M. Nagasawa

Semi-metallic spin density wave phase in (TMTSF)2X (X=AsF6 and PF6) under pressure

In the spin density wave (SDW) phase of (TMTSF) 2 AsF 6 and (TMTSF) 2 PF 6 , we measured the metal-insulator transition temperature T MI and the activation energy E A in the SDW insulating phase under pressure. The ratio 2 E A / k B T MI near the critical pressure ∼10.5 kbar, above which the SDW phase does not exist, is examined and compared with the mean-field theory developed by Yamaji. We conclude that T MI does not correspond to the normal-SDW transition temperature T SDW but is lower than T SDW . Non-linear conduction with finite threshold was observed above T MI .

Non-linear conduction in insulating phase of (DMTSA)BF4 under high pressure

Organic conductor (DMTSA)BF 4 is a compound composed of an electron donor DMTSA and a monovalent acceptor BF 4 The conductivity at room temperature is ∼400Ω -1 cm -1 and behaves metallic. At ambient pressure, the gradual metal-insulator transition was observed at ∼200K. We found that the sample shows non-linear conduction in the insulating state under 0.35GPa. We discuss the apparent similarity with the non-linear electric conduction by the density wave sliding.

Simultaneous measurement of heat capacity and internal relaxation time

A new steady-state method is proposed to simultaneously measure the heat capacity and internal relaxation time using superimposed constant and square-wave power. This method is appropriate for small sample systems with low thermal conductance. The accuracies of the heat capacity and internal relaxation time are almost the same as those determined using other similar methods, but correction for the effect of internal relaxation on the heat capacity is more flexible. The thermal diffusivity and thermal conductivity are also simultaneously determined for rod- and slab-shaped samples. We demonstrate the method for small slabs of synthetic silica in the 80-320 K temperature range.

Electronic properties of (DMTSA)BF4 at ambient pressure

In the organic conductor (DMTSA)BF 4 [DMTSA: 2,3-dimethyl-tetra-seleno-antracene], the electronic system is expected to be the Mott-Hubbard insulator. However, its conductivity at room temperature is 200-400Ω -1 cm -1 and behaves metallic above ∼200K. At ambient pressure we found anomalies both in the DC conductivity components and in the permittivity at ∼80K, similar to those in (TMTTF) 2 X associated with charge ordering at ∼100K. Possible origin of the anomalies is discussed.

Size effect on the spin-density wave dynamics in (TMTSF)2X (X=AsF6, PF6)

Abstract The threshold electric field (E T ) and the pinning lengths for the spin density wave (SDW) sliding were determined for (TMTSF) 2 AsF 6 samples of various cross-sectional area (S). Contrary to large S, E T in small S samples is independent of temperature and is as large as 100mV/cm at 4.2K, We obtained the pinning length ~ 8A in the thinnest sample, that is 4 times larger than that in thicker ones and is equal to that expected in the washboard model. Crystal size effect on the SDW dynamics is discussed.