749 research outputs found
The crossover between lasing and polariton condensation in optical microcavities
We study a model of a photon mode dipole-coupled to a medium of two-level
oscillators in a microcavity in the presence of dephasing processes introduced
by coupling to external baths. Decoherence processes can be classified as
pair-breaking or non-pair-breaking in analogy with magnetic or non-magnetic
impurities in superconductors. In the absence of dephasing, the ground state of
the model is a polariton condensate with a gap in the excitation spectrum.
Increase of the pair-breaking parameter reduces the gap, which becomes
zero at a critical value ; for large , the conventional
laser regime is obtained in a way that demonstrates its close analogy to a
gapless superconductor. In contrast, weak non-pair-breaking processes have no
qualitative effect on the condensate or the existence of a gap, although they
lead to inhomogeneous broadening of the excitations
Two-Component Fluid Membranes Near Repulsive Walls: Linearized Hydrodynamics of Equilibrium and Non-equilibrium States
We study the linearized hydrodynamics of a two-component fluid membrane near
a repulsive wall, via a model which incorporates curvature- concentration
coupling as well as hydrodynamic interactions. This model is a simplified
version of a recently proposed one [J.-B. Manneville et al. Phys. Rev. E, 64,
021908 (2001)] for non-equilibrium force-centres embedded in fluid membranes,
such as light-activated bacteriorhodopsin pumps incorporated in phospholipid
(EPC) bilayers. The pump/membrane system is modeled as an impermeable,
two-component bilayer fluid membrane in the presence of an ambient solvent, in
which one component, representing active pumps, is described in terms of force
dipoles displaced with respect to the bilayer midpoint. We first discuss the
case in which such pumps are rendered inactive, computing the mode structure in
the bulk as well as the modification of hydrodynamic properties by the presence
of a nearby wall. We then discuss the fluctuations and mode structure in steady
state of active two-component membranes near a repulsive wall. We find that
proximity to the wall smoothens membrane height fluctuations in the stable
regime, resulting in a logarithmic scaling of the roughness even for initially
tensionless membranes. This explicitly non-equilibrium result, a consequence of
the incorporation of curvature-concentration coupling in our treatment, also
indicates that earlier scaling arguments which obtained an increase in the
roughness of active membranes near repulsive walls may need to be reevaluated.Comment: 39 page Latex file, 3 encapsulated Postscript figure
Onset of Superfluidity in 4He Films Adsorbed on Disordered Substrates
We have studied 4He films adsorbed in two porous glasses, aerogel and Vycor,
using high precision torsional oscillator and DC calorimetry techniques. Our
investigation focused on the onset of superfluidity at low temperatures as the
4He coverage is increased. Torsional oscillator measurements of the 4He-aerogel
system were used to determine the superfluid density of films with transition
temperatures as low as 20 mK. Heat capacity measurements of the 4He-Vycor
system probed the excitation spectrum of both non-superfluid and superfluid
films for temperatures down to 10 mK. Both sets of measurements suggest that
the critical coverage for the onset of superfluidity corresponds to a mobility
edge in the chemical potential, so that the onset transition is the bosonic
analog of a superconductor-insulator transition. The superfluid density
measurements, however, are not in agreement with the scaling theory of an onset
transition from a gapless, Bose glass phase to a superfluid. The heat capacity
measurements show that the non-superfluid phase is better characterized as an
insulator with a gap.Comment: 15 pages (RevTex), 21 figures (postscript
Coherent Radio Pulses From GEANT Generated Electromagnetic Showers In Ice
Radio Cherenkov radiation is arguably the most efficient mechanism for
detecting showers from ultra-high energy particles of 1 PeV and above. Showers
occuring in Antarctic ice should be detectable at distances up to 1 km. We
report on electromagnetic shower development in ice using a GEANT Monte Carlo
simulation. We have studied energy deposition by shower particles and
determined shower parameters for several different media, finding agreement
with published results where available. We also report on radio pulse emission
from the charged particles in the shower, focusing on coherent emission at the
Cherenkov angle. Previous work has focused on frequencies in the 100 MHz to 1
GHz range. Surprisingly, we find that the coherence regime extends up to tens
of Ghz. This may have substantial impact on future radio-based neutrino
detection experiments as well as any test beam experiment which seeks to
measure coherent Cherenkov radiation from an electromagnetic shower. Our study
is particularly important for the RICE experiment at the South Pole.Comment: 44 pages, 29 figures. Minor changes made, reference added, accepted
for publication in Phys. Rev.
