Akane Sato

Origin of the resistivity anomalies of (EDT–TTF)[M(dmit)2](M = Ni, Pd)

α-(EDT–TTF)[Ni(dmit)2] comprises crossing columns of EDT–TTF and Ni(dmit)2 and exhibits unique metallic behaviour with a characteristic resistivity peak at around 14 K. The possibility of the development of the superstructure was examined down to 9 K by using a low-temperature imaging plate X-ray system, but no extra X-ray reflections could be found below 14 K. The 14 K anomaly is, therefore, thought not to originate from charge density wave (CDW) instability. New phases of γ-(EDT–TTF)[Pd(dmit)2] and γ-(EDT–TTF)[Ni(dmit)2] were found. γ-(EDT–TTF)[Pd(dmit)2] has parallel columns of EDT–TTF and Pd(dmit)2 and shows a resistivity anomaly around 80 K. Below 80 K, X-ray satellite reflections indicating CDW formation were observed. γ-(EDT–TTF)[Ni(dmit)2] undergoes a metal–insulator transition at around 100 K.

Glass transition in κ-(BEDT-TTF)2Cu[N(CN)2]X (X=Br and Cl)

Heat capacities of κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]X (X= Br and Cl) and deuterated salts were measured by chopped light are calorimetry. A glass transition due to freezing of the conformation of the ethylene moiety in the BEDT TTF molecule was deteeted around 100 K at 0.24 - 16 Hz on each salt. The glass transition temperature (ca. 80 K) at daily time scale (ca. 1 mHz) we corresponds to the temperature at which the ccoling rate or annealing sigmficantly alfect the physical properties of the superconducting transition temperature and Meissner volume on κ-(BEDT-TTF) 2 Cu(N(CN) 2 ]Br and κ-(BEDT-TTF-d 8 ) 2 Cu[N(CN) 2 ]Br.

New BETS Superconductors and Metals with Tetrahedral Non-Magnetic and Magnetic Anions (BETS = Bis(ethylenedithio)tetraselenafulvalene)

Abstract Two types of BETS conductors are reported. Needle crystals of λ-BETS2GaXxY4-x gave a series of new superconductors: λ-BETS2GaCl4 (Tc=7 K), λ-BETS2GaBrCl3 (8 K), λ-BETS2GaCl3F (3.5 K), λ-BETS2GaBr0.5Cl3.5 (6.5 K). λ-BETS2GaBr1.5Cl2.5, which is a semiconductor at 1 bar, exhibited a superconducting transition at 8 K and ca. 1 kbar. Pressure and temperature dependencies of the resistivities of λ-BETS2GaXxY4-x closely resemble those of κ-type BEDT-TTF superconductors. Many BETS metals with magnetic ions (Fe3+, Mn2+, Co2+, Cu2+) were prepared. Among them, κ-type MX4 (M=Fe, Co; X=Cl, Br) salts and θ-type Cu2Cl6 salts are metallic down to at least 4 K. The static susceptibility of λ-BETS2FeCl4 shows a characteristic magnetic field dependence below its metal-insulator transition temperature.

Electrical Resistivity under High Pressure and Upper Critical Magnetic Field of the Molecular Superconductor α-(EDT-TTF)[Ni(dmit) 2]

Electrical resistivity under a high pressure and the upper critical magnetic field ( H c2 ) in the superconducting state were investigated on the ambient-pressure superconductor α-(EDT-TTF)[Ni(dmit) 2 ], which exhibits a phase transition around 20 K and the superconducting transition at 1.3 K under ambient pressure. Above 9.9 kbar, the superconducting transition was not observed down to 0.5 K, while the former transition survived up to 13.7 kbar. The temperature dependence of H c2 was estimated: |d H c2 /d T | T = T c ≈-0.27 T/K ( B ∥ c * -axis); |d H c2 /d T | T = T c ≈-3.5 T/K ( B ∥ a b -plane). The anisotropy of H c2 within the a b -plane was found to be quite small indicating the two-dimensionally isotropic character within the a b -plane in the superconducting state of the salt.

M(dmit)2 and M(dmise)2(M = Ni, Pd) Compounds with One-, Two-and Three-Dimensional Metallic Bands

Abstract The band energy scheme of [(CH3)2(C2H5)2N][Pd(dmit)2]2 at high pressure was discussed and it might be possible that the insulating phase above 7 kbar is related to the 1D metal instability. The low-temperature crystal structures of 2D M(dmit)2(M=Ni, Pd) compounds, α-[(CH3)2(C2H5)2N]-[Ni(dmit)2]2 and (C7H16N)[Ni(dmit)2]2 were determined by imaging plate(IP) system equipped with a closed-cycle herium refrigirator. The X-ray satellite reflections of γ-(EDT-TTF)[Pd(dmit)2]2 indicating CDW formation below 80 K were also observed by IP system. [(CH3)3HN] [Ni(dmise)2]2 has a nearly three-dimensional Fermi surface owing to a strong intersheet interaction between their selone groups.

