High-pressure synthesis and properties of Ba2Can-1CunO2n(O,F)(2) (n=2-5) superconductors

Superconducting compounds of Ba2Can-1CunO2n(O,F)(2) (n = 2-5) whose apical oxygen is partially substituted by fluorine have been synthesized under hig h pressure. The nearly single-phase samples of n = 3 and 4 were prepared fr om almost stoichiometric compositions (delta is small) of Ba2Ca2Cu3O6(O(1-y +/- delta)6F(y))(2) (y = 0.6-0.8) and Ba2Ca3Cu4O8(O(1-y +/- delta)Fy)(2) ( y = 0.5-0.7), respectively. The samples showed sharp superconducting transi tions in both resistivity and susceptibility measurements. The samples show ed typical under-doped behavior (S-shape) in the temperature dependence of resistivity. The T, for the n = 3 phase monotonically increased with decrea sing a nominal fluorine content y, which shows the electronic state shifts from under-doped toward optimally one with increasing hole density by subst itution of apical O2- for F-. The highest Tc measured so far for n = 2, 3, 4 and 5 phases were 90, 120, 105 and 90 K, respectively. This compound show s a maximum Tc at n = 3 like in many other homologous series. In spite of a short spacing between superconducting blocks (approximate to7.36 Angstrom) , the critical current density (J(c)) and irreversibility field (H-irr) of the n = 3 phase decreased rapidly with increasing temperature provably due to the small carrier (holes) density. (C) 2001 Elsevier Science B.V. All ri ghts reserved.