Weak Electron Localization Research Articles

Dimensional Cross-Over in Thin Film Spin-Glasses

Measurements of the electrical transport in the presence of a magnetic field for very thin spin-glass films of AuMn, AgMn and AuFe alloys, are reported. The influence of the effective dimensionality on the spin-glass order is studied by varying the film thickness between 4nm and 40nm. Weak electron localization and spin-glass resistivity measurements only provide a qualitative picture of the transition from spin-glass to Kondo behaviour in the thinnest films. Measuring the temperature dependence of the Hall resistivity on the other hand directly reflects the thickness dependence of the freezing temperature Tf. Our results clearly indicate that dimensionality effects as well as the reduced elastic mean free path influence the spin-glass transition. The experimental correlation length exponent v = 0.8 is smaller than the theoretically predicted value.

Boundary scattering and weak localization of electrons in a magnetic field.

The influence of boundary scattering on one- and two-dimensional weak localization is studied both analytically and by numerical simulation. Diffuse and specular boundary scattering are considered for two geometries, a metal film in a parallel magnetic field and a laterally restricted two-dimensional electron gas in a perpendicular field. The results, which extend the Al'tshuler-Aronov and Dugaev-Khmel'nitskii theories, are relevant to recent magnetoresistance experiments on high-mobility channels in GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As heterostructures.

Fabrication of one-dimensional GaAs wires by focused ion beam implantation

Novel, simple techniques were developed to fabricate very narrow GaAs conducting wires by utilizing direct focused ion beam (FIB) implantation. We employed two methods for fabrication of the wires. The first method makes use of high-resistivity (HR) regions formed by FIB implantation without successive annealing to define a very narrow conducting wire (HR method). In the second one, focused Si ions are line implanted into p-type GaAs, forming an n-type conducting wire in the p-type region. Then, reverse-bias is applied across the pn junction in order to make the wire narrower by expanding the depletion layer (PN method). This structure has an advantage that the thickness of the wire can be varied by bias voltage. Magnetoconductances measured in all the fabricated wires show evidences of weak electron localization and conductance fluctuations due to a quantum interference effect. The minimum widths of the wires are evaluated to be ∼20 nm (HR method) and ∼100 nm (PN method) by fitting the theory of one-dimensional weak localization to the experimental data. The phase coherent length of the electronic waves in the wires is also derived to be ∼120 nm at 1.3 K.

Boundary scattering modified one-dimensional weak localization in submicron GaAs/AlGaAs heterostructures

The influence of boundary scattering on the one-dimensional weak localization of electrons is studied experimentally, in a submicron width GaAs/AlGaAs heterostructure. The weak field magnetoresistance is measured at temperatures between 100 mK and 14 K. It is shown that the usual Al'tshuler-Aronov theory is inapplicable because of boundary scattering effects in the high mobility material. The observed effects can be accounted for by extensions of a recent theory of Dugaev and Khmel'nitskii. The analysis shows that scattering from the channel boundaries is predominantly specular, rather than diffuse.

Weak localization of electrons in a classical gas.

Weak localization of electrons in a classical gas.

Aharonov-Bohm effect and weak localization in cylindrical Ag films.

Magnetoresistance measurements have been performed on thin cylindrical Ag films (diameter \ensuremath{\approxeq}1.2 \ensuremath{\mu}m) between 1.3 and 4.2 K, in order to investigate the Aharonov-Bohm effect. The measurements were analyzed in terms of the recent theories of two-dimensional weak electron localization. The observed oscillations in the magnetoresistance are periodic in the magnetic flux quantum h/2e and in excellent agreement with the Altshuler-Aronov-Spivak theory. Fourier analysis of the data was used to study the oscillatory behavior in detail. From the dependence of the magnetoresistance on temperature and magnetic field, values of the phase-breaking time ${\ensuremath{\tau}}_{\ensuremath{\varphi}}$ and of the spin-orbit scattering time ${\ensuremath{\tau}}_{\mathrm{s}.\mathrm{o}.}$ were obtained.

Resonant Kondo scattering of weakly localized electrons.

