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We present a fully first-principles method for superconducting thin films. The layer dependent phonon spectrum is calculated to determine the layer dependence of the electron-phonon coupling for such systems, which is coupled to the Kohn-Sham-Bogoliubov-de Gennes equations, and it is solved in a parameter-free way. The theory is then applied to different surface facets of niobium slabs and to niobium-gold heterostructures. We investigate the dependence of the transition temperature on the thickness of the slabs and the inverse proximity effect observed in thin superconducting heterostructures.

The dynamical behavior of magnetic flux in superconducting Tl-Ba-Ca-Cu-O films has been studied using the Nernst effect. Large temperature gradients of \ensuremath{\sim}${10}^{5}$ K/cm and, correspondingly, large thermal forces on flux lines were obtained by pulsed laser heating of films. Due to the large thermal driving forces it was possible to observe, at temperatures well below ${\mathit{T}}_{\mathit{c}}$, the transition from thermally assisted flux flow to flux creep and viscous flux flow.

We measure current-voltage characteristics as function of magnetic field and temperature in Nb strips of different thickness and width. The instability voltage of the flux-flow state related to the vortex critical velocity ${v}^{\ensuremath{\ast}}$ is studied and compared with the Larkin-Ovchinnikov theory. Beside the usual power-law dependence ${v}^{\ensuremath{\ast}}\ensuremath{\approx}{B}^{\ensuremath{-}1/2}$, in the low-field range a new crossover field, ${B}_{cr1}$, is observed below which ${v}^{\ensuremath{\ast}}$ decreases by further lowering the external magnetic field $B$. We ascribe this unexpected crossover to vortex channeling due to a fanlike penetration of the applied magnetic field as confirmed by magneto-optic imaging. The observation of ${B}_{cr1}$ becomes a direct evidence of a general feature in type-II superconducting films at low fields that is a channel-like vortex motion induced by the inhomogeneous magnetic state caused by the relatively strong pinning.

Via a direct coupling between the order parameter and the singlet Josephson supercurrent we detect spin-wave resonances and their dispersion in ferromagnetic Josephson junctions in which the usual insulating or metallic barrier is replaced with a weak ferromagnet. The coupling arises within the Fraunhofer interferential description of the Josephson effect because the magnetic layer acts as a time-dependent phase plate. A spin-wave resonance at a frequency ${\ensuremath{\omega}}_{s}$ implies a dissipation that is reflected as a depression in the current-voltage curve of the Josephson junction when $\ensuremath{\hbar}{\ensuremath{\omega}}_{s}=2eV$. We have thereby performed a resonance experiment on only ${10}^{7}$ Ni atoms.

While the time-dependent Ginzburg-Landau theory successfully describes a number of experimental results for homogeneous high-${T}_{c}$ superconductors, the proper value of the characteristic depairing electric field was recently found unreasonably small for Bi-Sr-Ca-Cu-O thin films. We present numerical simulations which show that these features can be the consequence of smooth spatial inhomogeneity of the local critical temperature. Because of the interplay of percolation and superconducting transition, even a small disorder affects strongly the nonlinear excess conductivity in the transition region, causing considerable rescaling of the characteristic electric field.

High-quality mono rare earth $R{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (R=Gd,Eu,Nd) and mixed rare earth $({\mathrm{Gd}}_{1/3}{\mathrm{Eu}}_{1/3}{\mathrm{Nd}}_{1/3}){\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (GEN123) films were prepared by off-axis laser deposition, and their transport and flux pinning characteristics were experimentally studied. Enhanced flux pinning due to rare earth mixing is evidenced by high transport critical current density ${(J}_{c})$ and an upward shifted irreversibility line defined by zero-resistance criterion. In contrast, the characteristic field, i.e. the accommodation field ${(H}_{\mathrm{acc}}),$ identified from the field dependence of transport ${J}_{c},$ shows no increase due to rare earth mixing. It is found that ${H}_{\mathrm{acc}}$ vs $T$ can be scaled by a negative exponential relation at a wide range of temperatures, and the field anisotropy of ${H}_{\mathrm{acc}}$ appears pronounced as the field direction varies from $H\ensuremath{\parallel}c$ to $H\ensuremath{\parallel}(a,b).$ According to the analysis on the temperature dependence of ${J}_{c},$ it is suggested that the flux pinning mechanism in mixed rare earth samples is dominated by stress-field centers which may be induced by dislocations and enhanced by lattice mismatch in GEN123 films. Finally, the fluctuation of vortex phases is discussed with respect to correlated and uncorrelated structure disorders in epitaxial 123 thin films.

In this paper an analytical and numerical study of a superconducting thin film with ferromagnetic textures of elliptical geometries in close proximity is presented. The screening currents induced in the superconductor dueto the magnetic texture are calculated. Close to the superconducting transition temperature I c the spontaneous creation of superconducting vortices becomes energy favorable depending on the value of the magnetization and the geometrical quantities of the magnetic texture. The creation of vortices by elliptic dots is more energy favorable than those created by circular ones. The superconductor covered by elliptic dots array exhibits anisotropic transport properties.

