Thermodynamic Study of an Incommensurate Organic Superconductor (MDT-TSF)(AuI2)0.436

Tetsushi ISHIKAWA, Yasuhiro NAKAZAWA, Satoshi YAMASHITA, Masaharu OGUNI, Kazuya SAITO, Kazuo TAKIMIYA and Tetsuo OTSUBO

Low-temperature heat capacity measurements are performed on the organic superconductor (MDT-TSF)(AuI2)0.436; its critical temperature determined by the transport measurement is 4.5 K. A broad but reproducible thermal anomaly related to the superconducting transition is observed during precise measurements of the heat capacity under magnetic fields of up to 6 T. An important finding is that almost 60% of the normal electron density of states remains even in the superconducting ground state. This phenomenon and the intrinsic broadening of the thermal anomaly are common to another organic superconductor kappa-(BEDT-TTF)4Hg2.89Br8, in which an incommensurate relationship exists between anion and donor lattices. However, it is emphasized that (MDT-TSF)(AuI2)0.436 exhibits the superconducting transition at a relatively high temperature despite the small electronic heat capacity coefficient gamma and non-dimerized molecular arrangement in the donor layers.
(J. Phys. Soc. Jpn., 75, 074606 (2006))

Anomalous Enhancement of Electronic Heat Capacity in Organic Conductors kappa-(BEDT-TTF)4Hg3-dBr8 (X = Br, Cl)

Akihito NAITO, Yasuhiro NAKAZAWA, Kazuya SAITO , Hiromi TANIGUCHI, Kazushi KANODA and Michio SORAI

Peculiar low-temperature electronic properties induced by the filling change from a half-filled band realized in kappa-(BEDT-TTF)4Hg2.89Br8 and kappa-(BEDT-TTF)4Hg2.78Cl8 which belong to the well-known kappa-type family of organic conductors were studies by thermodynamic measurements. We observed unusual behaviors both in the lattice and the electronic heat capacities. The former is associated with the dimensional crossover in the acoustic phonons, which is attributable to the peculiarity of mercury chains in the anion layers. The electronic heat capacity represented by the gamma term at low temperatures is enhanced up to 52-55 mJ K-2 mol-1 (for the formula (BEDT-TTF)2Hg1.5-0.5dX4)  due to anomalous growth of the antiferromagnetic spin fluctuations in these salts.
(Phys. Rev. B, 71, 054514 (2005))

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