High field alloy, thermoelectric, and mm wave studies of the field induced superconducting state in l-(BETS)2Fe xGa1-xCl4

Abstract

Alloy studies in the π -d organic conductor λ-(BETS) 2 Fe x Ga 1−x Cl 4 have given new insight into the nature of field induced superconductivity (FISC), since the mechanism of the FISC involves cancellation of the π − d exchange field by the external field. Alloying on the Fe x Ga 1−x site allows tuning of the exchange field, thereby influencing the FISC phase boundary. A brief review of the low temperature phases are given, and new high magnetic field thermoelectric and mm wave results that probe the low temperature ground state are presented.The discovery of magnetic field induced superconductivity in λ-(BETS) 2 FeCl 4 [1] (BETS= bis(ethylenedithio)tetraselenafulvalene) has drawn attention to the π−d electron spin exchange mechanisms [2] in molecular systems where magnetic order in the d electron system strongly influences the behavior of the conducting π electron system. The magnetic field dependent phase diagram of the λ-(BETS) 2 FeCl 4 material is shown in Fig. 1. For H =0, and below the metal-insulator transition temperature (T M I = 8.3K), λ-(BETS) 2 FeCl 4 enters a highly insulating antiferromagnetic (AFI) phase [3]. Below T M I , a spin-flop transition to a canted antiferromagnetic (CAF) phase occurs near 1 T, and above 11 T, a paramagnetic metallic (PM) phase appears.At higher magnetic fields , field induced superconductivity (FISC) is stabilized in λ-(BETS) 2 FeCl 4 below 5 K between 18 and 45 T [1,4]. The FISC state involves the cancellation of the exchange field B J by the external magnetic field B ext , i.e. the effective internal field in the conduction electron layer is B ef f = B ext − |B J | ( i.e. the JaccarinoPeter or J-P effect [5,6], where for the π − d discussed here, B J is negative.). Although the AFI-PM transition is nearly independent of field direction, the PM-FISC transition requires careful alignment of the field in the a-c molecular planes to avoid orbital dissipation in the superconducting phase. Recently a similar FISC state has been reported in κ-(BETS) 2 FeBr 4 by Tanatar and co-workers. [11] The more general λ-(BETS) 2 Fe x Ga 1−x Br y Cl 4−y class of organic conductors, with localized magnetic moments at the anion sites, and conduction electrons in the molecularcation layers, exhibit competition between magnetic, metallic, insulating, and superconducting ground states. This is exemplified in Fig. 2a and 2 When the FISC was first discovered, it was tempting to anticipate that in order to exist at high fields, the superconducting state might arise from unconventional mechanisms such as triplet pairing. (See Lebed and Yamaji [10] for a recent theoretical treatment of possible low-D, high field, ground states.) However, the subsequent discovery that the superconducting phase was re-entrant at high magnetic fields, and that it had a "symmetry" around 32 T at the center of the FISC state, suggested that the Jaccarino-Peter mechanism should be seriously considered. Here, in simple terms, an external field of 32 T would cancel the corresponding exchange field J of the same order....