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 d isorder induced by proton and electron irradiation on the upper and lower c ritical points in the vortex phase diagram of YBCO is presented. We find th at 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 f ields. Strong pinning point defects raise the lower critical point, while w eak pinning point defects have little or no effect on the lower critical po int. The upper critical point is lowered by point disorder, bur raised by L ine disorder. First order melting is suppressed by point disorder in two wa ys, by lowering of the upper critical point for weak point pins, or by merg ing of the upper and lower critical points for strong point pins. The diffe ring responses of the upper and lower critical points to line and point dis order can be understood in a picture of transverse and longitudinal spatial fluctuations. (C) 2000 Elsevier Science B.V. All rights reserved.