Susanne Viefers

A CARBON-NANOTUBE-BASED NANORELAY

We investigate the operational characteristics of a nanorelay based on a conducting carbon nanotube placed on a terrace in a silicon substrate. The nanorelay is a three-terminal device that acts as a switch in the gigahertz regime. Potential applications include logic devices, memory elements, pulse generators, and current or voltage amplifiers.

Quantum rings for beginners: energy spectra and persistent currents

Abstract Theoretical approaches to one-dimensional and quasi-one-dimensional quantum rings with a few electrons are reviewed. Discrete Hubbard-type models and continuum models are shown to give similar results governed by the special features of the one-dimensionality. The energy spectrum of the many-body states can be described by a rotation–vibration spectrum of a “Wigner molecule” of “localized” electrons, combined with the spin-state determined from an effective antiferromagnetic Heisenberg Hamiltonian. The persistent current as a function of the magnetic flux through the ring shows periodic oscillations arising from the “rigid rotation” of the electron ring. For polarized electrons the periodicity of the oscillations is always the flux quantum Φ 0 . For nonpolarized electrons the periodicity depends on the strength of the effective Heisenberg coupling and changes from Φ 0 first to Φ 0 /2 and eventually to Φ 0 / N when the ring gets narrower.

High frequency properties of a CNT-based nanorelay

We have theoretically investigated the high frequency properties of a carbon-nanotube-based three-terminal nanoelectromechanical relay. The intrinsic mechanical frequency of the relay is in the GHz regime, and the electromechanical coupling shows a non-linear resonant behaviour in this frequency range. We discuss how these resonances may be detected and show that the resonance frequencies can be tuned by the bias voltage. Also, we show that the influence of external electromagnetic fields on the relay is negligible at all frequencies.

Quantum Hall physics in rotating Bose–Einstein condensates

The close theoretical analogy between the physics of rapidly rotating atomic Bose condensates and the quantum Hall effect (i.e. a two-dimensional electron gas in a strong magnetic field) was first pointed out ten years ago. As a consequence of this analogy, a large number of strongly correlated quantum-Hall-type states have been predicted to occur in rotating Bose systems, and suggestions have been made for how to manipulate and observe their fractional quasiparticle excitations. Due to a very rapid development in experimental techniques over the past years, experiments on BEC now appear to be close to reaching the quantum Hall regime. This paper reviews the theoretical and experimental work done to date in exploring quantum Hall physics in cold bosonic gases. Future perspectives are discussed briefly, in particular the idea of exploiting some of these strongly correlated states in the context of topological quantum computing.

Effects of surface forces and phonon dissipation in a three-terminal nanorelay

We have performed a theoretical analysis of the operational characteristics of a carbon–nanotube-based three-terminal nanorelay. We show that short range and van der Waals forces have a significant impact on the characteristics of the relay and introduce design constraints. We also investigate the effects of dissipation due to phonon excitation in the drain contact, which changes the switching time scales of the system, decreasing the longest time scale by 2 orders of magnitude. We show that the nanorelay can be used as a memory element and investigate the dynamics and properties of such a device.

Gender Equity in Higher Education: Why and How? A Case Study of Gender Issues in a Science Faculty.

At a time when more and more natural science subjects are attracting an increasing number of women (chemistry for example) physics remains a male stronghold. It is not easy to understand this phenomenon or the anomaly that over-representation of males in physics faculties is more likely to occur in countries known for their attempts at equalizing opportunities for women. Sweden, for example, has a parliament in which 40% of its members are women and yet the average percentage of women lecturers in physics faculties is about half of that. In Sweden today women professors of physics (both appointed and promoted) typically represent 10% or less of the total professorial staff. In this paper we report on a qualitative case study of gender equity in a large physics faculty in a Swedish university. In order to locate our study in a more general social and political context we look at Swedish legislation that seeks to equalize opportunities for women in higher education. The rest of the study focuses on a brief review of research in the area of gender issues in higher education and an analysis of interviews with three women in physics: one a professor, one a lecturer and the third a PhD student. The analysis discusses why the current disproportion exists, if it is a good or bad thing for physics and physicists and how one might rectify any perceived problems in terms or gender relations and gender equity.

Many-body spectrum and particle localization in quantum dots and finite rotating bose condensates

The yrast spectra (i.e. the lowest states for a given total angular momentum) of quantum dots in strong magnetic fields, are studied in terms of exact numerical diagonalization and analytic trial wave functions. We argue that certain features (cusps) in the many-body spectrum can be understood in terms of particle localization due to the strong field. A new class of trial wavefunctions supports the picture of the electrons being localized in Wigner molecule-like states consisting of consecutive rings of electrons, with low-lying excitations corresponding to rigid rotation of the outer ring of electrons. The geometry of the Wigner molecule is independent of interparticle interactions and the statistics of the particles.

Quantum Hall physics : Hierarchies and conformal field theory techniques

The fractional quantum Hall effect, being one of the most studied phenomena in condensed matter physics during the past 30 years, has generated many ground-breaking new ideas and concepts. Very ear ...

Current-spin-density-functional study of persistent currents in quantum rings

We present a numerical study of persistent currents in quantum rings using current spin density functional theory (CSDFT). This formalism allows for a systematic study of the joint effects of both spin, interactions and impurities for realistic systems. It is illustrated that CSDFT is suitable for describing the physical effects related to Aharonov-Bohm phases by comparing energy spectra of impurity-free rings to existing exact diagonalization and experimental results. Further, we examine the effects of a symmetry-breaking impurity potential on the density and current characteristics of the system and propose that narrowing the confining potential at fixed impurity potential will suppress the persistent current in a characteristic way. PACS numbers: 73.20.Dx, 73.23.Ra, 71.15.Mb

The effect of interactions on Bose-Einstein condensation in a quasi two-dimensional harmonic trap

A dilute bose gas in a quasi two-dimensional harmonic trap and interacting with a repulsive two-body zero-range potential of fixed coupling constant is considered. Using the Thomas-Fermi method, it is shown to remain in the same uncondensed phase as the temperature is lowered. Its density profile and energy are identical to that of an ideal gas obeying the fractional exclusion statistics of Haldane.