Keith A. Benedict

Cryogenically enhanced magneto-Archimedes levitation

The application of both a strong magnetic field and magnetic field gradient to a diamagnetic body can produce a vertical force which is sufficient to counteract its weight due to gravity. By immersing the body in a paramagnetic fluid, an additional adjustable magneto-buoyancy force is generated which enhances the levitation effect. Here we show that cryogenic oxygen and oxygen–nitrogen mixtures in both gaseous and liquid form provide sufficient buoyancy to permit the levitation and flotation of a wide range of materials. These fluids may provide an alternative to synthetic ferrofluids for the separation of minerals. We also report the dynamics of corrugation instabilities on the surface of magnetized liquid oxygen.

Theory of phonon spectroscopy in the fractional quantum Hall regime

We describe a theoretical framework for the interpretation of time-resolved phonon absorption experiments carried out in the fractional quantum Hall regime of a magnetically quantized two-dimensional electron system. The only phonons which can be absorbed at low temperature are those whose energies exceed the magnetoroton gap predicted by Girvin, MacDonald and Platzman @Phys. Rev. B 33, 2481 ~1986!#. The rate of energy transfer from the phonons to the electron liquid is entirely controlled by the creation of these collective excitations. Using simple isotropic approximations for the phonon propagation and electron-phonon coupling, we obtain analytic results for the regime in which the electron temperature and the characteristic temperature of the phonons are much less than the gap, and identify the way in which the dispersion curve of the magnetorotons could be extracted from time- and angle-resolved experiments.

Many-body effects in a quantised 2DES probed by discrete-level tunnelling spectroscopy

Abstract We use the discrete state of a self-assembled InAs quantum dot for tunnelling spectroscopy of the local density of states of a 2DES over the full-energy range from the Fermi energy, EF, to the subband edge. In a magnetic field, B, applied parallel to the current we observe the formation of Landau levels and measure their width as a function of B. When the dot state is resonant with EF, we observe a Fermi-edge singularity in the current which enables us to study many-body effects in a magnetic field.

Dynamical excitations in the collision of two-dimensional Bose-Einstein condensates

We investigate the way in which the pattern of fringes in a coherent pair of two-dimensional Bose condensed clouds of ultra-cold atoms traveling in opposite directions subject to a harmonic trapping potential can seed the irreversible formation of internal excitations in the clouds, notably solitons and vortices. We identify under, over and critically damped regimes in the dipole oscillations of the condensates according to the balance of internal and centre-of-mass energies of the clouds. We carry out simulations of the collision of two clouds with respect to different initial phase differences in these regimes to investigate the creation of internal excitations. We distinguish the behaviour of this system from previous studies of quasi one-dimensional BEC’s. In particular we note that the nature of the internal excitations is only essentially sensitive to an initial phase difference between the clouds in the overdamped regime.

Model for the voltage steps in the breakdown of the integer quantum Hall effect.

In samples used to maintain the U.S. resistance standard the breakdown of the dissipationless integer quantum Hall effect occurs as a series of dissipative voltage steps. A mechanism for this type of breakdown is proposed, based on the generation of magnetoexcitons when the quantum Hall fluid flows past an ionized impurity above a critical velocity. The calculated generation rate gives a voltage step height in good agreement with measurements on both electron and hole gases. We also compare this model to a hydrodynamic description of breakdown.

Fermi edge singularities in high magnetic fields

Abstract The current voltage characteristics of a single barrier tunnelling device which contains self-organized InAs quantum dots within the barrier are studied in zero and strong magnetic field conditions (both perpendicular and parallel to the interfaces). It is observed that in all cases there are large peaks whenever the energy of the bound state of a dot approaches the chemical potential of the 2d electron system formed at an interface. These are identified as thermally broadened Fermi-edge singularities which arise from many-body contributions to the tunnelling amplitude. We note that the standard theory of such singularities appears to work well in situations when the spin states of the dot are Zeeman split but is qualitatively incorrect in zero field. In the quantum Hall regime the peak height is an oscillatory function of magnetic field in agreement with theoretical predictions.

