Vortex-lattice phase transitions in Bi2Sr2CaCu2 O8 crystals with different oxygen stoichiometry.

Abstract

The vortex-lattice phase transitions in Bi 2Sr2CaCu2O8 crystals with various oxygen stoichiometry are studied using local magnetization measurements. Three new findings are reported: The first-order phase transition line at elevated temperatures shifts upward for more isotropic overdoped samples. At lower temperatures another sharp transition is observed that results in enhanced bulk pinning in the second magnetization peak region. The two lines merge at a multicritical point at intermediate temperatures forming apparently a continuous phase transition line that is anisotropy dependent. PACS numbers: 74.60.Ec, 74.60.Ge, 74.60.Jg, 74.72.Hs The mixed state of high-temperature superconductors (HTSC) has a very complicated phase diagram. The nature of the different vortex phases and the thermodynamic transitions between them are of fundamental interest, and are the subject of substantial recent theoretical and experimental efforts [1 ‐ 10]. It is generally accepted that the high anisotropy plays a crucial role in the richness of the phase diagram of HTSC. Nevertheless, a systematic study of the anisotropy effects is quite complicated and was limited so far by the lack of a well defined phase boundary that could be monitored as a function of the anisotropy. A recent advance in local measurements has revealed a sharp step in magnetization due to a first-order vortex-lattice phase transition in Bi2Sr2CaCu2O8 (BSCCO) crystals [10]. Such a clearly defined fundamental transition is thus a natural candidate for an investigation of the anisotropy effects in HTSC, and this paper presents a first study in this direction. Another intriguing feature of the phase diagram of many HTSC crystals, and BSCCO in particular, is the anomalous second magnetization peak at lower temperatures. The associated increase of magnetization with magnetic field has been attributed to surface barrier effects [11], crossover from surface barrier to bulk pinning [12], sample inhomogeneities [13], dynamic effects [14], and 3D to 2D transitions [15‐ 17]. Our local measurements indicate that a very abrupt upturn in the bulk critical current occurs at the onset of the second peak in BSCCO. We postulate that this behavior is triggered by an underlying thermodynamic phase transition of the flux-line lattice. Furthermore, for the different anisotropy crystals the two phase transition lines are found to form, apparently, one continuous transition line that changes from first to possibly second order at