Injection and migration of charges in the oxide of a metal-oxide-semiconductor structure

Injection of charges in oxide of metal oxide semiconductor structur

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

A few-electron quadruple quantum dot in a closed loop

We report the realization of a quadruple quantum dot device in a square-like configuration where a single electron can be transferred on a closed path free of other electrons. By studying the stability diagrams of this system, we demonstrate that we are able to reach the few-electron regime and to control the electronic population of each quantum dot with gate voltages. This allows us to control the transfer of a single electron on a closed path inside the quadruple dot system. This work opens the route towards electron spin manipulation using spin-orbit interaction by moving an electron on complex paths free of electron

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)

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

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

Shot noise in coupled dots and the "fractional charges"

We consider the problem of shot noise in resonant tunneling through double quantum dots in the case of interacting particles. Using a many-body quantum mechanical description we evaluate the energy dependent transmission probability, the total average current and the shot noise spectrum. Our results show that the obtained reduction of the noise spectrum, due to Coulomb interaction, can be interpret in terms of non--interacting particles with fractional charge like behavior.Comment: some clarifications added, to appear in Phys. Lett.

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