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The crystal of \ensuremath{\lambda}-${(\mathrm{BETS})}_{2}{(\mathrm{F}\mathrm{e}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{)\mathrm{C}\mathrm{l}}_{4}$ with $x\ensuremath{\approx}0.45$ undergoes successive superconducting and superconductor-to-insulator transitions around liquid helium temperature. The large diamagnetic susceptibility (\ensuremath{\approx}-35 emu/mol at 4 K) observed in the superconducting phase of the system with $x\ensuremath{\approx}0.47,$ which exhibits a superconducting transition at 4.3 K and superconductor-to-insulator transition at 3.7 K, can be regarded as evidence for the bulk nature of the superconductor-to-insulator transition. The anisotropy of the susceptibility and spin-flop behavior indicate that the system takes a \ensuremath{\pi}-d coupled antiferromagnetic insulating ground state, which is essentially the same as that of \ensuremath{\lambda}-${(\mathrm{BETS})}_{2}{\mathrm{FeCl}}_{4}$ exhibiting a coupled metal-insulator and antiferromagnetic transition at 8.5 K. The easy axis is parallel to the c direction. Unlike the Ga-rich system $(xl0.35)$ with superconducting ground state, the Fe-rich system such as \ensuremath{\lambda}-${(\mathrm{BETS})}_{2}{(\mathrm{F}\mathrm{e}}_{0.7}{\mathrm{Ga}}_{0.3}{)\mathrm{C}\mathrm{l}}_{4}$ shows a metal-insulator transition at ambient pressure. At high pressure, however, the \ensuremath{\pi} and d electrons tend to be independent of each other and the system takes a superconducting ground state.

λ-(BETS)2GaBrxCl4-x [BETS = bis(ethylenedithio)tetraselenafulvalene; 0 ≤ x ≤ 2] is a molecular superconductor with strongly correlated conduction electrons. The electrical transport properties of λ-(BETS)2GaBrxCl4-x are drastically changed by varying the bromine content x or by applying pressure. At ambient pressure, the superconducting transition could be observed for x < 0.75. The pressure and x dependencies of Tc were examined. The M−H curve (M = magnetization; H = magnetic field) at 2 K indicated the almost perfect Meissner state of the superconducting phase of λ-(BETS)2GaCl4. The Hc1 is ∼8 Oe for H⊥ and 12 Oe for H∥, where H⊥ and H∥ are the magnetic fields perpendicular and parallel to the c axis, respectively. The magnetic susceptibility of λ-(BETS)2GaBrxCl4-x increases with decreasing temperature to ∼60 K, below which the susceptibility becomes x-dependent and tends to be suppressed with increasing x. The isotropic decrease of the static susceptibility at lower temperature observed in the insulatin...

β-[(CH3)4N][Pd(dmit)2]2 is isomorphic to [(CH3)4N][Ni(dmit)2]2, which is the first pure π acceptor molecular conductor exhibiting a superconducting transition. The crystal structure at low temperature, pressure dependence of the resistivity and the magnetic susceptibility of β-[(CH3)4N][Pd(dmit)2]2 were examined. The general features of the phase diagram of β-[(CH3)4N][Pd(dmit)2]2 resemble those of the organic superconductors. At ambient pressure, the resistivity increases gradually with lowering temperature. A superconducting transition was observed at 6—9 kbar. The highest Tc was 6.5 K. Between 5 and 6.3 kbar, a characteristic rapid resistivity decrease was observed at the small temperature range above the offset temperature of the superconducting transition. Although the resistivity is not zero, this temperature region can be regarded as a “pre-superconducting region.” Above 9 kbar the metallic state is stabilized down to low temperature. Above 5 kbar, most of the crystals exhibited resistivity anomali...

