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Emergence from Quantum Frustration and Topology

Periodic Reporting for period 2 - EQFT (Emergence from Quantum Frustration and Topology)

Reporting period: 2020-04-01 to 2021-09-30

When electrons are strongly interacting with one another, the rules of quantum mechanics dictate that they correlate or entangle their motion and spin orientation with each other and in cases of strong entanglement the resulting emergent behaviour cannot be understood in terms of individual electrons acting independently of one another, but is the result of all acting together in unison. Understanding the principles that govern such emergent quantum behaviour is one of the forefront intellectual challenges in physics, such understanding may also lead to development of new technologies based on controlling and manipulating quantum entanglement to achieve new functionality. The overall aim of the current research is to explore experimentally the manifestation and control of emergent properties of quantum materials in the presence of strong correlations and spin-orbit coupling, when the spin and orbital angular momentum of electrons are strongly entangled. This is a largely experimentally unexplored regime where theoretical guidance suggests a fertile ground to potentially discover completely new types of correlated quantum behaviour, ranging from quantum spin liquids, where a local spin flip creates multiple exotic quasiparticles with fractional quantum numbers, to novel forms of magnetic order, with counter-rotating spin spirals or spontaneously formed periodic arrangements of spin vortices, to magnetic quasiparticles with topological properties. Single crystals of spin-orbit dominated quantum materials, with key ingredients to exhibit correlated quantum behaviour, will be synthesized and their magnetic states will be probed using the latest advances in neutron and resonant x-ray scattering that allow unprecedented high-sensitivity mapping of the static and dynamic correlations in space and time (or momentum and energy). The results will be compared with the latest theoretical models of many-body correlated quantum states. This aim is to establish the experimental manifestation and manipulation of magnetic quasiparticles with topological character and help build a systematic understanding of the organizing principles that govern emergent quantum phases of matter in the unexplored regime of strong correlations and spin-orbit entanglement.
From the start of the project, work has been performed on several of the research themes outlined in the original proposal, including experimental exploration of topological magnetic quasiparticles, magnetic excitations in quantum magnets on strongly frustrated lattice geometries, exploration of new structural families of spin-orbit Mott insulators, experimental manifestation of quantum criticality, synthesis of new classes of rare-earth magnets with strong spin-orbit coupling as candidates to display novel correlated quantum behaviour. Main results obtained so far include direct visualization of the isospin texture of the quantum wavefunction of topological magnetic quasiparticles, experimental identification of a novel mechanism for ground state selection by quantum fluctuations in the strong spin orbit regime, new experimental and theoretical results on the role of glide symmetry breaking near quantum phase transitions, direct experimental observation of avoided quasiparticle decay due to strong quantum interactions and experimental observation of the complete momentum- and energy-dependent spectrum of coherent quantum fluctuations in a frustrated triangular quantum antiferromagnet, discovery of a new crystallographic superstructure in a doped spin-orbit Mott insulator that interpolates between honeycomb and triangular structures, first characterization of spin excitations in a Kitaev magnet with counter-rotating spin spirals, experimental characterization of spin dynamics in the proximity of quantum criticality for coupled spin-1/2 ladders, direct observation of electron-phonon couplings in a spin-orbit Mott insulator, experimental characterization of quantum entanglement in a quasi-one-dimensional magnet using inelastic neutron scattering, experimental observation of a transition from a spin-orbit entangled ground state to a spin-only state with quenched orbital moment, induced by applied pressure. Work has also started and is ongoing on the synthesis and experimental exploration of other candidate materials to display novel topological and/or correlated quantum behaviour in frustrated geometries for magnetic ions in the strong spin orbit regime.
The above results give an illustration of the new and rich physics discovered so far. Current and ongoing work will build on the results so far to search for new forms of emergent quantum behaviour from strong correlations and new forms of topological magnetic quasiparticles in quantum magnetic materials in the strong spin orbit regime.
Interplay of quantum fluctuations and frustration in a triangular quantum antiferromagnet.