C. Coccorese

Magnetic field depedennce of pinning mechanisms in Bi2Sr2Ca1Cu2O8+x thin films

Abstract We performed investigation of the pinning mechanisms in Bi 2 Sr 2 CaCu 2 O 8+ x (BSCCO) thin films as a function of the temperature, magnetic field and bias current. The specimens were prepared by molecular beam epitaxy deposition technique with a subsequent “ex-situ” annealing in air atmosphere. Transition temperatures T c exceeded 86 K and critical current densities J c were of the order of 10 5 A/cm 2 at T = 4.2 K. Analyzing the temperature dependence of the resistivity ϱ ( T ) data we show that they could be well described by an activation process. The absoute value of the activation energy U depends crucially on the elaboration procedure, while the obtained magnetic field dependence U ( H ) does not depend on the selected model. Introducing a pinning strength distribution we were able to describe the U ( H ) dependence taking into account a crossover from an individually pinned vortex regime to a collective behavior. This crossover was also confirmed by the current dependence of U . In fact, below the crossover field H 0 the curves U ( J ) were well described by a spatial washboard-type variation of the pinning energy, while for H > H 0 the U ( J ) curves showed a logarithmic behavior.

Temperature scaling of the flux pinning force in Bi2Sr2Ca1Cu2O8+x thin films

The electrical transport properties of Bi2Sr2Ca1Cu2O8+x thin films have been measured in the temperature range from 20 to 50 K and in external magnetic fields up to 4 T. The dependence of the normalized pinning force f=Fp/Fp,max on the reduced magnetic field h=H/H* (H* is the irreversibility field) exhibits a scaling law behavior, indicating that a single pinning mechanism is present in the investigated temperature range. The Fp(H) curves were analyzed within the framework of the classical Anderson–Kim theory of flux creep, taking into consideration the nonlinear current dependence of the pinning energy U and its dependence on the temperature T, the magnetic field H, and the current density J. Under these assumptions a generalized formula for Fp was obtained which, assuming a suitable U(J) dependence, was able to explain the f(h) curve in the observed temperature range.

Upper critical magnetic field and vortex pinning in superconducting/spin glass multilayers

Abstract We present a study of the upper critical magnetic field, H c2 , and pinning force, F p , in superconducting (Nb)/spin glass (CuMn) multilayers. Patterned samples with different CuMn thicknesses and Mn concentrations were measured in both the perpendicular and the parallel magnetic field orientations. The behavior of the perpendicular upper critical field H c2⊥ was in agreement with the anisotropic Ginzburg–Landau theory, while the parallel upper critical field H c2‖ temperature dependence for highly anisotropic samples was nonlinear in the whole investigated temperature range. Grain boundary pinning mainly influences the perpendicular critical current density values independently on the multilayer anisotropy. For parallel magnetic field orientation, instead, the intrinsic vortex pinning on CuMn layers was dominant.

Flux creep-flux flow crossover in disordered superconductors

Abstract We propose a method for the correct definition of the effective pinning potential U eff and the critical current density J ∗ in the case of large spread in the values of pinning energies (disordered superconductor). This physical situation corresponds to the high temperature superconductors. We show that, in the limit of small variances σ u of the pinning potential, the current dependence of the effective pinning energy, U eff ( J ), is determined by the interaction of vortices with microinhomogeneities under the deterministic spatial shape U(x) . At large σ u (i.e. σ u ⪢ U eff > k B T ) U eff is shown to have a logarithmic J -dependence in a wide range of J .