Measurements of the magnetic scattering time in very thin Cu films (thickness $t\ensuremath{\simeq}5$ nm), homogeneously doped with magnetic Cr atoms (concentration \ensuremath{\lesssim}100 ppm), are reported. The weak electron localization causes an anomalous magnetoresistance which is very sensitive to the spin-flip scattering of the conduction electrons. The maximum in the temperature dependence of the spin-flip scattering rate is in excellent agreement with the Suhl-Nagaoka approximation for the resonant Kondo scattering in the vicinity of the Kondo temperature ${T}_{\mathrm{K}}\ensuremath{\simeq}2$ K.

Coherent Electron Scattering in Mesoscopic Metal Films

In mesoscopic metal films (size smaller than the inelastic diffusion length for the electrons), ensemble averaging is incomplete. Direct interference between the electron waves gives rise to a sample specific magnetoresistance. The magnetoresistance of a mesoscopic loop oscillates with flux-period h/e. In series arrays of mesoscopic loops, the h/e amplitude is inversely proportional to the square-root of the number of loops. This is in agreement with the stochastic "self-averaging" of the direct interference in larger samples. In long metal cylinders, impurity averaging is complete and only h/2e oscillations caused by the weak electron localization, are present.

Weak localization of photons: Universal fluctuations and ensemble averaging.

We have carried out an experimental study of weak localization of photons in a rigid, nonabsorbing, disordered medium. Unlike recent results reported for particles in a fluid, we observe large-amplitude fluctuations in the scattering intensity as a function of angle. Ensemble averaging of these sample-specific fluctuations decreases their intensity and uncovers a sharp peak in the backscattering direction. These results emphasize the common origin of weak localization of electrons, as a wave phenomenon, and of photons which are inherently wavelike.

Weak electron localization and electron-electron interaction effects in metallic glasses at temperatures just above superconducting transition

Electrical resistance and magnetoresistance (MR) of metallic glasses Zr x Be 1− x ( x = 0.6; 0.7) and Zr 75Rh 25 in the temperature range of 4.2–12 K and in the magnetic field of 7.4T have been studied. It has experimentally been shown that in accordance with the theory of weak localization of electrons (WEL) and electron-electron interaction (EEI) temperature dependences of conductivity and magnetic conductivity (MC) are determined by localization effects and EEI in the Cooper and Diffusion channels. The comparison of theoretical and experimental values of coefficients at temperature laws of the conductivity variation points out to the need to take into account the Maki-Thompson (MT) temperature correction in the absence of a magnetic field.

Study of the Magnetic Scattering Time by Electron Localization

We have measured the anomalous magnetoresistance of Cr doped Cu films in a perpendicular magnetic field. Using the theory of weak electron localization to describe this magnetoresistance, we obtain detailed information about the temperature dependence of the magnetic scattering caused by the local magnetic moments. The temperature dependence of the spin-flip scattering rate in the vicinity of the Kondo temperature is much stronger than predicted by theory.

Quantum oscillations in the superconducting fluctuation regime of cylindrical A1 films

Magnetoresistance measurements in 2-..mu..m-diam hollow Al cylinders are analyzed within the framework of two-dimensional weak electron localization theories. The observed resistance oscillations, due to the normal quantum interference of electrons enhanced by superconducting fluctuations, are well described by the combined theories of Altshuler and co-workers and Larkin. Values for the different electron interaction times are comparable to the values obtained in plane Al films.

One-dimensional effects in low-temperature conductivity of ultrathin metallic filaments

The resistance of ultrathin filaments (UF) of Hg (with diameters 20–40 Å) increases as the temperature is lowered as ΔR/R ∼ T −3 2 (T < 60 K) and decreases with the applied electrical field as ΔR/R ∼ 1 E . The magnitude 1113 1104 of this effects increases as the diameter of UF is made smaller. Such temperature dependence is interpreted in terms of weak electron localization in one-dimension. The non-linearity of current-voltage characteristics of UF is due to electron-electron interactions in one-dimensional tunnel junctions.

Influence of Spin-Orbit Coupling on Weak Localization

The spin-orbit coupling modifies the weak localization of the conduction electrons in thin films and reverses the magnetoconductance. This is quantitatively demonstrated in Mg films covered with submonolayers of Au. The effect provides a new and direct method for measuring the spin-orbit coupling.