Vortex pinning by natural linear defects in thin films of YBa2Cu3O7-delta - art. no. 184523

The behavior of the superconducting current density j(s)(B,T) and the dynam ical relaxation rate Q (B, T) of YBa2Cu3O7-delta thin films exhibits a numb er of features typical for strong pinning of vortices by growth induced lin ear defects. At low magnetic fields j(s)(B) and Q(B) are constant up to a c haracteristic field B*, that is directly proportional to the linear defect density n(dis1). The pinning energy U-c(B = 0) approximate to 600 K can be explained by half-loop excitations determining the thermal activation of vo rtices at low magnetic fields. Extending the Bose glass theory [D. R. Nelso n and V. M. Vinokur, Phys. Rev. B 48, 13 060 (1993)], we derive a different expression for the vortex pinning potential epsilon (r)(R), which is valid for all defect sizes and describes its renormalization due to thermal fluc tuations. With this expression we explain the temperature dependence of the true critical current density j(c)(0,T) and of the pinning energy U-c(0,T) at low magnetic fields. At high magnetic fields mu H-0 >> B* the current d ensity experiences a power law behavior j(s)(B) similar to B-alpha, with al pha approximate to -0.58 for films with low n(dis1) and alpha approximate t o -0.8 to -1.1 for films with high n(dis1). The pinning energy in this regi me, U-c(high B) approximate to 60 -200 K is independent of magnetic field, but depends on the dislocation density. This implies that vortex pinning is still largely determined by the linear defects, even when the vortex densi ty is much larger than the linear defect density. Our results show that nat ural linear defects in thin films form an analogous system to columnar trac ks in irradiated samples. There are, however, three essential differences: (i) typical matching fields are at least one order of magnitude smaller, (i i) linear defects are smaller than columnar tracks, and (iii) the distribut ion of natural linear defects is nonrandom, whereas columnar tracks are ran domly distributed. Nevertheless the Bose glass theory, that has successfull y described many properties of pinning by columnar tracks, can be applied a lso to thin films. A better understanding of pinning in thin films is thus useful to put the properties of irradiated samples in a broader perspective .