Experimental Test of the Numerical Renormalization Group Theory for Inelastic Scattering from Magnetic Impurities

We present measurements of the phase coherence time \tauphi in quasi one-dimensional Au/Fe Kondo wires and compare the temperature dependence of \tauphi with a recent theory of inelastic scattering from magnetic impurities (Phys. Rev. Lett. 93, 107204 (2004)). A very good agreement is obtained for temperatures down to 0.2 $T_K$. Below the Kondo temperature $T_K$, the inverse of the phase coherence time varies linearly with temperature over almost one decade in temperature.Comment: 5 pages, 3 figure

Cohérence Quantique et Effet Kondo dans les Nanostructures

Quantum coherence effects lie at the heart of mesoscopic physics: they control the behaviour of conductors which size becomes comparable to the electron phase coherence length Lf.The experiments presented in this thesis deal with the effects of the phase coherence of electrons on the transport properties of metallic diffusive conductors. We have first concerned ourselves with electron dephasing in measuring, at very low temperature, the weak localisation magnetoresistance of quasi-1D gold wires containing iron magnetic impurities. The measured behaviour of the electron phase coherence time tf is well described in the framework of Kondo impurities, by the interplay between the single impurity Kondo effect and the RKKY impurity-impurity interactions. This result is an important contribution to the debate about the low temperature saturation of tf.Elsewhere, we have studied the quantum magnetoconductance oscillations, Altshuler-Aronov-Spivak (AAS) and Aharonov-Bohm (AB), in 2D networks of silver rings with different geometries. Notably, using a recent theory, we have extracted tf from the Fourier harmonics of the AAS oscillation. The measured temperature dependence, different from that extracted from a quasi-1D wire, suggests an effect of the topology on the dephasing. Finally, we have measured the size dependence of the amplitudes of both AB and AAS oscillations in networks of 10^6 to 10 loops: when one dimension of the network becomes smaller than Lf, the ensemble averaging of the quantum oscillations is non trivial, revealing that transport is dominated by subtle quantum interferences. This is a clear and convincing signature of the dimensional crossover leading to mesoscopic physics.Les effets de cohérence quantique sont au cœur de la physique mésoscopique : ils gouvernent le comportement des conducteurs dont la taille devient comparable à la longueur de cohérence de phase des électrons Lf.Les expériences présentées dans cette thèse concernent les effets de la cohérence de phase électronique sur les propriétés de transport de conducteurs métalliques diffusifs.Nous nous sommes d'abord intéressés aux mécanismes de déphasage électronique en mesurant la magnétorésistance de localisation faible de fils quasi-1D en or contenant des impuretés magnétiques de fer, à très basse température. Le comportement du temps de cohérence de phase électronique tf mesuré s'explique bien dans le cadre de la physique des impuretés Kondo, par la combinaison de l'effet Kondo à une impureté, et des interactions entre impuretés de type RKKY. Ce résultat est une contribution importante dans le débat sur la saturation de tf à très basse température.Ensuite, nous avons étudié les oscillations quantiques de magnétoconductance, Altshuler-Aronov-Spivak (AAS), et Aharonov-Bohm (AB), dans des réseaux 2D d'anneaux en argent présentant différentes géométries. Notamment, à partir d'une théorie récente, nous avons extrait tf à partir des harmoniques de Fourier de l'oscillation AAS. La dépendance en température mesurée, différente de celle extraite d'un fil quasi-1D, laisse supposer un effet de la topologie sur le déphasage. Enfin, nous avons mesuré la dépendance en taille des amplitudes des oscillations AB et AAS dans des réseaux de 10^6 à 10 anneaux : lorsqu'une dimension du réseau devient inférieure à Lf, la moyenne d'ensemble des oscillations quantiques est non triviale, révélant que des interférences quantiques subtiles dominent le transport. C'est une signature spectaculaire de la transition dimensionnelle vers la physique mésoscopique

Anomalous temperature dependence of the dephasing time in mesoscopic Kondo wires

4 pages, 5 figures.We present measurements of the magnetoconductance of long and narrow quasi one-dimensional gold wires containing magnetic iron impurities in a temperature range extending from $15$mK to $4.2$K. The dephasing rate extracted from the weak antilocalisation shows a pronounced plateau in a temperature region of $300$mK - $800$mK, associated with the phase breaking due to the Kondo effect. Below the Kondo temperature the dephasing rate decreases linearly with temperature, in contradiction with standard Fermi-liquid theory. Our data suggest that the formation of a spin glass due to the interactions between the magnetic moments are responsible for the observed anomalous temperature dependence

Dimensional crossover in quantum networks: from macroscopic to mesoscopic Physics

4 pagesWe report on magnetoconductance measurements of metallic networks of various sizes ranging from 10 to $10^{6}$ plaquettes, with anisotropic aspect ratio. Both Altshuler-Aronov-Spivak (AAS) $h/2e$ periodic oscillations and Aharonov-Bohm (AB) $h/e$ 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 $L_\phi$, 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

Quantum Hall effect in polycrystalline CVD graphene: grain boundaries impact

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Quantum Hall effect in polycrystalline CVD graphene: grain boundaries impact

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Unusual backscattering between quantum Hall edge states in CVD graphene

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