K. Kajita

A new type oscillatory phenomenon in the magnetotransport of θ-(BEDT―TTF)2I3

Abstract A new type oscillatory magnetotransport phenomenon has been observed in θ-type crystals of (BEDT-TTF) 2 I 3 at temperatures below 6 K and in the magnetic field above 3 T. The oscillation appears when the magnetic field of a fixed strength is rotated from the direction normal to the conductive two dimensional plane to a direction parallel to the plane. The period of the oscillation is described by an equation tan ( θ min )= sN ( s =0.39, N =0, 1, 2, 3,…), where θ min is the angle giving the position of the trough of the oscillation. The amplitude of the oscillation is primarily determined by the magnetic field component normal to the conductive plane. It is related to the temperature through the change in the resistivity ⪢ 0 .

Transport properties of ((CH3)4N) (Ni(dmit)2)2: A new organic superconductor

Abstract Transport properties of ((CH 3 ) 4 N) (Ni(dmit) 2 ) 2 crystals are studied in the temperature region from 300 to 1.5 K under hydrostatic pressure up to 15 kbar. A resistivity jump at around 100 K and a sharp rise of resistivity below 20 K are characteristic of this crystal. Hydrostatic pressure reduces both anomalies. In the sample where the low temperature anomaly was suppressed by high pressure, we have observed an onset of a superconducting transition under 3.2 kbar at 3.0 K. The transition temperature becomes high with increasing pressure. At 7 kbar, we find T c = 5.0 K .

New organic superconductors K- and θ-(BEDT-TTF)2I3: Transport property

Abstract Transport properties of newly synthesized organic conductors K - and θ-(BEDT-TTF)2I3 are investigated. We find that these materials are quasi-two dimensional metals. Both materials exhibit ambient pressure superconductivity with Tc around 3.6K. The upper critical magnetic field is determined as a function of temperature and the coherence length is estimated. We find that the coherence lengths for k -type in the two dimensional plane and normal to it are about 360A and 13.5A, respectively.

Transport properties of massless Dirac fermions in an organic conductor α-(BEDT-TTF)2I3 under pressure

A zero-gap state with the Dirac cone-type energy dispersion was found in an organic conductor α-(BEDT-TTF)2I3 under high hydrostatic pressures. This is the first two-dimensional zero-gap state discovered in bulk crystals with layered structures. In contrast to the case of graphene, the Dirac cone in this system is highly anisotropic. The present system, therefore, provides a new type of massless Dirac fermions with anisotropic Fermi velocity. From the galvano-magnetic measurements, the density and mobilities of electrons and holes were determined in the temperature region between 77u2009K and 2u2009K. In this region, the carrier density (n) depends on temperature (T) as n∝T2 and decreases by about four orders of magnitude. On the other hand, the sheet resistance per BEDT-TTF layer (RS) stays almost constant in the region. The value is written as RS=gh/e2 in terms of the quantum resistance h/e2=25.8 kΩ, where g is a parameter that depends weakly on temperature.

Reentrant behavior in the pressure-temperature dependence of the resistivity of (DMeO-DCNQI)2Cu

Abstract The electrical resistivities (ϱ) of (DMeO-DCNQI) 2 Cu were measured up to 12.5 kbar and down to 1.5 K. Similar to (DMe-DCNQI) 2 Cu, (DMeO-DCNQI) 2 Cu shows a pressure-induced metal instability. But the critical pressure ( P c = 8 kbar), above which the system exhibits the metal instability, is 10 2 times larger than that of (DMe-DCNQI) 2 Cu. Reentrance of the metallic phase was observed at 8–9.5 kbar. Above 10 kbar, the reentrant behavior disappears and the system undergoes a sharp metal-insulator transition. At the “reentrant pressure region”, ϱ begins to increase at T min with decreasing temperature (T) and reaches its maximum at T max . Between T min and T max , ϱ is linearly dependent on log T indicating the scattering of conduction electrons by the magnetic ions (Cu 2+ ): ρ = ρ 0 + c log T. The content of Cu 2+ seems to vary with pressure. This unique behavior suggests (DMeO-DCNQI) 2 Cu to be a new family of the dense Kondo materials.

