Pressure-tuning of the quantum spin Hamiltonian of a triangular lattice antiferromagnet

Quantum antiferromagnets on a triangular lattice are prototype materials to investigate the phenomena of geometrical frustration in quantum matter. Apart from highly unusual magnetic properties, they possess a very rich phase diagram ranging from a simple unfrustrated square lattice to a quantum spin liquid, yet to be confirmed experimentally. One major obstacle in this area of research is the lack of materials with appropriate (ideally tuned) exchange coupling parameters. Here, we demonstrate an alternative approach where, instead of the chemical composition, the spin Hamiltonian of a triangular lattice antiferromagnet is tuned by hydrostatic pressure. The combination of tunnel-diode-oscillator and electron spin resonance techniques allows us to accurately monitor the spin-Hamiltonian parameters in Cs₂CuCl₄, revealing a significant increase of its exchange coupling ratio from 0.3 to 0.42 at a pressure of 1.8 GPa. A number of emerging field-induced transitions were observed in the high-pressure phase.