Effect of defects on the critical points in YBa2Cu3O7-delta

The upper and lower critical points are investigated in untwinned YBa2Cu3O7 -delta single crystals with dilute columnar defects. Dilute columnar defect s raise the upper critical point, indicating that the transition near the u pper critical point is a vortex entanglement transition. The lower critical point is very sensitive to columnar defect disorder and its position can b e described by a Lindemann-like criterion similar to that for melting. Dilu te columnar defects induce non-linear behavior in the I-V curves of the vor tex liquid state above the lower critical point, which we interpret as a ve stige of the critical region associated with the Bose glass transition belo w the lower critical point.