Coherent Oscillations in an Exciton-Polariton Josephson Junction

We report on the observation of spontaneous coherent oscillations in a microcavity polariton bosonic Josephson junction. The condensation of exciton polaritons takes place under incoherent excitation in a disordered environment, where double potential wells tend to appear in the disordered landscape. Coherent oscillations set on at an excitation power well above the condensation threshold. The time resolved population and phase dynamics reveal the analogy with the AC Josephson effect. We have introduced a theoretical two-mode model to describe the observed effects, which allows us to explain how the different realizations of the pulsed experiment have a similar phase relation

Spontaneous Pattern Formation in a Polariton Condensate

Polariton condensation can be regarded as a self-organization phenomenon, where phase ordering is established among particles in the system. In such condensed systems, further ordering can possibly occur in the particle density distribution, under particular experimental conditions. In this work we report on spontaneous pattern formation in a polariton condensate under non-resonant optical pumping. The slightly elliptical ring-shaped excitation laser we employ is such to force condensation to occur in a single-energy state with periodic boundary conditions, giving rise to a multi-lobe standing wave patterned state

Penrose-Onsager Criterion Validation in a One-Dimensional Polariton Condensate

We perform quantum tomography on one-dimensional polariton condensates, spontaneously occurring in linear disorder valleys in a CdTe planar microcavity sample. By the use of optical interferometric techniques, we determine the first-order coherence function and the amplitude and phase of the order parameter of the condensate, providing a full reconstruction of the single particle density matrix for the polariton system. The experimental data are used as input to theoretically test the consistency of Penrose-Onsager criterion for Bose-Einstein condensation in the framework of nonequilibrium polariton condensates. The results confirm the pertinence and validity of the criterion for a non equilibrium condensed gas.Comment: 5 pages, 4 figure

Spontaneous self-ordered states of vortex-antivortex pairs in a Polariton Condensate

Polariton condensates have proved to be model systems to investigate topological defects, as they allow for direct and non-destructive imaging of the condensate complex order parameter. The fundamental topological excitations of such systems are quantized vortices. In specific configurations, further ordering can bring the formation of vortex lattices. In this work we demonstrate the spontaneous formation of ordered vortical states, consisting in geometrically self-arranged vortex-antivortex pairs. A mean-field generalized Gross-Pitaevskii model reproduces and supports the physics of the observed phenomenology

Two Meson Systems with Ginsparg-Wilson Valence Quarks

Unphysical effects associated with finite lattice spacing and partial quenching may lead to the presence of unphysical terms in chiral extrapolation formulae. These unphysical terms must then be removed during data analysis before physical predictions can be made. In this work, we show that through next-to-leading order, there are no unphysical counterterms in the extrapolation formulae, expressed in lattice-physical parameters, for meson scattering lengths in theories with Ginsparg-Wilson valence quarks. Our work applies to most sea quark discretization, provided that chiral perturbation theory is a valid approximation. We demonstrate our results with explicit computations and show that, in favorable circumstances, the extrapolation formulae do not depend on the unknown constant C_Mix appearing at lowest order in the mixed action chiral Lagrangian. We show that the I=1 KK scattering length does not depend on C_Mix in contrast to the I=3/2 K-pi scattering length. In addition, we show that these observables combined with f_K / f_pi and the I=2 pi-pi scattering length share only two linearly independent sets of counterterms, providing a means to test the mixed action theory at one lattice spacing. We therefore make a prediction for the I=1 KK scattering length.Comment: 21 pages, 2 figures, 2 tables. Version to be published in PRD. Improved discussion in Sec. III B. Added reference

Dynamics of long-range order in an exciton-polariton condensate

We report on time resolved measurements of the first order spatial coherence in an exciton polariton Bose-Einstein condensate. Long range spatial coherence is found to set in right at the onset of stimulated scattering, on a picosecond time scale. The coherence reaches its maximum value after the population and decays slower, staying up to a few hundreds of picoseconds. This behavior can be qualitatively reproduced, using a stochastic classical field model describing interaction between the polariton condensate and the exciton reservoir within a disordered potential.Comment: 7 pages, 4 figure

