Triangular-lattice quantum-spin-liquid candidates examined by low-temperature broadband ESR spectroscopy

几何挫折、无序和量子纠缠的相互作用可能会阻止强交换耦合局域自旋的长程有序，从而产生一种新的物质状态。各种有机电荷转移盐被认为是这种难以捉摸的量子自旋液态的最佳材料近似。尽管如此，它们的基态性质仍然令人困惑，需要进行仔细的实验。电子自旋共振（ESR）是一种直接探测相关自旋，解决其低能动力学问题的实验工具，并且可以调整到感兴趣的能量尺度。因此，我们建议在宽频率和磁场范围以及非常低的温度下进行ESR研究，以解决有机自旋液体候选者的紧迫开放性问题。本项目旨在探索和理解三角晶格上的量子自旋液体候选者，它们的基态和激发光谱，它们的磁相图，对有效相关的依赖和受挫程度。该项目专注于由S = 1/2电荷转移盐分子二聚体构建的有机量子自旋液体。利用并进一步改进我们最近开发的超低温宽带电子自旋共振技术，我们在前所未有的参数范围（T > 20 mK, B < 8 T, 0.5 GHz < f < 90 GHz）下研究了电子自旋系统的磁性。彻底的温度、场和角度相关实验使我们能够解卷积ESR信号的贡献。我们可以确定激发谱中可能存在的自旋间隙，获得有关价键固体形成和相关争议方面的信息，这对于发展三角晶格上量子自旋液体候选体的理论描述具有特别重要的意义。

The interplay of geometrical frustration, disorder and quantum entanglement may prevent long-range order of strongly exchange-coupled localized spins, resulting in a novel state of matter. Various organic charge-transfer salts are considered the best material approximation of this elusive quantum-spin-liquid state. Still, their ground-state properties remain puzzling and call for scrutinizing experiments. Electron spin resonance (ESR) is an experimental tool that directly probes the relevant spins, resolves their low-energy dynamics, and can be adjusted to the energy scales of interest. Therefore, we propose ESR investigations in broad frequency and magnetic-field ranges and down to very low temperatures to settle pressing open questions of organic spin-liquid candidates.This project aims at the exploration and understanding of quantum-spin-liquid candidates on a triangular lattice, their ground states and excitation spectra, their magnetic phase diagrams, dependences on effective correlations and degree of frustration. The project focusses on organic quantum spin liquids built from S = 1/2 molecular dimers of charge transfer salts. Utilizing and further improving our recently developed ultra-low-temperature broadband techniques of electron spin resonance, we examine the magnetic properties of the electron spin system in an unprecedented parameter range (T > 20 mK, B < 8 T, 0.5 GHz < f < 90 GHz). Thorough temperature-, field- and angular-dependent experiments enable us to deconvolute the contributions to the ESR signal. We can determine the possible existence of a spin gap in the excitation spectrum, gain information on the formation of valence bond solids and related controversial aspects, which are of particular importance for the development of a theoretical description of quantum-spin-liquid candidates on triangular lattices.