6,376 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
Density-wave phases of dipolar fermions in a bilayer
We investigate the phase diagram of dipolar fermions with aligned dipole
moments in a two-dimensional (2D) bilayer. Using a version of the
Singwi-Tosi-Land-Sjolander scheme recently adapted to dipolar fermions in a
single layer [M. M. Parish and F. M. Marchetti, Phys. Rev. Lett. 108, 145304
(2012)], we determine the density-wave instabilities of the bilayer system
within linear response theory. We find that the bilayer geometry can stabilize
the collapse of the 2D dipolar Fermi gas with intralayer attraction to form a
new density wave phase that has an orientation perpendicular to the density
wave expected for strong intralayer repulsion. We thus obtain a quantum phase
transition between stripe phases that is driven by the interplay between strong
correlations and the architecture of the low dimensional system.Comment: 5 pages, 3 figure
Dipolar fermions in a multilayer geometry
We investigate the behavior of identical dipolar fermions with aligned dipole
moments in two-dimensional multilayers at zero temperature. We consider density
instabilities that are driven by the attractive part of the dipolar interaction
and, for the case of bilayers, we elucidate the properties of the stripe phase
recently predicted to exist in this interaction regime. When the number of
layers is increased, we find that this "attractive" stripe phase exists for an
increasingly larger range of dipole angles, and if the interlayer distance is
sufficiently small, the stripe phase eventually spans the full range of angles,
including the situation where the dipole moments are aligned perpendicular to
the planes. In the limit of an infinite number of layers, we derive an analytic
expression for the interlayer effects in the density-density response function
and, using this result, we find that the stripe phase is replaced by a collapse
of the dipolar system.Comment: 9 pages, 8 figure
Phase Equilibrium of Binary Mixtures in Mixed Dimensions
We study the stability of a Bose-Fermi system loaded into an array of coupled
one-dimensional (1D) "tubes", where bosons and fermions experience different
dimensions: Bosons are heavy and strongly localized in the 1D tubes, whereas
fermions are light and can hop between the tubes. Using the 174Yb-6Li system as
a reference, we obtain the equilibrium phase diagram. We find that, for both
attractive and repulsive interspecies interaction, the exact treatment of 1D
bosons via the Bethe ansatz implies that the transitions between pure fermion
and any phase with a finite density of bosons can only be first order and never
continuous, resulting in phase separation in density space. In contrast, the
order of the transition between the pure boson and the mixed phase can either
be second or first order depending on whether fermions are allowed to hop
between the tubes or they also are strictly confined in 1D. We discuss the
implications of our findings for current experiments on 174Yb-6Li mixtures as
well as Fermi-Fermi mixtures of light and heavy atoms in a mixed dimensional
optical lattice system.Comment: 12 pages, 6 figure
Spontaneous patterns in coherently driven polariton microcavities
We consider a polariton microcavity resonantly driven by two external lasers
which simultaneously pump both lower and upper polariton branches at normal
incidence. In this setup, we study the occurrence of instabilities of the
pump-only solutions towards the spontaneous formation of patterns. Their
appearance is a consequence of the spontaneous symmetry breaking of
translational and rotational invariance due to interaction induced parametric
scattering. We observe the evolution between diverse patterns which can be
classified as single-pump, where parametric scattering occurs at the same
energy as one of the pumps, and as two-pump, where scattering occurs at a
different energy. For two-pump instabilities, stripe and chequerboard patterns
become the dominant steady-state solutions because cubic parametric scattering
processes are forbidden. This contrasts with the single-pump case, where
hexagonal patterns are the most common arrangements. We study the possibility
of controlling the evolution between different patterns. Our results are
obtained within a linear stability analysis and are confirmed by finite size
full numerical calculations.Comment: 15 pages, 9 figure
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
Non-equilibrium Berezinskii-Kosterlitz-Thouless Transition in a Driven Open Quantum System
The Berezinskii-Kosterlitz-Thouless mechanism, in which a phase transition is
mediated by the proliferation of topological defects, governs the critical
behaviour of a wide range of equilibrium two-dimensional systems with a
continuous symmetry, ranging from superconducting thin films to two-dimensional
Bose fluids, such as liquid helium and ultracold atoms. We show here that this
phenomenon is not restricted to thermal equilibrium, rather it survives more
generally in a dissipative highly non-equilibrium system driven into a
steady-state. By considering a light-matter superfluid of polaritons, in the
so-called optical parametric oscillator regime, we demonstrate that it indeed
undergoes a vortex binding-unbinding phase transition. Yet, the exponent of the
power-law decay of the first order correlation function in the (algebraically)
ordered phase can exceed the equilibrium upper limit -- a surprising
occurrence, which has also been observed in a recent experiment. Thus we
demonstrate that the ordered phase is somehow more robust against the quantum
fluctuations of driven systems than thermal ones in equilibrium.Comment: 11 pages, 9 figure
Tail States in Disordered Superconductors with Magnetic Impurities: the Unitarity Limit
When subject to a weak magnetic impurity distribution, the order parameter
and quasi-particle energy gap of a weakly disordered bulk s-wave superconductor
are suppressed. In the Born scattering limit, recent investigations have shown
that `optimal fluctuations' of the random impurity potential can lead to the
nucleation of `domains' of localised states within the gap region predicted by
the conventional Abrikosov-Gor'kov mean-field theory, rendering the
superconducting system gapless at any finite impurity concentration. By
implementing a field theoretic scheme tailored to the weakly disordered system,
the aim of the present paper is to extend this analysis to the consideration of
magnetic impurities in the unitarity scattering limit. This investigation
reveals that the qualitative behaviour is maintained while the density of
states exhibits a rich structure.Comment: 18 pages AMSLaTeX (with LaTeX2e), 6 eps figure
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