Strongly enhanced irreversibility fields and Bose-glass behaviour in bulk YBCO with discontinuous columnar irradiation defects

Flux pinning properties and irreversibility fields Birr(T) of melt-textured YBCO with discontinuous or multiple-in-line-damage (MILD) columnar defects produced by irradiation with high-energy U238 ions were studied at a constant matching field of B = 10?T and for several energy losses between Se = 1.67 and 2.4?keV???1. With increasing Se and increasing length of the MILD pins, the critical current density jc(H,T) strongly increases and Birr(T) for fields along the c axis progressively shifts upwards reaching 9?T at 77?K. For Se = 2.4?keV???1, a pronounced kink is observed in Birr(T) at 8?T which is a strong indication of Bose-glass behaviour. The jc(H) dependence of this sample shows a peak at a low applied field Bp. This peak effect is explained by the entanglement of vortices. It is argued that for MILD pins single vortices interact simultaneously with many short columnar defects in neighbouring ion trails resulting in an automatic splay in the vortex orientation. The observed decrease of Bp with increasing temperature is estimated taking the increasing thermal fluctuations into account.