Quantum frustration and criticality as new regimes of quantum matter

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

Electrons in solids do not behave independently of one another but correlate their motion to avoid the large repulsive Coulomb forces. Under certain conditions correlation effects can become very strong and lead to completely new properties for the material as a whole. An example is superconductivity with very important technological applications. Here electrical current flows with no resistance, an emergent quantum property of all electrons in the system acting together in unison. A major challenge of current research is to identify the key conditions under which electrons spontaneously organize themselves in such surprisingly robust ways and to determine the quantum rules that govern this behaviour. The research proposed here aims to obtain direct microscopic information about the internal structure and dynamics of unexplored quantum phases of electrons, in particular novel magnetic quantum phases. The research will exploit recent opportunities for high-resolution studies of electronic correlations opened up by new advances in neutron and x-ray instrumentation technology. A central issue of correlated systems is the physics at critical points separating different forms of electronic order. This will be explored experimentally by applying high magnetic fields to an Ising magnet to suppress the transition temperature to spontaneous magnetic order all the way down to zero temperature and thus drive the electrons into a new regime of critical quantum matter on the verge of order with unexplored properties. Here thermal fluctuations are absent but spins fluctuate strongly because of the Heisenberg uncertainty principle with the remarkable property that all ~10^23 spins fluctuate in unison as a single macroscopic quantum state. The energy spectrum of spin fluctuations at the critical point will be measured to compare with current theories of criticality in quantum matter. Of fundamental interest is the behaviour of electrons confined to lattices with strong geometric frustration effects, as realized for triangular layers where no spin arrangement can simultaneously satisfy all antiferromagnetic interactions between nearest neighbours; here many arrangements have comparable energies and quantum fluctuations become very important and can stabilize novel types of quantum order or quasiparticles. This problem of quantum frustration will be explored in a number of insulating and metallic frustrated quantum magnets.

Publications

10 25 50

 
Description We have experimentally realized for the first time in the laboratory one of the canonical models for a continuous quantum phase transition near zero temperature: quantum criticality in a chain of Ising spins in strong transverse magnetic field. We have directly probed the spin dynamics spectrum near criticality in the Ising material CoNb2O6 and observed key signatures of universality governed by a hidden symmetry in the many-body wavefunction, nor observed in nature before. This is an important result as it shows the power of symmetry to describe complex quantum behaviour.

We have experimentally discovered a novel electronic ground states for mobile electrons with orbital degeneracy in hexagonal layers, realized in the metallic antiferromagnet AgNiO2. Via resonant x-ray diffraction we have observed direct evidence for spontaneous charge order co-existing with metallic conductivity on a honeycomb latice. The results show a new mechanism for lifting orbital degeneracy in the close proximity of the Mott transition. In applied magnetic field we have observed evidence for a cascade of magnetic phase transition and have proposed that at high field the magnetic structure can be described as a magnetic supersolid.

We have observed the first direct experimental evidence for a new type of magnetic interaction between magnetic ions with strong spin-orbit coupling. Such Kitaev interactions occur for magnetic moments of Ir ions located at centres of edge-sharing Oxygen octahedra and consist of a strongly anisotropic magnetic exchange coupling only the components orthogonal to the plane of the Ir-Oxygen-Ir bond. In honeycomb and related three-fold coordinated lattices such interactions lead to strong quantum frustration effects and have been predicted to stabilize quantum spin liquid phases or unconventional forms of magnetic order. We have found direct evidence for such physics via our observation of highly complex incommensurate magnetic orders, non-coplanar and with counter-rotating moments in beta- and gamma-Li2IrO3, where Ir ions form lattices that share the same three-fold coordination of the planar honeycomb, but in three dimensions. Such magnetic orders cannot be explained by conventional (Heisenberg) magnetic interactions and require the presence of dominant Kitaev exchanges. The measurements are thus the first direct experimental evidence that such interactions exist in nature and that they can stabilize highly unconventional forms of magnetic order.
Exploitation Route Our experimental results on quantum criticality have stimulated the development of new theoretical models to describe many-body quantum bound states near critical points and the stability of novel spin-density wave magnetic order in the presence of geometric frustration and strong quantum fluctuations. Our breakthrough in experimentally realizing quantum criticality in the Ising chain magnet CoNb2O6 has been followed by several experimental groups around the world with complementary terahertz spectroscopy, nuclear magnetic resonance and specific heat measurements to characterize thermodynamics behaviour near quantum criticality.

Our experimental results revealing novel magnetic behaviour stabilized by Kitaev interactions in honeycomb iridates with three-fold coordinated, edge-shared octahedra have stimulated the development of new theoretical models for quantum spin liquid phases in three dimensional hyperhoneycomb and stipyhoneycomb lattices and for new forms of magnetic order with counter-rotating moments that can occur only in the close proximity of such spin-liquid phases.

