Periodic Reporting for period 4 - EQFT (Emergence from Quantum Frustration and Topology)
Reporting period: 2023-04-01 to 2024-12-31
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, this is important for society from a fundamental point of view, but also because it may lead to development of new technologies based on controlling and manipulating quantum entanglement to achieve new functionality. The overall aim of the research of this project was 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 and magnetic quasiparticles with topological properties. As part of this research project several quantum materials with strong spin-orbit coupling were investigated in detail, and some were synthesized in single crystal form for the very first time. Several new forms of correlated quantum behaviour were discovered and in several cases this led to new theoretical models of cooperative quantum effects in the presence of strong spin-orbit coupling.
The research performed during the project followed the 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. The main already published results include: direct visualization of the isospin texture of the quantum wavefunction of topological magnetic quasiparticles [reference 11 in the Publications List], experimental identification of a novel mechanism for ground state selection by quantum fluctuations in the strong spin orbit regime [11], new experimental and theoretical results on the role of glide symmetry breaking near quantum phase transitions [8], 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 [6], discovery of a new crystallographic superstructure in a doped spin-orbit Mott insulator that interpolates between honeycomb and triangular structures [4], first characterization of spin excitations in a Kitaev magnet with counter-rotating spin spirals [2], experimental characterization of spin dynamics in the proximity of quantum criticality for coupled spin-1/2 ladders [1], direct observation of electron-phonon couplings in a spin-orbit Mott insulator [3], experimental characterization of quantum entanglement in a quasi-one-dimensional magnet using inelastic neutron scattering [10], 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 [5,7], observation of quantum dispersion renormalization effects and two-magnon excitations in the spin-1 square lattice antiferromagnet La2NiO4 [17], experimental observation of tuning the confinement potential between spinons in an Ising chain using longitudinal fields [16], discovery of novel theoretical mechanisms for quantum bound state formation tuned by transverse field in an Ising chain [18], direct experimental observation of six-fold clock anisotropy arising from bond-dependent exchange interactions in strong spin-orbit coupled honeycomb magnets [12,14], breakthrough in the synthesis of single crystals of rare-earth honeycomb and hyperhoneycomb magnets that realize new platforms for quantum compass spin models in the extreme spin-orbit regime [13,15]. More results on the magnetic field dependence of the continuum excitations in an Ising-like triangular lattice quantum antiferromagnet and observation of a novel field induced phase are currently in the process of being written up.
Several of the above results represent significant advances beyond the state of the art in the research to understand emergent and topological properties of quantum magnetic materials, in particular in the experimental manifestation of magnetic topological quasiparticles, effects of spin-orbital fluctuations in the ground state selection, complete mapping of the spectrum of quantum fluctuations in the canonical triangular lattice antiferromagnet, discovery of novel mechanisms of bound state formation due to strong interactions in Ising-like chains, synthesis breakthrough that allowed discovery of new platforms for realizing quantum compass spin models in the extreme spin-orbit regime.