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Crystals 2012, 2(3), 741-761; doi:10.3390/cryst2030741

Thermodynamic Properties of κ-(BEDT-TTF)2X Salts: Electron Correlations and Superconductivity

1,*  and 2
1 Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama 1-1, Toyonaka, Osaka 560-0043, Japan 2 Condensed Molecular Materials Lab., RIKEN, Hirosawa 2-1, Wako, Saitama 351-0198, Japan
* Author to whom correspondence should be addressed.
Received: 11 April 2012 / Revised: 16 June 2012 / Accepted: 18 June 2012 / Published: 29 June 2012
(This article belongs to the Special Issue Molecular Conductors)
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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.
Keywords: heat capacity; electron correlation; Mott insulator; organic superconductors heat capacity; electron correlation; Mott insulator; organic superconductors
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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Nakazawa, Y.; Yamashita, S. Thermodynamic Properties of κ-(BEDT-TTF)2X Salts: Electron Correlations and Superconductivity. Crystals 2012, 2, 741-761.

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