Piezoelectric mechanism of orientation of a bilayer Wigner crystal in a GaAs matrix

A mechanism for orientation of bilayer classical Wigner crystals in a piezoelectric medium is considered. For the GaAs system the piezoelectric correction to the electrostatic interaction between electrons is calculated. It is shown that taking into account the correction due to the piezoelectric effect leads to a dependence of the total energy of the electron crystal on its orientation with respect to the crystallographic axes of the surrounding matrix. A generalization of Ewald's method is obtained for calculating the anisotropic interaction between electrons in a Wigner crystal. The method is used to calculate the energy of bilayer Wigner crystals in electron layers parallel to the crystallographic planes (001), (0-11), and (111) as a function of their orientation and the distance between layers, and the energetically most favorable orientation for all types of electron lattices in a bilayer system is found. It is shown that phase transitions between structures with different lattice symmetry in a Wigner crystal can be accompanied by a change of its orientation.Comment: 11 pages, 4 eps figures include

Suppression of Superconductivity in Mesoscopic Superconductors

We propose a new boundary-driven phase transition associated with vortex nucleation in mesoscopic superconductors (of size of the order of, or larger than, the penetration depth). We derive the rescaling equations and we show that boundary effects associated with vortex nucleation lowers the conventional transition temperature in mesoscopic superconductors by an amount which is a function of the size of the superconductor. This result explains recent experiments in small superconductors where it was found that the transition temperature depends on the size of the system and is lower than the critical Berezinsk\u{i}-Kosterlitz-Thouless temperature.Comment: To appear in Phys. Rev. Lett. Vol. 86 (15 Jan. 2001

Metastability and paramagnetism in superconducting mesoscopic disks

A projected order parameter is used to calculate, not only local minima of the Ginzburg-Landau energy functional, but also saddle points or energy barriers responsible for the metastabilities observed in superconducting mesoscopic disks (Geim et al. Nature {\bf 396}, 144 (1998)). We calculate the local minima magnetization and find the energetic instability points between vortex configurations with different vorticity. We also find that, for any vorticity, the supercurrent can reverse its flow direction on decreasing the magnetic field before one vortex can escape.Comment: Modified version as to appear in Phys. Rev. Let

Vortex structure of thin mesoscopic disks in the presence of an inhomogeneous magnetic field

The vortex states in a thin mesoscopic disk are investigated within the phenomenological Ginzburg-Landau theory in the presence of different ''model'' magnetic field profiles with zero average field which may result from a ferromagnetic disk or circulating currents in a loop near the superconductor. We calculated the dependences of both the ground and metastable states on the magnitude and shape of the magnetic field profile for different values of the order parameter angular moment, i.e. the vorticity. The regions of existence of the multi-vortex state and the giant vortex state are found. We analysed the phase transitions between these states and studied the contribution from ring-shaped vortices. A new transition between different multi-vortex configurations as the ground state is found. Furthermore, we found a vortex state consisting of a central giant vortex surrounded by a collection of anti-vortices which are located in a ring around this giant vortex. The limit to a disk with an infinite radius, i.e. a film, will also be discussed. We also extended our results to ''real'' magnetic field profiles and to the case in which an external homogeneous magnetic field is present.Comment: 17 pages, 23 figures. Submitted to PR

New combined PIC-MCC approach for fast simulation of a radio frequency discharge at low gas pressure

A new combined PIC-MCC approach is developed for accurate and fast simulation of a radio frequency discharge at low gas pressure and high density of plasma. Test calculations of transition between different modes of electron heating in a ccrf discharge in helium and argon show a good agreement with experimental data. We demonstrate high efficiency of the combined PIC-MCC algorithm, especially for the collisionless regime of electron heating.Comment: 6 paged, 8 figure

Perfectly Translating Lattices on a Cylinder

We perform molecular dynamics simulations on an interacting electron gas confined to a cylindrical surface and subject to a radial magnetic field and the field of the positive background. In order to study the system at lowest energy states that still carry a current, initial configurations are obtained by a special quenching procedure. We observe the formation of a steady state in which the entire electron-lattice cycles with a common uniform velocity. Certain runs show an intermediate instability leading to lattice rearrangements. A Hall resistance can be defined and depends linearly on the magnetic field with an anomalous coefficient reflecting the manybody contributions peculiar to two dimensions.Comment: 13 pages, 5 figure