Single-particle vs. pair superfluidity in a bilayer system of dipolar bosons

We consider the ground state of a bilayer system of dipolar bosons, where dipoles are oriented by an external field in the direction perpendicular to the parallel planes. Quantum Monte Carlo methods are used to calculate the ground-state energy, the one-body and two-body density matrix, and the superfluid response as a function of the separation between layers. We find that by decreasing the interlayer distance for fixed value of the strength of the dipolar interaction, the system undergoes a quantum phase transition from a single-particle to a pair superfluid. The single-particle superfluid is characterized by a finite value of both the atomic condensate and the super-counterfluid density. The pair superfluid phase is found to be stable against formation of many-body cluster states and features a gap in the spectrum of elementary excitations.Comment: 4 figure

Phonon transport effects in one-dimensional width-modulated graphene nanoribbons

We investigate the thermal conductance of one-dimensional periodic width-modulated graphene nanoribbons using lattice dynamics for the phonon spectrum and the Landauer formalism for phonon transport. We conduct a full investigation considering all relevant geometrical features, i.e., the various lengths and widths of the narrow and wide regions that form the channel. In all cases that we examine, we find that width-modulation suppresses the thermal conductance at values even up to ∼70% below those of the corresponding uniform narrow nanoribbon. We show that this can be explained by the fact that the phonon spectrum of the width-modulated channels acquires less dispersive bands with lower group velocities and several narrow bandgaps, which reduce the phonon transmission function significantly. The largest degradation in thermal conductance is determined by the geometry of the narrow regions. The geometry of the wider regions also influences thermal conductance, although modestly. Our results add to the ongoing efforts in understanding the details of phonon transport at the nanoscale, and our conclusions are generic and could also apply to other one-dimensional channel materials

Microscopic description of anisotropic low-density dipolar Bose gases in two dimensions

A microscopic description of the zero energy two-body ground state and many-body static properties of anisotropic homogeneous gases of bosonic dipoles in two dimensions at low densities is presented and discussed. By changing the polarization angle with respect to the plane, we study the impact of the anisotropy, present in the dipole--dipole interaction, on the energy per particle, comparing the results with mean field predictions. We restrict the analysis to the regime where the interaction is always repulsive, although the strength of the repulsion depends on the orientation with respect to the polarization field. We present a series expansion of the solution of the zero energy two-body problem which allows us to find the scattering length of the interaction and to build a suitable Jastrow factor that we use as a trial wave function for both a variational and diffusion Monte Carlo simulation of the infinite system. We find that the anisotropy has an almost negligible impact on the ground state properties of the many-body system in the universal regime where the scattering length governs the physics of the system. We also show that scaling in the gas parameter persists in the dipolar case up to values where other isotropic interactions with the same scattering length yield different predictions.Comment: 9 figures, 1 tabl

Hidden dimers and the matrix maps: Fibonacci chains re-visited

The existence of cycles of the matrix maps in Fibonacci class of lattices is well established. We show that such cycles are intimately connected with the presence of interesting positional correlations among the constituent `atoms' in a one dimensional quasiperiodic lattice. We particularly address the transfer model of the classic golden mean Fibonacci chain where a six cycle of the full matrix map exists at the centre of the spectrum [Kohmoto et al, Phys. Rev. B 35, 1020 (1987)], and for which no simple physical picture has so far been provided, to the best of our knowledge. In addition, we show that our prescription leads to a determination of other energy values for a mixed model of the Fibonacci chain, for which the full matrix map may have similar cyclic behaviour. Apart from the standard transfer-model of a golden mean Fibonacci chain, we address a variant of it and the silver mean lattice, where the existence of four cycles of the matrix map is already known to exist. The underlying positional correlations for all such cases are discussed in details.Comment: 14 pages, 2 figures. Submitted to Physical Review

FIBONACCI SUPERLATTICES OF NARROW-GAP III-V SEMICONDUCTORS

We report theoretical electronic structure of Fibonacci superlattices of narrow-gap III-V semiconductors. Electron dynamics is accurately described within the envelope-function approximation in a two-band model. Quasiperiodicity is introduced by considering two different III-V semiconductor layers and arranging them according to the Fibonacci series along the growth direction. The resulting energy spectrum is then found by solving exactly the corresponding effective-mass (Dirac-like) wave equation using tranfer-matrix techniques. We find that a self-similar electronic spectrum can be seen in the band structure. Electronic transport properties of samples are also studied and related to the degree of spatial localization of electronic envelope-functions via Landauer resistance and Lyapunov coefficient. As a working example, we consider type II InAs/GaSb superlattices and discuss in detail our results in this system.Comment: REVTeX 3.0, 16 pages, 8 figures available upon request. To appear in Semiconductor Science and Technolog

Obésité, facteurs socio-démographiques et indicateurs de santé chez les personnes âgées à Marseille

L’objectif de cette étude est triple : (1) déterminer si les facteurs socio-démographiques que sont l’âge, le niveau d’éducation et le statut marital ont le même impact sur l’obésité chez les femmes et chez les hommes, âgés de 60 ans et plus ; (2) évaluer les associations entre d’une part l’obésité et, d’autre part, l’auto-évaluation de la santé, les incapacités physiques et l’estime de soi ; (3) tester le rôle médiateur potentiel des maladies cardio-vasculaires et du diabète sur la relation entre obésité et auto-évaluation de la santé. Pour cela, 400 personnes âgées de 60 à 92 ans, vivant à Marseille, ont été interrogées. Ces rencontres individuelles comprenaient une prise de mesures anthropométriques et un entretien dirigé à l’aide d’un questionnaire (durée moyenne : 1h30). La prévalence de l’obésité dans notre échantillon de population est de 17 %, taux comparable à l’ensemble de la population française du même âge. Nos analyses ont permis de montrer que les corrélats de l’obésité sont fort différents entre les femmes et les hommes âgés. Chez les hommes, seul le statut marital est significativement associé à l’obésité alors que chez les femmes, l’âge, le niveau d’éducation, l’auto-évaluation de la santé et les incapacités physiques sont associés à l’obésité. Enfin, nous avons montré que la relation entre obésité et auto-évaluation de la santé (observée chez les femmes) est une association indirecte « médiée » par les maladies cardio-vasculaires.The purpose of this study is threefold: (1) to determine whether the socio-demographic factors of age, level of education and marital status have the same impact on obesity in women and in men aged 60 years and above; (2) to evaluate the associations between obesity on the one hand and self-rated health, physical incapacities and self-esteem on the other; (3) to test the mediatory potential of cardio-vascular diseases and diabetes on the relationship between obesity and self-rated health. For this, 400 persons aged from 60 to 92 years, living in Marseille, were questioned. These individual encounters included anthropometric measurement, and an interview carried out with the aid of a questionnaire (average duration: 1h30). The prevalence of obesity in our population sample is 17%, a rate comparable to the whole of this age group in the French population. Our analyses show that the correlates of obesity are very different between older women and men. For the men, only marital status is significantly associated with obesity while for the women, age, education level, self-rated health and physical incapacities are associated with obesity. Finally we have shown that the relation between obesity and self-rated health (for the women) is an indirect association affected by cardio-vascular diseases