Magnetic and transport properties of an ambient-pressure superconductor α-EDT-TTF[Ni(dmit)2]

The anisotropy of the upper critical field, Meissner and diamagnetic shielding effect, temperature dependence of magnetic susceptibility were examined for an ambient pressure superconductor, α-EDT-TTF[Ni(dmit)2]. The upper critical field was found to be almost isotropic within conductive ab-plane. The Meissner and diamagnetic shielding effect revealed that this salt is a bulk superconductor that has a diamagnetic transition of 1.39 K. The magnetic susceptibility does not exhibit any drop suggesting CDW formation.

A novel superconductor with insulating magnetic ground state

Abstract The BETS conductor with mixed anions, λ-BETS 2 Fe x Ga 1− x Cl 4 (BETS=bis(ethylenedithio)tetraselenafulvalene) exhibited an unprecedented superconductor-to-insulator (SC–I) transition around x =0.43 at ambient pressure. The large drop in magnetic susceptibility at T c (≈4.2 K) and its recovery at T SC–I (≈3.2 K) clearly showed that this SC–I transition reflects an intrinsic nature of the crystal. The magnitude of ac susceptibility (≈−40 emu mol −1 ) at 4 K, where the system is in the superconducting state, gave the Meissner volume fraction of about 75%. The anisotropy and field dependence of magnetization below T SC–I indicate the insulating ground state to be antiferromagnetic with easy axis parallel to c . The systematic examination of the resistivities of λ-BETS 2 Fe x Ga 1− x Cl 4 showed that the system undergoes a MI transition at x > x c1 (≈0.5), SC transition at x x c2 (≈0.35) and SC–I transition at x c2 x x c1 . The SC–I transition was also observed in λ-BETS 2 Fe x Ga 1− x Cl 4 ( x ≈0.55) at 1 kbar, which exhibited a metal–insulator (M–I) transition at ambient pressure. The high-pressure resistivity measurements revealed a superconducting transition in λ-BETS 2 Fe 0.7 Ga 0.3 Cl 4 . Based on these resistivity measurements, the preliminary P-T- x phase diagram of λ-BETS 2 Fe x Ga 1− x Cl 4 was obtained. The anisotropy and field dependence of the susceptibilities of λ-BETS 2 Fe x Ga 1− x Cl 4 ( x =1.0, 0.7, 0.55 and 0.43) suggested the characteristic π-d coupled antiferromagnetic structure of the insulating ground state. The Br-containing system, λ-BETS 2 Fe x Ga 1− x BrCl 3 was semiconducting at ambient pressure. But λ-BETS 2 Fe x Ga 1− x BrCl ( x =0.5) exhibited a SC–I transition ( T c =7 K and T SC–I =3 K) at 2 kbar. While λ-BETS 2 Fe x Ga 1− x BrCl ( x ≈0.1–0.2) showed a SC–M (metal) transition around 1.5 kbar. It was quite surprising that the susceptibility of λ-BETS 2 Fe x Ga 1− x BrCl ( x ≈0.1) showed the antiferromagnetic ordering of the diluted Fe 3+ spins around 4 K at ambient pressure. The recovery of the metallic state below T SC–M in this system seems to be connected with the magnetic ordering of Fe spins.

Development and low-temperature crystal structures of M(dmiX)2(X=S,Se) conductors

Abstract The low-temperature crystal structures of α-[(CH 3 ) 2 (C 2 H 5 ) 2 N][Ni(dmit) 2 ] 2 (11 K), (C 7 H 16 N)[Ni(dmit) 2 ] 2 (20 K) and γ-[(CH 3 ) 2 (C 2 H 5 ) 2 N][Ni(dmit) 2 ] 2 (4.3 K) were determined by use of new imaging plate(IP) system equipped with He refrigerator; γ-[(CH 3 ) 2 (C 2 H 5 ) 2 N][Ni(dmit) 2 ] 2 was found to be metallic down to 2K with the Pauli-like magnetic susceptibility and to have the round 2D Fermi surfaces at 4.3 K. [(CH 3 ) 3 NH][NI(dmise) 2 ] 2 and [(CH 3 ) 2 H 2 N][Ni(dmit) 2 ] 2 gave 3D Fermi surfaces owing to the strong intersheet interactions between their selenone groups.

The new synthetic metals of M(dmise)2: [Me3HN][Ni(dmise)2]2 and (EDT–TTF)[Ni(dmise)2]

Both of the newly synthesized [Me3HN][Ni(dmise)2]2 and (EDT–TTF)[Ni(dmise)2] exhibit metallic behaviour around room temperature at ambient pressure; tight-binding band calculation indicates that the former has a nearly three-dimensional Fermi surface owing to a strong intersheet interaction between their selone groups.

Calorimetric study of metal-insulator transition in (DIMET)2I3

Heat capacity of a quasi-one-dimensional organic conductor (DIMET) 2 I 3 has been measured by chopped-light ac calorimetry below room temperature. An anomaly due to a metal-insulator transition has been clearly detected around 40 K. The magnitude of the anomaly is analyzed through comparison with other Q1D conductors. A possible SDW mechanism is suggested.