8,634 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
Imperfect rationality, macroeconomic equilibrium and price rigidities
We introduce some elements of Prospect Theory into a general equilibrium model with monoolistic competition in the good market and real wage rigidities due to (right to manage or efficient) wage bargaining, or to efficiency wages. We show that, under these types of labor market frictions, an increase in workers’ loss aversion: (i) reduces the equilibrium wage and in this way increases potential output; (ii) induces workers to work and consume less and in this way decreases potential output. If the former effect is greater (smaller) than the latter one, loss aversion increases (decreases) potential output. We also show that, under all the types of labor market frictions we consider, if loss aversion reduces equilibrium output, it also enhances the plausibility of nominal price rigidities
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
Renormalization Group Flow of the Two-Dimensional Hierarchical Coulomb Gas
We consider a quasilinear parabolic differential equation associated with the
renormalization group transformation of the two-dimensional hierarchical
Coulomb system in the limit as the size of the block L goes to 1. We show that
the initial value problem is well defined in a suitable function space and the
solution converges, as t goes to infinity, to one of the countably infinite
equilibrium solutions. The nontrivial equilibrium solution bifurcates from the
trivial one. These solutions are fully described and we provide a complete
analysis of their local and global stability for all values of inverse
temperature. Gallavotti and Nicolo's conjecture on infinite sequence of
``phases transitions'' is also addressed. Our results rule out an intermediate
phase between the plasma and the Kosterlitz-Thouless phases, at least in the
hierarchical model we consider.Comment: 34pages,2figures, to appear in CM
Anderson-like Transition for a Class of Random Sparse Models in d >= 2 Dimensions
We show that the Kronecker sum of d >= 2 copies of a random one-dimensional
sparse model displays a spectral transition of the type predicted by Anderson,
from absolutely continuous around the center of the band to pure point around
the boundaries. Possible applications to physics and open problems are
discussed briefly.Comment: 19 pages, 1 figure. New version corrects misprints and adds
pertaining reference
Superfluidity, Sound Velocity and Quasi Condensation in the 2D BCS-BEC Crossover
We study finite-temperature properties of a two-dimensional superfluid made
of ultracold alkali-metal atoms in the BCS-BEC crossover. We investigate the
region below the critical temperature of the
Berezinskii-Kosterlitz-Thouless phase transition, where there is
quasi-condensation, by analyzing the effects of phase and amplitude
fluctuations of the order parameter. In particular, we calculate the superfluid
fraction, the sound velocity and the quasi-condensate fraction as a function of
the temperature and of the binding energy of fermionic pairs.Comment: 7 pages, 4 figures, improved version to be published in Phys. Rev.
Non-BCS superconductivity for underdoped cuprates by spin-vortex attraction
Within a gauge approach to the t-J model, we propose a new, non-BCS mechanism
of superconductivity for underdoped cuprates. The gluing force of the
superconducting mechanism is an attraction between spin vortices on two
different N\'eel sublattices, centered around the empty sites described in
terms of fermionic holons. The spin fluctuations are described by bosonic
spinons with a gap generated by the spin vortices. Due to the no-double
occupation constraint, there is a gauge attraction between holon and spinon
binding them into a physical hole. Through gauge interaction the spin vortex
attraction induces the formation of spin-singlet (RVB) spin pairs with a
owering of the spinon gap. Lowering the temperature the approach exhibits two
crossover temperatures: at the higher crossover a finite density of incoherent
holon pairs are formed leading to a reduction of the hole spectral weight, at
the lower crossover also a finite density of incoherent spinon RVB pairs are
formed, giving rise to a gas of incoherent preformed hole pairs, and magnetic
vortices appear in the plasma phase. Finally, at a even lower temperature the
hole pairs become coherent, the magnetic vortices become dilute and
superconductivity appears. The superconducting mechanism is not of BCS-type
since it involves a gain in kinetic energy (for spinons) coming from the spin
interactions.Comment: 4 pages, 3 figures, accepted by the proceedings of SNS2010 conferenc
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
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