Synchronized and Desynchronized Phases of Exciton-Polariton Condensates in the Presence of Disorder

Condensation of exciton-polaritons in semiconductor microcavities takes place despite in plane disorder. Below the critical density the inhomogeneity of the potential seen by the polaritons strongly limits the spatial extension of the ground state. Above the critical density, in presence of weak disorder, this limitation is spontaneously overcome by the non linear interaction, resulting in an extended synchronized phase. This mechanism is clearly evidenced by spatial and spectral studies, coupled to interferometric measurements. In case of strong disorder, several non phase-locked (independent) condensates can be evidenced. The transition from synchronized phase to desynchronized phase is addressed considering multiple realizations of the disorder.Comment: 11 pages, 4 figures,corrected typos, added figure

Magneto-elastic coupling and unconventional magnetic ordering in triangular multiferroic AgCrS2

The temperature evolution of the crystal and magnetic structures of ferroelectric sulfide AgCrS2 have been investigated by means of neutron scattering. AgCrS2 undergoes at TN = 41.6 K a first-order phase transition, from a paramagnetic rhombohedral R3m to an antiferromagnetic monoclinic structure with a polar Cm space group. In addition to being ferroelectric below TN, the low temperature phase of AgCrS2 exhibits an unconventional collinear magnetic structure that can be described as double ferromagnetic stripes coupled antiferromagnetically, with the magnetic moment of Cr+3 oriented along b within the anisotropic triangular plane. The magnetic couplings stabilizing this structure are discussed using inelastic neutron scattering results. Ferroelectricity below TN in AgCrS2 can possibly be explained in terms of atomic displacements at the magneto-elastic induced structural distortion. These results contrast with the behavior of the parent frustrated antiferromagnet and spin-driven ferroelectric AgCrO2

Origin of Spin Incommensurability in Hole-doped S=1 Y2−xCaxBaNiO5\rm Y_{2-x}Ca_x Ba Ni O_5 Chains

Spin incommensurability has been recently experimentally discovered in the hole-doped Ni-oxide chain compound $\rm Y_{2-x}Ca_x Ba Ni O_5$ (G. Xu {\it al.}, Science {\bf 289}, 419 (2000)). Here a two orbital model for this material is studied using computational techniques. Spin IC is observed in a wide range of densities and couplings. The phenomenon originates in antiferromagnetic correlations ``across holes'' dynamically generated to improve hole movement, as it occurs in the one-dimensional Hubbard model and in recent studies of the two-dimensional extended t-J model. The close proximity of ferromagnetic and phase-separated states in parameter space are also discussed.Comment: RevTex, 4 pages, 4 figures (eps

Ab initio study of reflectance anisotropy spectra of a sub-monolayer oxidized Si(100) surface

The effects of oxygen adsorption on the reflectance anisotropy spectrum (RAS) of reconstructed Si(100):O surfaces at sub-monolayer coverage (first stages of oxidation) have been studied by an ab initio DFT-LDA scheme within a plane-wave, norm-conserving pseudopotential approach. Dangling bonds and the main features of the characteristic RAS of the clean Si(100) surface are mostly preserved after oxidation of 50% of the surface dimers, with some visible changes: a small red shift of the first peak, and the appearance of a distinct spectral structure at about 1.5 eV. The electronic transitions involved in the latter have been analyzed through state-by-state and layer-by-layer decompositions of the RAS. We suggest that new interplay between present theoretical results and reflectance anisotropy spectroscopy experiments could lead to further clarification of structural and kinetic details of the Si(100) oxidation process in the sub-monolayer range.Comment: 21 pages, 8 figures. To be published in Physical Rev.