The ambient-pressure superconductor a-(EDT-TTF)[Ni(dmit)2], which is a charge transfer salt formed between EDT-TTF and Ni(dmit)2, exhibits a superconducting transition at 1.3 K and a phase transition around 20 K featuring a characteristic resistance hump. We estimated that the room-temperature electrical conductivity was 140–370 and 14–36 S cm−1 for the current parallel and perpendicular to the conducting plane respectively. We measured the upper critical magnetic field (Hc2) in the superconducting phase. The temperature dependence of Hc2 was estimated |d Hc2/dT| T = Te = −0.27 T K−1 (B||c-axis); |dHc2/d T| T = Tc = −3.5 T K−1 (B||ab-plane). The anisotropy of Hc2 within the ab-plane was found to be quite small, indicating a two-dimensionally isotrope character within the ab-plane in the superconducting state of the salt.

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 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.

We have examined time-averaged charges of the molecular donors around the phase boundary between the insulating, superconducting, and metal phases for ${\ensuremath{\beta}}^{\ensuremath{''}}$-type ET [$\mathrm{ET}=\text{bis}$-ethelenedithio-tetrathiafulvalene] salts with one (two) hole(s) per two (three) molecules by using vibrational spectroscopy. Around the phase boundary, the site charges are neither those expected for a well-developed charge-ordered (CO) state nor a homogeneous distribution. The molecular charges exhibit an inhomogeneous distribution just above the insulator-superconductor transition temperature. We have analyzed the distribution of the site charges from the viewpoint of the alternation of intersite Coulomb interactions along the stacking direction. The closeness in the energy of several CO patterns mostly contributes to the inhomogeneous distribution, whereas a large difference (no difference) in the energy levels contributes to the CO (homogeneous) state. Our observations indicate that the instability due to the closeness in the energy of several distributions correlates with the insulator-superconductor transition in nondimerized or weakly dimerized molecular conductors.

We report the anisotropy of the upper critical magnetic field and the pressure-temperature phase diagram for a molecular superconductor, α-EDT-TTF[Ni (dmit) 2], which is a charge-transfer salt formed between EDT-TTF and Ni (dmit) 2. The critical field was found to be almost isotropic within the conductive ab-plane. The pressure-temperature (P-T) phase diagram shows decreasing of a superconducting transition temperature by increasing an applied pressure. These results are discussed in connection with the electrical-anisotropy studies reported for the normal-conducting state of this salts.

Meissner and diamagnetic shielding effects were investigated for an organic superconductor, α-EDT-TTF[Ni(dmit) 2 ] (dmit = 4,5-dimercapto-1,3-dithiole-2-thione, EDT-TTF=ethylenedithio-tetrathiafulvalene), under ambient pressure. The onset of the diamagnetic transition occurred at 1.39 K. From the measurements of the Meissner effect, the volume fraction of the superconducting phase was estimated to be more than 62%. From the measurements of the diamagnetic shielding effect, this fraction was estimated to be about 93%. This high level of volume fraction shows that the superconducting phase is the main phase in α-EDT-TTF[Ni(dmit) 2 ].

The λ-type BETS alloys (BETS=bis(ethylenedithio)tetrathiafulvalene), λ-BETS2FexGa1 − xCl4 exhibited an unprecedented superconductor-to-insulator (SC-I) transition around x = 0.43 at ambient pressure. The large susceptibility drop at Tc and its recovery at tsc-i clearly show this SC-I transition reflects an intrinsic property of the crystals. The anisotropic field dependence of magnetization at 2 K indicated the insulating ground state to be antiferromagnetic. λ-BET2FexGa1 − xCl4 undergoes a MI transition at x>xc1 (≈0.5), SC transition at x

The electrical properties of a series of λ-type BETS conductors, λ-BETS2(FexGa1−x)Cl4 can be controlled continuously by varying the x-value. The system shows a metal-insulator transition for x > x1 (x1 ≈ 0.5) and a superconducting transition for x < x1. The T-x phase diagram indicates that the system shows the superconductor-insulator transition for x2 < x < x1 (x2 ≈ 0.35).

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.