Anomalous dephasing of two-dimensional electrons in an AlGaAs/GaAs parabolic quantum well structure

Magnetoconductivity measurements are performed on a parabolic quantum well structure. The weak localization effect is observed at a low magnetic field for both single-subband and double-subband occupation regimes. Applying weak-localization theory, we have extracted the dephasing rate. The extracted dephasing rate increases with increasing conductivity in the small-energy-transfer regime and shows a similar trend as the electron density is increased in the large-energy-transfer regime. This is in conflict with Fermi-liquid theory, and cannot be attributed to electron–phonon scattering.     

Nature of coherent enhancement of inelastic electron scattering from disordered media

The mechanism of weak localization of relatively fast electrons scattered with a fixed energy loss from disordered media is examined. The main focus of this paper is to put forward an explanation for why coherent enhancement of electron scattering in the inelastic-scattering channel takes place at angles which differ from π. A simplified kinematic model is proposed to determine the basic properties of the weak localization of electrons in the inelastic scattering channel. The model easily reproduces the range of scattering angles typical for the weak localization of electrons with fixed energy loss. The procedure does not require calculation of the contribution from the crossed diagrams. The results agree with those based on the dynamical theory associated with the calculation of the crossed and ladder diagrams. It is possible to follow the transition from the weak localization of the new type to the ordinary weak localization with decreasing energy loss. The weak localization of the new type is in accord with the weak localization of regular type if the energy loss is about the energy of an optical phonon.     

Deterministic weak localization in periodic structures

In some perfect periodic structures classical motion exhibits deterministic diffusion. For such systems we present the weak localization theory. As a manifestation for the velocity autocorrelation function a universal power law decay is predicted to appear at four Ehrenfest times. This deterministic weak localization is robust against weak quenched disorders, which may be confirmed by coherent backscattering measurements of periodic photonic crystals.     

Electronic properties of grains and grain boundaries in graphene grown by chemical vapor deposition

We synthesize hexagonal shaped single-crystal graphene, with edges parallel to the zig-zag orientations, by ambient pressure CVD on polycrystalline Cu foils. We measure the electronic properties of such grains as well as of individual graphene grain boundaries, formed when two grains merged during the growth. The grain boundaries are visualized using Raman mapping of the D band intensity, and we show that individual boundaries between coalesced grains impede electrical transport in graphene and induce prominent weak localization, indicative of intervalley scattering in graphene.     

On the theory of quantum interference between inelastic and elastic electron scattering events

The mechanism of weak localization of relatively fast electrons scattered with a fixed energy loss from disordered media is examined. The main focus of this paper is to put forward an explanation why coherent enhancement of electron scattering in the inelastic-scattering channel takes place at angles which differ from π. A simplified kinematic model is proposed to determine the basic properties of the weak localization of electrons in the inelastic scattering channel. The model reproduces easily the range of scattering angles typical of the weak localization of electrons with a fixed energy loss. The procedure does not require calculation of the contribution from the crossed diagrams. The results agree with those based on the dynamical theory associated with the calculation of the crossed and ladder diagrams. It is possible to follow the transition from the new type of weak localization to the ordinary weak localization with decreasing energy loss. The new-type weak localization is in agreement with the regular weak localization if the energy loss is approximately equal to the energy of an optical phonon. Zh. éksp. Teor. Fiz. 111, 1001–1015 (March 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Anomalous magnetoresistance of two-dimensional systems in the presence of spin-orbit scattering

A theory of weak localization in two-dimensional semiconductor structures and metal films is developed for spin relaxation by the Elliott-Yafet mechanism. The theory is valid in the entire range of classically weak magnetic fields. It is shown that effects due to spin-orbit interaction substantially modify magnetoresistance in both diffusive and ballistic regimes.     

Ehrenfest-time dependence of weak localization in open quantum dots

Semiclassical theory predicts that the weak localization correction to the conductance of a ballistic chaotic cavity is suppressed if the Ehrenfest time exceeds the dwell time in the cavity [I. L. Aleiner and A. I. Larkin, Phys. Rev. B 54, 14423 (1996)]. We report numerical simulations of weak localization in the open quantum kicked rotator that confirm this prediction. Our results disagree with the "effective random matrix theory" of transport through ballistic chaotic cavities.     

Role of orbital dynamics in spin relaxation and weak antilocalization in quantum dots

We develop a semiclassical theory for spin-dependent quantum transport to describe weak (anti)localization in quantum dots with spin-orbit coupling. This allows us to distinguish different types of spin relaxation in systems with chaotic, regular, and diffusive orbital classical dynamics. We find, in particular, that for typical Rashba spin-orbit coupling strengths, integrable ballistic systems can exhibit weak localization, while corresponding chaotic systems show weak antilocalization. We further calculate the magnetoconductance and analyze how the weak antilocalization is suppressed with decreasing quantum dot size and increasing additional in-plane magnetic field.     

