Electric and magnetic properties and phase diagram of a series of organic superconductorsλ−BETS2GaXz

Abstract

The physical properties of a series of organic superconductors with fourfold quasistacking structures, -BETS 2 GaX z Y 4Ϫz ͓BETSϭbis͑ethylenedithiotetraselenafulvalene; X,Y ϭF, Cl, Br; 0ϽzϽ2)] are changed continuously by tuning the value of z. The compound -BETS 2 GaBr z Cl 4Ϫz , has a superconducting ground state for 0ϽzϽz c ͑Ϸ0.8͒ and a semiconducting state for z c ϽzϽ2.0. The maximum T c is about 10 K, showing the existence of a 10-K class of non-ET organic superconductors ͓ETϭbis͑ethylenedithio͒tetrathiafulvalene͔. The temperature dependence of the resistivity closely resembles that for -type ET superconductors but the susceptibility behavior is different. The susceptibility of -BETS 2 GaBr 1.5 Cl 2.5 is isotropic and decreases fairly sharply below 30 K. No indication of a spin-flop transition was observed at 10 K, suggesting that the insulating phase that neighbors the superconducting phase is not antiferromagnetic at least down to 10 K.Since the discovery of the organic superconductor TMTSF 2 PF 6 , 1 a number of molecular superconductors have been developed by using planar molecules such as TMTSF, BEDT-ITF, 2 M (dmit) 2 , 3 etc. Bechgaard salts TMTSF 2 X (XϭPF 6 ,ClO 4 ,...͒ and -type BEDT-TTF ͑or ET͒ conductors are representative organic superconductors whose low-temperature properties have been intensively investigated. In spite of the large difference in the dimensionality of -metal bands, the phase diagrams of these two organic superconductors seem to share a common nature, that is, the superconducting phase neighbors in the antiferromagnetic insulating phase. In Bechgaard salts, the magnetic insulating ground state originates from the one dimensionality of the metallic band, whereas the strong electron correlation in twodimensional metal is considered to be responsible for the appearance of the magnetic phase of -type ET superconductors. At high pressure, the magnetic insulating states of TMTSF and ET systems are changed to superconducting states. A typical example of the phase diagram of organic conductors can be seen in the series of the isomorphous Bechgaard salts of TMTCF 2 X, ͓C͑chalcogen atom͒ϭS or Se͔. With increasing pressure, the ground state of TMTCF 2 X system tends to change in the following manner: spin-Peierls state→spin-density wave ͑SDW͒ state→superconducting state→metallic state. 4 The antiferromagnetic insulating phase was also found in the dmit superconductor, ␤-͓(CH 3 ͒ 4 N͔͓Pd(dmit) 2 ͔ 2 . 5,6 Contrary to a Bechgaard salt, this system is composed of the closed-shell cations and -acceptor molecules ͓Pd(dmit) 2 ]. ␤-͓(CH 3 ͒ 4 N͔͓Pd(dmit) 2 ͔ 2 exhibits a superconducting transition above 5 kbar (T c Ϸ6 K͒, 5 while NMR and ESR studies gave an evidence of the antiferromagnetic ground state at ambient pressure. 7 The dimeric molecular arrangement of ␤-͓(CH 3 ͒ 4 N͔͓Pd(dmit) 2 ͔ 2 resembles that of TMTCF 2 X and is quite different from the dimeric arrangement of the so-called -type ET superconductor. But the weakly semiconducting behavior of ␤-͓(CH 3 ͒ 4 N͔ ͓Pd(dmit) 2 ͔ 2 at ambient p...

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.…”

The influence of Sm doping in the electron-doped manganites La0.9Te0.1MnO3

“…The structural parameters are refined by the standard Rietveld technique [18] and the fitting between the experimental spectra and the calculated values is relatively good based on the consideration of lower RP values as shown in Table 1. The electronic bandwidth W has been used to discuss magnetic and transport properties of perovskites with varied A-site doping [15,19]. The empirical formula of the bandwidth W for ABO 3 -type perovskites using the tight binding approximation [20] is W ∝ cos ω/(d Mn-O ) 3 Table 2.…”

Fermi surface studies in the magnetic-field-induced superconductorλ−(BETS)2FeCl4

“…At zero magnetic field, -(BETS) 2 FeCl 4 shows the metal-insulator transition at ϳ8 K, which is associated with the antiferromagnetic order of the Fe ions. [1][2][3][4][5][6][7][8] In contrast, the isostructural -͑BETS͒ 2 GaCl 4 remains metallic and has a superconducting transition at ϳ6 K. 5 The insulating phase for -(BETS) 2 FeCl 4 is destabilized by the magnetic field above ϳ10 T, 3,4,7 where the paramagnetic state of the Fe moments is induced by the fields. In the insulating phase, a spin-flip transition from the uniaxial antiferromagnetic ͑AF͒ state to the canted antiferromagnetic ͑CAF͒ state is present around 1 T. 3,4,7 Quite recently, it was reported that superconductivity is induced above 17 T below 1 K when the magnetic field is applied exactly parallel to the conduction plane ͑ac plane͒.…”