Superconducting-coil--resistor circuit with electric field quadratic in the current

It is shown for the first time that the observed [Phys. Lett. A 162 (1992) 105] potential difference Phi_t between the resistor and the screen surrounding the circuit is caused by polarization of the resistor because of the kinetic energy of the electrons of the superconducting coil. The proportionality of Phi_t to the square of the current and to the length of the superconducting wire is explained. It is pointed out that measuring Phi_t makes it possible to determine the Fermi quasimomentum of the electrons of a metal resistor.Comment: 2 pages, 1 figur

Magnetic Pinning of Vortices in a Superconducting Film: The (anti)vortex-magnetic dipole interaction energy in the London approximation

The interaction between a superconducting vortex or antivortex in a superconducting film and a magnetic dipole with in- or out-of-plane magnetization is investigated within the London approximation. The dependence of the interaction energy on the dipole-vortex distance and the film thickness is studied and analytical results are obtained in limiting cases. We show how the short range interaction with the magnetic dipole makes the co-existence of vortices and antivortices possible. Different configurations with vortices and antivortices are investigated.Comment: 12 pages, 12 figures. Submitted to Phys. Rev.

Flux-Induced Vortex in Mesoscopic Superconducting Loops

We predict the existence of a quantum vortex for an unusual situation. We study the order parameter in doubly connected superconducting samples embedded in a uniform magnetic field. For samples with perfect cylindrical symmetry, the order parameter has been known for long and no vortices are present in the linear regime. However, if the sample is not symmetric, there exist ranges of the field for which the order parameter vanishes along a line, parallel to the field. In many respects, the behavior of this line is qualitatively different from that of the vortices encountered in type II superconductivity. For samples with mirror symmetry, this flux-induced vortex appears at the thin side for small fluxes and at the opposite side for large fluxes. We propose direct and indirect experimental methods which could test our predictions.Comment: 6 pages, Latex, 4 figs., uses RevTex, extended to situations far from cylindrical symmetr

Superconductivity in multi-phase Mg-B-O compounds

Structures of MgB2-based materials manufactured under pressure (up to 2 GPa) by different methods having high superconducting performance and connectivity are multiphase and contain different Mg-B-O compounds. Some oxygen can be incorporated into MgB2 and boron into MgO structures, MgBx (X=4-20) inclusions contain practically no oxygen. Regulating manufacturing temperature, pressure, introducing additions one can influence oxygen and boron distribution in the materials and thus, affect the formation, amount and sizes of Mg-B-O and MgBx inclusions and changing type of pinning, pinning force and so affect critical current density jc. The boron concentration increase in initial Mg and B mixture allows obtaining sample containing 88.5 wt% of MgB12 with Tc of 37.4 K (estimated magnetically)

The Flux-Line Lattice in Superconductors

Magnetic flux can penetrate a type-II superconductor in form of Abrikosov vortices. These tend to arrange in a triangular flux-line lattice (FLL) which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-$T_c$ supercon- ductors (HTSC's) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSC's the FLL is very soft mainly because of the large magnetic penetration depth: The shear modulus of the FLL is thus small and the tilt modulus is dispersive and becomes very small for short distortion wavelength. This softness of the FLL is enhanced further by the pronounced anisotropy and layered structure of HTSC's, which strongly increases the penetration depth for currents along the c-axis of these uniaxial crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may melt the FLL and cause thermally activated depinning of the flux lines or of the 2D pancake vortices in the layers. Various phase transitions are predicted for the FLL in layered HTSC's. The linear and nonlinear magnetic response of HTSC's gives rise to interesting effects which strongly depend on the geometry of the experiment.Comment: Review paper for Rep.Prog.Phys., 124 narrow pages. The 30 figures do not exist as postscript file