Electron Coherence in Mesoscopic Kondo Wires

We present measurements of the magnetoresistance of long and narrow quasi one-dimensional gold wires containing magnetic iron impurities. The electron phase coherence time 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 phase coherence time increases, as expected in the framework of Kondo physics. At much lower temperatures, the phase coherence time saturates again, in contradiction with standard Fermi liquid theory. In the same temperature regime, the resistivity curve displays a characteristic maximum at zero magnetic field, associated with the formation of a spin glass state. We argue that the interactions between the magnetic moments are responsible for the low temperature saturation of the phase coherence time.Comment: To appear in Advances in Solid State Physics, Vol 43, edited by B. Kramer (Springer Verlag, Berlin 2003

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

Conductance and persistent current in quasi-one-dimensional systems with grain boundaries: Effects of the strongly reflecting and columnar grains

We study mesoscopic transport in the Q1D wires and rings made of a 2D conductor of width W and length L >> W. Our aim is to compare an impurity-free conductor with grain boundaries with a grain-free conductor with impurity disorder. A single grain boundary is modeled as a set of the 2D-$\delta$-function-like barriers positioned equidistantly on a straight line and disorder is emulated by a large number of such straight lines, intersecting the conductor with random orientation in random positions. The impurity disorder is modeled by the 2D $\delta$-barriers with the randomly chosen positions and signs. The electron transmission through the wires is calculated by the scattering-matrix method, and the Landauer conductance is obtained. We calculate the persistent current in the rings threaded by magnetic flux: We incorporate into the scattering-matrix method the flux-dependent cyclic boundary conditions and we introduce a trick allowing to study the persistent currents in rings of almost realistic size. We mainly focus on the numerical results for L much larger than the electron mean-free path, when the transport is diffusive. If the grain boundaries are weakly reflecting, the systems with grain boundaries show the same (mean) conductance and the same (typical) persistent current as the systems with impurities, and the results also agree with the single-particle theories treating disorder as a white-noise-like potential. If the grain boundaries are strongly reflecting, the typical persistent currents can be about three times larger than the results of the white-noise-based theory, thus resembling the experimental results of Jariwala et al. (PRL 2001). We extend our study to the 3D conductors with columnar grains. We find that the persistent current exceeds the white-noise-based result by another one order of magnitude, similarly as in the experiment of Chandrasekhar et al. (PRL 1991)

Quantum and Boltzmann transport in the quasi-one-dimensional wire with rough edges

We study quantum transport in Q1D wires made of a 2D conductor of width W and length L>>W. Our aim is to compare an impurity-free wire with rough edges with a smooth wire with impurity disorder. We calculate the electron transmission through the wires by the scattering-matrix method, and we find the Landauer conductance for a large ensemble of disordered wires. We study the impurity-free wire whose edges have a roughness correlation length comparable with the Fermi wave length. The mean resistance and inverse mean conductance 1/ are evaluated in dependence on L. For L -> 0 we observe the quasi-ballistic dependence 1/ = = 1/N_c + \rho_{qb} L/W, where 1/N_c is the fundamental contact resistance and \rho_{qb} is the quasi-ballistic resistivity. As L increases, we observe crossover to the diffusive dependence 1/ = = 1/N^{eff}_c + \rho_{dif} L/W, where \rho_{dif} is the resistivity and 1/N^{eff}_c is the effective contact resistance corresponding to the N^{eff}_c open channels. We find the universal results \rho_{qb}/\rho_{dif} = 0.6N_c and N^{eff}_c = 6 for N_c >> 1. As L exceeds the localization length \xi, the resistance shows onset of localization while the conductance shows the diffusive dependence 1/ = 1/N^{eff}_c + \rho_{dif} L/W up to L = 2\xi and the localization for L > 2\xi only. On the contrary, for the impurity disorder we find a standard diffusive behavior, namely 1/ = = 1/N_c + \rho_{dif} L/W for L < \xi. We also derive the wire conductivity from the semiclassical Boltzmann equation, and we compare the semiclassical electron mean-free path with the mean free path obtained from the quantum resistivity \rho_{dif}. They coincide for the impurity disorder, however, for the edge roughness they strongly differ, i.e., the diffusive transport is not semiclassical. It becomes semiclassical for the edge roughness with large correlation length

