers:
- Dr. Tsutomo Momoi, RIKEN
- Prof. Nic Shannon, OIST
- Dr. Andrew Smerald, OIST
- Prof. Keisuke Totsuka, Yukawa Institute for Theoretical Physics, Kyoto
- Dr. Hiroaki Ueda, OIST
- Theme: Magnetism in 3He
- Type of collaboration: Joint research
- Researchers:
- Dr. Andrew Casey, Royal Holloway, University of London, UK
- Prof. Brian Cowan, Royal Holloway, University of London, UK
- Dr. M. Neumann, Royal Holloway, University of London, UK
- Prof. John Saunders, Royal Holloway, University of London, UK
- Prof. Nic Shannon, OIST
- Theme: Optical response of graphene quantum dots
- Type of collaboration: Joint research
- Researchers:
- Prof. Keshav Dani, OIST
- Dr. Eleftheria Kavousanaki, OIST
- Mr. Rico Pohle, OIST
- Prof. Nic Shannon, OIST
- Theme: Spin ice and statistical mechanics
- Type of collaboration: Joint research
- Researchers:
- Dr. Simon Banks, University College London, UK
- Dr. Laura Bovo, London Centre for Nanotechnology and University College London, UK
- Prof. Steve Bramwell, London Centre for Nanotechnology and University College London, UK
- Ms. Brooks-Bartlett, University College London, UK
- Dr. Karim Essafi, OIST
- Dr. Adam Harman-Clarke, Harman-Clarke Consulting & Geneix start-up, London, UK
- Prof. Peter Holdsworth, Ecole Normale Supérieure de Lyon, France
- Dr. Ludovic Jaubert, OIST
3. Activities and Findings
3.1 Quantum and classical spin liquids on the pyrochlore lattice
Rare-earth pryochlore oxides, a family of materials which includes the spin-ices Dy2Ti2O7 and Ho2Ti2O7, have proved a fabulous resource for exploring the many different types of novel phonomena which occur in frustrated magnets. The TQM unit continues to work intensively on these systems, with research falling into two major sub-themes.
The first major sub-theme these is the construction of a unified theory of magnetic order and spin liquids states in rare-earth pyrochlore oxides with anisotropic exchange interactions. Building on work carried out by Mr Han Yan during his Internship in the TQM unit, Mr Owen Benton has been able to unify the treatment of all known classical phases in a the most-general model of nearest-neighbour exchange interactions on the pyrochlore lattice, discovering a new spin-liquid phase in the process. These results have been validated by very detailed and careful simulations of microscopic models, carried out by Dr Ludovic Jaubert and Dr Matthieu Taillefumier.
The first instalment of these results is now available as a preprint [H. Yan et al., arXiv:1311.3501].
The second major sub-theme is the study of quantum spin ice, a project carried out in long-term collaboration with Prof. Karlo Penc, (SZFKI, Budapest), Dr Frank Pollmann (MPI-PKS Dresden), Prof. Roderich Moessner (MPI-PKS Dresden), Dr Paul McClarty (ISIS, UK), and the former TQM postdoc, Dr Olga Sikora (NTU, Taiwan), and involving both Mr Owen Benton and Prof. Shannon. During FY2013, significant progress was made towards the ambitious goal of establishing the complete ground state phase diagram of a quantum spin ice with both short-range exchange and long-range dipolar interactions.
In particular, during his visit to OIST in March 2014, Prof. Penc was able to confirm an earlier conjecture that the problem of minimizing dipolar interactions on a pyrochlore lattice reduces to the much easier problem of finding the ground state of a two-dimensional Ising model, with anisotropic, short-range, interactions. This insight permits the complete solution for ordered grounds states, with arbitrary competing exchange interactions.
These results have been cross-checked against classical and quantum Monte Carlo simulation, carried out by Dr Paul McClarty and Dr Olga Sikora, and are now being prepared for publication.
Figure 1: Quantum and classical spin liquids on the pyrochlore lattice: New insights into frustrated magnets : probability distribution function showing how the order-by-disorder system Er2Ti2O7 selects its ordered ground state. This work is described in the preprint [Han Yan et al., arXiv:1311.3501v1].
3.2 Quantum spin nematics
Liquid crystals, states in which molecules select a common axis while retaining the ability to flow, are a key ingredient of modern display technology. Quantum spin-nematics, the magnetic analogues of the liquid crystals, remain an important research theme in the TQM unit.
