Small Bipolarons in the 2-dimensional Holstein-Hubbard Model. II Quantum Bipolarons

We study the effective mass of the bipolarons and essentially the possibility to get both light and strongly bound bipolarons in the Holstein-Hubbard model and some variations in the vicinity of the adiabatic limit. Several approaches to investigate the quantum mobility of polarons and bipolarons are proposed for this model. It is found that the bipolaron mass generally remains very large except in the vicinity of the triple point of the phase diagram, where the bipolarons have several degenerate configurations at the adiabatic limit (single site (S0), two sites (S1) and quadrisinglet (QS)), while the polarons are much lighter. This degeneracy reduces the bipolaron mass significantly. The triple point of the phase diagram is washed out by the lattice quantum fluctuations which thus suppress the light bipolarons. We show that some model variations, for example a phonon dispersion may increase the stability of the (QS) bipolaron against the quantum lattice fluctuations. The triple point of the phase diagram may be stable to quantum lattice fluctuations and a very sharp mass reduction may occur, leading to bipolaron masses of the order of 100 bare electronic mass for realistic parameters. Thus such very light bipolarons could condense as a superconducting state at relatively high temperature when their interactions are not too large, that is, their density is small enough. This effect might be relevant for understanding the origin of the high Tc superconductivity of doped cuprates far enough from half filling.Comment: accepted Eur. Phys. J. B (january 2000) Ref. B960

Small Bipolarons in the 2-dimensional Holstein-Hubbard Model. I The Adiabatic Limit

The spatially localized bound states of two electrons in the adiabatic two-dimensional Holstein-Hubbard model on a square lattice are investigated both numerically and analytically. The interplay between the electron-phonon coupling g, which tends to form bipolarons and the repulsive Hubbard interaction $\upsilon \geq 0$, which tends to break them, generates many different ground-states. There are four domains in the $g,\upsilon$ phase diagram delimited by first order transition lines. Except for the domain at weak electron-phonon coupling (small g) where the electrons remain free, the electrons form bipolarons which can 1) be mostly located on a single site (small $\upsilon$, large g); 2) be an anisotropic pair of polarons lying on two neighboring sites in the magnetic singlet state (large $\upsilon$, large g); or 3) be a "quadrisinglet state" which is the superposition of 4 electronic singlets with a common central site. This quadrisinglet bipolaron is the most stable in a small central domain in between the three other phases. The pinning modes and the Peierls-Nabarro barrier of each of these bipolarons are calculated and the barrier is found to be strongly depressed in the region of stability of the quadrisinglet bipolaron

Many-polaron states in the Holstein-Hubbard model

A variational approach is proposed to determine some properties of the adiabatic Holstein-Hubbard model which describes the interactions between a static atomic lattice and an assembly of fermionic charge carriers. The sum of the electronic energy and the lattice elastic energy is proved to have minima with a polaron structure in a certain domain of the phase diagram. Our analytical work consists in the expansion of these energy minima from the zero electronic transfer limit which remarkably holds for a finite amplitude of the onsite Hubbard repulsion and for an unbounded lattice size.Comment: submitted to Journal of Statistical Physic

Château-Gontier – Chemin de Montplours

Découverte d’un ensemble de structures fossoyées masqué en partie par un apport secondaire de terre charbonneuse. Le mobilier recueilli, fragments de pot ou de pichet, date du bas Moyen Âge (xiiie s. environ). La présence de ces quelques vestiges montre que nous sommes probablement en périphérie d’un habitat localisé hors de l’emprise du diagnostic. Fig. 1 – Pot globulaire médiéval DAO : S Jean

Paule – Route nationale 164, aqueduc gallo-romain de Carhaix

L’évaluation archéologique réalisée sur la future 2 x 2 voies entre le hameau de la Pie et la commune du Moustoir durant l’été 1998, avait permis de repérer très précisément les parties de l’aqueduc gallo-romain de Carhaix concernées par l’emprise de la déviation. La fouille de sauvetage entreprise sur cet ouvrage a été réalisée en deux phases entre la Direction départementale de l’équipement des Côtes d’Armor (maître d’œuvre), le Service régional de l’archéologie de Bretagne et l’Afan chargé..

Domloup – ZAC du Tertre (tranches 1 et 2)

Le diagnostic archéologique entrepris sur une partie de l'emprise de la future ZAC économique du tertre de Domloup (35) a été réalisé du 14 septembre au 6 novembre 2009 par une équipe de l'Inrap. La mise en oeuvre des deux premières tranches a permis d'explorer une superficie d'environ 36 ha. Le maillage des tranchées de sondage, environ 10 % de la surface concernée, a favorisé la détection de nombreuses structures fossoyées appartenant à diverses périodes chronologiques (du Néolithique aux T..

Domloup – ZAC du Tertre (tranches 1 et 2)

Le diagnostic archéologique entrepris sur une partie de l'emprise de la future ZAC économique du tertre de Domloup (35) a été réalisé du 14 septembre au 6 novembre 2009 par une équipe de l'Inrap. La mise en oeuvre des deux premières tranches a permis d'explorer une superficie d'environ 36 ha. Le maillage des tranchées de sondage, environ 10 % de la surface concernée, a favorisé la détection de nombreuses structures fossoyées appartenant à diverses périodes chronologiques (du Néolithique aux T..

Le Moustoir – Route nationale 164, « déviation La Pie »

Le projet de déviation en tracé 2 x 2 voies du village du Moustoir et du hameau de La Pie se développe sur 7,350 km entre la future déviation sud de Carhaix-Plouguer et l’actuel doublement déjà réalisé à l’est du lieu-dit « Belle-Vue », situé sur la commune de Glomel. La présence confirmée de l’aqueduc antique de Carhaix-Plouguer sur certaines parties du tracé ainsi que la proximité de cette ancienne capitale de cité gallo-romaine (Vorgium) a conduit le Service régional de l’archéologie de Br..

Negative reflection of elastic guided waves in chaotic and random scattering media

The propagation of waves in complex media can be harnessed either by taming the incident wave-field impinging on the medium or by forcing waves along desired paths through its careful design. These two alternative strategies have given rise to fascinating concepts such as time reversal or negative refraction. Here, we show how these two processes are intimately linked through the negative reflection phenomenon. A negative reflecting mirror converts a wave of positive phase velocity into its negative counterpart and vice versa. In this article, we experimentally demonstrate this phenomenon with elastic waves in a 2D billiard and in a disordered plate by means of laser interferometry. Despite the complexity of such configurations, the negatively reflected wave field focuses back towards the initial source location, thereby mimicking a phase conjugation operation while being a fully passive process. The super-focusing capability of negative reflection is also highlighted in a monochromatic regime. The negative reflection phenomenon is not restricted to guided elastic waves since it can occur in zero-gap systems such as photonic crystals, chiral metamaterials or graphene. Negative reflection can thus become a tool of choice for the control of waves in all fields of wave physics.Comment: 9 pages, 6 figure