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

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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.