Response of Landau energy levels to the change of magnetic flux in non-isotropic impurities

We have studied a practical approach for the inclusion of non-isotropic impurities in the systems showing integer quantum Hall effect. We observe that if the impurity distribution in the direction which the electric field applys, then the localized levels goes out from the origin with respect to the isotropic case, and the Hall step will the clearly observed

Current-induced organization of vortex motion in type-II superconductors

When a magnetic field is applied to a type-II superconductor, it penetrates the sample in localized tubes of magnetic flux associated with quantized current vortices; under appropriate conditions, these vortices form an ordered lattice. In a material free of crystal defects, transport currents force this lattice to move and dissipate energy, giving the material a non-zero resistance. The presence of defects, however, can inhibit vortex motion, or even pin vortices to specific locations. Thus, to engineer materials with improved properties it is important to understand the motion of a driven vortex lattice in the presence of different kinds of pinning defects1,2. Recent research has investigated vortex-lattice dynamics in the cases of weak, uniform pinning and strong but non-uniform pinning. Here we consider a different regime, in which the barriers to vortex motion at sample surfaces3 also play a crucial role. Our experiments on clean, detwinned YBa2Cu3O7–delta crystals at temperatures around 80–90K reveal an interplay between surface pinning and weak bulk pinning that leads to the formation of a defect superstructure in the vortex lattice. This current-induced organization is similar to phenomena observed in the dynamics of sliding charge-density waves, and represents a fundamentally new kind of vortex dynamics