Theory of finite temperature crossovers near quantum critical points close to, or above, their upper-critical dimension
A systematic method for the computation of finite temperature () crossover
functions near quantum critical points close to, or above, their upper-critical
dimension is devised. We describe the physics of the various regions in the
and critical tuning parameter () plane. The quantum critical point is at
, , and in many cases there is a line of finite temperature
transitions at , with . For the relativistic,
-component continuum quantum field theory (which describes lattice
quantum rotor () and transverse field Ising () models) the upper
critical dimension is , and for , is the control
parameter over the entire phase diagram. In the region , we obtain an expansion for coupling constants which then are
input as arguments of known {\em classical, tricritical,} crossover functions.
In the high region of the continuum theory, an expansion in integer powers
of , modulo powers of , holds for all
thermodynamic observables, static correlators, and dynamic properties at all
Matsubara frequencies; for the imaginary part of correlators at real
frequencies (), the perturbative expansion describes
quantum relaxation at or larger, but fails for or smaller. An important principle,
underlying the whole calculation, is the analyticity of all observables as
functions of at , for ; indeed, analytic continuation in is
used to obtain results in a portion of the phase diagram. Our method also
applies to a large class of other quantum critical points and their associated
continuum quantum field theories.Comment: 36 pages, 4 eps figure
The types of Mott insulator
There are two classes of Mott insulators in nature, distinguished by their
responses to weak doping. With increasing chemical potential, Type I Mott
insulators undergo a first order phase transition from the undoped to the doped
phase. In the presence of long-range Coulomb interactions, this leads to an
inhomogeneous state exhibiting ``micro-phase separation.'' In contrast, in Type
II Mott insulators charges go in continuously above a critical chemical
potential. We show that if the insulating state has a broken symmetry, this
increases the likelihood that it will be Type I. There exists a close analogy
between these two types of Mott insulators and the familiar Type I and Type II
superconductors
Structure Formation, Melting, and the Optical Properties of Gold/DNA Nanocomposites: Effects of Relaxation Time
We present a model for structure formation, melting, and optical properties
of gold/DNA nanocomposites. These composites consist of a collection of gold
nanoparticles (of radius 50 nm or less) which are bound together by links made
up of DNA strands. In our structural model, the nanocomposite forms from a
series of Monte Carlo steps, each involving reaction-limited cluster-cluster
aggregation (RLCA) followed by dehybridization of the DNA links. These links
form with a probability which depends on temperature and particle
radius . The final structure depends on the number of monomers (i. e. gold
nanoparticles) , , and the relaxation time. At low temperature, the
model results in an RLCA cluster. But after a long enough relaxation time, the
nanocomposite reduces to a compact, non-fractal cluster. We calculate the
optical properties of the resulting aggregates using the Discrete Dipole
Approximation. Despite the restructuring, the melting transition (as seen in
the extinction coefficient at wavelength 520 nm) remains sharp, and the melting
temperature increases with increasing as found in our previous
percolation model. However, restructuring increases the corresponding link
fraction at melting to a value well above the percolation threshold. Our
calculated extinction cross section agrees qualitatively with experiments on
gold/DNA composites. It also shows a characteristic ``rebound effect,''
resulting from incomplete relaxation, which has also been seen in some
experiments. We discuss briefly how our results relate to a possible sol-gel
transition in these aggregates.Comment: 12 pages, 10 figure
Recent developments in planet migration theory
Planetary migration is the process by which a forming planet undergoes a
drift of its semi-major axis caused by the tidal interaction with its parent
protoplanetary disc. One of the key quantities to assess the migration of
embedded planets is the tidal torque between the disc and planet, which has two
components: the Lindblad torque and the corotation torque. We review the latest
results on both torque components for planets on circular orbits, with a
special emphasis on the various processes that give rise to additional, large
components of the corotation torque, and those contributing to the saturation
of this torque. These additional components of the corotation torque could help
address the shortcomings that have recently been exposed by models of planet
population syntheses. We also review recent results concerning the migration of
giant planets that carve gaps in the disc (type II migration) and the migration
of sub-giant planets that open partial gaps in massive discs (type III
migration).Comment: 52 pages, 18 figures. Review article to be published in "Tidal
effects in Astronomy and Astrophysics", Lecture Notes in Physic
Quasars and their host galaxies
This review attempts to describe developments in the fields of quasar and
quasar host galaxies in the past five. In this time period, the Sloan and 2dF
quasar surveys have added several tens of thousands of quasars, with Sloan
quasars being found to z>6. Obscured, or partially obscured quasars have begun
to be found in significant numbers. Black hole mass estimates for quasars, and
our confidence in them, have improved significantly, allowing a start on
relating quasar properties such as radio jet power to fundamental parameters of
the quasar such as black hole mass and accretion rate. Quasar host galaxy
studies have allowed us to find and characterize the host galaxies of quasars
to z>2. Despite these developments, many questions remain unresolved, in
particular the origin of the close relationship between black hole mass and
galaxy bulge mass/velocity dispersion seen in local galaxies.Comment: Review article, to appear in Astrophysics Update
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