We have developed novel inelastic neutron scattering experimental techniques for measuring the spin dynamics in highly-absorbing materials and observed for the first time dispersive spin-wave excitations in any material containing iridium. Materials containing iridium ions are currently the subject of much research in pursuit of novel forms of magnetic behaviour associated with the strong spin-orbit coupling for the iridium electrons. Our experiments have offered a breakthrough in this field in being able to observe spin gaps and magnetic dispersion relations in such materials using high-resolutrion inelastic neutron scattering.

We have demonstrated the power of magnetic resonant x-ray diffraction on extremely small single crystals (17 micron diameter) combined with magnetic symmetry analysis to provide complete solutions to highly-complex incommensurate magnetic structures, non-coplanar and with counter-rotating magnetic moments. Our work is right at the boundary of what is technically experimentally achievable using the technique of single-crystal magnetic resonant x-ray diffraction and we believe our results will stimulate new research groups in using this technique for probing complex magnetic structures that cannot be solved by any other experimental technique.
Sectors Education

URL http://www2.physics.ox.ac.uk/research/quantum-magnetism
 
Description European Union's Horizon 2020 research and innovation program Advanced Grant
Amount € 2,500,000 (EUR)
Funding ID European Union's Horizon 2020 research and innovation program Grant Agreement Number 788814, Emergence from Quantum Frustration and Topology (EQFT) 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 10/2018 
End 09/2023
 
Description Platform Grant
Amount £1,736,109 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 04/2015 
End 03/2020
 
Description Royal Society Newton International Fellowship for Dr Amir Abbas Haghighirad to hold in our research group
Amount £101,000 (GBP)
Funding ID "Synthesis and properties of novel highly frustrated quantum materials" 
Organisation The Royal Society 
Sector Charity/Non Profit
Country United Kingdom
Start 01/2013 
End 12/2014
 
Description Band structure calculations - nickelates and iridates 
Organisation United States Naval Research Laboratory
Country United States 
Sector Public 
PI Contribution We characterized the crystal structure, charge order and magnetic order in various materials and our collaborators performed complementary band structure calculations.
Collaborator Contribution Our theory collaborators have performed ab initio. band structure calculations on layered nickelates and iridates that we explored with neutron and resonant x-ray diffraction techniques.
Impact http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.106.157206 http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.245113
Start Year 2010
 
Description Band structure calculations of iridates and rhodates 
Organisation Goethe University Frankfurt
Country Germany 
Sector Academic/University 
PI Contribution We investigated the crystals and magnetic structure of layered honeycomb 4d and 5d materials and our theory collaborators performed complementary band-structure calculations.
Collaborator Contribution Our collaborators have performed ab initio. band-structure calculations to determine the electronic properties and structural stability of materials we investigated experimentally.
Impact http://journals.aps.org/prb/abstract/10.1103/PhysRevB.92.235119 http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.245113
Start Year 2013
 
Description Novel iridates - Berkeley 
Organisation University of California, Berkeley
Department Department of Physics
Country United States 
Sector Academic/University 
PI Contribution Our research group used resonant and non-resonant x-ray diffraction techniques to solve the crystal structure and magnetic order in single crystal samples synthesized by our collaborators.
Collaborator Contribution Our collaborators synthesized and characterized the single crystal sample of a novel iridium oxide material, which realised a Kitaev material in three-dimensions.
Impact http://www.nature.com/ncomms/2014/140627/ncomms5203/full/ncomms5203.html http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.197201
Start Year 2013
 
Description Novel magnetism in the strong spin orbit regime - Augsburg 
Organisation University of Augsburg
Department Center for Electronic Correlations and Magnetism
Country Germany 
Sector Academic/University 
PI Contribution My research group have performed structural and magnetic properties measurements on powder and single crystal samples synthesized by our collaborators.
Collaborator Contribution Our partners have synthesizes and characterized powder and single crystal samples of various iridium oxide materials.
Impact http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.108.127204 http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.245113 http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.205116
Start Year 2011
 
Description Spiral magnetism in Kitaev materials - Berkeley 
Organisation University of California, Berkeley
Department Department of Physics
Country United States 
Sector Academic/University 
PI Contribution We solved experimentally the incommensurate magnetic structure of several Kitaev materials.
Collaborator Contribution Our theory collaborators developed a theoretical model to describe the magnetic structure we observed experimentally.
Impact http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.197201 http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.245134 http://www.nature.com/ncomms/2014/140627/ncomms5203/full/ncomms5203.html
Start Year 2013