Dr Andrew Smerald continued the research on spin-excitations in Quantum spin nematics which he began during his PhD thesis at the University of Bristol, where he was supervised by Prof. Shannon. The first of a series of papers detailing this work was published in Physical Review B [Andrew Smerald and Nic Shannon, Phys. Rev. B 88, 184430 (2013)]. This paper sheds new light on the physical character of spin excitations, and their connection with the symmetries broken by spin-nematic order, using properties of the SU(3) group famous from the theory of Quarks. Animation of spin excitations based on this work can be found here.
Figure 2: Quadrupolar wave excitation in a quantum spin-nematic, as described by the theory developed in [Andrew Smerald and Nic Shannon, Phys. Rev. B 88, 184430 (2013)]. This theory makes specific predictions for inelastic neutron scattering, which can be used to test for this elusive state of matter.
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The PhD thesis of Dr Smerald, "Theory of the Nuclear Magnetic 1/T1 Relaxation Rate in Conventional and Unconventional Magnets " was also published in FY2013, having been singled out for by Springer Verlag as an outstanding graduate thesis. It is now available as an Ebook, in hardback, or for Kindle. |
Dr. Hiroaki Ueda joined the TQM unit at the beginning of FY 2013, bringing with him considerable expertise on diagrammatic approaches to frustrated magnets. Since then, Dr Ueda has developed an exciting new research theme in the TQM unit - topological excitations in spin nematics. At the same time, he has continued his independent work with Dr Tsutomu Momoi (RIKEN) and Prof. Keisuke Totsuka (YITP), publishing results for the high-field behaviour of frustrated ferromagnets [H. Ueda and T. Momoi, Phys. Rev. B 87, 13, 144417 (2013)].
3.3 Magnetism in 3He
Thin films of 3He offer a unique opportunity to study frustrated magnetism in the extreme quantum limit, with tunable multiple-spin exchange interactions between spin-1/2 3He nuclei
In collaboration with members of the experimental Quantum fluids and solids group of Prof. John Saunders at Royal Holloway, the University of London, Prof. Shannon has explored how such thin films can be used as a model system to study the statistical mechanics of two-dimensional quantum magnets [A. Casey et al., Phys. Rev. Lett. 111, 125302 (2013)].
Figure 3: 3He atoms absorbed onto the surface of graphite form a model system for studying two-dimensional magnetism：The role of competing exchange interactions in two-dimensional ferromagnets was explored in [A. Casey et al., Phys. Rev. Lett. 111, 125302 (2013) ].
This work was singled out as both an Editor's Suggestion in Physical Review Letters, and a Research Highlight "Frustrating Helium" in Nature Materials.
3.4 Optical response of graphene quantum dots
Before joining the TQM unit in September 2013, Mr Rico Pohle spent three months working in OIST's Femtosecond Spectrocopy unit, with Prof. Keshav Dani. There he studied the optical properties of graphene quantum dots. Mr Pohle's return to the TQM unit sparked a series of discussions, which have grown into a collaboration between the two units.
This project, undertaken in collaboration with Dr Eleftheria Kavousanaki, has provided new insight into the real-space character of the electron wave functions in both graphene quantum dots and extended 2D graphene sheets. A paper is now in preparation.
Figure 4: Electronic wave functions in qraphene quantum dots with hexagonal and triangular boundaries.
3.5 Spin Ice and Statistical Mechanics [Jaubert]
A central question in frustrated magnetism is about what happens when we cool down a crystal to very low temperature. Does the system order ? Does it remain disordered and fluctuate ? A bit of both maybe ? This property takes a new flavour when it is mediated by exotic excitations such as magnetic monopoles in spin ice materials.
In collaboration with researchers from University College London and Ecole Normale Supérieure de Lyon, members of the TQM unit have shown how these magnetic charges, even though they emerge from correlations between spins, can effectively decouple from the underlying spin structure to crystallize over a magnetic fluid background with all the characteristics of a Coulomb phase. These results have been published and discussed in the context of recent experiments on pyrochlore oxides and nano-lithographic lattices in Physical Review X 4, 011007 (2014).
Depending on the experimental conditions of magnetic field, physical and chemical pressure, this magnetic fluid background is strongly multi-facet. As for any fluid, interactions between atoms give rise to short-range correlations. In spin ice, they take the form of topological constraints and loop degrees of freedom, whose statistical properties are currently investigated by the TQM unit with Dr. Karim Essafi. The fluctuations of these topological sectors can be measured experimentally and have shown a non-trivial influence of the demagnetisation [Bovo et al. Journal of Physics: Condensed Matter 25, 386002 (2013)].
