Geometrical frustration, quantum entanglement and disorder may prevent long-range order of localized spins with strong exchange interactions, resulting in a novel state of matter. Organic charge-transfer salts are considered the best approximation of this elusive quantum-spin-liquid state, although its ground-state properties remain puzzling. Within this PhD project, we want to conduct ESR investigations in a broad frequency range and down to low temperatures because it is the only spectroscopic method capable to probe the low-energy dynamics of spin liquid candidates.
This PhD 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 recently developed ultra-low-temperature broadband techniques of electron spin resonance, we directly probe 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 .
 Miksch, et al.Science 372, 276 (2021)