Transition of a pristine Mott insulator to a correlated Fermi liquid: Pressure-dependent optical investigations of a quantum spin liquid

Systematic pressure- and temperature-dependent infrared studies on the quasi-two-dimensional organic quantum spin liquid β ′ − EtMe 3 Sb [ Pd ( d m i t ) 2 ] 2 disclose the electronic and lattice evolution across the Mott insulator-metal transition. Increasing hydrostatic pressure continuously suppresses the insulating ground state; for p > 0.6 GPa, a Drude-like component develops indicating the appearance of coherent quasiparticles at the Fermi level. In the vicinity of the Mott transition, not only the electronic state changes rapidly, but also the vibration modes exhibit pronounced changes in frequency and Fano constant, underlining the strong coupling between lattice and electrons. The anisotropy of the in-plane optical response becomes inverted above 0.6 GPa. The findings are discussed in detail and summarized in a phase diagram comprising different experimental approaches.