Abstract: When a condensate is sheared by imparting a velocity to a part of the condensate, phase singularities must appear at the interface between the region that is still at rest and the region that has acquired a velocity. For helium, Feynman argued that these phase singularies will arrange themselves in the form of a vortex row. BoseEinstein condensates of ultracold atomic gases differ from helium in that the healing length is generally much larger and is, in fact, tunable. Another difference is that multicomponent condensates can be created, where the two components forming the mixture are usually two different hyperfine states of the condensed atoms. These two components can be manipulated separately and can be interconverted. In this contribution, we investigate how these additional degrees of freedom, available in quantum gases, change what happens in sheared condensates. In particular, we consider skyrmion rows as an alternative to vortex rows, and we also consider phase slip lines filled with the second, unmoving component, in a condensate mixture. We show that depending on the ratios of the interaction strengths between the components, and depending on the shear velocity, skyrmion rows and phase slip lines can become lower in energy than vortex rows, and hence should be observable in quantum gases. Moreover, we find that the velocity field affects the stability region of the condensate with respect to phase separation.

Abstract: Spatially nonhomogeneously spin polarized nuclei are proposed as a new mechanism to monitor electron states in a nanostructure, or as a means to create and, if necessary, reshape such nanostructures in the course of the experiment. We found that a polarization of nuclear spins may lift the spin polarization of the electron states in a nanostructure and, if sufficiently strong, leads to a polarization of the electron spins. Polarized nuclear spins may form an energy landscape capable of binding electrons with energy up to several meV and the localization radius > 100 Angstrom. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: The stable and metastable states of different configurations of a mesoscopic loop in the form of an eight is studied in the presence of a magnetic field. We find that for certain configurations the current is equal to zero for any value of the magnetic field leading to a magnetic field independent superconducting state. The state with fixed phase circulation becomes unstable when the momentum of the superconducting electrons reaches a critical value. At this moment the kinetic energy of the superconducting condensate becomes of the same order as the potential energy of the Cooper pairs and it leads to an instability. Numerical analysis of the time-dependent Ginzburg-Landau equations shows that the absolute value of the order parameter changes gradually at the transition from a state with one phase circulation to another although the vorticity change occurs abruptly. (C) 2003 Elsevier B.V. All rights reserved.

Abstract: A numerical approach to GinzburgLandau (GL) theory is demonstrated and we review its applications to several examples of current interest in the research on superconductivity. This analysis also shows the applicability of the two-dimensional approach to thin superconductors and the re-defined effective GL parameter κ. For two-gap superconductors, the conveniently written GL equations directly show that the magnetic behavior of the sample depends not just on the GL parameter of two bands, but also on the ratio of respective coherence lengths.

Abstract: We report a totally CMOS-compatible fabrication technique to assemble horizontally suspended single-walled carbon nanotube (SWCNT) resonators. Individual SWCNTs are assembled in parallel at multiple sites by a technique called dielectrophoresis. The mechanical resonance frequencies of the suspended SWCNTs are in the range of 2035 MHz as determined from the piezoresistive response of the resonators during electrostatic actuation. The resistance of the suspended SWCNT either remains unchanged or increases or decreases significantly as a function of the actuation frequency. This can be explained by the effect the nanotube chirality has on the piezoresistive gauge factor.

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.