Abstract
Heat capacity measurements of κ-(BEDT-TTF)2X (BEDT-TTF: Bis(ethylendithio) tetrathiafulvalene, X: counteranions) which are classified as two-dimensional (2D) dimer-Mott system are reported. At first, we explain structural and electronic features originated from rigid dimerization in donor arrangement in 2D layers. The antiferromagnetic Mott insulating phase located at low-pressure region in the phase diagram shows vanishing γ electronic heat capacity coefficient in the heat capacity, which claims opening of a charge-gap in this insulating state. Then, a systematic change of the γ around the Mott boundary region is reported in relation to the glass freezing of ethylene dynamics. The thermodynamic parameters determined by ∆Cp/γTc of 10 K class superconductors, κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTF)2Cu[N(CN)2]Br demonstrate that a rather large gap with a strong coupling character appears around the Fermi-surface. On the other hand, the low temperature heat capacity clearly shows a picture of nodal-gap structure due to an anisotropic pairing. The comparison with lower Tc compounds in the κ-type structure is also performed so as to discuss overall features of the κ-type superconductors. The heat capacity measurements of hole-doped systems containing mercury in the counteranions show an anomalous enhancement of γ, which is consistent with the T1−1 of NMR experiments etc. The results of heat capacity measurements under high pressures are also reported.
Highlights
Organic charge transfer salts consisting of donor/acceptor molecules with their counter ions provide us with interesting research stages to study various electronic properties peculiar for molecular systems.In these salts, the constitutive donor/acceptor molecules stack in segregated sheet/column structures from those of counter ions
Thermodynamic investigations by low temperature heat capacity measurements in κ-(BEDT-TTF)2X are reported, where X = Cu(NCS)2, Cu[N(CN)2]Br including partially and fully deuterated compounds, Cu[N(CN)2]Cl, Ag(CN)2H2O and κ-(BEDT-TTF)4Hg2.89Br8, κ-(BEDT-TTF)4Hg2.78Cl8 and κ-(MDT-TTF)2AuI2 which are known as 2D strongly correlated electron systems
The Mott insulating salts of κ-(BEDT-TTF)2Cu[N(CN)2]Cl and deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br shows vanishing γ, which is insensitive to magnetic fields in heat capacity, which claims the opening of the charge-gap in the Mott insulating state
Summary
Organic charge transfer salts consisting of donor/acceptor molecules with their counter ions provide us with interesting research stages to study various electronic properties peculiar for molecular systems. The appearance of superconductivity with relatively high transition temperatures, antiferromagnetism with localized spin characters, and a kind of spin liquid states are extensively studied in dimerized salts of κ-(BEDT-TTF)2X [4,5]. On the other hand, deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br (eight hydrogen atoms in ethylene group of BEDT-TTF molecule were deuterated) and κ-(BEDT-TTF)2Cu[N(CN)2]Cl located in the low pressure region show insulating behaviors Thermodynamic measurements on these salts have been performed by several groups with the purpose of characterizing the 10 K class superconductivity and electronic structure in the phase diagram surrounding it. We discuss low temperature heat capacity results of insulating, and metallic compounds with κ-type salts focusing on the electronic states and the phase diagram
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