Chemical Control of Electrical Properties and Phase Diagram of a Series of λ-Type BETS Superconductors, λ-(BETS)₂GaBr_xCl_4-x

Abstract

λ-(BETS)2GaBr x Cl4 - x [BETS = bis(ethylenedithio)tetraselenafulvalene; 0 ≤ x ≤ 2] is a molecular superconductor with strongly correlated conduction electrons. The electrical transport properties of λ-(BETS)2GaBr x Cl4 - 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 T c 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 H c1 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)2GaBr x Cl4 - 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 insulating system with x > 1.0 indicates the insulating ground state seems not to be antiferromagnetic but probably nonmagnetic. The crystal structure determinations of a series of λ-(BETS)2GaBr x Cl4 - x and the calculations of the intermolecular overlap integrals of the highest occupied molecular orbital of BETS were made to elucidate a key factor of the superconducting transition mechanism. The x-dependence of intermolecular overlap integrals seems to suggest that the magnitude of the “spin gap” of the nonmagnetic insulating state tends to be diminished with decreasing x. There exists one intermolecular overlap integral exhibiting a large temperature and x-dependence, which seems to play a crucial role in determining the nature of the ground state.

Universal scaling relations in molecular superconductors

“…This behaviour suggests that the molecular systems are in the low-scattering-rate limit where Γ < (2/π)2∆/h and Γ e follows Γ. We note that the fitted power law for Γ e is also broadly consistent with the temperature dependence of the scattering rate deduced from the temperature dependent resistance of individual examples of the molecular metals; measurements for molecular metals just above T c generally show power-law exponents in the region 1.5 to 2 [13,14,15,16,17,18,19]. is seen (solid line).…”

“…1(a) shows ρ s /c 2 (= ne 2 /m * ǫ 0 c 2 ) and T c derived from µSR measurements in the vortex state [5,6], plotted against σ 0 (T c ) in the highest conductivity direction for a series of molecular superconductors. The materials range from a highly anisotropic quasi-one-dimensional (q1D) organic superconductor ((TMTSF) 2 ClO 4 ), through systems of two-dimensional (2D) layered organic superconductors (BETS and ET salts) to examples of three-dimensional (3D) fulleride superconductors; full details are listed in Table 1 [7,8,9,10,11,12,13,14,15,16,17,18,19,20]. Note that the parameter values vary over several orders of magnitude, which is important for successful determination of scaling properties.…”

Organic Antiferromagnetic Metals Exhibiting Superconducting Transitions κ-(BETS)2FeX4 (X=Cl, Br): Drastic Effect of Halogen Substitution on the Successive Phase Transitions

“…As reported in -type BETS conductors, the decrease of unit cell volume induced by the BrPCl exchange in MX anions produces &&chemical pressure'' in the crystal lattice (20,21). Similar to the real pressure, the chemical pressure enhances the metallic nature of the system.…”

Effects of Localized Spins in Quasi-Two Dimensional Organic Conductors

“…Many experiments have been reported on the λ-type BETS compounds λ-BETS 2 MX 4 (M = Ga, Fe, X = Cl, Br); λ-BETS 2 GaCl 4 undergoes a superconducting transition at 8 K, whereas its Fe analogue exhibit a metalinsulator(M-I) transition at almost the same temperature, [3][4][5][6] though the resistivity of these two is similar above 10 K. This M-I transition of the FeCl 4 salt is accompanied by the antiferromagnetic ordering of Fe spins with high spin state (S = 5/2), which was…”