Depinning Of Flux Lines Research Articles

Vortex Activation Energy and Normalized Magnetization Relaxation Rate in the YBa2Cu3O7−δ /YSZ Quasi-multilayer

The quasi-multilayer films of YBa2Cu3O7−δ/YSZ are prepared by means of pulsed laser deposition (PLD) and a systematic study of I–V characteristic curves for the YBCO/YSZ quasi-multilayer film are presented, with magnetic fields up to 7 T and 15 K<T<80 K. The activation energy E(H,T) for flux line depinning is determined by fitting the nonlinear of the curves up to the flux flow region. And magnetization relaxation rate S(T) is also approximately obtained by a convenient way. The results show the S(T) plateau-like domain for the YBa2Cu3O7−δ/YSZ quasi-multilayer film develops at high H (∼ 7 T) at intermediate temperatures 25 K<T<55 K, while the S(T) plateau-like domain for the optimally doped YBa2Cu3O7−δ films disappears at H=2 T, which implies the defects introduced in the quasi-multilayer serve as strong pinning centers and influence the thermal activated state of the system at low bias current values.

Localization in non-Hermitian quantum mechanics and flux-line pinning in superconductors

A recent development in studies of random non-Hermitian quantum systems is reviewed. Delocalization was found to occur under a sufficiently large constant imaginary vector potential even in one and two dimensions. The phenomenon has a physical realization as flux-line depinning in type-II superconductors. Relations between the delocalization transition and the complex energy spectrum of the non-Hermitian systems are described. Analytical and numerical results obtained for a non-Hermitian Anderson model are shown.

Spiral depinning from linear and planar defects

The present paper gives an explicit calculation of preexponential factors in the Arrhenius law for thermally activated depinning of flux lines from linear and planar defects in the presence of a longitudinal current J. Analytic results are found in the asymptotic limit J → 0 using an instanton technique. From this the longitudinal electric field generated in a cylindrical sample with columnar defects is obtained. We also discuss briefly the dynamics of the spiral nucleation process in the case of a line defect.

Quantum depinning of flux lines from columnar defects.

The depinning of a flux line from a columnar defect is studied within the path-integral approach. Instantons of the quantum field theory in 1+1 dimensions are computed for the flux line whose dynamics is dominated by the Magnus force. The universal temperature dependence of the decay rate in the proximity of the critical current is obtained. This problem provides an example of macroscopic quantum tunneling, which is accessible to the direct comparison between theory and experiment.

Flux creep through columnar defects.

We discuss thermal depinning of flux lines from single-line defects in the presence of an external current J. Starting from Langevin dynamics the prefactor in the Arrhenius law for the escape rate scrR is derived. From this, we show the existence of a sharp depinning temperature in the limit J\ensuremath{\rightarrow}0. Moreover, the effective activation energy and time scale involved in magnetic relaxation for a sample with columnar defects are calculated. The results allow a quantitative discussion of experiments in the strong-pinning limit.

Flux dynamics in submicron superconducting NbTi wires

We find that the diameter of a superconducting filament plays a major role in the pinning and depinning of flux lines as the diameter is reduced to the submicron scale. We attribute this behavior to a finite-size effect of flux pinning.

Depinning of flux lines in Bi 2Sr 2CaCu 2O 8+y

We measured the temperature T and the magnetic field H dependences of the critical current density J c ( T, H), and the current I vs. the voltage V characteristics for Bi 2Sr 2CaCu 2O 8+y films with the preferred c-axis orientation prepared by laser ablation technique. Characteristic features were as follows: (1) J c ( T, H) vs. 1 T or H for H / / the c-axis showed a sharp drop at a threshold temperature T t ( H) or a threshold field T t ( H), respectively. (2) A line formed by T t ( H) vs. H or T t ( H) vs. T on a T− H plane for H // the c-axis would indicate a beginning of depinning of flux lines from pinning centers. (3) The I vs. V characteristics gave also some useful informations on the depinning of the flux lines.

Drift of levitated YBCO superconductor induced by both a variable magnetic field and a vibration

It is shown that both a variable magnetic field, applied to a levitated superconductor, and a vibration of a permanent magnet induce vertical drift of a magnetically levitated YBCO superconductor. The drift is observed only when a field amplitude exceeds the threshold value. Depinning of flux lines by a variable magnetic field is a probable origin of the superconductor drift. Possible causes of the origin of a variable magnetic field in a superconducting bearing are discussed.

Dynamics of a permanent magnet levitating above a high-Tc superconductor

The oscillation modes of a small spherical permanent magnet ( 1.7 mm diameter) above a flat high-T, superconductor are investigated. From the attenuation and nonlinearity of the vertical oscillation we obtain the viscosity of the vortex lattice and the onset of hysteretic damping, which indicates depinning of flux lines. The shape of the potential well in which the magnet floats, rotates, and oscillates is found to depend on the magnetic history of the superconductor.

Flux line pinning in YBa 2Cu 3O 7–δ observed with a vibrating bar

A study of flux pinning has been performed using an oscillator consisting of a bar of sintered YBa 2Cu 3O 7–δ ceramic, clamped at both ends, which was made to perform flexural vibrations in a transversely applied magnetic field. Vibrations were driven by an alternating current flowing along the bar. The viscous motion of flux lines produced a current- and field-dependent damping of the oscillator, which was observed in the resonance response of the voltage induced along the bar. Flux line de-pinning was observed as a well defined damping peak in the temperature range 30–80 K. The de-pinning temperature was found to vary with current and magnetic field. The device has been studied in magnetic fields of 0.5 to 3T, at temperatures from 10 to 130 K, and with current densities in the range 5–500 mA cm −2.