Figure 5: Fragmentation of the magnetic moments: The spin configuration on the left (a) can be seen in two equivalent ways. On one hand, it is a topological defect carrying a magnetic charge (b), while on the other hand, it is also the sum of two magnetic fields with divergence-full (c) and divergence-free (d) properties, respectively responsible for the charge ordering and the magnetic fluid properties of our model.
4. Publications
4.1 Journals
- Brooks-Bartlett, M. E., Banks, S. T., Jaubert, L. D. C., Harman-Clarke, A. & Holdsworth, P. C. W. "Magnetic-Moment Fragmentation and Monopole Crystallization". Physical Review X 4, 011007, doi:10.1103/PhysRevX.4.011007 (2014).
- Smerald, A. & Shannon, N. "Theory of spin excitations in a quantum spin-nematic state". Physical Review B 88, 22, doi:10.1103/PhysRevB.88.184430 (2013).
- Casey, A., Neumann, M., Cowan, B., Saunders, J. & Shannon, N. "Two-Dimensional Ferromagnetism of a He3 Film: Influence of Weak Frustration". Physical Review Letters 111, 5, doi:10.1103/PhysRevLett.111.125302 (2013).
- Bovo, L., Jaubert, L., Holdsworth, P. C. W. & Bramwell, S. T. "Crystal shape-dependent magnetic susceptibility and Curie law crossover in the spin ices Dy2Ti2O7 and Ho2Ti2O7". Journal of Physics: Condensed Matter 25, 386002, doi:10.1088/0953-8984/25/38/386002 (2013).
- Sasaki, Y. & Ueda, H. T. "Reflection and Refraction Process of Spinwave in a Ferromagnet/Frustrated Ferromagnet Junction System". Journal of the Physical Society of Japan 82, 9, doi:10.7566/JPSJ.82.074604 (2013).
- Shannon, N. "Magnetic monopoles: Entropy lost". Nature Physics 9, 326-327, doi:10.1038/nphys2611 (2013).
- Ueda, H. & Momoi, T. "Nematic phase and phase separation near saturation field in frustrated ferromagnets". Physical Review B 87, 13, doi:10.1103/PhysRevB.87.144417 (2013).
4.2 Books and Other One-Time Publications
- Smerald, A. "Theory of the Nuclear Magnetic 1/T1 Relaxation Rate in Conventional and Unconventional Magnets". Springer, 2013. (University of Bristol PhD thesis, singled out for publication as a Springer Thesis)
4.3 Oral and Poster Presentations
- Shannon, N. "Quantum spin ice and beyond - towards a general theory of Kramers pyrochlores ?", in International workshop on frustration and topology in condensed matter physics, National Cheng Kung University, Taiwan (2014).
- Shannon, N. "Magnetization plateaux in Cr Spinels", in Joint ISSP-OIST Symposium: Lighting Up New Frontiers From Tokyo to Okinawa From Materials to Neurons, OIST, Japan (2014).
- Ueda, H. T. "Topological defects in a spin-nematic phase on the triangular lattice", in JPS Spring Meeting 2014, Tokai University, Japan (2014).
- Jaubert, L. "Spin liquids in pyrochlores", in Topological Matter out of Equilibrium Workshop, Dresden, Germany (2014).
- Jaubert, L. "Spin liquids and magnetic ordering in pyrochlores", The University of Tokyo, Japan (2014).
- Essafi, K. "Nonperturbative Renormalization Group Approach to Polymerized Membranes", University of Orsay, France (2014).
- Shannon, N., Sikora, O., Pollmann, F., Penc, K., McClarty, P. & Moessner, R. "Quantum effects in a realistic model of spin ice", in RIKEN-APW Joint Workshop“Highlights in condensed matter physics”, RIKEN, Japan (2014).
- Taillefumier, M., Robert, J., Canals, B., Henley, C. L. & Moessner, R. "Spin dynamics in the classical kagome antiferromagnet: Theory versus experiments", in APS March Meeting, Denver, USA (2014).
- Jaubert, L. "Spin liquids and magnetic ordering in pyrochlores", in APS March Meeting, Denver, USA (2014).
