Robert James Olsson

Modification of vortex behavior through heavy ion lithography

Columnar defects induced by high-energy heavy ion irradiation constitute one of the most effective vortex pinning sites in high temperature superconductors. We demonstrate a novel method of effectively patterning vortex pinning sites in single crystals of the high temperature superconductors YBa 2 Cu 3 O 7-d and Bi 2 Sr 2 CaCu 2 O x using heavy ion irradiation. These patterns include linear channels, a lattice of squares, and asymmetric pinning gradient sites where vortices can either flow unimpeded or are trapped in spatially periodic arrangements.

Effects of successive proton irradiation on the peak effect in YBa2Cu3O7−δ single crystals

The effect of increasing the density of uncorrelated defects on the peak effect is studied in a detwinned YBa2Cu3O7−δ single crystal. The defects were introduced with 9 MeV proton irradiation where the total dose ranged from 0.25×1015 to 2×1015 p/cm2. With each successive irradiation, the peak effect shifts down in field and is eventually suppressed in both the electrical resistivity and the dc magnetization by a sufficiently high irradiation dose. The peak effect reappears after a high temperature anneal, suggesting that it is primarily the point defects introduced by the irradiation which interfere with the formation of the peak effect.

Effect of defects on the critical points in YBa2Cu3O7−δ

Abstract The upper and lower critical points are investigated in untwinned YBa 2 Cu 3 O 7−δ single crystals with dilute columnar defects. Dilute columnar defects raise the upper critical point, indicating that the transition near the upper critical point is a vortex entanglement transition. The lower critical point is very sensitive to columnar defect disorder and its position can be described by a Lindemann-like criterion similar to that for melting. Dilute columnar defects induce non-linear behavior in the I-V curves of the vortex liquid state above the lower critical point, which we interpret as a vestige of the critical region associated with the Bose glass transition below the lower critical point.

Disordered Vortex Phases in YBa2Cu3O7−δ

The disordered vortex phases induced by line and point pinning in YBa2Cu3O7−δ are explored. At high defect densities, only a single disordered solid separated from the liquid phase by a melting line is observed. At low defect densities the topology of the phase diagram changes dramatically, with a vortex lattice phase adjoining disordered phases at high or low field. Critical points at the termination of the first-order melting line separate the lattice and the disordered phases. The line defect disordered phases follow the expected Bose glass behavior, while the point defect disordered phases do not exhibit the expected vortex glass behavior.