Thermodynamics of a Liquid-like Spin State in Molecule-based Magnets with Geometric Frustrations

Abstract

Abstract We review novel thermodynamic properties of κ-(BEDT-TTF)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2, where BEDT-TTF stands for bisethylenedithiotetrathiafulvalene and dmit stands for 1,3-dithiole-2-thione-4,5-dithiolate unveiled via single-crystal calorimetry. These compounds are organic dimer-based Mott insulators with a two-dimensional triangular lattice, where electron correlations produce localized radical spins on each molecular dimer. Néel-type magnetic orderings are prohibited by geometric frustrations and strong quantum mechanical fluctuations. The spin orientation of localized π electron fluctuates like a liquid even at zero energy, and the so-called spin-liquid (SL) ground state appears in them. We have performed heat capacity measurements down to extremely low temperatures and observed that the low-temperature heat capacities show a gapless character, evidenced by the finite electronic heat capacity coefficient, γ. Although the charge-transport properties of these compounds are insulating, the thermodynamic parameters, which reflect the low-energy excitations from the liquid ground state, resemble those of typical metallic compounds with Fermi-liquid characteristics. The magnitude of the γ was scaled with the magnetic susceptibility extrapolated down to T = 0. The realization of the unusual magnetic state coupled with the charge degree of freedom is suggested by several other experiments in addition to the heat capacity measurements. Herein we review thermodynamic discussions based on the experiments reported up to now to clarify the novelty of this magnetic ground state.