W. Sasaki

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.

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.

Magnetic susceptibilities of (DMe-DCNQI)2Cu and (DBr-DCNQI)2Cu

The static magnetic susceptibilities (χ) of two molecular conductors, (DMe-DCNQI)2Cu and (DBr-DCNQI)2Cu, are reported. In (DBr-DCNQI)2Cu, χ jumps at the metal-insulator (MI) transition temperature (TMI), suggesting that the disappearance of 2pπ conduction electrons and the appearance of the local magnetic moments on Cu sites take place simultaneously. An anomaly corresponding to the antiferromagnetic phase transition is observed at 13 K. In (DMe-DCNQI)2Cu which is metallic down to very low temperature, χ is almost temperature-independent from room temperature to 150K., and increases in the region of 50 K < T < 120 K. Below 40 K, χ is expressed as, χ = χ0 + C/T. This temperature dependence indicates that the paramagnetism originating from metal electrons is enhanced at low temperature.

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.

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.

Anomalous magnetotransport phenomena in θ-(BEDT-TTF)2I3

Abstract Anomalous magnetotransport phenomena have been observed in θ-(BEDT-TTF)2I3 crystals at temperatures below 15 K. The magnetoresistance M : (1) is a linear function of the magnetic field H, (2) is not affected by the angle between the electric current and the magnetic field, (3) but depends on the magnetic field orientation with respect to the crystal axis. Magnetoresistance is expressed as M = (aH 2 a + bH 2 b + cH 2 c )ϱ 0 -3 2 /H in terms of H = (H a , H b , H c ) , the zero field resistivity ϱ0, and parameters a, b, and c which are independent of temperature and magnetic field. We have found that b ⪢ a >c. Magnetoresistance up to 40 is observed for H = 7T along the b-axis at T = 1.5K.

Transport property of a newly synthesized organic conductor, β''-(BEDT-TTF)2AuBr2

Abstract Transport phenomena of a newly synthesized organic conductor, β″-(BEDT-TTF)2AuBr2 were studied. All the crystals examined were found to be metallic down to very low temperatures. For samples which contain impurities, an increase in the resistivity with decreasing temperature was observed below 13 K. Galvanomagnetic effect was measured at liquid helium temperatures. Negative magnetoresistance at low magnetic field and positive magnetoresistance at high field were found to coexist in this temperature region. Kochlers rule applies to the positive component of the magnetoresistance.