A.V. Volkozub

Growth of high-quality 2212 BSCCO crystals in Pt crucibles and characterisation

Abstract We report the growth of pure, high-quality Bi2Sr2CaCu2Ox (BSCCO 2212) crystals, grown by a novel flux method in Pt crucibles. The growth process involves repeated remelting and recrystallisation of the same melt. The crystals appear to be chemically pure, contain no Pt contamination and demonstrate a very high as-grown Tc (onset), above 92 K, and the majority in the range 93–95 K with transition widths (10–90% level) between 4 and 8 K. The lattice parameters of a typical crystal are a=5.414(2) A , b=5.413 (2) A , c=30.893 (12) A . We discuss the growth process and the qualities of these crystals.

Superconductivity in the Y-Pd-B-C system

Abstract The discovery of superconductivity at 12 K in the single-phase YNi2B2C compound and at 23 K in the multiphase YPd5B3C0.2 compound has renewed the interest in the field of intermetallic superconductors. We have made an extensive study of superconductivity in YTBC (TPd, Ru and Pt) systems, starting with nine different compositions of Y, Pd, B, C and using transport and low-field magnetization measurements. We observed an onset of superconductivity at 23 K and zero resistance at 20 K in the multiphase YPd3B2Cy(y

Growth of doped and undoped BSCCO 2212 crystals in platinum crucibles by repeated remelting and recrystallisation

Abstract We describe the growth of Y-doped and undoped Bi 2 Sr 2 CaCu 2 O y (BSCCO 2212) crystals by a repeated melting and recrystallisation technique in platinum crucibles. The conditions for the growth of high-quality, chemically pure (undoped) and highly Y-doped crystals in platinum crucibles have been optimised. The as-grown, undoped crystals indicate a very high T c (onset) between 92–95 K and the Y-doped ones between 90 and 92 K with transition widths (10–90% level) between 4 and 10 K. It appears that, in the case of doped batches, the crystals with a high level of Y-doping are produced homogeneously and reproducibly by this method, but the low-level doping produces inhomogeneous distribution of Y within the crystal layers. The reasons are discussed.

Growth of BSCCO 2212 crystals in flat dish-shaped crucibles

Abstract We report the growth of undoped and Y- and Gd-doped Bi 2 Sr 2 CaCu 2 O 8 (BSCCO 2212) crystals by the flux method in flat dish-shaped crucibles where the growth mechanism appears to be different from that observed in standard high form or conical crucibles. The crystals exhibited two different morphologies with two different properties. Thin plate crystals grew at the surface of the melt (type ‘A’), and others grew under the top layer as polygonal thick blocks (type ‘B’). The orientation and the habit of the ‘A’ crystals were dependent on the growth conditions, they were free from any detectable Al contamination or flux inclusions and were good quality 2212 crystals exhibiting high as-grown T c (onset) between 90 and 93 K with transition widths between 2 and 10 K. On the other hand, the polygonal blocks were mixed with flux and indicated a T c (onset) of 86 K with a broader transition width. The influence of the shape of the crucible and the possible growth processes are discussed.

Novel vortex dynamics in untwinned YBa2Cu3O7−δ single crystals

Magnetotransport measurements on a clean, untwinned YBa2Cu3O7−δ single crystal show that the vortex dynamics at temperatures just below the vortex lattice melting transition are highly dependent on the type of modulation of the probing current. While in the case of DC current the flux flow is disordered, the “shaking” of the vortex lattice by a square-wave current leads to a more uniform vortex motion. A small asymmetry (∼10%) in the durations of the positive and negative parts of the square-wave period induces periodical oscillations of the voltage response amplitude. The period of oscillations (∼100 s) is the same order of magnitude as the time needed for vortices to cross the sample (transit time). We relate the observed voltage oscillations to a periodically ordered vortex motion.

Current-induced resistivity hysteresis in detwinned YBa2Cu3O7−δ single crystals

Abstract Resistivity-temperature and voltage-current characteristics of clean detwinned YBa 2 Cu 3 O 7−δ single crystals have been measured using a SQUID picovoltmeter in magnetic fields of up to 5 T. We have observed hysteretic behaviour near T m in the ϱ(T) dependence: the resistivity when warming the sample up (the “superheated” state) was lower than when cooling it down (the “supercooled” state). The V-I characteristics for the sample in the “superheated” and “supercooled” states were also found to be different. We have found that the “superheated” state is unstable and can be switched to the “supercooled” state by applying a comparatively low probing current density ( j ∼ 0.4A/cm 2 ). We relate the current-induced resistivity hysteresis to the rearrangement of the solid vortex domains due to the probing current.

Hysteretic flux dynamics in a detwinned YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// crystal from SQUID picovoltmeter measurements

We report measurements of resistivity hysteresis in both temperature and magnetic field performed on a detwinned YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// single crystal in the region of the vortex lattice melting transition. We have found evidence that the observed hysteresis cannot be explained by current-induced nonequilibrium effects. The measured voltage-current characteristics for the sample in the overheated state also display hysteretic behaviour. This strongly supports the idea that the vortex solid can be melted by transport current.

Large-scale correlated vortex motion in an untwinned YBa2Cu3O7-δ single crystal

The vortex dynamics at temperatures below the vortex lattice melting transition has been found to be highly dependent on the type of modulation of the probing current. While in the case of dc current the vortices flow nonuniformly, the periodical ordering was observed for an asymmetrically square-wave modulated current.

Field-cooled magnetization of high-Tc single crystals: Thermal history effects

Abstract Field-cooled magnetization m FC of YBa 2 Cu 3 O 7− x , Bi 2 Sr 2 CaCu 2 O y and Tl 2 Ba 2 CaCu 2 O z single-crystal superconductors has been studied as a function of thermal history. For the first time, a dependence of the Meissner fraction on the rate of cooling the sample was found. After a more rapid cooling, m FCW (T) on warming shows a pronounced minimum just before the transition into the normal state. This behaviour has been explained within a generalized critical state model, taking into account nonlinear flux diffusion.

Magnetic measurements on Tl-2212 and Bi-2212 single crystals: A comparative study

We have compared the magnetic behaviour of two identicaly shaped single crystals, Tl2Ba2CaCu2O8 and Bi2Sr2CaCu2O8. The critical current density is found to decrease more rapidly with temperature in Bi-2212 although it is the highest in this material at low temperatures (T<10K). I–V characteristic curves have been obtained from measurements of magnetic sweep rate dependencies of the hysteries loops. We have found that the characteristic temperature at which flux motion becomes important is significantly higher in Tl-2212 than in Bi-2212. Hence the Tl-2212 has a larger effective pinning.