Quasi-Continuous Mott Transition from a Spin Liquid to a Fermi Liquid at Low Temperatures

For a long time, the Mott transition has been believed to be the first-order transition with a clear discontinuity at low temperatures, as shown by various experiments and the celebrated dynamical mean-field theory. Recent theoretical works, however, suggest that the transition can be continuous if the Mott insulator carries a spin liquid with a spinon Fermi surface[1,2]. Indeed, recent several experimental studies have suggested the existence of such spin liquids[3]; an organic triangular-lattice system κ-(ET)₂Cu₂(CN)₃ is one of the spin-liquid candidates with a spinon Fermi surface. In the present study, we demonstrate the case of a quasi-continuous Mott transition from a Fermi liquid to a spin liquid in κ-(ET)₂Cu₂(CN)₃[4]. We performed electric transport experiments under fine pressure tuning and found that as the Mott transition is approached, the Fermi-liquid coherence temperature continuously falls to the scale of kelvins, with a divergent quasi-particle decay rate on the metal side, and the charge gap gradually closes on the insulator side. A Clausius-Clapeyron analysis of the pressure-temperature phase diagram provides thermodynamic evidence for the extremely weak first-order nature of the Mott transition. These findings provide additional support for the existence of a spinon Fermi surface, which becomes an electron Fermi surface when charges are delocalized.
[1]T. Senthil, Phys. Rev. B 78, 045109(2008).
[2]R. V. Mishmash et al., Phys. Rev. B 91, 235140 (2015).
[3]Y. Zhou, K. Kanoda, T.-K. Ng Rev. Mod. Phys. 89, 025003 (2017).
[4]T. Furukawa et al., Nat. Commun. 9, 307 (2018).