Signature of the matching field in Bose-glass melting of untwinned YBa2Cu3O7-delta single crystals

We map out the phase boundary separating the Bose-glass and vortex-liquid p hases in an irradiated twin-free YBa2Cu3O7-delta single crystal. We take th e phase boundary to be the temperature T-g and magnetic field H at which th e crystal begins to screen a small ac magnetic field, h(ac). There is a sig nificant change in slope dT(g)/dH of the phase boundary at the matching fie ld B-Phi (similar or equal to 0.5 T) indicating that interstitial vortices significantly weaken pinning in the Bose-glass state. There is also a prono unced peak in the slope dT(g)/dH just below B-Phi at higher h(ac). Both fea tures disappear when the field is tilted away from the columns. [S0163-1829 (99)51318-0].