Ergodic vs diffusive decoherence in mesoscopic devices

We report on the measurement of phase coherence length in a high mobility two-dimensional electron gas patterned in two different geometries, a wire and a ring. The phase coherence length is extracted both from the weak localization correction in long wires and from the amplitude of the Aharonov-Bohm oscillations in a single ring, in a low temperature regime when decoherence is dominated by electronic interactions. We show that these two measurements lead to different phase coherence lengths, namely $L_{\Phi}^\mathrm{wire}\propto T^{-1/3}$ and $L_{\Phi}^\mathrm{ring}\propto T^{-1/2}$. This difference reflects the fact that the electrons winding around the ring necessarily explore the whole sample (ergodic trajectories), while in a long wire the electrons lose their phase coherence before reaching the edges of the sample (diffusive regime).Comment: LaTeX, 5 pages, 4 pdf figures ; v2: revised versio

Magnetic dephasing in mesoscopic spin glasses

We have measured Universal Conductance Fluctuations in the metallic spin glass Ag:Mn as a function of temperature and magnetic field. From this measurement, we can access the phase coherence time of the electrons in the spin glass. We show that this phase coherence time increases with both the inverse of the temperature and the magnetic field. From this we deduce that decoherence mechanisms are still active even deep in the spin glass phase

Quantum Coherence at Low Temperatures in Mesoscopic Systems: Effect of Disorder

We study the disorder dependence of the phase coherence time of quasi one-dimensional wires and two-dimensional (2D) Hall bars fabricated from a high mobility GaAs/AlGaAs heterostructure. Using an original ion implantation technique, we can tune the intrinsic disorder felt by the 2D electron gas and continuously vary the system from the semi-ballistic regime to the localized one. In the diffusive regime, the phase coherence time follows a power law as a function of diffusion coefficient as expected in the Fermi liquid theory, without any sign of low temperature saturation. Surprisingly, in the semi-ballistic regime, it becomes independent of the diffusion coefficient. In the strongly localized regime we find a diverging phase coherence time with decreasing temperature, however, with a smaller exponent compared to the weakly localized regime.Comment: 21 pages, 30 figure

Low temperature dephasing in irradiated metallic wires

We present phase coherence time measurements in quasi-one-dimensional Ag wires implanted with Ag$^{+}$ ions with an energy of $100 keV$. The measurements have been carried out in the temperature range from $100 mK$ up to $10 K$; this has to be compared with the Kondo temperature of iron in silver, i.e. $T_{K}^{Ag/Fe} \approx 4 K$, used in recent experiments on dephasing in Kondo systems\cite{mallet_prl_06,birge_prl_06}. We show that the phase coherence time is not affected by the implantation procedure, clearly proving that ion implantation process by itself \emph{does not lead to any extra dephasing} at low temperature.Comment: 4 pages, 4figure

Quantum transport in spin glasses

Manuscrit rédigé en langue anglaiseThe spin glass is a state of matter in which the magnetic disorder is quenched. Being considered as a model system for glasses in general, it has been extensively studied, both theoretically and experimentally. The research have converged towards two main descriptions of the fundamental state of the system that are clearly antagonist. On the one hand, the "mean-field" solution has a non trivial broken symmetry, and the ground state is composed of multiple valleys in a hierarchical structure. On the other hand, a magnetic "droplet" model, based on the off-equilibrium dynamics of a unique ground state. The experimental validation of one of these two theories requires a detailed observation of the sample at the microscopic level. Mesoscopic physics, which deals with interference effects of the electrons, proposes a unique tool to access to this microscopic configuration of the impurities: the universal conductance fluctuations. Indeed, these fluctuations represent a unique fingerprint of the sample disorder. This work presents the implementation of universal conductance fluctuations measurements in spin glasses. The electron interference effects being sensitive to the decoherence processes of the spin glass, they give access experimentally to new quantities related to the excitations of the system. The measurement of correlations between the disorder fingerprints allow to explore under a new perspective the non conventional order of this glassy state.Le verre de spin est une phase de la matière dans laquelle le désordre magnétique est gelé. Étant considéré comme un système modèle des verres en général, il a fait l'objet de nombreux travaux théoriques et expérimentaux. Les recherches ont convergé vers deux principales descriptions de l'état fondamental du système diamétralement opposées. D'une part, la solution " champ-moyen " nécessite une brisure de symétrie non triviale, et l'état fondamental est composé de multiples états organisés en une structure hiérarchique. D'autre part, une approche de " gouttelettes ", fondée sur la dynamique hors-équilibre d'un état fondamental unique. La validation expérimentale d'une de ces deux théories nécessite une observation détaillée de l'échantillon au niveau microscopique. La physique mésoscopique, basée sur les effets d'interférences électroniques, propose un outil unique pour accéder à cette configuration microscopique des impuretés: les fluctuations universelles de conductance. En effet, ces fluctuations représentent une empreinte unique du désordre dans l'échantillon. Ce travail présente la mise en œuvre de mesures de fluctuations de conductance universelles dans les verres de spin. Les effets d'interférences électroniques étant sensibles aux processus de décohérence du verre de spin, ils donnent accès expérimentalement à de nouvelles quantités concernant les excitations du système. La mesure des corrélations entre les empreintes du désordre permet quant à elle d'explorer sous un angle nouveau l'ordre non conventionnel de cet état vitreux

