The Labusch Parameter of a Driven Flux Line Lattice in YBa2_2Cu3_3O7_7 Superconducting Films

We have investigated the influence of a driving force on the elastic coupling (Labusch parameter) of the field-cooled state of the flux line lattice (FLL) in 400 nm thick YBa$_2$Cu$_3$O$_7$ superconducting films. We found that the FLL of a field-cooled state without driving forces is not in an equilibrium state. Results obtained for magnetic fields applied at $0^\circ$ and 30$^\circ$ relative to CuO$_2$ planes, show an enhancement of the elastic coupling of the films at driving current densities several orders of magnitude smaller than the critical one. Our results indicate that the FLL appears to be in a relatively ordered, metastable state after field cooling without driving forces.Comment: 4 Figure

Drastic improvement of surface structure and current-carrying ability in YBa2Cu3O7 films by introducing multilayered structure

Much smoother surfaces and significantly improved superconducting properties of relatively thick YBa2Cu3O7 (YBCO) films have been achieved by introducing a multilayered structure with alternating main YBCO and additional NdBCO layers. The surface of thick (1 microm) multilayers has almost no holes compared to YBCO films. Critical current density (Jc) have been drastically increased up to a factor > 3 in 1 microm multilayered structures compared to YBCO films over entire temperature and applied magnetic filed range. Moreover, Jc values measured in thick multilayers are even larger than in much thinner YBCO films. The Jc and surface improvement have been analysed and attributed to growth conditions and corresponding structural peculiarities.Comment: Accepted to Appl. Phys. Lett. 88, June (2006), in press 4 pages, 3 figure

Vortex matter in superconductors

The behavior of the ensemble of vortices in the Shubnikov phase in biaxially oriented films of the high-temperature superconductor YBa2Cu3O7−δ(YBCO) in an applied magnetic field is investigated for different orientations of the field. The techniques used are the recording of the current–voltage characteristics in the transport current and of resonance curves and damping of a mechanical oscillator during the passage of a transport current. It is shown that the behavior of the vortex ensemble in YBCOfilms, unlike the case of single crystals, is determined by the interaction of the vortices with linear defects—edge dislocations, which are formed during the pseudomorphic epitaxialgrowth and are the dominant type of defect of the crystal lattice, with a density reaching 1015 lines/m2. The effective pinning of the vortices and the high critical current density (Jc⩾3×1010 A/m2 at 77 K) in YBCOfilms are due precisely to the high density of linear defects. New phase states of the vortex matter in YBCOfilms are found and are investigated in quasistatics and dynamics; they are due to the interaction of the vortices with crystal defects, to the onset of various types of disordering of the vortex lattice, and to the complex depinning process. A proposed H–T phase diagram of the vortex matter for YBCOfilms is proposed

Overcritical state in superconducting round wires sheathed by iron

Magnetic measurements carried out on MgB_2 superconducting round wires have shown that the critical current density J_c(B_a) in wires sheathed by iron can be significantly higher than that in the same bare (unsheathed) wires over a wide applied magnetic field B_a range. The magnetic behavior is, however, strongly dependent on the magnetic history of the sheathed wires, as well as on the wire orientation with respect to the direction of the applied field. The behavior observed can be explained by magnetic interaction between the soft magnetic sheath and superconducting core, which can result in a redistribution of supercurrents in the flux filled superconductor. A phenomenological model explaining the observed behavior is proposed.Comment: 9 pages, 7 figure

Exact asymptotic behavior of magnetic stripe domain arrays

The classical problem of magnetic stripe domain behavior in films and plates with uniaxial magnetic anisotropy is treated. Exact analytical results are derived for the stripe domain widths as function of applied perpendicular field, $H$, in the regime where the domain period becomes large. The stripe period diverges as $(H_c-H)^{-1/2}$, where $H_c$ is the critical (infinite period) field, an exact result confirming a previous conjecture. The magnetization approaches saturation as $(H_c-H)^{1/2}$, a behavior which compares excellently with experimental data obtained for a $4 \mu$m thick ferrite garnet film. The exact analytical solution provides a new basis for precise characterization of uniaxial magnetic films and plates, illustrated by a simple way to measure the domain wall energy. The mathematical approach is applicable for similar analysis of a wide class of systems with competing interactions where a stripe domain phase is formed.Comment: 4 pages, 4 figure

Drastic improvement of surface structure and current-carrying ability in YBa2Cu3O7 films by introducing multilayered structure

Much smoother surfaces and significantly improved superconducting properties of relatively thick YBa2Cu3O7 (YBCO) films have been achieved by introducing a multilayered structure with alternating main YBCO and additional NdBCO layers. The surface of thick (1 µm) multilayers has almost no holes compared to YBCO films. Critical current density (Jc) has been drastically increased up to a factor \u3e3 in 1 µm multilayered structures compared to YBCO films over entire temperature and applied magnetic field range. Moreover, Jc values measured in thick multilayers are even larger than in much thinner YBCO films. The Jc and surface improvement have been analyzed and attributed to growth conditions and corresponding structural peculiarities

Virgin magnetization of a magnetically shielded superconductor wire: Theory and experiment

On the basis of exact solutions to the London equation, the magnetic moment of a type II superconductor filament surrounded by a soft-magnet environment is calculated and the procedure of extracting the superconductor contribution from magnetic measurements is suggested. A comparison of theoretical results with experiments on MgB2/Fe wires allows the estimation of the value of critical current for the first magnetic flux penetration

Direct visualization of iron sheath shielding effect in MgB_2 superconducting wires

Local magneto-optical imaging and global magnetization measurement techniques were used in order to visualize shielding effects in the superconducting core of MgB_2 wires sheathed by ferromagnetic iron (Fe). The magnetic shielding can provide a Meissner-like state in the superconducting core in applied magnetic fields up to ~1T. The maximum shielding fields are shown to correlate with the saturation fields of magnetization in Fe-sheaths. The shielding has been found to facilitate the appearance of an overcritical state, which is capable of achieving a critical current density (J_c) in the core which is larger than J_c in the same wire without the sheath by a factor of ~2. Other effects caused by the magnetic interaction between the sheath and the superconducting core are discussed.Comment: 4 pages, 3 figure

Organic MgB2-xCx superconductor with high performance enabled by liquid mixing approach

Comparative analysis of structural and electromagnetic characteristics have been performed on nano SiC- and polycarbosilane-doped MgB2 samples prepared by the dry and liquid mixing approaches. The total benefit of liquid mixing approach for fabrication of organic MgB2 superconductor with excellent electromagnetic performance has been demonstrate