W. Benoit

Nonlinear AC susceptibility, surface and bulk shielding

Abstract We calculate the nonlinear AC response of a thin superconducting strip in perpendicular field, shielded by an edge current due to the geometrical barrier. A comparison with the results for infinite samples in parallel field, screened by a surface barrier, and with those for screening by a bulk current in the critical state, shows that the AC response due to a barrier has general features that are independent of geometry, and that are significantly different from those for screening by a bulk current in the critical state. By consequence, the nonlinear (global) AC susceptibility can be used to determine the origin of magnetic irreversibility. A comparison with experiments on a Bi 2 Sr 2 CaCu 2 O 8+δ crystal shows that in this material, the low-frequency AC screening at high temperature is mainly due to the screening by an edge current, and that this is the unique source of the nonlinear magnetic response at temperatures above 40 K.

Low temperature internal friction spectrum of YBa2Cu3Ox

Abstract The elastic and anelastic behaviours of polycrystalline YBa2Cu3Ox specimens have been studied between 80 and 300 K, by means of a resonant bar technique. Three damping peaks have been observed at 90, 115 and 220 K. The stability of these peaks during annealing in vacuum has been carefully examined. The 90 K peak is not related to the superconducting transition because it is still observed after thermal treatments leading to the disappearance of superconductivity. The 115 K is due to a relaxation mechanism. The activation energy is ∼0.17 eV and the frequency factor is ∼1012s-1. Also this peak is certainly correlated with the excess oxygen atoms, because it disappears with annealing in vacuum. The 220 K maximum, which was also observed by ultrasonic measurements, seems to be affected by the sample preparation, i.e. by the sintering conditions. Finally, a frequency hysteresis has been observed in every superconducting sample, which accounts for an anomalous behaviour of the elastic modulus.

Pinning in the flux-line-cutting regime of Bi2Sr2Ca1Cu2O8 single crystals at high field

Abstract Using a low-frequency torsion pendulum we show that in a Bi2Sr2Ca1Cu2O8 single crystal the irreversibility line Birr(T) is frequency dependent down to 10-5 Hz in the high-field regime. The activation energy has a logarithmic field dependence, U0(B)=U∗ 1n(B∗/B). A microscopic model for flux-line-cutting and pancake collision yields quantitative expressions for U0 and for Birr(T)=B∗ exp(−T/T∗), which reproduce the experimental data very well.

Edge-shape barrier irreversibility and decomposition of vortices in Bi2Sr2CaCu2O8

Abstract Magnetic flux dynamics is studied in Bi 2 Sr 2 CaCu 2 O 8 single crystals by means of magneto-optical technique. It is clearly demonstrated that the magnetic irreversibility of these crystals in a magnetic field perpendicular to the basal plane at temperatures higher than approximately 35 K is governed by an edge-shape barrier and its disappearance determines the high temperature part of the magnetic irreversibility line which is commonly associated in the literature with vortex lattice melting. We argue that this barrier exists because of the non ellipsoidal shape of the samples and can disappear only when the flux lines lose their rigidity decomposing into pancakes, which is the only true magnetic phase transition on the B-T diagram for Bi 2 Sr 2 CaCu 2 O 8 .

Kinks and vortex-twister dynamics in type-II superconductors

Abstract We report magneto-optical observations of moving helicoidal vortex structures in high purity YBa2Cu3O7−δ single cyrstals. We found that the dynamics of these ‘vortex-twisters’ is mainly controlled by localized instabilities (kinks) which stream along the helices. The kinks allow the motion of the twisters, or the annihilation of twisters with opposite chirality.

Pinning in BSCCO above the ordinary irreversibility line

Frequency-dependent observations of magnetic flux structures are used to show that pinning plays a principal role in the whole mixed state in Bi2Sr2CaCu2O8 (BSCCO) single crystals. We speculate that the random pinning force on the moving vortices may dominate over thermal fluctuations and considerably modify the position of the vortex lattice phase transition.

AC losses in (Bi,Pb)2Sr2Ca2Cu3Ox tapes

Abstract A double peak structure is observed in the AC losses of (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed tapes using a torsion-pendulum oscillator. The low-temperature peak is associated to the intragrain flux expulsion, while the high-temperature peak results from a macroscopic current path around the whole sample due to a well-coupled fraction of the grains. The flux pinning by the dislocations forming small-angle grain boundaries is suggested to control the transport current.

Spatially resolved resistivity near the vortex lattice phase transition in Bi2Sr2CaCu2O8+δ single crystals

Abstract Using a multiterminal contact configuration, we investigate the local variations of the resistivity drop near the vortex lattice first order phase transition in a very homogeneous Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO) single crystal.

Investigation of flux penetration in YBa2Cu3O7−δ filaments

Abstract ac Susceptibility measurements using a low frequency torsion pendulum and an ac susceptometer were conducted on YBa2Cu3O7−δ filaments in low magnetic dc fields (≤1T). Different dissipation peaks are observed, dependent on the temperature and the applied amplitude. The peak at low temperatures, which is of intergranular nature is studied in detail. The penetration follows the Bean model and the intergranular creep is observed.

Peak effect in untwinned YBa2Cu3O7−δ single crystals

Abstract We report on the observation of a weak effect of the critical current density in untwinned YBa2Cu3O7−δ single crystals of different purity, using a low frequency torsion pendulum. We construct the peak effect line and the irreversibility line.