Reizo Kato

Strain-induced superconductor/insulator transition and field effect in a thin single crystal of molecular conductor

The strain in an organic field-effect channel was controlled using a pressure-sensitive organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br [BEDT-TTF=bis(ethylenedithio)tetrathia-fulvalene]. The difference in thermal properties between the channel and the substrate induced strain with cooling, resulting in a phase transition. An unshrinkable SiO2∕Si substrate was used to strain the subject material to the insulating state at low temperature, whereas a shrinkable polymer-based substrate allowed it to be a superconductor. The strain-induced insulating phase on SiO2∕Si is at the vicinity of the phase boundary with the superconducting phase, actualizing field-effect transistor behavior.

Field-Induced Carrier Delocalization in the Strain-Induced Mott Insulating State of an Organic Superconductor

We report the influence of the field effect on the dc resistance and Hall coefficient in the strain-induced Mott insulating state of an organic superconductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br. Conductivity obeys the formula for an activated transport sigma(square)=sigma(0)exp(-W/k(B)T), where sigma(0) is a constant and W depends on the gate voltage. The gate-voltage dependence of the Hall coefficient shows that, unlike in conventional field-effect transistors, the effective mobility of dense hole carriers ( approximately 1.6x10(14) cm(-2)) is enhanced by a positive gate voltage. This implies that carrier doping involves delocalization of intrinsic carriers that were initially localized due to electron correlation.

Temperature Dependence of Internal Field by Analysis of Specific Heat on an Organic Conductor λ-BETS2FeCl4

uf06c-BETS2FeCl4 (BETS = bis (ethylenedithio) tetraselenafulvalene) system exhibits a mysterious paramagnetic metal (PM)–antiferromagnetic insulator (AFI) phase transition. Just after AF ordering, the large entropy of the Fe 3d localized spin still remains. To investigate why the mysterious free 3d spin exists in the uf070 spin AF ordered state, we measured the specific heat in the vicinity of the PM–AFI transition. The formation process of this AF ordering is discussed in the context of a low-dimensional spin network. It is proposed that the FeCl4 crystal field plays a crucial role in this magnetic ordering via a uf070–d interaction.uf020