Abstract
The Mott metal-insulator transition—a manifestation of Coulomb interactions among electrons—is known as a discontinuous transition. Recent theoretical studies, however, suggest that the transition is continuous if the Mott insulator carries a spin liquid with a spinon Fermi surface. Here, we demonstrate the case of a quasi-continuous Mott transition from a Fermi liquid to a spin liquid in an organic triangular-lattice system κ-(ET)2Cu2(CN)3. Transport experiments performed under fine pressure tuning have 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 continuously closes on the insulator side. A Clausius-Clapeyron analysis provides thermodynamic evidence for the extremely weak first-order nature of the transition. These results provide additional support for the existence of a spinon Fermi surface, which becomes an electron Fermi surface when charges are delocalized.
Highlights
The Mott metal-insulator transition—a manifestation of Coulomb interactions among electrons—is known as a discontinuous transition
Antiferromagnetic interactions are self-conflicting for spins on a triangle-based lattice, which may fall into exotic states called quantum spin liquids[1,2]
Spin liquids were substantiated in κ(ET)2Cu2(CN)[3] and EtMe3Sb[Pd(dmit)2]2 (ET and dmit denote BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene and 1,3dithiole-2-thione-4,5-dithiolate, respectively), which are organic materials with triangular lattices[3,4]; they exhibit unconventional spin excitations of a gapless or marginally gapped nature[3,4,5,6,7,8,9]
Summary
The Mott metal-insulator transition—a manifestation of Coulomb interactions among electrons—is known as a discontinuous transition. Spin liquids were substantiated in κ(ET)2Cu2(CN)[3] and EtMe3Sb[Pd(dmit)2]2 (ET and dmit denote BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene and 1,3dithiole-2-thione-4,5-dithiolate, respectively), which are organic materials with triangular lattices[3,4]; they exhibit unconventional spin excitations of a gapless or marginally gapped nature[3,4,5,6,7,8,9] These properties lead to an intriguing proposal of delocalized spin excitations—spinons with a Fermi surface10,11—which has recently been discussed in a strong spin–orbit-coupled triangularlattice YbMgGaO4 system as well[12,13]. A ClausiusClapeyron analysis of the P−T phase diagram yields thermodynamic evidence for the extremely weak first-order nature of the transition These findings provide additional support for the existence of a spinon Fermi surface in the spin liquid phase
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