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

Abstract

Systematic pressure-and temperature-dependent infrared studies on the two-dimensional organic quantum spin-liquid β ′ -EtMe3Sb[Pd(dmit)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 a jump both 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.

Molecular quantum materials: electronic phases and charge dynamics in two-dimensional organic solids

“…We can define 'bad metals' by the loss of coherence. Dynamical mean field calculations [311,342,390,391] predict the modification of the Drude peak that extends well above the Ioffe-Regel-Mott limit, in full agreement with experiments on several κ-(BEDT-TTF) 2 X salts [364,392,398,420,428], as shown in Figure 54(a), but also θ-(BEDT-TTF) 2 I 3 .…”

“…interpreted as the closing of a small spin gap in the quantum-spin-liquid state [546,578]. A negligible field effect in β -EtMe 3 Sb[Pd(dmit) 2 ] 2 quantum spin liquid was recently reported by two independent groups [428,560], in contrast to previous measurements [559]. The absence of reproducible experimental data on the thermal conductivity κ(T ) represents a challenge to understanding quantum-spin-liquid nature in organic solids.…”

Low-temperature dielectric anomaly arising from electronic phase separation at the Mott insulator-metal transition

“…The Mott IMT is based on the physical picture that a reduction of electronic correlations (i.e., the rise of W/U) gradually closes the Mott-Hubbard gap: a coherent charge response develops, causing a finite metallic conductivity. Pressure-dependent optical studies on several organic Mott insulators indeed observe this behavior 13,35 . Concerning dielectric response, it was pointed out 36 that even in the case of certain continuous IMTs, the reduction of the charge gap might induce an enhancement in ε 1 , perhaps leading to a divergence at low T. Probing the optical response at THz frequencies is actually a convenient method to monitor the gap contribution to the permittivity.…”

“…From a theoretical point of view, the single-band Hubbard model is well understood 6,7 , and is found to be in excellent agreement with experiments 5,[8][9][10] . While commonly concealed by antiferromagnetism, recent development in the field of organic QSLs enabled investigation of the lowtemperature Mott IMT in absence of magnetic order [11][12][13][14] , revealing finite-frequency precursors of the metal already on the insulating side 15 .…”

“…Below T crit , the IMT is of first order and comprises an insulator-metal coexistence regime 6,16,17 . Figure 1 displays the phase diagram of genuine Mott systems at half filling without magnetic order 15 , as realized in triangular-lattice organic compounds subject to strong geometrical frustration 5,[11][12][13][14]21 . Note, many oxides are charge-transfer insulators 22 and involve a pronounced coupling of the IMT to the lattice 10,23 , whereas the molecular materials studied here are ideal model systems to elucidate single-band Mott insulators without significant structural modifications at the transition to the metallic state.…”