Papers by Keyword: Proximity Effect

Abstract: We have performed structure analysis and electrical conductivity measurements of Pb ultrathin films of different thicknesses grown on Ge (111) at low temperature by using electron diffraction, scanning tunneling microscopy, and in-situ four-point probe method in ultrahigh vacuum. Three samples with different deposition amounts of Pb corresponding to 1, 3 and 10 monolayer (ML) were revealed to have different structures. The 1 ML-Pb sample, having a wetting layer and tiny clusters on it, did not show superconductivity. The 10-ML-Pb sample, consisted of continuous Pb (111) thin film structure, showed thin-film superconductivity around 6 K. The 3-ML-Pb sample, consisted of the wetting layer with unconnected Pb (111) islands on it, also showed superconductivity around 4 K. This superconductivity is thought to be induced in the wetting layer by proximity effect from superconducting Pb (111) islands. Thus, it is important to study the detailed growth structures for understanding atomic-layer superconductivity.

Abstract: We study the superconducting transition temperature T_c of F2/F1/S trilayers (Fi is a metallic ferromagnet, S is a s-superconductor), where the long-range triplet superconducting component is generated at canted magnetizations of the F layers. In this paper we show that it is possible to realize different spin-valve effect modes - the standard switching effect, the triplet spin-valve effect, reentrant T_c(α) dependence or reentrant T_c(α) dependence with the inverse switching effect - by variation of the F2/F1 interface transparency or the exchange splitting energy. In addition, we show that positionof the T_c minimum can be changed by joint variation of the F2/F1 interface transparency and the layerthicknesses.

Abstract: Superconducting Nb/Au/Ca_1-xSr_xCuO₂/YBa₂Cu₃O₇ mesa-heterostructures were investigated. Dependencies of electrical parameters versus inverse capacitance were measured. A band diagram which takes into account an accumulation of holes in Ca_1-xSr_xCuO₂ interlayer and band bending due to difference of work functions was proposed. The dependencies of electrical parameters were analyzed by examining the quasipartical and superconducting currents.

Abstract: The proximity effect and competition between the BCS and LOFF states are studied in the Cooper limit for thin F/S and F/S/F nanostructures, where F is a ferromagnet and S is a superconductor. The dependences of the critical temperature T_c on the exchange field I, electron correlations λ_f, and the thickness d_f of the F layer are derived for F/S bilayers and F/S/F trilayers. Two new π-phase superconducting states with electronelectron repulsion in the F layers of F/S/F trilayers are predicted. A 2D LOFF state in F/S/F trilayers is possible only in the presence of a weak magnetic field and the appropriate parameters of the F and S layers. The absence of the suppression of 3D superconductivity in short-period Gd/La superlattices is explained and the electronelectron coupling constant in gadolinium is predicted. A method of superconducting probe spectroscopy based on the proximity effect is proposed for determining the symmetry of the order parameter, the magnitude and sign of electron correlations, and the exchange field in various nanomagnets F.

Abstract: We study asymmetrical three-layers F1F2S and F1SF2 structures in an external parallelmag-netic field. Assuming that all F and S layers are dirty, we solve the boundary value problem for theUsadel function. We calculate the critical temperature of in the F1F2S and F1SF2 systems as functionof the F layers thicknesses and external magnetic field.

Abstract: We consider the Josephson effect in a three-layer symmetric superconductor-ferromagnet-superconductor (SFS) system. In the framework of the proximity effect theory we calculate the Josephson current as a function of the ferromagnet thickness, taking into account the Umklapp processes at the internal boundary and the critical temperature dependence of the order parameter phase difference. The results of the calculations are in good agreement with experimental data.

Abstract: A high frequency coaxial transformer (HFCT) with a split winding structure and litz wire has been developed for high frequency (HF) and high power DC/DC converter applications. A method combined numerical analysis of magnetic field and analytical calculation of litz wire winding losses, taking into account conduction losses and proximity effect losses, is proposed for the designed HFCT. The experimental results validate the winding loss calculation method. The measured results demonstrate that the voltage ratio has a good agreement with the turn ratio over the frequency range from 0.1MHz to 1MHz, indicating that the high coupling efficiency has been obtained.

Abstract: Besides weldable component geometries for the high-frequency welding process also possible process and system induced activated material reactions during discontinuous high-frequency welding are presented in this paper. Among others such material reactions can be a locally limited thermal influence on the base metal, defined plastic derformations during the upsetting process as well as grain refinement in the weld seam, comparable to thermomechanical treatment during rolling for increasing strength or ductility.

Abstract: On the base of new boundary-value problem for the Eilenberger function we investigate the superconducting and magnetic states in ferromagnet/superconductor (FM/S) nanostructures, where superconductivity is a superposition of the BCS pairing with zero total momentum in the S layers and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing with nonzero 3D coherent momentum k in the FM layers. We originally study the interplay between the BCS and FFLO states in the pure thin FM/S/FM trilayers and two novel -phase superconducting states with electron-electron repulsion in the FM layers are predicted. The modulated FFLO states are possible in such trilayers only in presence of external magnetic field at suitable parameters of the FM and S layers. In the FM/S superlattices there are also two -phase magnetic states (0 and ) with compensation of the exchange field paramagnetic effect. This fact allows us to explain a surprisingly high Tc ~ 5K in the short period Gd/La superlattice and to predict the sign and value of the electron-electron interaction in the ferromagnetic Gd metal.

Abstract: The proximity effect of the ferromagnetic metal/superconductor (FM/S) bilayers in а external longitudinal magnetic field is considered in the dirty limit. The critical temperature and the superconducting current distribution versus applied magnetic field’s magnitude, film’s thicknesses and a transparency of the contact is calculated, with taking into account аn umklapp processes possibility on the interface of the FM/S contact. It is shown that superconducting current is strictly inhomogeneous and asymmetrical. It is shown also that the current inhomogeneity depends heavily from the FM/S interface transparency.