Thermodynamic investigation by heat capacity measurements of κ-type dimer-Mott organic compounds with chemical pressure tuning

Abstract

Thermodynamic discussion on the physical properties around the Mott boundary of the dimer-Mott system of [Formula: see text]-type (BEDT-TTF)2X system, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and X denotes monovalent counter anions is performed by heat capacity measurements of the [Formula: see text]-(d[n,n] BEDT-TTF)2Cu[N(CN)2]Br of which donor molecules are partially deuterated. We compare temperature dependences of heat capacity of d[2,2], d[3,3] and d[4,4] compounds (defined as shown in Fig. 1) obtained in rapidly cooled and slowly cooled conditions. While the heat capacity of d[2,2] compound shows cooling rate dependence with the slight change of the absolute values Cp between 25 K and 50 K, the temperature dependences of the other compounds do not give drastic change by changing the cooling rate. The compounds of d[2,2] and d[3,3] located very close to the Mott boundary show peculiar hump structure in the C[Formula: see text]T[Formula: see text] that is related to the existence of a kind of lattice instability region around the first-order phase boundary. The temperature and chemical pressure dependences of the anomalous region are discussed in terms of the phase diagram in which the Mott boundary has peculiar rounding at low-temperature region.