Epitaxial YBa2Cu3O7-δ films nucleate in c-axis oriented single-crystalline islands. The surface of the single-crystalline SrTiO3 substrates exhibit steps of one third of the YBa2Cu3O7-δ c-axis. These steps generate a mismatch in the island boundaries between the CuO2 superconducting blocks. We show that these defect regions are strong candidates for being the pinning centers responsible for the large critical currents observed in Laser Ablated and Sputtered thin films.@en

Thin films of the high-temperature superconductor YBa2Cu3O(7-δ) exhibit both a large critical current (the superconducting current density generally lies between 10 and 10 A m-2 at 4.2 K in zero magnetic field) and a decrease in such currents with magnetic field that point to the importance of strong vortex pinning along extended defects. But it has hitherto been unclear which types of defect - dislocations, grain boundaries, surface corrugations and anti-phase boundaries - are responsible. Here we make use of a sequential etching technique to address this question. We find that both edge and screw dislocations, which can be mapped quantitatively by this technique, are the linear defects that provide the strong pinning centres responsible for the high critical currents observed in these thin films. Moreover, we find that the superconducting current density is essentially independent of the density of linear defects at low magnetic fields. These natural linear defects, in contrast to artificially generated columnar defects, exhibit self-organized short-range order, suggesting that YBa2Cu3O(7-δ) thin films offer an attractive system for investigating the properties of vortex matter in a superconductor with a tailored defect structure.@en

In an attempt to clarify the origin of the large critical current densities Jc observed in Laser Ablated and Sputtered YBa2Cu3O7-δ (YBCO) thin films, we make a systematic study of the low temperature Jc in samples carefully analysed using STM and AFM. Jc (B) is determined from torque-magnetometry performed in ring-patterned thin films. Epitaxial YBCO films nucleate in e-axis oriented single-crystalline islands with sizes ranging between 200 and 700 nm. We show that Jc can be mainly explained by vortex pinning loealised in the island boundaries.@en

We studied the superconducting current density js (B, T) and dynamical relaxation rate Q(B, T) of laser ablated thin films of YBa2Cu3O7-δ with various dislocation densities (ndisl ∼ 7 - 70 μm-2). In these films dislocations act as strong pinning centers like columnar defects in heavy ion-irradiated crystals. The matching field clearly manifests itself in a constant js(B) ∼ 5.1011 - 1.1012 Am-2 at T = 4.2 K and μ0H ≲ 0.7·BΦ and a constant Q(B). In contrast to irradiated crystals, in films we do not observe a peak in the relaxation rate Q versus T below the matching field BΦ = ndisl · Φ0. Instead, in thin films Q(T) increases monotonously with increasing temperature. We conclude that the influence of vortex-vortex interactions in thin films is greatly reduced due to the non-random distribution of pinning sites and the low matching field.@en

The field dependence of the superconducting current density js(B)and the pinning energy UC(B) in thin films of YBa2Cu3O7-δindicates three different pinning regimes. At low fields, μH < B* ≈ 0.7nd,Φ0s the density of dislocations), jsB) is constant and Uc = 600 K at T 0 for all YBa2Cu3O7-δfilms, independent of the density of dislocations. This is much lower than the pinning energy of a vortex in a single dislocation, for which Uc ≈ 6000 K. At intermediate fields, B* < μ0H < 0.5 T, a sharp decrease of Uc is observed. For μ0H > 0.5 T, Uc(B) is constant again, with UC(B) ≈ 80 K for sputtered films and UC(B) ≈ 200 K for laser ablated films.@en

To investigate the quantum tunneling of almost macroscopic vortex segments we measured the normalized relaxation rate Q of superconducting currents for various high- Tc superconductors (HTS’s) down to 100 mK in magnetic fields up to 7 T. At fields below ?0.5 T, Q is essentially independent of T in the temperature regime between thermally activated and quantum motion as theoretically expected. However, at higher fields, we find an unexpected linear T dependence of Q persisting down to the lowest temperatures in all investigated samples. Since these compounds were chosen to represent the distinct classes of dirty and clean HTS’s, the extrapolated Q(T=0) values are used to discuss the theoretical models for quantum creep in the clean and dirty limits. Based on this discussion we propose a semiempirical interpolation formula for Q(T=0) which is also valid for compounds in between the dirty and clean limits.@en

The behavior of the superconducting current density (formula presented) and the dynamical relaxation rate (formula presented) of (formula presented) thin films exhibits a number of features typical for strong pinning of vortices by growth induced linear defects. At low magnetic fields (formula presented) and (formula presented) are constant up to a characteristic field (formula presented) that is directly proportional to the linear defect density (formula presented) The pinning energy (formula presented) can be explained by half-loop excitations determining the thermal activation of vortices at low magnetic fields. Extending the Bose glass theory [D. R. Nelson and V. M. Vinokur, Phys. Rev. B 48, 13 060 (1993)], we derive a different expression for the vortex pinning potential (formula presented) which is valid for all defect sizes and describes its renormalization due to thermal fluctuations. With this expression we explain the temperature dependence of the true critical current density (formula presented) and of the pinning energy (formula presented) at low magnetic fields. At high magnetic fields (formula presented) the current density experiences a power law behavior (formula presented) with (formula presented) for films with low (formula presented) and (formula presented) to -1.1 for films with high (formula presented) The pinning energy in this regime, (formula presented) 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 density is much larger than the linear defect density. Our results show that natural linear defects in thin films form an analogous system to columnar tracks in irradiated samples. There are, however, three essential differences: (i) typical matching fields are at least one order of magnitude smaller, (ii) linear defects are smaller than columnar tracks, and (iii) the distribution of natural linear defects is nonrandom, whereas columnar tracks are randomly distributed. Nevertheless the Bose glass theory, that has successfully described many properties of pinning by columnar tracks, can be applied also 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.@en

The behavior of the superconducting current density j s(B,T) and the dynamical relaxation rate Q(B,T) of YBa2Cu3O7-δ thin films exhibits a number of features typical for strong pinning of vortices by growth induced linear defects. At low magnetic fields js(B) and Q(B) are constant up to a characteristic field B*, that is directly proportional to the linear defect density ndisl. The pinning energy Uc(B=0)≈600 K can be explained by half-loop excitations determining the thermal activation of vortices at low magnetic fields. Extending the Bose glass theory [D. R. Nelson and V. M. Vinokur, Phys. Rev. B 48, 13 060 (1993)], we derive a different expression for the vortex pinning potential εr(R), which is valid for all defect sizes and describes its renormalization due to thermal fluctuations. With this expression we explain the temperature dependence of the true critical current density jc(O,T) and of the pinning energy Uc(0,T) at low magnetic fields. At high magnetic fields μ0H≫B* the current density experiences a power law behavior js(B)∼Bα with α≈-0.58 for films with low ndisl and α≈-0.8 to -1.1 for films with high ndisl. The pinning energy in this regime, Uc(high B)≈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 density is much larger than the linear defect density. Our results show that natural linear defects in thin films form an analogous system to columnar tracks in irradiated samples. There are, however, three essential differences: (i) typical matching fields are at least one order of magnitude smaller, (ii) linear defects are smaller than columnar tracks, and (iii) the distribution of natural linear defects is nonrandom, whereas columnar tracks are randomly distributed. Nevertheless the Bose glass theory, that has successfully described many properties of pinning by columnar tracks, can be applied also 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.@en