Transport quantique dans les verres de spin

Le verre de spin est une phase de la matière dans laquelle le désordre magnétique est gelé. Étant considéré comme un système modèle des verres en général, il a fait l'objet de nombreux travaux théoriques et expérimentaux. Les recherches ont convergé vers deux principales descriptions de l'état fondamental du système diamétralement opposées. D'une part, la solution champ-moyen nécessite une brisure de symétrie non triviale, et l'état fondamental est composé de multiples états organisés en une structure hiérarchique. D'autre part, une approche de gouttelettes , fondée sur la dynamique hors-équilibre d'un état fondamental unique. La validation expérimentale d'une de ces deux théories nécessite une observation détaillée de l'échantillon au niveau microscopique. La physique mésoscopique, basée sur les effets d'interférences électroniques, propose un outil unique pour accéder à cette configuration microscopique des impuretés: les fluctuations universelles de conductance. En effet, ces fluctuations représentent une empreinte unique du désordre dans l'échantillon. Ce travail présente la mise en œuvre de mesures de fluctuations de conductance universelles dans les verres de spin. Les effets d'interférences électroniques étant sensibles aux processus de décohérence du verre de spin, ils donnent accès expérimentalement à de nouvelles quantités concernant les excitations du système. La mesure des corrélations entre les empreintes du désordre permet quant à elle d'explorer sous un angle nouveau l'ordre non conventionnel de cet état vitreux.The spin glass is a state of matter in which the magnetic disorder is quenched. Being considered as a model system for glasses in general, it has been extensively studied, both theoretically and experimentally. The research have converged towards two main descriptions of the fundamental state of the system that are clearly antagonist. On the one hand, the mean-field solution has a non trivial broken symmetry, and the ground state is composed of multiple valleys in a hierarchical structure. On the other hand, a magnetic droplet model, based on the off-equilibrium dynamics of a unique ground state. The experimental validation of one of these two theories requires a detailed observation of the sample at the microscopic level. Mesoscopic physics, which deals with interference effects of the electrons, proposes a unique tool to access to this microscopic configuration of the impurities: the universal conductance fluctuations. Indeed, these fluctuations represent a unique fingerprint of the sample disorder. This work presents the implementation of universal conductance fluctuations measurements in spin glasses. The electron interference effects being sensitive to the decoherence processes of the spin glass, they give access experimentally to new quantities related to the excitations of the system. The measurement of correlations between the disorder fingerprints allow to explore under a new perspective the non conventional order of this glassy state.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

In vivo characterization of rodent cyclic myocardial perfusion variation at rest and during adenosine-induced stress using cine-ASL cardiovascular magnetic resonance

Assessment of cyclic myocardial blood flow (MBF) variations can be an interesting addition to the characterization of microvascular function and its alterations. To date, totally non-invasive in vivo methods with this capability are still lacking. As an original technique, a cine arterial spin labeling (ASL) cardiovascular magnetic resonance approach is demonstrated to be able to produce dynamic MBF maps across the cardiac cycle in rats

COPD beyond proximal bronchial obstruction: phenotyping and related tools at the bedside

International audienceChronic obstructive pulmonary disease (COPD) is characterised by nonreversible proximal bronchial obstruction leading to major respiratory disability. However, patient phenotypes better capture the heterogeneously reported complaints and symptoms of COPD. Recent studies provided evidence that classical bronchial obstruction does not properly reflect respiratory disability, and symptoms now form the new paradigm for assessment of disease severity and guidance of therapeutic strategies. The aim of this review was to explore pathways addressing COPD pathogenesis beyond proximal bronchial obstruction and to highlight innovative and promising tools for phenotyping and bedside assessment. Distal small airways imaging allows quantitative characterisation of emphysema and functional air trapping. Micro-computed tomography and parametric response mapping suggest small airways disease precedes emphysema destruction. Small airways can be assessed functionally using nitrogen washout, probing ventilation at conductive or acinar levels, and forced oscillation technique. These tests may better correlate with respiratory symptoms and may well capture bronchodilation effects beyond proximal obstruction. Knowledge of inflammation-based processes has not provided well-identified targets so far, and eosinophils probably play a minor role. Adaptative immunity or specific small airways secretory protein may provide new therapeutic targets. Pulmonary vasculature is involved in emphysema through capillary loss, microvascular lesions or hypoxia-induced remodelling, thereby impacting respiratory disability

Cardiovascular Magnetic Resonance of Myocardial Structure, Function, and Perfusion in Mouse and Rat Models

This review summarizes small-animal cardiovascular magnetic resonance (CMR) techniques that are being actively developed at present. Taking into account with few exceptions only literature of the past 2 years it shows that small-animal CMR has become an important and versatile analysis tool in many biomedical studies. The relatively complex signal formation and detection in magnetic resonance offers numerous ways of creating and modulating image contrast as a function of the specific needs. Although most new small-animal CMR developments are done within the scientific MR community, the MR manufacturers have readily contributed in making these techniques robust and available for routine application studies. Unlike other cardiovascular imaging techniques, CMR is used in many facets to assess morphology, global and regional function, blood flow, myocardial structure, cell damage, metabolism, and other molecular processes for studying mouse and rat models of human disease as well as general biochemical mechanisms in vivo