Stephen Powell

Quench dynamics across quantum critical points

We study the quantum dynamics of a number of model systems as their coupling constants are changed rapidly across a quantum critical point. The primary motivation is provided by the recent experiments of Greiner et al. [Nature (London) 415, 39 (2002)] who studied the response of a Mott insulator of ultracold atoms in an optical lattice to a strong potential gradient. In a previous work, it had been argued that the resonant response observed at a critical potential gradient could be understood by proximity to an Ising quantum critical point describing the onset of density wave order. Here we obtain numerical results on the evolution of the density wave order as the potential gradient is scanned across the quantum critical point. This is supplemented by studies of the integrable quantum Ising spin chain in a transverse field, where we obtain exact results for the evolution of the Ising order correlations under a time-dependent transverse field. We also study the evolution of transverse superfluid order in the three-dimensional case. In all cases, the order parameter is best enhanced in the vicinity of the quantum critical point.

Physics-based numerical modeling and characterization of 6H-silicon-carbide metal–oxide–semiconductor field-effect transistors

A detailed analysis of silicon-carbide (SiC) metal–oxide–semiconductor field-effect-transistor (MOSFET) physics is performed. Measurements of current–voltage characteristics are taken. A device simulator is developed based on the drift–diffusion equations. The model accounts for incomplete ionization. Comprehensive mobility and interface state models are developed for SiC MOSFETs. The mobility model accounts explicitly for bulk transport, as well as for interface states, surface phonons and surface roughness. Agreement between simulated and measured terminal characteristics is obtained. The results provide values for interface state occupation as a function of energy and position along the surface. Results giving values for surface mobility as a function of position along the channel indicate that interface states have an especially strong effect on SiC operation. Our investigation indicates that substantial reduction of interface states can give rise to a fivefold increase in transconductance.

Chiral Rashba spin textures in ultracold Fermi gases

Spin-orbit (SO) coupling is an important ingredient in many recently discovered phenomena such as the spin-Hall effect and topological insulators. Of particular interest is topological superconductivity, with its potential application in topological quantum computation. The absence of disorder in ultracold atomic systems makes them ideal for quantum computation applications; however, the SO coupling schemes proposed thus far are experimentally impractical owing to large spontaneous emission rates in the alkali fermions. In this paper, we develop a scheme to generate Rashba SO coupling with a low spontaneous emission extension to a recent experiment. We show that this scheme generates a Fermi surface spin texture for

Higgs transitions of spin ice

^{40}\mathrm{K}

Classical to quantum mappings for geometrically frustrated systems: Spin-ice in a [100] field

atoms, which is observable in time-of-flight measurements. The chiral spin texture, together with conventional

Classical to quantum mapping for an unconventional phase transition in a three-dimensional classical dimer model

s

Critical behavior in the cubic dimer model at nonzero monomer density

-wave interactions, leads to topological superconductivity and non-Abelian Majorana quasiparticles.

Interacting Hofstadter spectrum of atoms in an artificial gauge field.

Frustrated magnets such as spin ice exhibit Coulomb phases, where correlations have power-law forms at long distances. Applied perturbations can cause ordering transitions which cannot be described by the usual Landau paradigm, and are instead naturally viewed as Higgs transitions of an emergent gauge theory. Starting from a classical statistical model of spin ice, it is shown that a variety of possible phases and transitions can be described by this approach. Certain cases are identified where continuous transitions are argued to be likely; the predicted critical behavior may be tested in experiments or numerical simulations.

SU(2)-Invariant Continuum Theory for an Unconventional Phase Transition in a Three-Dimensional Classical Dimer Model

Certain classical statistical systems with strong local constraints are known to exhibit Coulomb phases, where long-range correlation functions have power-law forms. Continuous transitions from these into ordered phases cannot be described by a naive application of the Landau-Ginzburg-Wilson theory, since neither phase is thermally disordered. We present an alternative approach to a critical theory for such systems, based on a mapping to a quantum problem in one fewer spatial dimensions. We apply this method to spin ice, a magnetic material with geometrical frustration, which exhibits a Coulomb phase and a continuous transition to an ordered state in the presence of a magnetic field applied in the [100] direction.

Magnetic phases and transitions of the two-species Bose-Hubbard model

We study the transition between a Coulomb phase and a dimer crystal observed in numerical simulations of the three-dimensional classical dimer model, by mapping it to a quantum model of bosons in two dimensions. The quantum phase transition that results, from a superfluid to a Mott insulator at fractional filling, belongs to a class that cannot be described within the Landau-Ginzburg-Wilson paradigm. Using a second mapping, to a dual model of vortices, we show that the long-wavelength physics near the transition is described by a U(1) gauge theory with SU(2) matter fields.