521 research outputs found
Flux quantization for a vortex in two-gap superconductor
Contrary to recent theoretical prediction, we show that the magnetic flux of
a vortex in SU(2) model of two-gap superconductor is quantized in units of
2\pi/g, not 4 \pi/g. For the U(1) version of this model, the flux is quantized
in units of 2 \pi\alpha/g where 0 < \alpha < 1. The parameter \alpha depends on
the masses and concentrations of the Cooper pairs in the two condensates.Comment: 7 page
Soluble `Supersymmetric' Quantum XY Model
We present a `supersymmetric' modification of the -dimensional quantum
rotor model whose ground state is exactly soluble. The model undergoes a
vortex-binding transition from insulator to metal as the rotor coupling is
varied. The Hamiltonian contains three-site terms which are relevant: they
change the universality class of the transition from that of the ()--- to
the -dimensional classical XY model. The metallic phase has algebraic ODLRO
but the superfluid density is identically zero. Variational wave functions for
single-particle and collective excitations are presented.Comment: 12 pages, REVTEX 3.0, IUCM93-00
Quantum State Sensitivity of an Autoresonant Superconducting Circuit
When a frequency chirped excitation is applied to a classical high-Q
nonlinear oscillator, its motion becomes dynamically synchronized to the drive
and large oscillation amplitude is observed, provided the drive strength
exceeds the critical threshold for autoresonance. We demonstrate that when such
an oscillator is strongly coupled to a quantized superconducting qubit, both
the effective nonlinearity and the threshold become a non-trivial function of
the qubit-oscillator detuning. Moreover, the autoresonant threshold is
sensitive to the quantum state of the qubit and may be used to realize a high
fidelity, latching readout whose speed is not limited by the oscillator Q.Comment: 5 pages, 4 figure
Analytic Coulomb matrix elements in the lowest Landau level in disk geometry
Using Darling's theorem on products of generalized hypergeometric series an
analytic expression is obtained for the Coulomb matrix elements in the lowest
Landau level in the representation of angular momentum. The result is important
in the studies of Fractional Quantum Hall effect (FQHE) in disk geometry.
Matrix elements are expressed as simple finite sums of positive terms,
eliminating the need to approximate these quantities with slowly-convergent
series. As a by-product, an analytic representation for certain integals of
products of Laguerre polynomials is obtained.Comment: Accepted to J. Math. Phys.; 3 pages revtex, no figure
Liouvillian Approach to the Integer Quantum Hall Effect Transition
We present a novel approach to the localization-delocalization transition in
the integer quantum Hall effect. The Hamiltonian projected onto the lowest
Landau level can be written in terms of the projected density operators alone.
This and the closed set of commutation relations between the projected
densities leads to simple equations for the time evolution of the density
operators. These equations can be used to map the problem of calculating the
disorder averaged and energetically unconstrained density-density correlation
function to the problem of calculating the one-particle density of states of a
dynamical system with a novel action. At the self-consistent mean-field level,
this approach yields normal diffusion and a finite longitudinal conductivity.
While we have not been able to go beyond the saddle point approximation
analytically, we show numerically that the critical localization exponent can
be extracted from the energetically integrated correlation function yielding
in excellent agreement with previous finite-size scaling
studies.Comment: 9 pages, submitted to PR
Cooling and squeezing via quadratic optomechanical coupling
We explore the physics of optomechanical systems in which an optical cavity
mode is coupled parametrically to the square of the position of a mechanical
oscillator. We derive an effective master equation describing two-phonon
cooling of the mechanical oscillator. We show that for high temperatures and
weak coupling, the steady-state phonon number distribution is non-thermal
(Gaussian) and that even for strong cooling the mean phonon number remains
finite. Moreover, we demonstrate how to achieve mechanical squeezing by driving
the cavity with two beams. Finally, we calculate the optical output and
squeezing spectra. Implications for optomechanics experiments with the
membrane-in-the-middle geometry or ultracold atoms in optical resonators are
discussed.Comment: 4 pages, 3 figure
Observability of radiation pressure shot noise in optomechanical systems
We present a theoretical study of an experiment designed to detect radiation
pressure shot noise in an optomechanical system. Our model consists of a
coherently driven optical cavity mode that is coupled to a mechanical
oscillator. We examine the cross-correlation between two quadratures of the
output field from the cavity. We determine under which circumstances radiation
pressure shot noise can be detected by a measurement of this cross-correlation.
This is done in the general case of nonzero detuning between the frequency of
the drive and the cavity resonance frequency. We study the qualitative features
of the different contributions to the cross-correlator and provide quantitative
figures of merit for the relative importance of the radiation pressure shot
noise contribution to other contributions. We also propose a modified setup of
this experiment relevant to the "membrane-in-the-middle" geometry, which
potentially can avoid the problems of static bistability and classical noise in
the drive.Comment: 12 pages + 4 page appendix, 10 figure
Vanishing Hall Resistance at High Magnetic Field in a Double Layer Two-Dimensional Electron System
At total Landau level filling factor a double layer
two-dimensional electron system with small interlayer separation supports a
collective state possessing spontaneous interlayer phase coherence. This state
exhibits the quantized Hall effect when equal electrical currents flow in
parallel through the two layers. In contrast, if the currents in the two layers
are equal, but oppositely directed, both the longitudinal and Hall resistances
of each layer vanish in the low temperature limit. This finding supports the
prediction that the ground state at is an excitonic superfluid.Comment: 4 pages, 4 figure
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