Large anisotropy in magnetic field induced superconductors k-(BETS) 2 Fe x Ga 1x Cl 4 S. Uji a, * , C. Terakura a , T. Terashima a , T. Yakabe a , Y. Terai a , Y. Imanaka a , S. Yasuzuka a , M. Tokumoto b , F. Sakai c , A. Kobayashi d , H. Tanaka e , H. Kobayashi e , L. Balicas f , J.S. Brooks f a National Research Institute for Mets, NIMS, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan b AIST, Tsukuba, Ibaraki 305-8568, Japan c ISSP, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan d Research Centre for Spectrochemistry, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan e IMS, Okazaki, Aichi 444-8585, Japan f NHMFL, Florida State University, Tallahassee, FL 32306, USA Abstract Resistance measurements have been performed to investigate the field induced superconductivity in k- (BETS) 2 Fe x Ga 1x Cl 4 (x ¼ 0:47). In magnetic fields parallel to the conduction layers (c-axis), the superconducting (S) phase is induced between 5 and 24 T at 1.6 K. However, when the field is tilted from the c-axis to the b  -axis (normal to the conduction layers), the S phase disappears at around 6°. This strong anisotropic behavior shows that the orbital effect plays a significant role in the destructive mechanism of the superconductivity. The orbital critical field normal to the conduction layers is estimated to be 1.5 T at 1.6 K. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Organic superconductor; Anisotropy; k-(BETS) 2 Fe x Ga 1x Cl 4 For conventional superconductors, Cooper pairs are destabilized under magnetic fields by both the Zeeman and orbital effects. However, it was recently found that a magnetic two-dimensional (2D) organic conductor k- (BETS) 2 FeCl 4 , where BETS is bis(ethylenedithio)tetra- selenafulvalene, shows a superconducting phase only under high magnetic fields [1,2]. At a zero magnetic field, k-(BETS) 2 FeCl 4 shows a metal–insulator transition around 8 K, which is associated to the antiferromagnetic order of the Fe 3þ spins (S ¼ 5=2) [3,4]. The antiferro- magnetic insulating (AFI) phase is removed by the ap- plication of a field of the order of 10 T which stabilizes a paramagnetic metallic (PM) phase due to the gain of Zeeman energy of Fe 3þ moments. Below 1 K, and when a magnetic field is applied parallel to the conduction layers, superconductivity (S) is induced above 17 T and then destroyed above 42 T [1,2]. This field-induced S phase does not appear for fields applied perpendicularly to the 2D conducting layers. In contrast, the iso-struc- tural non-magnetic salt k-(BETS) 2 GaCl 4 , which has a similar Fermi surface [5], remains metallic and shows a superconducting transition at T c  6 K. The supercon- ductivity is destroyed under a magnetic field of 13 T (3 T) parallel (perpendicular) to the conduction layers [6]. In order to investigate the field induced anisotropic superconductivity, we have performed the resistance measurements in k-(BETS) 2 Fe x Ga 1x Cl 4 (x ¼ 0:47). Fig. 1 shows T –H phase diagram for H kc. We have the field induced S phase above 5 T. The maximum T c is 3 K at 14 T. * Corresponding author. Tel.: +81-298-59-5078; fax: +81- 298-59-5010. E-mail address: uji.shinya@nims.go.jp (S. Uji). 0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02765-X Physica C 388–389 (2003) 611–612 www.elsevier.com/locate/physc