Thermal and magnetic properties in organic metals (DMe-DCNQI)2Cu, (DMeO-DCNQI)2Cu and (DMe1-xMeBrx-DCNQI)2Cu: Enhancement of density of states

We have studied the electronic state in new type of organic conductors (DMe-DCNQI)2Cu, (DMeO-DCNQI)2Cu and (DMe1-xMcBrx-DCNQI)2 Cu (0<x<0.10), measuring the low temperature specific heat and magnetic susceptibility. For the sample of (DMe0.945MeBr0.055-DCNQI)2Cu, which reveals puzzling reentrant behavior in resistivity (metal- insulator- metal) with lowering temperature, an extraordinary enhancement of density of states at Fermi level is observed. Such remarkable properties of these Cu salts are discussed based on the mixed-valency of Cu atoms, which implies the mixing of 3d electrons of Cu into 2pπ conduction band of DCNQI through Cu-N coordination bond.

The first molecular superconductor based on the π-acceptor molecule and the closed shell cation, [(CH3)4N][Ni(dmit)2]2

Abstract Molecular designing analyses and electrical properties of [(CH3)4N][Ni(dmit)2]2 and α- and β-[(CH3)4N][Pd(dmit)2]2 are presented. In [(CH3)4N][Ni(dmit)2]2, the resistivity jump at around 100 K and the sharp rise of resistivity below 20 K were observed. In the sample where the low temperature anomaly was suppressed by high pressure, we have observed a superconducting transition: 3.0 K at 3.2 kbar and 5.0 K at 7 kbar. The 100 K anomaly is accompanied by large hysteresis. Non-linear conducting behavior was observed in this temperature region.

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.

Discontinuous Change of Metal-Insulator Transition in the Alloyed θ-system: θ-(BEDT-TTF) 2(Rb 1-xCsx)Zn(SCN) 4

The behaviors of the metal-insulator (MI) transitions for θ-(BEDT-TTF) 2 (Rb 1- x Cs x )Zn(SCN) 4 [ x =0.00, 0.07, 0.25, 0.34, 0.47, 0.61, 1.00] were investigated by transport and magnetic measurements. Although the unit cell volume ( V ) increases and the dihedral angle between the donor columns (θ) decreases in proportion to the Cs content ( x ), the MI transition temperature does not change continuously. For x ≤0.07, the MI transitions occur around 200 K with a large hysteresis due to the structural modulation coupled with charge ordering. Magnetic behaviors in slow and rapid cooling runs are distinctly different. These transitions ( x ≤0.07) are called the Rb-type. The MI transition temperature at x =0.25 drops to 35 K abruptly and the transition temperature decreases slowly with increasing x further. The transport and magnetic behaviors show a reduced hysteresis and no hysteresis is observed at x =1. These features characterize the Cs-type transition (0.25≤ x ). These results indicate that the mechan...

Molecular design and solid state properties of new superconductors and molecular metals with ordered spin structures

Abstract We have examined several new types of molecular metals including three new superconductors, θ-(BEDT-TTF) 2 I 3 , κ-(BEDT-TTF) 2 I 3 and [(CH 3 ) 4 N] [Ni(dmit) 2 ] 2 . According to our molecular designing analyses based on simple tight-binding band calculations, there are two ways to avoid the metal-insulator transition encountered in one-dimensional (1D) systems. They are to design (1) 2D metals and (2) “multi-Fermi surfaces systems”. The 2D layered molecular superconductors, θ- and κ-(BEDT-TTF) 2 I 3 , Ni(dmit) 2 superconductors and magnetic molecular metals, R 1 ,R 2 -DCNQI 2 Cu(R 1 , R 2 =CH 3 , CH 3 O, Br, Cl) are described from the molecular designing point of view.