5,200 research outputs found
Collective pairing of resonantly coupled microcavity polaritons
We consider the possible phases of microcavity polaritons tuned near a
bipolariton Feshbach resonance. We show that, as well as the regular polariton
superfluid phase, a "molecular" superfluid exists, with (quasi-)long-range
order only for pairs of polaritons. We describe the experimental signatures of
this state. Using variational approaches we find the phase diagram (critical
temperature, density and exciton-photon detuning). Unlike ultracold atoms, the
molecular superfluid is not inherently unstable, and our phase diagram suggests
it is attainable in current experiments.Comment: paper (4 pages, 3 figures), Supplemental Material (7 pages, 8
figures
Topological Sound and Flocking on Curved Surfaces
Active systems on curved geometries are ubiquitous in the living world. In
the presence of curvature orientationally ordered polar flocks are forced to be
inhomogeneous, often requiring the presence of topological defects even in the
steady state due to the constraints imposed by the topology of the underlying
surface. In the presence of spontaneous flow the system additionally supports
long-wavelength propagating sound modes which get gapped by the curvature of
the underlying substrate. We analytically compute the steady state profile of
an active polar flock on a two-sphere and a catenoid, and show that curvature
and active flow together result in symmetry protected topological modes that
get localized to special geodesics on the surface (the equator or the neck
respectively). These modes are the analogue of edge states in electronic
quantum Hall systems and provide unidirectional channels for information
transport in the flock, robust against disorder and backscattering.Comment: 15 pages, 6 figure
Models of plastic depinning of driven disordered systems
Two classes of models of driven disordered systems that exhibit
history-dependent dynamics are discussed. The first class incorporates local
inertia in the dynamics via nonmonotonic stress transfer between adjacent
degrees of freedom. The second class allows for proliferation of topological
defects due to the interplay of strong disorder and drive. In mean field theory
both models exhibit a tricritical point as a function of disorder strength. At
weak disorder depinning is continuous and the sliding state is unique. At
strong disorder depinning is discontinuous and hysteretic.Comment: 3 figures, invited talk at StatPhys 2
Absorption, Photoluminescence and Resonant Rayleigh Scattering Probes of Condensed Microcavity Polaritons
We investigate and compare different optical probes of a condensed state of
microcavity polaritons in expected experimental conditions of non-resonant
pumping. We show that the energy- and momentum-resolved resonant Rayleigh
signal provide a distinctive probe of condensation as compared to, e.g.,
photoluminescence emission. In particular, the presence of a collective sound
mode both above and below the chemical potential can be observed, as well as
features directly related to the density of states of particle-hole like
excitations. Both resonant Rayleigh response and the absorption and
photoluminescence, are affected by the presence of quantum well disorder, which
introduces a distribution of oscillator strengths between quantum well excitons
at a given energy and cavity photons at a given momentum. As we show, this
distribution makes it important that in the condensed regime, scattering by
disorder is taken into account to all orders. We show that, in the low density
linear limit, this approach correctly describes inhomogeneous broadening of
polaritons. In addition, in this limit, we extract a linear blue-shift of the
lower polariton versus density, with a coefficient determined by temperature
and by a characteristic disorder length.Comment: 16 pages, 11 figures; minor correction
Contractile stresses in cohesive cell layers on finite-thickness substrates
Using a minimal model of cells or cohesive cell layers as continuum active
elastic media, we examine the effect of substrate thickness and stiffness on
traction forces exerted by strongly adhering cells. We obtain a simple
expression for the length scale controlling the spatial variation of stresses
in terms of cell and substrate parameters that describes the crossover between
the thin and thick substrate limits. Our model is an important step towards a
unified theoretical description of the dependence of traction forces on cell or
colony size, acto-myosin contractility, substrate depth and stiffness, and
strength of focal adhesions, and makes experimentally testable predictions.Comment: 5 pages, 3 figure
Driven depinning of strongly disordered media and anisotropic mean-field limits
Extended systems driven through strong disorder are modeled generically using
coarse-grained degrees of freedom that interact elastically in the directions
parallel to the driving force and that slip along at least one of the
directions transverse to the motion. A realization of such a model is a
collection of elastic channels with transverse viscous couplings. In the
infinite range limit this model has a tricritical point separating a region
where the depinning is continuous, in the universality class of elastic
depinning, from a region where depinning is hysteretic. Many of the collective
transport models discussed in the literature are special cases of the generic
model.Comment: 4 pages, 2 figure
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