We study the depinning transition of a driven chain-like system in the presence of frustration and quenched disorder. The analysis is motivated by recent transport experiments on artificial vortex-flow channels in superconducting thin films. We start with a London description of the vortices and then map the problem onto a generalized Frenkel-Kontorova model and its continuous equivalent, the sine-Gordon model. In the absence of disorder, frustration reduces the depinning threshold in the commensurate phase, which nearly vanishes in the incommensurate regime. Depinning of the driven frustrated chain occurs via unstable configurations that are localized at the boundaries of the sample and evolve into topological defects which move freely through the entire sample. In the presence of disorder, topological defects can also be generated in the bulk. Further, disorder leads to pinning of topological defects. We find that weak disorder effectively reduces the depinning threshold in the commensurate phase, but increases the threshold in the incommensurate phase., 16 pages, 12 figures

Measurements of the in-plane magnetic-field penetration depth for the n-doped high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ are reported. The measurements were performed at 4 GHz by a microstrip resonator technique on thin films obtained by laser ablation. The temperature dependence of the penetration depth is carefully analyzed and the energy gap-ratio is estimated. The results confirm that ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ behaves as a BCS-like superconductor.

We have used a scanning superconducting quantum interference device microscope to image individual vortices near lithographically patterned surface steps in weak-pinning superconducting thin films of amorphous MoGe. The field-cooled vortex distributions are strongly influenced by the surface steps, with an enhanced vortex density along the thin side of the steps and a wide vortex-free region along the thick side of the steps. The surface steps induce orientational order that persists for many intervortex spacings away from the steps. We study the effects of surface step pinning for different magnetic-field strengths and step heights by analyzing the intervortex spacing and computing vortex correlation functions. For certain sample configurations, we are able to apply a Lorentz force to the vortices and observe an asymmetric vortex response near the surface steps. We discuss the interaction between a vortex and a surface step and consider possible mechanisms for generating the vortex distributions which we observe upon field cooling.

Magneto-optical investigation of flux penetration into high-temperature superconducting thin films allows the determination of the local critical current density j c by an inversion scheme of Biot-Savart's law. This method is used to examine the influence of silver sheeting on j c in thin films of YBa 2 Cu 3 O 7 - Φ (YBCO) quantitatively. It can be found that a feasible silver covering layer on top of a YBCO thin film can enhance the critical current density by up to 50%. Spatially resolved measurements of the magnetic-flux density distribution in partly silver covered YBCO films show the influence of the cover layer on the current pattern in the superconductor. The measured enhancement of the critical current density, that is induced by the silver layer, has its origin in a spatially varying proximity effect between superconductor and silver layer which leads to a strong variation of the flux-line energies on a small length scale. This variation is directly related to an additional pinning force density on the flux lines. A detailed model is developed that can explain the measured enhancement of the critical current density by considering this additional pinning.

The vortex dynamics in YBa2Cu3O7-δ thin films have been studied at microwave frequencies. A pronounced peak in the surface resistance Rs is observed in these films at frequencies of 4.88 and 9.55 GHz for magnetic fields varying from 0.2 to 0.8 T. The occurrence of the peak in Rs is crucially dependent on the depinning frequency ωp and on the nature and concentration of growth defects present in these films. Introduction of artificial defects by swift heavy ion irradiation with a 200 MeV Ag ion at a fluence of 4×10^10 ions/cm2 enhances ωp and suppresses the peak at 4.88 GHz but the peak at 9.55 GHz remains unaffected. The peak can be associated with a transformation of the flux line lattice (FLL) from a pinned solid to a depinned liquid in a depinning frequency domain similar to the order-disorder transformation of the FLL associated with the peak effect in critical currents in dc or low frequencies as the temperature or field is increased. A second peak at lower temperature has also been observed at 9.55 GHz. This could be related to twin boundaries from angular dependence studies of Rs. Based on the temperature variation of Rs, vortex phase diagrams have been constructed at 9.55 GHz.

We consider a thin-film normal metal/superconductor junction in the presence of an externally applied in-plane magnetic field for several symmetries of the superconducting order parameter. For p-wave superconductors, a strongly spin-polarized current emerges due to an interplay between the nodal structure of the superconducting order parameter, the existence or non-existence of zero-energy surface states, and the Zeeman-splitting of the bands which form superconductivity. Thus, the polarization depends strongly on the orbital symmetry of the superconducting state. Our findings suggest a mechanism for obtaining fully spin-polarized currents crucially involving zero-energy surface states, not present in s-wave superconductors., Comment: 5 pages, 5 figures. Accepted for publication in Phys. Rev. B

We have explored the effects of ion beam irradiation on the electrical and structural properties of superconducting thin films of YBa/sub 2/Cu/sub 3/O/sub 7/..sqrt../sub delta/. Damage created by nuclear-energy loss processes degrades the superconducting transitions. Data from resistance measurements as well as from Rutherford backscattering and channeling measurements suggest that ion bombardment is not changing the volume fraction of superconducting material substantially. Instead, it appears that islands of good superconductor are becoming progressively decoupled, destroying the phase coherence

A straightforward method for the self-consistent solution of the Bogoliubov equations for a class of geometries which includes the case of superconducting thin films is presented. Solutions of the gap equation obtained from the self-consistency condition imposed on the pair amplitude are studied and the temperature and thickness dependence of the gap is obtained. The results obtained are compared, where appropriate, with the corresponding results in Ginzburg-Landau theory and the McMillan tunneling model.