P. Mathieu

Phenomenological Theory of Vortex Motion in Type-II Superconductors

Based on the analogy of the Bekarevich-Khalatnikov theory of He II, a continuum description of vortex motion in the mixed state is presented. The possibility of critical surface currents and associated surface Joule effects is inferred, in agreement with experiment.

DEPINNING TRANSITION IN TYPE-II SUPERCONDUCTORS

The surface impedance Z(f) of conventional isotropic materials has been carefully measured for frequencies f ranging from 1 kHz to 3 MHz, allowing a detailed investigation of the depinning transition. Our results exhibit the irrelevance of classical ideas to the dynamics of vortex pinning. We propose a new picture, where the linear ac response is entirely governed by disordered boundary conditions of a rough surface, whereas in the bulk vortices respond freely. The universal law for Z(f) thus predicted is in remarkable agreement with experiment, and tentatively applies to microwave data in YBaCuO films.

Metastable states of a flux-line lattice studied by transport and small-angle neutron scattering

Flux-line lattice (FLL) states have been studied using transport measurements and small-angle neutron scattering in low-T{sub c} materials. In Pb-In, the bulk dislocations in the FLL do not influence the transport properties. In Fe-doped NbSe{sub 2}, transport properties can differ after a field cooling (FC) or a zero field cooling (ZFC) procedure, as previously reported. The ZFC FLL is found ordered with narrow Bragg peaks and is linked to a linear V(I) curve and to a superficial critical current. The FC FLL pattern exhibits two Bragg peaks and the corresponding V(I) curve shows an S-shape. This can be explained by the coexistence of two ordered FLLs slightly tilted from the applied field direction by different superficial currents. These currents are wiped out when the transport current is increased.

Persistence of an ordered flux line lattice above the second peak in Bi2Sr2CaCu2O8+δ

We report Small Angle Neutron Scattering measurements of the flux lines lattice (FLL) in

Small angle neutron scattering and vortex lattice dynamical phase diagram

Bi_2Sr_2CaCu_2O_{8+ \delta}

Flux-lines lattice order and critical current studied by time-of-flight small-angle neutron scattering

. As previously reported, the scattered intensity decreases strongly when the magnetic field is increased, but it remains measurable far above the second peak. The direct observation of Bragg peaks proves that the characteristics of a lattice are still present. No structural features related to a symmetry breaking, such as a liquid like or an amorphous state, can be observed. However, the associated scattered intensity is very low and is difficult to explain. We discuss the coexistence between two FLL states as a possible interpretation.

Structure of the flux lines lattice in NbSe2: Equilibrium state and influence of the magnetic history

We report a detailed neutron diffraction study of both pinned and moving magnetic Flux Line Lattice (FLL) in NbTa and PbIn samples. In NbTa, the FLL presents a hexagonal lattice even in the absence of current, meanwhile PbIn presents a strongly dislocated phase. In PbIn, dislocations are eliminated by the application of transport current in agreement with theoretical predictions. On the other hand, the absence of curvature of the flux lines for subcritical currents confirm the presence of surface pinning of the FLL.

Evidence for vortex pinning by surface irregularities in untwinned YBaCuO crystals

Small angle neutron scattering (SANS) is an historical technique to study the flux lines lattice (FLL) in a superconductor. Structural characteristics of the FLL can be revealed, providing fundamental information for the physics of vortex lattice. However, the spatial resolution is limited and all the correlation lengths of order are difficult to extract with precision. We show here that a time of flight technique reveals the Bragg peak of the FLL, and also its translational order with a better resolution. We discuss the implication of these results for pinning mechanisms in a Niobium sample.

La réponse à un échelon de champ d'un supraconducteur de type II: un moyen simple de tester l'ancrage des vortex en volume The magnetic-field step response of a type II superconductor as a simple test of the vortex bulk pinning

We have performed small-angle neutron scattering of the flux line lattice (FLL) in a Fe-doped

Observation of the ideal low-frequency response of the mixed state and the diamagnetism of a type II superconductor

{\text{NbSe}}_{2}