Ž . Physica C 331 2000 292–296 www.elsevier.nlrlocaterphysc Magnetization study of organic superconductor ž / ž / k- BEDT-TTF Cu NCS 2 2 Hyeong-Jin Kim a, ) , Jae-Hyuk Choi a , Mun-Seog Kim a , Sung-Ik Lee a , Jin-Tae Kim b a National CreatiÕe Research InitiatiÕe Center for SuperconductiÕity, Department of Physics, Pohang UniÕersity of Science and Technology, Pohang, Kyungbuk 790-784, South Korea b Korea Research Institute of Standards and Science, P.O. Box 102, Yusong, Taejon 305-600, South Korea Received 9 November 1999; received in revised form 29 November 1999; accepted 13 January 2000 Abstract Ž . Ž . Ž Ž . . Organic superconductor k- BEDT-TTF Cu NCS BEDT-TTF denotes bis ethylenedithia tetrathiafulvalene with T 2 2 c ,9.5 K has a highly anisotropic nature similar to the high-T superconductor. Temperature dependence of magnetization c was measured in the field range of 1 kOe FH F5 kOe perpendicular to the conducting layers. Near T , the reversible c magnetization deviates from the mean-field behavior due to the positional distortion of vortices, which is reflected by the crossover of the magnetization curves at T ,8.75 K. Far below T where the fluctuation is diminished, the magnetization c was analyzed using Hao and Clem’s model, and the thermodynamic parameters, such as the Ginzburg–Landau parameter k , w Ž. Ž.x w Ž. Ž.x the critical fields H 0 and H 0 and the characteristic lengths j 0 and l 0 , were obtained. q 2000 Elsevier Science c c2 B.V. All rights reserved. PACS: 74.25.Bt; 74.25.Ha; 74.60.Ec; 74.70.Kn Keywords: Organic superconductor; Thermodynamic parameters; Vortex fluctuation 1. Introduction Ž The organic superconductors of the k- BEDT- . Ž TTF X family, where BEDT-TTF denotes bis eth- 2 . y ylenedithia tetrathiafulvalene and X is a monova- lent anion, have a layered structure composed of insulating anion layers and superconducting layers formed by BEDT-TTF molecules. As a member of ) Corresponding author. Tel.: q 82-562-279-5498; fax: q 82- 562-279-5299. Ž . E-mail address: hjkim72@postech.ac.kr H.-J. Kim . Ž . Ž . the k- BEDT-TTF X family, k- BEDT-TTF - 2 2 Ž . Cu NCS with T , 9 K is known to be a strong 2 c Ž . wx anisotropic material g ; 200 1 like the high-T c cuprate superconductors. The distance between su- ˚ Ž . perconducting layers s ; 15 A is nearly the same as that of Bi Sr CaCu O which is a typical two-di- 2 2 2 8 Ž . mensional 2D material. The strong anisotropy com- bined with this structural feature might lead to the weak interlayer coupling, i.e., 2D nature of the or- ganic superconductor. Thus, various phenomena, wx such as the vortex fluctuation 2 and the dimen- w x sional crossover in the vortex structure 3,4 , re- vealed in high-T superconductors and, also, a quan- c 0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0921-4534 00 00026-5