- Essafi, K. "Nonperturbative Renormalization Group Approach to Polymerized Membranes", in APS March Meeting, Denver, USA (2014).
- Benton, J. O. "An anisotropic Coulomb liquid on the pyrochlore lattice", in APS March Meeting, Denver, USA (2014).
- Ueda, H. T. & Shannon, N. "Topological defects in a spin-nematic phase on the triangular lattice", in APS March Meeting, Denver, USA (2014).
- Pohle, R., Kavousanaki, E., Dani, K. & Shannon, N. "Optics of Graphene from Dot to Sheet", in APS March Meeting, Denver, USA (2014).
- Yan, H. "Effective electromagnetism in rare-earth pyrochlore oxides", in APS March Meeting, Denver, USA (2014).
- Yan, H. "Emergent Gauge Field in Spin Ice", in RIKEN-OIST Mathematical Physics Workshop, OIST, Japan (2014).
- Benton, J. O. "An anisotropic Coulomb liquid on the pyrochlore lattice", in International Workshop on Frustration and Topology in Condensed Matter Physics, National Cheng Kung University, Taiwan (2014).
- Taillefumier, M., Robert, J., Canals, B., Henley, C. L. & Moessner, R. "Semi classical spins dynamics on the kagome lattice", in International Workshop on Frustration and Topology in condensed matter physics, National Cheng Kung University, Taiwan (2014).
- Yan, H., Benton, J. O., Jaubert, L. D. C. & Shannon, N. "Effective electromagnetism in rare-earth pyrochlore oxides", in International Workshop on Frustration and Topology in Condensed Matter Physics, National Cheng Kung University, Taiwan (2014).
- Shannon, N., Sikora, O., Benton, J. O., Pollmann, F., McClarty, P., Penc, K. & Moessner, R. "Quantum spin ice", in RIKEN-APW Joint Workshop“Highlights in condensed matter physics”, RIKEN, Japan (2014).
- Jaubert, L. "Living on the edge : ground-state selection" in “quantum spin-ice” pyrochlores, Institut Néel, Grenoble, France (2013).
- Jaubert, L. "Living on the edge: towards a general theory for pyrochlores", Ecole Normale Supérieure de Lyon, France (2013).
- Jaubert, L. "Towards a general theory for pyrochlores", EPFL, Switzerland (2013).
- Jaubert, L. "Towards a general theory for highly frustrated rare earth pyrochlores", Ecole Normale Supérieure de Paris, France (2013).
- Jaubert, L. "Living on the edge: towards a general theory for pyrochlores", in GDR Mico Workshop, Gif-sur-Yvette, France (2013).
- Ueda, H. T. "Hidden chiral symmetry breaking in the quasi J1-J2 chain under saturation field", in The Physical Society of Japan: Fall-time Meeting 2013, Kochi University, Japan (2013).
- Shannon, N. "Rare Earth Pyrochlores with Anisotropic Exchange Interactions - Spin Ice Goes Quantum ?", in Special Symposium "Quantum Spin Ice", Fall Meeting of Japanese Physical Society, Tokushima University, Japan (2013).
- Shannon, N. "Rare Earth Pyrochlores with Anisotropic Exchange Interactions - Spin Ice Goes Quantum ?", Max Planck Institute for Solid State Research, Stuttgart, Germany (2013).
- Shannon, N. "Rare Earth Pyrochlores with Anisotropic Exchange Interactions - Spin Ice Goes Quantum ?", University Stuttgart, Germany (2013).
- Jaubert, L. "Spin liquids in 2D and 3D pyrochlores", in XXV IUPAP International Conference on Statistical Physics, Seoul National University, Korea (2013).
- Jaubert, L. "Towards a general theory for pyrochlores", Cavendish Laboratory, University of Cambridge, UK (2013).
- Jaubert, L. "Towards a general theory for pyrochlores", London Centre for Nanotechnology, University College London, UK (2013).
- Shannon, N. "Rare earth pyrochlores with anisotropic exchange interactions - spin ice goes quantum?", in PSI Condensed Matter Colloquium Paul Scherrer Institut, Switzerland (2013).
- Shannon, N. "Rare earth pyrochlores with anisotropic exchange interactions - spin ice goes quantum?", Clarendon Laboratory, Oxford, UK (2013).
5. Intellectual Property Rights and Other Specific Achievements
Nothing to report
6. Meetings and Events
6.1 Research Visits and Seminars
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