Reversible magnetization and irreversibility line of tri-layer superconductor Ba 2 Ca 2 Cu 3 O 6 (O,F) 2 with T c w108 K * Mun-Seog Kim a, * , Kyu-Tae Kim a , Heon-Jung Kim b , Kyung-Hee Kim b , Jin-Ju Ko b , Jung-Dae Kim b , Sung-Ik Lee b , A. Iyo c , Y. Tanaka c a Division of Electromagnetic Metrology, Korea Research Institute of Standards and Science (KRISS), P.O. BOX 102, Yuseong, Daejeon 305-600, South Korea b National Creative Research Initiative Center for Superconductivity and Department of Physics, Pohang University of Science and Technology, Pohang 790-784, South Korea c National Institute of Advanced Industrial Science and Technology (AIST), Central 2, Tsukuba, Ibaraki 305-8568, Japan Received 28 October 2004; accepted 30 November 2004 by A.H. MacDonald Available online 13 December 2004 Abstract We report magnetization measurements of grain-aligned Ba 2 Ca 2 Cu 3 O 6 (O,F) 2 with T c x108 K. The interlayer distance of the material is the shortest among known tri-layer superconductors. Unexpectedly, the magnetization data show that the coupling strength between CuO 2 layers is rather weak. A direct reflection of the weak coupling is highly suppressed irreversibility line, i.e. a broad reversible region in H–T plane. The decoupling field obtained from the irreversibility line is less than 0.1 T, which is comparable with that of quasi two-dimensional superconductor Bi 2 Sr 2 CaCu 2 O 8Cd . Comparison of data with the Hao–Clem model gives characteristic parameters [x ab (0) and l ab (0)] and the critical fields [H c (0) and H c2 c (0)]. A large value of penetration depth, l ab (0)Z240 nm reflects a small carrier concentration in CuO 2 planes, and explains the reason of the weak interlayer coupling. q 2004 Elsevier Ltd. All rights reserved. PACS: 74.25.Bt; 74.62.Bf; 74.25.Op; 74.72.Jt Keywords: A. Tri-layer superconductor; A. Ba 2 Ca 2 Cu 3 O 6 (O,F) 2 ; D. Irreversibility line; D. Interlayer coupling; D. Anisotropy ratio 1. Introduction For high-temperature superconductors (HTSC), charge carriers in the con-ducting CuO 2 planes are supplied from insulating charge-reservoir block (CRB). For instance, YBa 2 Cu 3 O 7Kd (Y-123) can be doped by the oxygen O d in YBa 2 O 2Cd block. However, the function of the CRB, beyond carrier supplier, is not yet completely clear. One can presume that CRB plays role to reduce the layer-by-layer coupling, since the physical extension of CRB widens the space between the CuO 2 planes. The strong anisotropic nature of HTSC is known to come from a weak interlayer coupling. Previously, a number of group synthesized a new homologous series Ba 2 (or Sr 2 )Ca nK1 Cu n O y (02 (nK 1)n).[1–11] Structurally, 02 (nK1)n is akin to Hg-based homologous series HgBa 2 Ca nK1 Cu n O 2nC2Cd (Hg-12(nK 1)n). The cardinal difference in the two series is structure of CRB. For Hg-based superconductors, the CRB consists of double rocksalt-like block HgBa 2 O 2Cd and the hole concentration in CuO 2 planes is determined by the 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.11.044 Solid State Communications 133 (2005) 459–463 www.elsevier.com/locate/ssc * This work was supported by the Creative Research Initiatives of the Korean Ministry of Science and Technology. * Corresponding author. Tel.: C82 428685725; fax: C82 428685018. E-mail address: msk2003@kriss.re.kr (M.-S. Kim).