Theory of strongly correlated electron systems
Members: Prof. Joseph Betouras, Prof. Feo Kusmartsev, Dr Ioannis Rousochatzakis
The overarching goal of this research is the fundamental understanding of novel collective phases of matter, such as high-Tc superconductivity and quantum spin liquids, starting from microscopic principles and using modern analytical and computational quantum many-body methods.
1. Quantum criticality, Fermi-surface topological transitions and higher-order van-Hove singularities
Project members: Prof. Joseph Betouras, Prof. Feo Kusmartsev
Designing material properties for technological breakthroughs can be only achieved via the complete understanding of the basic mechanisms that govern the collective behaviour of correlated electrons driving different phases (magnetic, superconducting, multiferroic) and desired characteristics (thermodynamic, transport). Quantum matter, either in equilibrium or out-of-equilibrium, organises itself following principles that modern experimental methods can now uncover. Theoretical efforts should lead to the understanding with predictive power and require modern techniques. There have been many challenges for the theory of correlated electrons, but recent developments and new ideas provide a fresh way forward to explain experimental data that had been a mystery for decades.
The main focus of our group is to study the yet uncovered principles of correlated systems and, at the same time, identify new quantum materials and understand experimental results on carefully chosen existing materials. We achieve this, through a unique blend of highly complementary expertise: many-body analytic field-theory approaches and modelling, ab-initio numerical methods, in close collaboration with our experimental groups and international partners. In particular, we study systems that display rich quantum critical behaviour, and/or systematic effects of topological changes— due to tuning of external parameters—of the Fermi surface (FS) in the correlated regime accompanied by singularities in the density of states (van Hove). We investigate the possibilities of new phases with unexpected physical behaviour. The expectation is that apart from a deep understanding of existing experimental puzzles, our studies will also lead to the discovery of new quantum materials that exhibit this physics.
Papers:
-Catastrophe theory classification of Fermi surface topological transitions in two dimensions,
Anirudh Chandrasekaran, Alex Shtyk, Joseph J. Betouras, and Claudio Chamon
Phys. Rev. Research 2, 013355 (2020)
https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.2.013355
-Multicritical Fermi surface topological transitions,
Dmitry V. Efremov, Alex Shtyk, Andreas W. Rost, Claudio Chamon, Andrew P. Mackenzie, and Joseph J. Betouras,
Phys. Rev. Lett. 123, 207202 (2019)
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.207202
-Novel phases in twisted bilayer graphene at magic angles as a result of van Hove singularities and interactions,
Yury Sherkunov and Joseph J. Betouras,
Phys. Rev. B 98, 205151 (2018)
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.98.205151
-Effects of Lifshitz transitions in ferromagnetic superconductors: the case of URhGe,
Yury Sherkunov, Andrey V. Chubukov, and Joseph J. Betouras,
Phys. Rev. Lett. 121, 097001 (2018)
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.097001
-Magnetic fluctuations and specific heat in NaxCoO2 near a Lifshitz transition,
Sergey Slizovskiy, Andrey V. Chubukov, and Joseph J. Betouras,
Phys. Rev. Lett. 114, 066403 (2015)
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.066403
-Holographic antiferromagnetic quantum criticality and AdS2 scaling limit,
Rong-Gen Cai, Run-Qiu Yang, and F. V. Kusmartsev,
Physical Review D 92, 046005, (2015)
https://journals.aps.org/prd/abstract/10.1103/PhysRevD.92.046005
Grants:
-Designing and exploring new quantum materials based on Fermi surface topological transitions
EPSRC, Standard grant, 1/4/2021 – 30/9/2024
PI: Joseph Betouras, co-I: Mark Greenaway
2. Strongly correlated magnets and spin liquids
Project members: Dr Ioannis Rousochatzakis, Prof. Joseph Betouras
The notion that strongly interacting spins in a solid can evade ordering down to zero temperature by condensing into a correlated spin fluid – the quantum spin liquid (QSL) – has a long history. While initially proposed as parent phases of high-Tc superconductors, QSLs are now established as one of the prime exponents of intrinsic topological orders, states of matter that cannot be described in the conventional Landau paradigm of symmetry breaking orders. QSLs exhibit a range of remarkable quantum phenomena, including long-range entanglement, topological degeneracy, and emergent fermions and gauge fields out of bosonic degrees of freedom. Controlling such properties holds the promise for quantum information advances and computing applications.
After decades of efforts, the search for QSLs in condensed matter systems has seen an unprecedented progress in the last years, with the discovery of a series of strongly correlated magnets that are fertile grounds for QSL physics. Prominent examples include the layered kagome magnets ZnCu3(OH)6Cl2 and Cu4(OH)6FBr, the 3D pyrochlore spin ice Dy2Ti2O7, the Kitaev ruthenate α-RuCl3 and iridate H3LiIr2O6, and the most recent triangular rare-earth delafossites (such as NaYbO2), which open a new arena for exploring spin-ice physics in two spatial dimensions.
Work in the department focuses on modeling such materials from microscopic principles, building from insights from experiment and ab initio calculations and using analytical and large-scale computational many-body methods. A major theme that transcends this research is how we can unambiguously detect QSLs in real materials.
