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

Abstract

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. We 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 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.