Cross-Over between universality classes in a magnetically disordered metallic wire

In this article we present numerical results of conduction in a disordered quasi-1D wire in the possible presence of magnetic impurities. Our analysis leads us to the study of universal properties in different conduction regimes such as the localized and metallic ones. In particular, we analyse the cross-over between universality classes occurring when the strength of magnetic disorder is increased. For this purpose, we use a numerical Landauer approach, and derive the scattering matrix of the wire from electron's Green's function.Comment: Final version, accepted for publication in New Journ. of Physics, 27 pages, 28 figures. Replaces the earlier shorter preprint arXiv:0910.427

Kondo decoherence: finding the right spin model for iron impurities in gold and silver

We exploit the decoherence of electrons due to magnetic impurities, studied via weak localization, to resolve a longstanding question concerning the classic Kondo systems of Fe impurities in the noble metals gold and silver: which Kondo-type model yields a realistic description of the relevant multiple bands, spin and orbital degrees of freedom? Previous studies suggest a fully screened spin $S$ Kondo model, but the value of $S$ remained ambiguous. We perform density functional theory calculations that suggest $S = 3/2$. We also compare previous and new measurements of both the resistivity and decoherence rate in quasi 1-dimensional wires to numerical renormalization group predictions for $S=1/2,1$ and 3/2, finding excellent agreement for $S=3/2$.Comment: 4 pages, 4 figures, shortened for PR

A microstructural study of superconductive nanocrystalline diamond

A transmission electron microscopy (TEM) study of superconducting nanocrystalline diamond (NCD) continuous layers is reported. The high resolution transmission electron microscopy (HREM) and the diffraction contrast modes of observations are used to reveal the nanograins configuration. Three types of them are observed: first, close to the interface with the Si/SiO2 substrate, 10 to 20 nm-sized diamond 16 seeds resulting from the 5nm size diamond powder deposition before growth that show some regrowth during CVD process, second a diamond overgrown layer, quasi-epitaxially by coalesced columnar NCD grains, and finally, up to the free surface, a thin disordered region composed of nanocrystallites smaller than 6 nm. This last layer was not nominally expected and is attributed to a renucleated-like (RND) diamond layer embedding ultra nanocrystalline grains. Diffraction contrast observations confirm this HREM observed behaviour.6 page

Phase diagram of boron-doped diamond revisited by thickness-dependent transport studies

International audienceWe report on a detailed study of the electronic properties of a series of boron-doped diamond epilayers with dopant concentrations ranging from 1.10^ 20 to 3.10^21 cm −3 and thicknesses (d ⊥) ranging from 2 µm to 8 nm. By using well-defined mesa patterns that minimize the parasitic currents induced by doping inhomogeneities, we have been able to unveil a new phase diagram differing from all previous reports. We show that the onset of superconductivity does actually not coincide with the metal-insulator transition in this system. Moreover a dimensional crossover from 3D to 2D transport properties could be induced by reducing d ⊥ in both the metallic non-superconducting and superconducting epilayers, without any reduction of Tc with d ⊥ in the latter

Transport through side-coupled double quantum dots: from weak to strong interdot coupling

We report low-temperature transport measurements through a double quantum dot device in a configuration where one of the quantum dots is coupled directly to the source and drain electrodes, and a second (side-coupled) quantum dot interacts electrostatically and via tunneling to the first one. As the interdot coupling increases, a crossover from weak to strong interdot tunneling is observed in the charge stability diagrams that present a complex pattern with mergings and apparent crossings of Coulomb blockade peaks. While the weak coupling regime can be understood by considering a single level on each dot, in the intermediate and strong coupling regimes, the multi-level nature of the quantum dots needs to be taken into account. Surprisingly, both in the strong and weak coupling regimes, the double quantum dot states are mainly localized on each dot for most values of the parameters. Only in an intermediate coupling regime the device presents a single dot-like molecular behavior as the molecular wavefunctions weight is evenly distributed between the quantum dots. At temperatures larger than the interdot coupling energy scale, a loss of coherence of the molecular states is observed.Comment: 9 pages, 5 figure