Abstract: Results of dielectric and resonance frequency (f(r)) measurements below room temperature are presented for Pb(ZrxTi1-x)O-3, x = 0.515 and 0.520. It is shown that the temperature coefficient of f(r) changes sign from negative to positive around 210 and 265 K for x = 0.520 and 200 and 260 K for x = 0.515. Anomalies in the real part of the dielectric constant (epsilon') are observed around the same temperatures at which the temperature coefficient of f(r) changes sign because of the electrostrictive coupling between the elastic and dielectric responses. Low-temperature powder x-ray-diffraction (XRD) data, however, reveal only one transition from the tetragonal to monoclinic phase similar to that reported by Noheda et al. [Phys. Rev. B, 61, 8687 (2000)]. Electron-diffraction data, on the other hand, reveal yet another structural transition at lower temperatures corresponding to the second anomaly in the epsilon' vs T and f(r) vs T curves. This second transition is shown to be a cell-doubling transition not observed by Noheda et al. in their XRD studies. The observation of superlattice reflections raises doubts about the correctness of the Cm space group proposed by Noheda et al. for the monoclinic phase of Pb(ZrxTi(1-x))O-3 below the second transition temperature.

Abstract: We study an exciton in a type-II quantum dot, where the electron is confined in the dot, but the hole is located in the barrier material. The exciton properties are studied as a function of a perpendicular magnetic field using a Hartree-Fock mesh calculation. Our model system consists of a planar quantum disk. Angular momentum (l) transitions are predicted with increasing magnetic field. We, also study the transition from a type-I to a type-H quantum dot which is induced by changing the confinement potential of the hole. For sufficiently large magnetic fields a reentrant behavior is found from l(h) = 0 to l(h) not equal 0 and back to l(h) = 0, which results in a transition from type II to type I.

Abstract: An overview is given of our investigation of the energy levels and of the correlation functions of the negatively and positively charged excitons (also called trions) in quantum wells in the presence of a perpendicular magnetic field, A detailed comparison is made with available experimental data in III-V and II-VI semiconductor quantum wells. (C) 2001 Elsevier Science B.V. All rights reserved.

Abstract: The magnetic coupling between two concentric mesoscopic superconductors with nonzero thickness is studied using the nonlinear Ginzburg-Landau theory. We calculated the free energy, the expelled field, the total field profile, the Cooper-pair density, and the current density distribution. By putting a smaller superconducting disk or ring in the center of a larger ring, the properties change drastically. Extra ground-state transitions are found, where the total vorticity stays the same, but the vorticity of the inner superconductor changes by 1. Due to the magnetic coupling, the current in the external ring exhibits extra jumps at the transition fields where the vorticity of the inner superconductor changes. In this case, for certain temperatures, re-entrant behavior and switching on and off of the superconducting behavior of the rings are found as a function of the magnetic field. A H-T phase diagram is obtained for the situation where the inner ring has a higher critical temperature than the outer ring. An analytic expression for the magnetic coupling is obtained for thin rings and extreme type-II superconductors.

Abstract: The influence of the geometry of a thin superconducting sample on the penetration of the magnetic field lines and the arrangement of vortices are investigated theoretically. We compare the vortex state of superconducting disks, squares, and triangles with the same surface area having nonzero thickness. The coupled nonlinear Ginzburg-Landau equations are solved self-consistently and the important demagnetization effects are taken into account. We calculate and compare quantities such as the free energy, the magnetization, the Cooper-pair density, the magnetic field distribution, and the superconducting current density for the three geometries. For given vorticity the vortex lattice is different for the three geometries, i.e., it tries to adapt to the geometry of the sample. This also influences the stability range of the different vortex states. For certain magnetic field ranges we found a coexistence of a giant vortex placed in the center and single vortices towards the corners of the sample. The H-T phase diagram is obtained for the three investigated geometries and we found that the critical magnetic field is substantially enhanced for the triangle geometry.

Abstract: We calculate the equilibrium properties and the dynamic response of two vertically coupled circular quantum dots populated by particles of different electrical charge sign, i.e., electrons and holes. The equilibrium density profiles are obtained and used to compute the frequencies and oscillator strengths of magnetoplasma excitations. We find a strong coupling between the modes derived from the center-of-mass modes of the individual dots which leads to an anticrossing with a pronounced oscillator strength transfer from the “acoustic” to the “optical” branch. Also, due to the breaking of the generalized Kohn theorem a number of other than center-of-mass modes are excited whose oscillator strengths, however, are rather weak.