Phase Transition of Spin Crossover Complex [Fe(2-pic)3]Cl2 · H2O :
A Very Large Hysteresis?

Heat capacities of a spin-crossover complex [Fe(2-pic)3]Cl2 · H2O, for which a very large hysteresis (∼90 K) in its phase transition has been reported, were measured. Upon cooling, the complex shows a phase transition from the high-spin state to a low-spin state at ca. 200 K. Upon heating, when the sample was held at a temperature around 200 K, a stabilization to another low-spin state occurred. The satbilized sample shows a phase transtition from the low-spin state to the initial high-spin state at ca. 280 K upon heating. It is proved that the large hysteresis is originated in a hysteresis-like phenomenon in which the metastable phase is involved.

(by T. Nakamoto & M. Sorai)

Quasi-Particle Excitations and
Anisotropy of Energy Gap
in the Superconductive States of
Organic Conductor κ-(BEDT-TTF)2X

The low-temperature heat capacity of a single crystal of κ-(BEDT-TTF)2Cu[N(CN)2]Br was studied in the temperature range between 0.11 and 4.5 K. The electronic heat capacity, Cel, in the superconductive state seems to show a quadratic temperature dependence at low temperatures below 2 K and residual γ of about 1.2 mJ K−2 mol−1 is also observed. These results are most reasonably attributed to the quasiparticle excitations of the d-wave superconductor with line-nodes in the gap structure. The magnetic field dependence of temperature-linear term γ, in Cel is also consistent with this node structure. The d-wave symmetry of superconductive electrons is strongly suggested.

(by Y. Nakazawa)

Electronic Heat Capacity Coefficient
around the Metal-Insulator Transition
in Two-Dimensional Electron System of
κ-(BEDT-TTF)2X

The electronic heat capacity of partially deuterated organic conductors, κ-(BEDT-TTF)2Cu[N(CN)2]Br, which are situated around the critical boundary of a metal-insulator transition in two-dimension, are studied. It was found that when the system has access to the metal-insulator boundary with the progressive deuteration of BEDT-TTF molecules, the normal-state γ term decreases monotonously. The enhancement of electron mass predicted by the Brinkmann-Rice theory was not found in this system. This result is marked as a basis for the future study of electronic features related to the nature of superconductivity and also on the universality and variation of a possible Mott transition in two-dimensional systems.

(by Y. Nakazawa)

Metal-Insulator Transition Due to SDW Formation in a Quasi-One-Dimensional Organic Conductor (DIMET)2I3 at 40 K

Heat capacity of a quasi-one-dimensional (Q1D) organic conductor (DIMET)2I3 has been measured by chopped-light ac calorimetry below room temperature. An anomaly due to a metal-insulator transition has been clearly detected around 40 K. This proves that the M-I transition in (DIMET)2I3 is not a crossover between metal and insulator due to carrier localization but a phase transition in thermodynamics. The magnitude of the anomaly is analyzed through comparison with other Q1D conductors in the TMTSF and TMTTF families. A possible SDW mechanism is suggested. The high transition temperature puts (DIMET)2I3 outside the Jérome phase diagram, which has been widely accepted for the TMTSF and TMTTF families.

(by K. Saito)