T. W. Li

Equilibrium magnetic properties and Meissner expulsion of magnetic flux in Bi2Sr2CaCu2Ox single crystals

Abstract The unusual spatial transformation of the magnetic-flux structure in Bi2Sr2CaCu2Ox single crystals during magnetization of zero-field cooled samples and during field cooling (the Meissner flux expulsion) is directly visualized by means of the magneto-optical technique. In both cases similar inhomogeneous flux distributions are observed at high temperatures in a magnetic field applied perpendicularly to the basal plane: The magnetic field in the crystal center is approximately equal to the applied field, but at the crystal perimeter a belt of lower field occurs. A model of such an equilibrium flux structure is developed taking into account the real shape of the crystals. The distribution of the Meissner screening current in this geometry is directly measured for the first time.

Anisotropic oxygen diffusion in Bi2Sr2CaCu2O8+x single crystals

Abstract The weight change of Bi-2212 single-crystalline samples as a function of temperature, time and surrounding atmosphere was investigated by a classical thermogravimetric technique. Clear evidence for a correlation between T c and the c -axis lattice parameter was obtained. A plateau was observed in the dependence of T c and the c -axis lattice parameter on the oxygen contents. From the size dependence it was concluded that oxygen out-diffusion takes place in the a-b plane in a quite anisotropic way, i.e. mainly along the a -direction. At 500°C D a / D b ≥5–6, D ab ≈6.12×10 -8 cm 2 / s .

Enhanced flux pinning in Bi-2212 single crystals by planar defects introduced via Ti-substitution

Abstract Ti-doped Bi-2212 single crystals were grown by the travelling solvent floating zone technique. Concentrations of about 1 at% and 2 at% (with respect to Cu) were shown to be incorporated in the structure which leads to lowering of T c to about 73 K from 85 K. High resolution microscopy revealed high densities of planar defects parallel to the a , c -planes. The flux pinning properties at elevated temperatures are significantly improved with respect to undoped single crystals.

Flux pinning in Bi-2212 single crystals with various oxygen contents

Abstract Systematic variation of the oxygen concentration proved that oxygen vacancies in the CuO 2 planes are indeed the predominant pinning centers in Bi 2 Sr 2 CaCu 2 O 8 + δ (Bi-2212) single crystals. Both the critical current density and the pinning energy increase proportional to the square root of the vacancy concentration. The change of the superconducting parameters, predominantly λ L (0), in going from the overdoped to the optimally doped regime, gives rise to a considerable decrease of the critical current.

Anisotropy of perpendicular field penetration into high-Tc superconductors induced by strong longitudinal field

Abstract The geometry of penetration of a magnetic field oriented perpendicular to the surface of a superconductor is changed by the presence of a longitudinal field. This is studied directly by means of the magneto-optical technique in different high-Tc materials. In YBa2Cu3O7 single crystals Brandts mode of magnetization is observed, namely, penetration of an AC field only along the longitudinal field even if the corresponding dimension of the sample is much longer than the other two. This anisotropy is the evidence that the force-free configuration of current and vortices cannot be destroyed by cutting and reconnection of the vortices in this 3D superconductor. On the contrary, the absence of the induced anisotropy in Bi2Sr2CaCu2O8 single crystals reveals the independence of pancake vortices in CuO layers from longitudinal field which makes a realization of the force-free configuration impossible in the layered 2D superconductor. An intermediate effect in thin films indicates that vortices are almost normal to the film surface even if the applied field is nearly parallel to it. The described experiment can be used as a new tool to study the internal vortex structure in superconductors.

TEM observation of neutron-induced collision cascades in Bi-2212 single crystals

Abstract Several high-quality single crystals of the superconductor Bi 2 Sr 2 CaCu 2 O 8 (Bi-2212) were exposed to three different neutron fluences in the central irradiation facility of the Triga Mark II reactor in Vienna in the form of pre-prepared and pre-characterized transmission electron microscopy (TEM) samples as well as in full crystalline form. We find that the radiation damage produced in Bi-2212 is very similar to that found previously in YBa 2 Cu 3 O 7− δ (Y-123) single crystals. The diameter of the amorphous cascade volume is 3.5 nm, the inwardly directed strain field is of approximately the same size, the density of cascades is 3.7×10 22 m −3 per 10 22 neutrons m −2 ( E >0.1 MeV) and their concentration scales linearly with neutron fluence. Based on these results and in view of the similar displacement cross-sections of the constituents of other high temperature superconductors, we conclude that the neutron-induced defects in high- T c materials will generally have a diameter (including the strain field) of 5–7 nm.

Unconventional angular dependence of the microscopic magnetic field distribution in single crystal Bi2Sr2CaCu2O8 from muon-spin rotation studies

Abstract The angular dependence of the microscopic magnetic field distribution in single-crystal Bi 2 Sr 2 CaCu 2 O 8 has been measured for a range of applied fields using the muon-spin rotation technique. The results show significant deviations from the predictions of the anisotropic 3D model, which may reflect the extremely anisotropic, quasi-2D nature of the flux structure. The temperature dependence of the field distribution at 400 mT has also been measured for a range of orientations, showing disagreement with the predictions of a simple two-fluid model.

TEM analysis of planar defects induced by Ti doping in Bi-2212 single crystals

Abstract An electron microscopy study is performed on Ti doped Bi-2212 single crystals. High resolution electron microscopy reveals that the Ti doping leads to incorporation of (010) planar defects in the Bi-2212 lattice. These defects consist of pairs of antiphase boundaries, which are separated along the b-axis by one modulation distance. The atomic shift at the boundary is such that one of the two CuO2 planes is continuous throughout the defect. Energy dispersive X-ray analysis clearly reveals the presence of Ti in the planar defects, whereas no Ti could be detected in defect free areas. This evidences the fact that these defects are induced by the Ti doping.

Observation of the Flux-Line Lattice by Neutron Diffraction and Muon-Spin Rotation

Small-angle neutron scattering and muon spin rotation have been used to obtain direct information about flux structures in the mixed state within the bulk of crystals of superconductors, including BSCCO-2212, niobium and YBCO. In highly anisotropic BSCCO, we see evidence of vortex fluctuations, melting, and decomposition of flux lines into ‘pancake’ vortices, which give a more twodimensional vortex structure. Our results from pure niobium show no sign of flux-lattice melting — in disagreement with recent claims. In YBCO the flux lattice is much more stable than in BSCCO, due to a much lower anisotropy, but it shows interesting flux structures as a function of field direction.

The equilibrium vortex state in heavy-ion irradiated Bi2Sr2CaCu2O8+δ

Abstract We obtain the field dependence of the free energy gain due to vortex line pinning by columnar defects in a Bi 2 Sr 2 CaCu 2 O 8+δ single crystal. Results are discussed in the light of the theoretically proposed Bose-glass phase.