Secondary ion mass spectrometry and x-ray analysis of superconducting Nb/Pd multilayers

We report on accurate structural investigations of sputtered Nb/Pd multilayers by means of high-resolution secondary ion mass spectrometry and x-ray reflectivity. The combined use of secondary ion mass spectrometry and x-ray specular reflectivity techniques allows us to study the chemical configuration of the interfaces and to relate it to the observed superconducting properties. Secondary ion mass spectrometry analyses reveal a distinct Nb and Pd modulation and very sharp profiles with abrupt interfaces indicating a negligible interdiffusion of Nb and Pd at the interfaces. Moreover, analyzing the features in the Nb and Pd profiles and correlating them to the oxygen distribution in the multilayers and to the low-angle x-ray patterns, thin layers (3–4 nm thick) of niobium oxide were noticed at the Nb/Pd interfaces, while no oxide layers at the Pd/Nb interfaces could be detected. The role of this oxide layer in the determination of the crossover between three- and two-dimensional superconducting behavior in...

Glancing-incidence X-ray characterization of Nb/Pd multilayers

SummaryThe study of periodic metallic multilayers in which one of the two constituent layers is a superconductor has attracted considerable interest. The structural configuration and quality of the interfaces is of fundamental importance because it influences the phases of the superconductive wave function in the interface region and, hence, the coupling between the nearest superconductive layers. In this work we present a structural investigation of Nb/Pd multilayers by using high- and low-angle X-ray diffraction measurements. All the samples were grown on Si(100) substrates by de-triode sputtering. We investigated two samples consisting of 10 stacks of nominally 19 nm Nb, 4 nm Pd and 18 nm Nb, 8 nm Pd, respectively. The high-angle analyses reveal that the Nb layer is oriented in the [110] direction and the Pd in the [111] direction. Off-specular reflectivity measurements show the presence of a (partially) correlated roughness across the interfaces. From specular reflectivity it was found how the rms roughness increases from the substrate to the surface.

NB LIFT-OFF PROCEDURE FOR MICROPATTERNING BI2SR2CA1CU2O8+X THIN FILMS

We developed an in situ micropatterning process for Bi2Sr2Ca1Cu2O8+x thin films using the large volumetric increase of refractory metals such as Nb, V, and Ta, during the hard regime of oxidation. After the lift‐off procedure, our films showed nonreduced critical currents and temperatures. This technique is particularly suitable for multilayer in situ lithographic process.

SUPERCONDUCTING BSCCO THIN FILMS OBTAINED BY MBE

SummaryWe realized BSCCO thin films withTc(R=0)=88 K using the Molecular Beam Epitaxy (MBE) technique. The stoichiometric control is reached by a multilayer deposition technique in which Ca−Cu−Sr−Bi are sequentially deposited using the two effusive cells and the two e-guns present in the system. EDS and X-ray analyses are used to cross-check the obtained results, after theex situ in air annealing. The recent acquisition of an atomic oxygen source will allow in the close future the completelyin situ realization of BSCCO thin films.

Structure and Superconducting Properties of Nb/Pd Multilayers

We have studied the structure and superconducting properties of Nb/Pd multilayers deposited by Molecular Beam Epitaxy system (MBE). The multilayer was grown on the Si(lll) substrate with a 40A Cu(111) buffer layer. During the deposition, electron microscopy was employed in-situ and showed the epitaxial growth of both Nb and Pd layers. The resistivity of each layer was calculated by the resistivities obtained from the multilayers and the resistivities caused by size effect in each layer to evaluate the mean free path of each layer. The transition temperature was calculated using the de — Gennes Werthamer theory and the results showed good agreement with the experimental results.

BSCCO thin films obtained by MBE coevaporation method

SummaryWe obtained as-grown Bi2Sr2Cu1Ox (2201) superconducting thin films using a MBE system consisting of two electron guns and two effusion cells. The samples were obtained evaporating simultaneously the single elements on heated substrates and oxygenating them during the growth by using an atomic oxygen plasma source (OPS). Sticking coefficients of each element were determined as a function of the substrate temperature and oxygen partial pressure. In order to monitor the surface structure, Reflection High Energy Electron Diffraction (RHEED) analysis was employed during the deposition process. The chemical composition was obtained by Energy Dispersion Spectroscopy (EDS) while the final structure was studied by X-ray diffraction patterns.