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

这一提议的重点是研究具有竞争相互作用的量子材料。对各种量子力学相的测量提供了基本抽象物理的最直接的表现，如量子自旋液体、拓扑行为和量子纠缠。对候选量子材料的涌现现象进行深入的实验和理论研究一直是当前凝聚态物理研究的一个主要主题。了解这些新的量子材料中复杂的磁相互作用对于以更高d过渡金属氧化物的现代理论的形式发展基础科学至关重要，对于设计新的和奇异的材料和功能器件的替代途径的坚实基础也是至关重要的，这为未来一代技术应用提供了真正的希望。对新颖的5D硫酸盐和4D硫酸盐的研究揭示了一系列新的理论建议以及对不寻常状态的预测，如Jeff Half Mott绝缘态、量子自旋液体相、Kitaev量子磁学、非传统超导、Weyl半金属、相关拓扑绝缘体等，这些确实是非常了不起和令人兴奋的。显然，从理论和实验上讲，Irate、Ruthenate和Rigate的物理性质都需要严峻的智力挑战，因此，在这个提议中，我们将重点放在4d Rh/Ru-和5d Ir基氧化物中新的候选量子材料的设计、合成和表征上。量子自旋液体(QSL)是一种新型的量子磁学状态，在这种状态下，由于强烈的量子涨落，长程磁序被抑制到最低温度。这种无缝隙QSL表现出长程量子纠缠和被称为Majorana费米子的分数级自旋激发。这种分数自旋激发不同于在具有长程磁有序的化合物中观察到的常规磁子。因此，本提案旨在通过实施详细的实验研究(基于实验室和最先进的中子、Muon和x射线同步加速器的先进测量)和从头算电子结构计算，研究不同晶体结构和晶格几何结构中的Irate、Ruthenate和Rhate的奇异和非传统磁性基态。这些深入的研究将有助于理解各种相互竞争的相互作用的重要性，例如自旋-轨道相互作用、现场库仑U、晶场、洪德耦合、跳跃和电子带宽。量子材料的非凡结构敏感性的本质也要求非常高质量的单晶，我们正计划利用哈威尔的UCL晶体生长实验室合成这种单晶，并使用各种中央设施对其进行研究。

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.