Naoya Tajima

Supramolecular organic conductor θ-(DIETS)2[Au(CN)4]: unique crystal structure and superconductivity under uniaxial strain (DIETS=diiodo(ethylenedithio)diselenadithiafulvalene)

Abstract The I⋯N type intermolecular interaction is highly useful for the control of the molecular arrangement in organic crystals. One of the strategies for the application of the iodine-based interaction to organic conductors is the utilization of iodine-bonded TTFs and metal-cyanate anions. Among them, the Au(CN)4 salt of an exotic donor molecule “diiodo(ethylenedithio)diselenadithiafulvalene” (DIETS) crystallizes in a so-called θ-type molecular packing of the conducting layer tailored by the I⋯N type donor⋯anion interaction. In contrast to the common θ-salts of BEDT-TTF, the temperature of the metal–insulator transition was lowered under hydrostatic pressure. On the other hand, under the uniaxial strain up to 10xa0kbar, we observed an abrupt drop of the resistivity around 8xa0K and the recovery of the resistance in a magnetic field is characteristic of the superconducting state.

Strange electric/magnetic behaviour of new (BEDT-TTF)(TCNQ)

Electric and magnetic properties for the third phase of (BEDT-TTF)(TCNQ) are investigated. The resistivity exhibits strange hysteresis in a wide range of temperature, with anomalies at 20 K, 80 K, and 175 K. The resistivity under pressure implies that the anomaly at 80 K originates from a kind of phase separation. The anomalies disappear when only 5 % of F1-TCNQ is doped. The magnetic susceptibility measured by SQUID showed apparent change at 20 K and 175 K, but no significant change was observed at 80 K. The spin susceptibility measured by ESR, on the other hand, exhibited gradual increase at the temperature from 80 K down to 20 K.

Organic semiconductors with extremely narrow energy gap

Abstract This paper describes the discovery and investigation of a group of organic conductors that behave as semiconductors with extremely narrow energy gaps when placed under high hydrostatic pressures. The most prominent character of them is that they have strongly temperature dependent carrier density and mobility and yet the resistance is constant over a wide temperature region. Up to now, we confirmed α-type crystals of (BEDT-TTF) 2 I 3 , (BEDT-TSeF) 2 I 3 , (BEDT-STF) 2 I 3 and θ-type crystal of (BEDT-TTF) 2 I 3 belong to this group. Among them, the typical one is α-(BEDT-TTF) 2 I 3 . Under high-pressures, it is a semiconductor with the energy gap of about 1xa0meV. The carrier density decreases by about six orders of magnitude between 300 and 1xa0K. The carrier mobility, on the other hand, increases by six orders of magnitude in the same temperature region. These two effects just cancel out and give rise to the temperature independent resistance. At low temperatures, the mobility goes up to a value as high as 10 6 xa0cm 2 /(Vxa0s).

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

Superconductivity induced by uniaxial strain in the Pd(dmit)2 system

Novel uniaxial strain effect in the strongly correlated two-dimensional system β-(cation)[Pd(dmit) 2 ] 2 [dmit = 2-thioxo-1,3-dithiol-4,5-dithiolate] with the dimer-based crystal structure is described. When cation = Me 4 As, the strain along the crystallographic b axis readily suppresses the non-metallic behavior and induces a superconductivity at 4 K under 7 kbar. On the other hand, small strain around 2 kbar along both the a- and c-axis directions effectively enhances the non-metallic behavior. These unusual results suggest the critical role of intra- and inter-dimer interactions that affects the band width and the effective on-site Coulomb interaction.

New Te-based counter anions for conducting cation radical salts: tetraiodotellurate(II) and hexaiododitellurate(II)

Abstract Synthesis, structure, and resistivity measurements of several cation radical salts formed by the organic donors TMTTF, EDT-TTF, ET and BETS with new Te-based planar dianions, TeI42− and Te2I62− are reported. The anion sheets in these salts consist of discrete mono- or dinuclear anions. Crystal structure of (ET)4TeI4 (1) is based on the “herring bone” arrangement of ET molecules, similar to the α-type. Compound 1 is a semiconductor (ρ r . t . =0.5 Ω cm ) and shows a transition to the insulating state at 210xa0K. (ET)5Te2I6 (2) consists of alternating arrangement of the θ-type donor layer and the checker-like anion sheet. With decreasing temperature, the resistivity of 2 (ρ r . t . =2 Ω cm ) shows a gradual increase followed by a sharp upturn at 110xa0K. (BETS)5Te2I6 (3) is isostructural with 2. This salt is metallic down to 120xa0K (ρ r . t . =0.02 Ω cm ) , and shows a gradual increase of the resistivity followed by a clear transition to the insulating state at about 60xa0K.

Transport properties of organic conductor α-(BEDT-TTF)2I3 and θ-(BEDT-TTF)2I3 under hydrostatic pressure or uniaxial strain

Abstract Pressure controlled switching between a metallic state and a semiconducting state was successfully realised on organic conductors α-(BEDT-TTF) 2 I 3 and θ-(BEDT-TTF) 2 I 3 . Under high hydrostatic pressures, α-(BEDT-TTF) 2 I 3 behaves as an ultra-narrow gap (about 1xa0meV at p =18xa0kbar) semiconductor. It is characterised by strongly temperature( T )-dependent carrier density. It decreases by about 10 6 times between 300 and 1xa0K. When strained in b -axis, on the other hand, this material was found to behave as a quasi-two-dimensional (Q2D) metal with a large Fermi surface. The carrier density is independent of T while conductivity increases by about 10 2 times between 300 and 4xa0K. θ-(BEDT-TTF) 2 I 3 exhibits a similar change. Under the ambient pressure, it is a typical Q2D metal and changes to a narrow gap semiconductor under pressures above about 5xa0kbar.

Anomalous magnetotransport of a highly mobile electron system in α-(BEDT-TTF)2I3

We found that α-(BEDT-TTF)2I3 crystal under high pressures and at low temperatures has a carrier system with high mobility and extremely low density. This system was found to be very sensitive to the magnetic field. Three stepwise changes in the transport properties were observed, when the magnetic field is increased or the temperature were decreased. We ascribed the changes in the state to the change in the mass of the carriers.

Finding of a new organic semiconductor with an extremely narrow gap

We found that α-(BEDT-TTF)2I3 placed under the pressure above 15kbar is a semiconductor with an extremely narrow gap (gap energy might be as low as several kelvins). The Hall coefficient which varies 6 orders of magnitude between 300K and 1K, and a temperature independent conductivity are key properties to understand this material. No such organic conductors nor inorganic conductors are ever known. Anomalous magnetotransport phenomena were observed at low temperatures.