Distortion of Interference Fringes and the Resulting Vortex Production of Merging Bose-Einstein Condensates

We investigate the distortion of interference fringes in colliding and merging Bose-Einstein Condensates due to inter-atomic interaction effects. We also consider the effect of the distortion on the spontaneous formation of vortices. The interference pattern depends crucially on two relevant parameters, the relative center-of-mass velocity and peak density of the initial state, which comprises a pair of initially well-separated, but coherent, condensate clouds. We identify three qualitatively distinct regimes of behavior during the subsequent evolution of the condensates: collision; expansion; merging. Using a comprehensive set of numerical simulations based on the Gross-Pitaevskii equation, we specify the cross-overs between these regimes and identify the system parameters required for dynamical instabilities and vortex creation. Interference fringes, which would be planar in the absence of interactions, are distorted by the combined effects of interaction and the nonuniform density profile of a harmonically trapped condensate. In appropriate parameter regimes, the distortion is sufficiently large to produce a net circulation and thereby a dynamical instability occurs by which vortices form with their cores on the interference minima.

Magnetoroton scattering by phonons in fractional quantum Hall liquids

Motivated by recent phonon-spectroscopy experiments in the fractional quantum Hall regime we consider processes in which thermally excited magnetoroton excitations are scattered by low-energy phonons. We show that such scattering processes can never give rise to dissociation of magnetorotons into unbound charged quasiparticles as had been proposed previously. In addition, we show that scattering of magnetorotons to longer wavelengths by phonon absorption is possible because of the shape of the magnetoroton-dispersion curve and it is shown that there is a characteristic cross-over temperature above which the rate of energy transfer to the electron gas changes from an exponential ~activated! to a power-law dependence on the effective phonon temperature.

Magneto-tunnelling spectroscopy of a two-dimensional electron system

We use a single, InAs self-assembled quantum dot to probe the local density of states of a two-dimensional electron system (2DES) at all energies from the sub-band edge to the Fermi energy. The dot is incorporated as part of an AlAs barrier in a single barrier tunnel diode. Variation of the bias across the device changes the energy of the dot ground state relative to the 2DES. For magnetic field, B, applied parallel to the current, we observe peaks in the current‐voltage characteristic, I(V), corresponding to the formation of Landau levels (LLs) in the 2DES although the lowest energy levels are not well resolved at low B due to the eAect of the quasiparticle lifetime. At higher B, at filling factor ma 1 and beyond, we observe a number of eAects. First, we observe directly the exchange enhancement of the Land e g-factor; the lower energy spin polarised LL moves to lower energy with increasing B. Second, close to ma 1, the current from the lowest LL is suppressed although the current is restored as the temperature, T, is increased from 100 mK to 2 K. Finally, for m6 1, reproducible fine structure appears in I(V), which is very sensitive to both B and T. ” 1998 Elsevier

Bogoliubov excitation spectrum of an elongated condensate throughout a transition from quasi-one-dimensional to three-dimensional

The quasiparticle excitation spectra of a Bose gas trapped in a highly anisotropic trap is studied with respect to varying total number of particles by numerically solving the effective one-dimensional (1D) Gross?Pitaevskii (GP) equation proposed recently by Mateo et?al. We obtain the static properties and Bogoliubov spectra of the system in the high energy domain. This method is computationally efficient and highly accurate for a condensate system undergoing a 1D to three-dimensional (3D) cigar-shaped transition, as is shown through a comparison of our results with both those calculated by the 3D-GP equation and analytical results obtained in limiting cases. We identify the applicable parameter space for the effective 1D-GP equation and find that this equation fails to describe a system with a large number of atoms. We also identify that the description of the transition from 1D Bose?Einstein condensate (BEC) to 3D cigar-shaped BEC using this equation is not smooth, which highlights the fact that for a finite value of a?/as the junction between the 1D and 3D crossover is not perfect.