Spin-orbit coupling and dimensionality at the heart of quantum magnetism of heavy transition metal oxides

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This proposal is focused on the study of quantum materials with competing interactions. Measurements of the various quantum-mechanical phases provide the most direct manifestation of the underlying abstract physics, such as quantum spin liquid, topological behaviour and quantum entanglement. The in-depth experimental and theoretical investigations of emergent phenomena of the candidate quantum materials have been serving as a major theme of recent condensed matter physics research. Understanding the complex magnetic interactions in these novel quantum materials is crucial for the development of fundamental science in the form of modern theory of higher d transition metal oxides, as well as for the strong foundation of alternative pathways towards the design of new and exotic materials and functional devices, which hold true promise for future generation of technological applications. The investigations on the novel 5d Iridates and 4d rhodates reveal a burgeoning list of theoretical proposals as well as predictions of unusual states, such as Jeff half Mott-insulating state, the quantum spin liquid phase, Kitaev quantum magnetism, unconventional superconductivity, Weyl semimetals, correlated topological insulators, etc., which are indeed truly remarkable and stimulating. The physics of iridates, ruthenates and rhodates clearly warrants serious intellectual challenges both theoretically and experimentally, and hence, in this proposal we focus on design, synthesis and characterisation of the new candidate quantum materials within 4d Rh/Ru- and 5d Ir-based oxides. Quantum spin liquid (QSL) is a novel state of quantum magnetism where long range magnetic order is suppressed due to strong quantum fluctuations down to the lowest temperature. The gapless QSLs exhibit long-range quantum-entanglement and fractionalised spin excitations named as Majorana Fermions. Such fractionalised spin excitations are different from conventional magnons observed in compounds with long-range magnetic ordering. The present proposal is therefore aimed to investigate the exotic and unconventional magnetic ground states of iridates, ruthenates and rhodates within a variety of crystal structures and lattice geometries by implementing detailed experimental study (both laboratory based and state-of-art neutron, muon and x-ray synchrotron based advanced measurements) and ab-initio electronic structure calculations. These in-depth investigations will help in understanding the importance of various competing interactions, e.g. spin-orbit interaction, on-site Coulomb U, crystal field, Hund's coupling, hopping and electronic bandwidth. The nature of the extraordinary structural sensitivity of quantum materials also calls for extraordinarily high-quality single crystals and we are planning to synthesise such single crystals using UCL crystal growth lab at Harwell and investigate them using various central facilities.