Catégorie : Collaborations

Abstract : We compare the ferroelectricity for two families of synthetic conductors: the experimentally found ferroelectricity in organic crystal like (TMTTF)2X and a plausilble ferroelectricity in specially designed polyene chains. The difference of the polymer with respect to the organic conductor is the reverse of the build-in (now the sites) and spontaneous (now the bonds) effects of dimerizations. The theory predicts the existence of solitons (dimerization kinks) with non-integer variable charges, both with and without the spin. With today’s understanding, we see them as walls separating domains with opposite electric polarization. Their physics will serve to relate transient ferroelectric processes and the visible range optics.

N. Kirova 1 S. Brazovskii 2
1 LPS – Laboratoire de Physique des Solides
2 LPTMS – Laboratoire de Physique Théorique et Modèles Statistiques

Anne Débarre 1 Mathias Kobylko 2 Anne-Marie Bonnot 3 A. Richard 1 V. N. Popov Luc Henrard Mathieu Kociak 2
1 LAC – Laboratoire Aimé Cotton
2 LPS – Laboratoire de Physique des Solides
3 SC2G – Semi-conducteurs à large bande interdite
NEEL – Institut Néel

Abstract : The charge density wave (CDW) can be strained by applied voltage or by charge transfer at junctions, or by an electric field which can also put the CDW into sliding. Such strains can give rise to local or long-range modulational instabilities. We consider and perform a numerical modeling for two cases of strained CDW, the case of voltage is applied along and across conducting chains. We consider the effect of random and periodic dislocation array on the X-ray profile and present the plausible scenario of a very long-wave coherent periodic unharmonic superstructure. For the voltage allied across the chains, we confirm the reconstruction of the junction by creating an array of dislocations, whose number increases by steps with increasing voltage. The dislocation cores concentrate most of the voltage drop across the junction, giving rise to an effective tunneling. We compare the results with the experiments on coherent micro-diffraction in blue bronzes and with tunneling experiments in NbSe3 and TaS3.

N. Kirova 1 A. Rojo-Bravo 2 S. Brazovskii 2
1 LPS – Laboratoire de Physique des Solides
2 LPTMS – Laboratoire de Physique Théorique et Modèles Statistiques

Abstract : We investigate weak localization in metallic networks etched in a two dimensional electron gas between 2525mK and 750750mK when electron-electron (e-e) interaction is the dominant phase breaking mechanism. We show that, at the highest temperatures, the contributions arising from trajectories that wind around the rings and trajectories that do not are governed by two different length scales. This is achieved by analyzing separately the envelope and the oscillating part of the magnetoconductance. For T≳0.3T≳0.3K we find \Lphienv∝T−1/3\Lphienv∝T−1/3 for the envelope, and \Lphiosc∝T−1/2\Lphiosc∝T−1/2 for the oscillations, in agreement with the prediction for a single ring \cite{LudMir04,TexMon05}. This is the first experimental confirmation of the geometry dependence of decoherence due to e-e interaction.

M. Ferrier 1 A. C. H. Rowe 1 S. Gueron 1, 2 H. Bouchiat 1 C. Texier 1, 3 G. Montambaux 1
1 LPS – Laboratoire de Physique des Solides
2 QUANTRONICS – Quantronics Group
SPEC – UMR3680 – Service de physique de l’état condensé, IRAMIS – Institut Rayonnement Matière de Saclay
3 LPTMS – Laboratoire de Physique Théorique et Modèles Statistiques

2008

Abstract : We consider a metric graph GG made of two graphs G1G1 and G2G2 attached at one point. We derive a formula relating the spectral determinant of the Laplace operator SG(γ)=det(γ−Δ)SG(γ)=det(γ−Δ) in terms of the spectral determinants of the two subgraphs. The result is generalized to describe the attachment of nn graphs. The formulae are also valid for the spectral determinant of the Schrödinger operator det(γ−Δ+V(x))det(γ−Δ+V(x)).

Auteur

Christophe Texier 1, 2
1 LPTMS – Laboratoire de Physique Théorique et Modèles Statistiques
2 LPS – Laboratoire de Physique des Solides

M. Kobylko 1 Anne-Marie Bonnot 2 A.R. Hightwalker A. Richard 3 Mathieu Kociak 1 Anne Débarre 3
1 LPS – Laboratoire de Physique des Solides
2 SC2G – Semi-conducteurs à large bande interdite
NEEL – Institut Néel
3 LAC – Laboratoire Aimé Cotton

Abstract : We consider the correction ΔσeeΔσee due to electron-electron interaction to the conductivity of a weakly disordered metal (Al’tshuler-Aronov correction). The correction is related to the spectral determinant of the Laplace operator. The case of a large square metallic network is considered. The variation of Δσee(LT)Δσee(LT) as a function of the thermal length LTLT is found very similar to the variation of the weak localization ΔσWL(Lϕ)ΔσWL(Lϕ) as a function of the phase coherence length. Our result for ΔσeeΔσee interpolates between the known 1d and 2d results, but the interaction parameter entering the expression of ΔσeeΔσee keeps a 1d behaviour. Quite surprisingly, the result is very close to the 2d logarithmic behaviour already for LT∼a/2LT∼a/2, where aa is the lattice parameter.