Papers:
-Quantum spin liquids at finite temperature: proximate dynamics and persistent typicality,
I. Rousochatzakis, S. Kourtis, J. Knolle, N. B. Perkins, R. Moessner,
Phys. Rev. B 100, 045117 (2019)
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.100.045117
-Quantum-classical crossover in the spin-1/2 Heisenberg-Kitaev kagome magnet,
Yang Yang, Natalia B. Perkins, Fulya Koc ̧, Chi-Huei Lin, Ioannis Rousochatzakis,
Phys. Rev. Research 2, 033217 (2020)
https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.2.033217
-Quantum spin liquid in the semiclassical regime,
Ioannis Rousochatzakis, Yuriy Sizyuk, Natalia B. Perkins,
Nat. Commun. 9, 1575 (2018) Editors’ Highlights
https://www.nature.com/articles/s41467-018-03934-1
-Classical spin liquid instability driven by off-diagonal exchange in strong spin-orbit magnets,
Ioannis Rousochatzakis, Natalia B. Perkins,
Phys. Rev. Lett. 118, 147204 (2017)
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.147204
-Resonating valence bond physics is not always governed by the shortest tunneling loops,
Arnaud Ralko, Ioannis Rousochatzakis,
Phys. Rev. Lett. 115, 167202 (2015)
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.167202
-Microscopic theory of the nearest-neighbor valence bond sector of the spin-1/2 kagome antiferromagnet,
Arnaud Ralko, Fre ́de ́ric Mila, Ioannis Rousochatzakis,
Phys. Rev. B 97, 104401 (2018)
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.104401
-Quantum dimer model for the spin-1/2 kagome Z2 spin liquid,
Ioannis Rousochatzakis, Yuan Wan, Oleg Tchernyshyov, Fre ́de ́ric Mila,
Phys. Rev. B 90, 100406(R) (2014).
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.100406
-Phase diagram and quantum order by disorder in the Kitaev K1-K2 honeycomb magnet,
Ioannis Rousochatzakis, Johannes Reuther, Ronny Thomale, Stephan Rachel, N. B. Perkins,
Phys. Rev. X 5, 041035 (2015)
https://journals.aps.org/prx/abstract/10.1103/PhysRevX.5.041035
-Entanglement scaling and spatial correlations of the transverse-field Ising model with perturbations
Richard Cole, Frank Pollmann, and Joseph J. Betouras
Phys. Rev. B 95, 214410 (2017)
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.214410
-Classical correlations of defects in lattices with geometrical frustration in the motion of a particle,
Frank Pollmann, Joseph J. Betouras, and Erich Runge,
Phys. Rev. B 73, 174417 (2006)
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.73.174417
Grants:
-Detecting fractionalization in strongly correlated magnets
EPSRC, New Investigator Award, 1/6/2021-1/6/2024
PI: Ioannis Rousochatzakis
-Spectroscopies of fractionalized excitations in topological strongly correlated matter
U.S. Department of Energy, Office of Basic Energy Sciences, 2017-2019
PI: Natalia B. Perkins, co-I: Ioannis Rousochatzakis
3. Unconventional and high-temperature superconductivity
Project members: Prof. Joseph Betouras, Prof. Feo Kusmartsev
The 1980s’ discovery of high-temperature superconductivity in copper oxides has marked a new era of condensed matter research, culminated in the more recent discovery of iron-based and nearly ferromagnetic superconductors. One of the most crucial goals in this quest has been to understand the “unconventional” mechanisms responsible for this phenomenon -- beyond the electron-phonon mechanism of Bardeen, Cooper, and Schrieffer (BCS). Besides its fundamental significance, a reliable control of the onset temperature Tc for superconductivity above room temperature will revolutionise modern technology. It will enable new means of power transmission and storage, currently infeasible electric and electronic devices, and possibly a material base for quantum computers, similar to what a transistor has been to classical computers.
Work in the department focuses on modelling unconventional superconducting materials of current interest, using analytical and computational quantum many-body techniques, along with insights from experiments and ab initio calculations. The department fosters collaborations with research centres around the world, including Oxford, London, Amherst, Toulouse, Dresden and Minneapolis.
Papers:
-Effects of Lifshitz Transitions in Ferromagnetic Superconductors: The Case of URhGe
Yury Sherkunov, Andrey V. Chubukov, and Joseph J. Betouras,
Phys. Rev. Lett. 121, 097001 (2018)
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.097001
- Non-Landau Damping of Magnetic Excitations in Systems with Localized and Itinerant Electrons
Andrey V. Chubukov, Joseph J. Betouras, and Dmitry V. Efremov
Phys. Rev. Lett. 112, 037202 (2014)
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.037202
-Anomalous magnetoresistance in the spinel superconductor LiTi2O4
K. Jin, G. He, X. Zhang, S. Maruyama, S. Yasui, R. Suchoski, J. Shin, Y. Jiang, H. S. Yu, J. Yuan, L. Shan, F. Kusmartsev, R. L. Greene, I. Takeuchi,
Nature Communications, 6, 7183 (2015)
https://www.nature.com/articles/ncomms8183