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

Abstract

We review novel thermodynamic properties of κ-(BEDT-TTF) 2 Cu 2 (CN) 3 and EtMe 3 Sb[Pd(dmit) 2 ] 2 , where BEDT-TTF stands for bisethylenedithiotetrathiafulvalene and dmit stands for 1,3-dithiole-2-thione-4,5-dithiolate unveiled via singlecrystal 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, £. Although the chargetransport 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 £ 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. Ç IntroductionThermodynamic measurements of molecular compounds can give macroscopic information of materials. The accurate measurements of the heat capacity, C p , as a function of temperature give information on the enthalpy, H, and the entropy, S, by integrating C p and C p T ¹1 with respect to temperature. Not only the detection of phase transitions by observing thermal anomalies but also possible phase relations including short range correlations can be investigated. 1,2Measurements under tunable external parameters such as pressure, magnetic field, and electric field can give thermodynamics information as a function of these parameters, leading to the detection of external pressure-and field-induced phase transitions and the understanding of their mechanism. Usually, thermodynamic quantities are determined by ensemble averages obeying statistical principles, which are related to microscopic structures, energies, and quantum mechanical properties of component molecules.For molecular assembly systems, such as organic conductors, organic magnets, metal complexes, liquid crystals, etc., physical properties are determined by molecular structures, electronic state of consisting molecules, molecular arrangements, and also molecular dynamics with various time scales. The heat capacity measurements can give information on changes in these microscopic degrees of freedom, and they are effective in detecting phase transitions and cr...

From charge- and spin-ordering to superconductivity in the organic charge-transfer solids

“…Thermodynamic measurements are consistent with a mix of gapped and gapless behavior at low temperature [260,[490][491][492][493]. The magnetic susceptibility measured using a torque magnetometer, which cancels impurity Curie contributions, remains paramagnetic to very low temperatures (30 mK) [493].…”

“…Spin liquid phase. κ-(ET) 2 Cu 2 (CN) 3 (κ-CN) is considered as one of the best candidates for a quantum spin liquid (QSL) [215,260]. As shown in Fig.…”

Thirty-Year Anniversary of κ-(BEDT-TTF)2Cu2(CN)3: Reconciling the Spin Gap in a Spin-Liquid Candidate

“…It is unlikely that such a small entropy reflects a bulk QSL phase and, instead, it was suggested that C ∝ T reflects domain wall fluctuations of a valence-bond glass [89]. Certainly, the rich insights from the specific heat measurements by Yamashita, Nakazawa, and coworkers [26,76] aroused large attention over the past decade and will remain subject of vivid discussions also in the upcoming years.…”

“…The abovementioned organic candidate compounds were suggested to host a gapless QSL state, based on a finite susceptibility χ(T → 0) > 0 [34,36,39] and non-zero Sommer-feld coefficient observed in specific heat measurements [26,38,39,76]. However, many other Mott-insulating systems with magnetic order or a spin gap are known to date.…”

Rotational tunneling of methyl groups and the electronic heat capacity of EtMe3Sb[Pd(dmit)2]2 under magnetic fields

“…13 As a matter of fact, the γ value of the spin liquid systems give similar magnitude as conducting compounds based on BEDT-TTF molecule and other donors/acceptors. 25,26 Although the gapless feature is commonly observed for spin liquid compounds, there still remains a question whether the gapless characters are stable up to high magnetic field as is true in the Fermi liquid systems. The thermal conductivity measurements for κ-(BEDT-TTF) 2 Cu 2 (CN) 3 in the dilution temperature region revealed possibility of tiny gap opening with Δ = 0.016 J/k B 15 and the μSR measurement detected a very weak internal magnetic field at extremely low temperature region.…”