Electronic Heat Capacity and Lattice Softening of Partially Deuterated Compounds of κ-(BEDT-TTF)2Cu[N(CN)2]Br

Abstract

Thermodynamic investigation by calorimetric measurements of the layered organic superconductors, κ-(BEDT-TTF)2Cu[N(CN)2]Br and its partially deuterated compounds of κ-(d[2,2]-BEDT-TTF)2Cu[N(CN)2]Br and κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br, performed in a wide temperature range is reported. The latter two compounds were located near the metal–insulator boundary in the dimer-Mott phase diagram. From the comparison of the temperature dependences of their heat capacities, we indicated that lattice heat capacities of the partially deuterated compounds were larger than that of the pristine compound below about 40 K. This feature probably related to the lattice softening was discussed also by the sound velocity measurement, in which the dip-like structures of the Δv/v were observed. We also discussed the variation of the electronic heat capacity under magnetic fields. From the heat capacity data at magnetic fields up to 6 T, we evaluated that the normal-state γ value of the partially deuterated compound, κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br, was about 3.1 mJ K−2 mol−1. Under the magnetic fields higher than 3.0 T, we observed that the magnetic-field insulating state was induced due to the instability of the mid-gap electronic state peculiar for the two-dimensional dimer-Mott system. Even though the volume fraction was much reduced, the heat capacity of κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br showed a small hump structure probably related to the strong coupling feature of the superconductivity near the boundary.

Thermodynamic properties of the Mott insulator-metal transition in a triangular lattice system without magnetic order

“…The upturn observed at 0 T disappears with increasing magnetic field, while a broad hump structure in the temperature dependence of C p T −1 appears at relatively high magnetic fields of 5-6 T. Since this behavior indicates that the low-temperature entropy shifts to higher temperatures in magnetic fields, the origin of this field dependence cannot be attributed to the 6 K anomaly and percolative superconductivity which is often observed near the 1st-order Mott transition. Considering the magnetic field dependence of the Mott boundary [23] and the bent quantum phase boundary [38,39] (Fig. 1(b)), the origin of the hump structure is also attributed to the critical behavior.…”

“…Furthermore, this deviation cannot be reproduced by extrinsic Schottky anomaly arising from magnetic impurities (the green curve). In the case of the AFI-FL Mott transition, such behavior is absent, and the simple C p T −1 =γ+βT 2 relation is observed even very near the first-order Mott boundary [23,24,26]. The lowtemperature gradual divergence is reminiscent of quantum critical behavior near a quantum critical point (QCP) because of the −lnT -like behavior (the red curve).…”

“…Br, which can access the 1st-order Mott transition between the AFI and the FL states [23,24]. As the peculiarity of the NMI salt is the persistence of the low-energy excitations related to the spin part, the present critical behavior may be induced by the instability of the fractionalization of the electron into the spin and charge sectors.…”

“…Based on the variation in the electronic heat capacity coefficient γ of the normal state shown in Fig. 1(c) [14,[23][24][25][26], the low-temperature FL state can be understood by the electron-mass enhancement with increasing electron correlations (the green arrow) and the decrease in the metallic portion due to the growth of phase coexistence near the Mott boundary (the orange arrow). Although slight percolative superconductivity is left in the AF Mott insulating salts very near the boundary, its γ is almost zero because the volume fraction of the FL is negligible.…”

New Spin on Metal-Insulator Transitions

“…Kawasugi et al achieved simultaneous control of band filling and bandwidth via in situ strain and gate tuning on κ-(BEDT-TTF) 2 X crystals [2]. Another powerful tuning method is partial chemical substitution, as applied in κ-type systems [3] and quasi one-dimensional (TMTTF) 2 X [4]. In the latter case, the Fabre salts with quarter-filled bands exhibit textbook-like charge-ordered states driven by inter-site Coulomb interactions, which also give rise to unconventional superconductivity in β -(BEDT-TTF) 2 X, as reviewed by Ihara and Imajo [5].…”