Bose glass melting and the transverse Meissner effect in YBa2Cu3O7-delta single crystals - art. no. 064514

We map out the phase boundary separating the vortex solid and liquid phases in YBa2Cu3O7-delta (YBCO) single crystals with irradiation-induced columna r defects. These randomly distributed, extended defects are expected to loc alize vortices into a "Bose glass" phase. The transition from the vortex li quid into the Bose glass is predicted to exhibit two fundamental signatures : a vanishing of the linear resistivity and, concomitantly, a screening of de magnetic fields applied perpendicular to the defect axis, the transverse Meissner effect. We have investigated both aspects by systematic measureme nts on two YBCO single crystals with different defect densities (matching f ields of 0.25 and 0.5 T), as well as on an unirradiated control sample. The melting line determined by the temperature, T-m, of vanishing resistance u ndergoes a 30% decrease in slope as the magnetic field is ramped through th e matching field. This is evidence that interstitial vortices are pinned mu ch more weakly than originally thought. If we associate the melting tempera ture with the Bose glass transition temperature, we obtain static critical exponents of nu (perpendicular to) = 1.7+/-0.2 and nu (perpendicular to) = 1.9+/-0.1 for the crystals with matching fields of 0.25 and 0.5 T, respecti vely. Simultaneously, we use a ten-element, linear array of microfabricated Hall probe magnetometers to observe directly the flux screening associated with the transverse Meissner state. We find the temperature above which th e Meissner state breaks down, T-s, to decrease linearly as the magnetic hel d applied perpendicular to the columnar defect axis increases. This linear trend, found in both irradiated crystals to cover a range of at least 40 K in T-s, is closely in line with the current theoretical expectation nu (per pendicular to) similar or equal to1. However, already for angles as small a s one degree, T-s(H-perpendicular to) falls below T-m(H-perpendicular to) b y more than 10 K. Thus, between T-s(H-perpendicular to) and T-m(H-perpendic ular to) we observe a large regime characterized by zero resistivity in the absence of a transverse Meissner effect: vortices remain effectively local ized even when rotated off the columnar defects.