Doping-dependence of the vortex phase diagram of Bi 2 Sr 2 Ca 2 Cu 3 O 10+d A. Piriou * , Y. Fasano, E. Giannini, Ø. Fischer Condensed Matter Physics Department, University of Geneva, Switzerland Available online 27 March 2007 Abstract We study the eﬀect of oxygen overdoping in the vortex phase diagram of Bi 2 Sr 2 Ca 2 Cu 3 O 10+d single crystals. In the overdoped regime we do ﬁnd a smooth variation of T c with the annealing pressure, indicating that Bi 2 Sr 2 Ca 2 Cu 3 O 10+d is less sensitive to oxygen-doping than the bilayer Bi 2 Sr 2 CaCu 2 O 8+d compound. Magnetic measurements reveal that oxygen overdoping produces a reduction of the liquid vortex region and increases the second-peak transition ﬁeld. These ﬁndings imply that the coupling between CuO 2 planes increases upon overdoping. We ﬁnd that this anisotropy decrease is steeper in Bi 2 Sr 2 Ca 2 Cu 3 O 10+d than in Bi 2 Sr 2 CaCu 2 O 8+d for the same doping level. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Bi 2 Sr 2 Ca 2 Cu 3 O 10+d ; Vortex phase diagram; Oxygen-doping The study of the superconducting properties of the tri- layer material Bi 2 Sr 2 Ca 2 Cu 3 O 10+d have attracted strong interest due to its high critical temperature, reaching a max- imum T c = 111 K at optimal doping, and higher CuO 2 planes coupling than in Bi 2 Sr 2 CaCu 2 O 8+d [1,2]. The recently acquired capability in growing pure and large Bi 2 Sr 2 Ca 2 Cu 3 O 10+d single crystals [1] triggered the investi- gation on its vortex phase diagram and pinning properties. A comparative study with Bi 2 Sr 2 CaCu 2 O 8+d for the case of slightly overdoped samples reports that the trilayer com- pound presents an enlarged vortex solid region, higher crit- ical currents and a smaller magnetic relaxation rate [2]. These results indicate that for Bi 2 Sr 2 Ca 2 Cu 3 O 10+d the inter- layer coupling between CuO 2 planes is enhanced. This was conﬁrmed by in and out-of-plane H C1 measurements reveal- ing an anisotropy reduction c 2223 =(m c /m ab ) 1/2  50  0.3c 2212 [2]. The intermediate anisotropy of Bi 2 Sr 2 Ca 2 - Cu 3 O 10+d makes this compound a suitable candidate for the investigation of changes induced in the vortex phase dia- gram when varying the interlayer coupling and eventually changing its nature from electromagnetic (large c) to Josephson (small c) [3]. In this work, we present the ﬁrst results on the eﬀects of oxygen overdoping on the Bi 2 Sr 2 - Ca 2 Cu 3 O 10+d vortex phase diagram. In particular, we report on the evolution of the irreversibility line, H IL (T), the ﬁeld at which pinning sets-in (vortex solid at lower ﬁelds), and the second-peak line, H SP (T), identiﬁed in other cuprates as an order–disorder phase transition in the vortex solid [3,4]. Single crystals were grown by means of the travelling- solvent ﬂoating-zone method [1] and post-anneals were carried out at temperatures between 500 and 550 °C at dif- ferent oxygen pressures, p(O 2 ). Considering T c as a param- eter for the evaluation the oxygen content, we developed a reliable method to control the doping of samples by tuning p(O 2 ). Critical temperatures and transition widths were determined from AC susceptibility measurements. We found a smooth dependence of T c on oxygen pressure, ranging from (100.0 ± 0.2) K for the underdoped regime (p(O 2 ) = 10 4 bar) to (108.5 ± 0.2) K for the overdoped regime (p(O 2 ) = 400 bar) and achieving the optimal doping with T c = (111.0 ± 0.2) K (p(O 2 ) = 20 bar), see for example inset of Fig. 1a. After annealing, transition widths of 1–2 K were obtained. The evolution of T c vs. p(O 2 ) indicates that Bi 2 Sr 2 Ca 2 Cu 3 O 10+d is less sensitive to oxygen pressure than Bi 2 Sr 2 CaCu 2 O 8+d [5]. However, and contrary to previous reports [6], we do detect a decrease of T c in the overdoped regime. 0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.03.102 * Corresponding author. Tel.: +41 22 379 6287. E-mail address: piriou@physics.unige.ch (A. Piriou). www.elsevier.com/locate/physc Physica C 460–462 (2007) 408–409