4,671 research outputs found
Short note on the excitonic Mott phase
An exciton gas on a lattice is analyzed in terms of a convergent hopping
expansion. For a given chemical potential our calculation provides a sufficient
condition for the hopping rate to obtain an exponential decay of the exciton
correlation function. This result indicates the existence of a Mott phase in
which strong fluctuations destroy the long range correlations in the exciton
gas at any temperature, either by thermal or by quantum fluctuations.Comment: 5 pages, 1 figur
Sensitive linear response of an electron-hole superfluid in a periodic potential
We consider excitons in a two-dimensional periodic potential and study the
linear response of the excitonic superfluid to an electromagnetic wave at low
and high densities. It turns out that the static structure factor for small
wavevectors is very sensitive to a change of density and temperature. It is a
consequence of the fact that thermal fluctuations play a crucial role at small
wavevectors, since exchanging the order of the two limits, zero temperature and
vanishing wavevector, leads to different results for the structure factor. This
effect could be used for high accuracy measurements in the superfluid exciton
phase, which might be realized by a gated electron-hole gas. The transition of
the exciton system from the superfluid state to a non-superfluid state and its
manifestation by light scattering are discussed.Comment: 9 pages, 5 figure
Graphene-based one-dimensional photonic crystal
A novel type of one-dimensional (1D) photonic crystal formed by the array of
periodically located stacks of alternating graphene and dielectric stripes
embedded into a background dielectric medium is proposed. The wave equation for
the electromagnetic wave propagating in such structure solved in the framework
of the Kronig-Penney model. The frequency band structure of 1D graphene-based
photonic crystal is obtained analytically as a function of the filling factor
and the thickness of the dielectric between graphene stripes. The photonic
frequency corresponding to the electromagnetic wave localized by the defect of
photonic crystal formed by the extra dielectric placed on the place of the
stack of alternating graphene and dielectric stripes is obtained.Comment: 8 pages, 2 figure
A Double Sigma Model for Double Field Theory
We define a sigma model with doubled target space and calculate its
background field equations. These coincide with generalised metric equation of
motion of double field theory, thus the double field theory is the effective
field theory for the sigma model.Comment: 26 pages, v1: 37 pages, v2: references added, v3: updated to match
published version - background and detail of calculations substantially
condensed, motivation expanded, refs added, results unchange
Dynamics of Open Bosonic Quantum Systems in Coherent State Representation
We consider the problem of decoherence and relaxation of open bosonic quantum
systems from a perspective alternative to the standard master equation or
quantum trajectories approaches. Our method is based on the dynamics of
expectation values of observables evaluated in a coherent state representation.
We examine a model of a quantum nonlinear oscillator with a density-density
interaction with a collection of environmental oscillators at finite
temperature. We derive the exact solution for dynamics of observables and
demonstrate a consistent perturbation approach.Comment: 7 page
Multi-Stability of Electromagnetically Induced Transparency in Atom-Assisted Optomechanical Cavities
We study how an oscillating mirror affects the electromagnetically induced
transparency (EIT) of an atomic ensemble, which is confined in a gas cell
placed inside a micro-cavity with an oscillating mirror in one end. The
oscillating mirror is modeled as a quantum mechanical harmonic oscillator. The
cavity field acts as a probe light of the EIT system and also produces a light
pressure on the oscillating mirror. The back-action from the mirror to the
cavity field results in several (from one to five) steady-states for this
atom-assisted optomechanical cavity, producing a complex structure in its EIT.
We calculate the susceptibility with respect to the few (from one to three)
stable solutions found here for the equilibrium positions of the oscillating
mirror. We find that the EIT of the atomic ensemble can be significantly
changed by the oscillating mirror, and also that the various steady states of
the mirror have different effects on the EIT.Comment: 10 pages, 9 figure
Quantum Measurement of a Single Spin using Magnetic Resonance Force Microscopy
Single-spin detection is one of the important challenges facing the
development of several new technologies, e.g. single-spin transistors and
solid-state quantum computation. Magnetic resonance force microscopy with a
cyclic adiabatic inversion, which utilizes a cantilever oscillations driven by
a single spin, is a promising technique to solve this problem. We have studied
the quantum dynamics of a single spin interacting with a quasiclassical
cantilever. It was found that in a similar fashion to the Stern-Gerlach
interferometer the quantum dynamics generates a quantum superposition of two
quasiclassical trajectories of the cantilever which are related to the two spin
projections on the direction of the effective magnetic field in the rotating
reference frame. Our results show that quantum jumps will not prevent a
single-spin measurement if the coupling between the cantilever vibrations and
the spin is small in comparison with the amplitude of the radio-frequency
external field.Comment: 16 pages RevTeX including 4 figure
Massive Type II in Double Field Theory
We provide an extension of the recently constructed double field theory
formulation of the low-energy limits of type II strings, in which the RR fields
can depend simultaneously on the 10-dimensional space-time coordinates and
linearly on the dual winding coordinates. For the special case that only the RR
one-form of type IIA carries such a dependence, we obtain the massive
deformation of type IIA supergravity due to Romans. For T-dual configurations
we obtain a `massive' but non-covariant formulation of type IIB, in which the
10-dimensional diffeomorphism symmetry is deformed by the mass parameter.Comment: 21 page
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