Wai K. Kwok

Structural and superconducting properties of oxygen-deficient and doped YBa2Cu3O7−x

Abstract Neutron powder diffraction studies of YBa2Cu3O7−x, Nd1+xBa2−xCu3O7+y, and YBa2Cu3−x FexO7+y show that symmetry or average formal Cu charge alone are not sufficient to explain superconductivity in these compounds. The data can be understood in terms of a model based on charged transfer from insulating reservoirs of charge, the Cu-O chains, to a two-dimensional Cu-O conduction plane.

4f Hybridization in the Cyclotron Mass of PrB6 and PrCu6

Hybldlzatlon effects of the 4f electrons with the conduction electrons in PrB5 and PrCu6 are reflected in the Fermi surface and the cyclotron mass determined from the de Haas-van Alphen effect. The masses of PrB6 and PrCu6 are three times and twice larger than those of LaB6 and LaCu6, respectively.

Characterization of off-axis MgB2 epitaxial thin films for planar junctions

Using scanning tunneling spectroscopy, we perform a full mapping of the quasiparticle density of states of magnesium diboride (MgB2) epitaxial thin films grown on (110) yttrium stabilized zirconia (YSZ) by hybrid physical-chemical vapor deposition. The films have critical temperatures of 40 K. X-ray measurements show an epitaxial MgB2 growth having the c-axis tilted by 32° with respect to the normal to the substrate, consistent with the atomic force microscopy images of the sample. Scanning tunneling spectroscopy clearly finds that the spectroscopic peak associated to the π gap is reduced on most of the film surface and the feature representative of the σ gap is present, with different intensity, on the majority of the sample’s surface, which is consistent with x-ray measurements.

Magnetization anomalies above the vortex lattice melting transition

Abstract A weak irreversible behavior was found in magnetization hysteresis curves in pristine as well as heavy-ion-irradiated Bi2Sr2CaCu2O8+δ single crystals measured by a SQUID magnetometer both above the vortex lattice melting transition in lower fields and the vortex-glass transition in higher fields. The onset of this anomaly on magnetic field HF strongly depends on sample disorder level, and begins to appear in the middle of the first-order phase transition. HF systematically shifts to higher temperature and field region with increasing the number of columnar defects. The results are interpreted by a disorder-induced pinning, which is effective below HF and becomes ineffective above HF.

Weak pinning phenomena in liquid state in Bi2Sr2CaCu2O8+δ with columnar defects

Abstract Anomalous weak pinning phenomena were found in the vortex liquid state of Bi 2 Sr 2 CaCu 2 O 8+δ single crystals with and without columnar defects up to B Φ = 1 T between H irr and H F , at which the M-T curves show a sharp step-like feature. With increasing B Φ H F as well as H irr is considerably pushed up. It is shown that the region defined by H irr and H F is very sensitive to the disorder and is greatly expanded by introducing columnar defects. These results strongly suggest appearance of the new phase in the vortex liquid region, which may be induced by the pinning effect.

Anomalous Magnetization Behavior in the Vortex Liquid State in Bi2Sr2CaCu2O8+δ

Weak irreversible behavior was found in magnetization curves in the vortex liquid state above the irreversibility line, H in, but below a characteristic field, H F, in as-grown Bi2Sr2CaCu2O8+δ single crystals and in columnar defected ones by using a SQUID magnetometer. H F was also determined by temperature sweep measurements, and is strongly influenced by disorders in the samples. With increasing the number of columnar defects H F systematically shifts higher temperature and field region. These results strongly suggest existence of a disorder-induced new phase between H in and H F.