Thallium and lead NMR studies of Tl 0:5 Pb 0:5 )Ba 0:2 Sr 0:8 ) 2 Ca 2 Cu 3 O y superconductors A.M. Panich a , S.D. Goren a , L. Frenkel Ben-Yakar a , M. Eder b , G. Gritzner b, * a Department of Physics, Ben-Gurion University of the Negev, P.O. Box 653, Beer Sheva 84105, Israel b Institut f ur Chemische Technologie Anorganischer Stoe, Johannes Kepler Universit at, A-4040 Linz, Austria Received 25 August 2000; received in revised form 18 January 2001; accepted 1 February 2001 Abstract Tl 0:5 Pb 0:5 )Ba 0:2 Sr 0:8 ) 2 Ca 2 Cu 3 O y Tl,Pb-1223) has been prepared, characterized and oriented for NMR studies. For comparison, we also fabricated Tl 0:5 Pb 0:5 )Ba 0:1 Sr 0:9 ) 2 CaCu 2 O y Tl,Pb-1212). In the case of Tl 0:5 Pb 0:5 )Ba 0:2 Sr 0:8 ) 2 - Ca 2 Cu 3 O y the 205 Tl NMR spectra and the spin±lattice relaxation time were measured in the temperature range from 68 to 300 K. For Tl 0:5 Pb 0:5 )Ba 0:1 Sr 0:9 ) 2 CaCu 2 O y the 205 Tl spectra were measured at room temperature only. The 207 Pb NMR spectra for both compounds were also obtained at room temperature. From the chemical shift measurements it was concluded that the valence states of Tl and Pb were 3 and 4, respectively. In addition, a linear relationship was found between the shift and the critical temperatures. At low temperatures an anisotropic melting of the vortex state was observed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.72.Fq; 74.25Nf Keywords: Tl,Pb-1223) superconductors; 205 Tl NMR spectroscopy; 207 Pb NMR spectroscopy; Vortex states 1. Introduction Among the dierent types of oxidic supercon- ducting materials with critical temperatures T c ) above the boiling point of nitrogen, the super- conductors of the TlBa,Sr) 2 Ca n1 Cu n O y family are considered to be promising candidates for ap- plications in external magnetic ®elds. In particular the TlBa,Sr) 2 Ca 2 Cu 3 O y Tl-1223) superconduct- ing phase has critical temperatures well above 110 K, yields high critical currents and shows a rela- tively high irreversibility ®eld. Undoped Tl-1223 specimens, however, are dicult to prepare in a pure phase form. Partial substitution of Ba 2 by Sr 2 in the spacing layer [1] and doping with PbO [2], with Bi 2 O 3 [3] or with both PbO and Bi 2 O 3 [4] into the insulating Tl-layer provided considerable improvements in the fabrication of Tl-1223 super- conducting material [5]. Additional information on the properties of superconductors, especially on the vortex state which appears in type II superconductors, is available from NMR studies. The vortex state creates an inhomogeneous magnetic ®eld inside the sample. The pro®le of the inhomogeneous mag- netic ®eld is re¯ected by the NMR spectrum. Pb- doped Tl-based superconductors are excellent candidates for NMR-studies since 203 Tl, 205 Tl and Physica C 356 2001) 129±140 www.elsevier.nl/locate/physc * Corresponding author. Tel.: +43-732-2468-8703; fax: +43- 732-2468-8905. E-mail address: gerhard.gritzner@ictas.uni-linz.ac.at G. Gritzner). 0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0921-453401)00147-2