Valentina Tobos

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.

The effect of disorder on the critical points in the vortex phase diagram of YBCO

The effect of line disorder induced by heavy ion irradiation and of point disorder induced by proton and electron irradiation on the upper and lower critical points in the vortex phase diagram of YBCO is presented. The authors find that dilute line disorder induces a Bose glass transition at low fields which is replaced at the lower critical point by first order melting at higher fields. Strong pinning point defects raise the lower critical point, while weak pinning point defects have little or no effect on the lower critical point. The upper critical point is lowered by point disorder, but raised by line disorder. First order melting is suppressed by point disorder in two ways, by lowering of the upper critical point only for weak point pins, or by merging of the upper and lower critical points for strong point pins. The differing responses of the upper and lower critical points to line and point disorder can be understood in a picture of transverse and longitudinal spatial fluctuations.

Irradiation-induced changes in the vortex phase diagram of YBCO

Abstract We explored the vortex phase diagram of YBCO using electrical transport and AC specific heat measurements. The evolution of the phase diagram with increasing defect density and varying defect geometry is discussed, with particular emphasis on the lower critical point Hlcp of the first-order vortex melting line.

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.