Christophe Texier 1 Gilles Montambaux 2
1 LPTMS – Laboratoire de Physique Théorique et Modèles Statistiques
2 LPS – Laboratoire de Physique des Solides

Abstract : We report on magnetoconductance measurements of metallic networks of various sizes ranging from 10 to 106106 plaquettes, with anisotropic aspect ratio. Both Altshuler-Aronov-Spivak (AAS) h/2eh/2e periodic oscillations and Aharonov-Bohm (AB) h/eh/e periodic oscillations are observed for all networks. For large samples, the amplitude of both oscillations results from the incoherent superposition of contributions of phase coherent regions. When the transverse size becomes smaller than the phase coherent length LϕLϕ, one enters a new regime which is phase coherent (mesoscopic) along one direction and macroscopic along the other, leading to a new size dependence of the quantum oscillations.

Auteurs

Félicien Schopfer François Mallet D. Mailly 1 C. Texier 2, 3 G. Montambaux 3 Christopher Bäuerle 4 Laurent Saminadayar 5
1 LPN – Laboratoire de photonique et de nanostructures
2 LPTMS – Laboratoire de Physique Théorique et Modèles Statistiques
3 LPS – Laboratoire de Physique des Solides
4 QuantECA – Circuits électroniques quantiques Alpes
NEEL – Institut Néel
5 TPS – Thermodynamique et biophysique des petits systèmes
NEEL – Institut Néel

Abstract : We present an x-ray diffuse scattering study of the charge-density wave (CDW) structure in the doped blue bronzes K0.3(Mo1−xVx)O3 (x=2.8%), Rb0.3(Mo1−xVx)O3 (x=1.44% and 0.28%), and K0.3(Mo1−xWx)O3 (x=2%). At low temperature, the ±2kF satellite reflections are broadened in all four compounds, and shifted in V-doped compounds. Moreover, we have observed an intensity asymmetry of the ±2kF satellite reflections relative to the pure compound, and a profile asymmetry of each satellite reflections. We show that both effects, intensity and profile asymmetry, give access to the local properties of CDW in disordered systems, including the pinning and the phase shifts around impurities. This leads us to propose a complete scenario of the pinning in doped blue bronzes. In the V-doped compounds, we show that the profile asymmetry is due to Friedel oscillations around the charged V substituent and that the intensity asymmetry is related to the strong pinning of the CDW. In the W-doped crystal, the CDW is weakly pinned in regions containing a few tens of neutral W substituents and the CDW phase is slightly distorted around the W impurities. Exact calculations are presented to sustain these ideas.

Auteurs

S. Ravy 1 S. Rouzière 2 J.-P. Pouget 2 S. Brazovskii 3 J. Marcus 4 J.-F. Bérar 5 E. Elkaim 1
1 SSOLEIL – Synchrotron SOLEIL
2 LPS – Laboratoire de Physique des Solides
3 LPTMS – Laboratoire de Physique Théorique et Modèles Statistiques
4 LEPES – Laboratoire d’Etudes des Propriétés Electroniques des Solides
5 Laboratoire de Cristallographie

Abstract : We review several results related to the problem of a quantum particle in a random environment. In an introductory part, we recall how several functionals of the Brownian motion arise in the study of electronic transport in weakly disordered metals (weak localization). Two aspects of the physics of the one-dimensional strong localization are reviewed : some properties of the scattering by a random potential (time delay distribution) and a study of the spectrum of a random potential on a bounded domain (the extreme value statistics of the eigenvalues). Then we mention several results concerning the diffusion on graphs, and more generally the spectral properties of the Schr\\ödinger operator on graphs. The interest of spectral determinants as generating functions characterizing the diffusion on graphs is illustrated. Finally, we consider a two-dimensional model of a charged particle coupled to the random magnetic field due to magnetic vortices. We recall the connection between spectral properties of this model and winding functionals of the planar Brownian motion.

Alain Comtet 1, 2 Jean Desbois 1 Christophe Texier 1, 3
1 LPTMS – Laboratoire de Physique Théorique et Modèles Statistiques
2 IHP – Institut Henri Poincaré
3 LPS – Laboratoire de Physique des Solides