Nonuniversality of anderson localization in short-range correlated disorder

We provide an analytic theory of Anderson localization on a lattice with a weak short-range correlated disordered potential. Contrary to the general belief, we demonstrate that already next-neighbor statistical correlations in the potential can give rise to strong anomalies in the localization length and the density of states, and to the complete violation of single-parameter scaling. Such anomalies originate in additional symmetries of the lattice model in the limit of weak disorder. The results of numerical simulations are in full agreement with our theory, with no adjustable parameters.     

Anisotropic magnetoresistance in thin inhomogeneous gold films

We investigate the magnetoresistance (MR) of thininhomogeneous gold films applying the magnetic field perpendicular as well as parallel to the film plane. The MR-data show a strong anisotropy which can be well explained within the theory of weak electron localization (WEL) in 2d for both field orientations. The important results is that fitting the MR-data for both orientations by the corresponding theoretical expressions we obtain nearly identical values for the phase coherence lengthsL and its temperature dependence. This confirms (i) the correctness of the fitting process and (ii) the validity of the theory. From this we conclude that WEL in 2d can also be used to describe the MR ofinhomogeneous films.     

Weak localization: The quasiclassical theory of electrons in a random potential

The recent subject of weak localization is an important area of research in condensed matter physics which has received a considerable amount of experimental and theoretical attention. In its existing form, the theory is founded entirely on the impurity technique of Green's function. In this paper, we present a theory of weak localization which is put rigorously on a quasiclassical basis thus providing a more intuitive understanding. For example, it becomes apparent that much of the underlying physics is a result of quantum mechanical interference effects in a very elementary sense. It is one of those unique cases where the superposition principle of quantum mechanics leads to observable consequences in the properties of macroscopic systems. In addition, all the important quantitative results obtained so far are recovered in the present quasiclassical approach.     

Weak localization of light in superdiffusive random systems

Lévy flights constitute a broad class of random walks that occur in many fields of research, from biology to economy and geophysics. The recent advent of Lévy glasses allows us to study Lévy flights-and the resultant superdiffusion-using light waves. This raises several questions about the influence of interference on superdiffusive transport. Superdiffusive structures have the extraordinary property that all points are connected via direct jumps, which is expected to have a strong impact on interference effects such as weak and strong localization. Here we report on the experimental observation of weak localization in Lévy glasses and compare our results with a recently developed theory for multiple scattering in superdiffusive media. Experimental results are in good agreement with theory and allow us to unveil the light propagation inside a finite-size superdiffusive system.     

Hamiltonian map approach to 1D Anderson model

A one-dimensional diagonal tight binding electronic system is analyzed with the Hamiltonian map approach to study analytically the inverse localization length of an infinite sample. Both the uncorrelated and the dichotomic correlated random potential sequences are considered in the evaluations of the inverse localization length. Analytical expressions for the invariant measure or the angle density distribution are the main motivation of this work in order to derive analytical results. The well-known uncorrelated weak disorder result of the inverse localization length is derived with a clear procedure. In addition, an analytical expression for high disorder is obtained near the band edge. It is found that the inverse localization length goes to 1 in this limit. Following the procedure used in the uncorrelated situation, an analytical expression for the inverse localization length is also obtained for the dichotomic correlated sequence in the small disorder situation.     

Monte Carlo simulation of the coherent backscattering of electrons in a ballistic system

We study weak localization effects in the ballistic regime as induced by man-made scatterers. Specular reflection of the electrons off these scatterers causes backscattered trajectories to occur, which interfere with their time-reversed path resulting in weak localization corrections to the resistance. Using a semi-classical theory, we calculate the change in resistance due to these backscattered trajectories. We found that the inclusion of the exact shape of the scatterers is very important in order to explain the experimental results of Katine et al.[Superlattices and Microstructures 20 , 337 (1996)].     

Propagation and localization of polaritons in disordered organic microcavities

The effect of disorder on the polaritonic states in organic microcavities utilizing J aggregates of cyanine dyes is examined. The comparison between the elastic mean free path, the phase breaking length and the wavelength of polaritons shows that, by varying two control parameters, one can achieve different regimes of cavity polariton propagation and localization (including weak and strong localization) in one sample at room temperature. We analyze the role of different parameters of the sample in the possibility of realization of each regime.     

Localization and interaction effects in CdTeBi superlattices

The transport properties of CdTeBi superlattice have been measured as a function of the modulation wavelength. A logarithmic dependence with temperature was observed for the resistance at low temperature. The resistivity varied from 0.1 to 3.9 mΩcm at 1.7 K as the modulation wavelength varied from 177 to 81 A. A logarithmic behavior with H was observed for the transverse (longitudinal) magnetoresistance for low (high) fields. Samples were studied at temperatures down to 1.7 K and in magnetic fields up to 5 Tesla. No negative magnetoresistance as predicted by weak localization theory was observed. Hall coefficient measurements showed a logarithmic dependence with temperature which is indicative of Coulomb correlation effects (interaction theory). All measurements indicate that the transport behavior of CdTeBi superlattices is dominated by spin-orbit and/or interaction effects while weak localization plays a minimal role.     

Prediction of a bcc-hcp phase transition for Sn: A first-principles study

The high-pressure structural transformation of elemental Sn is studied using an ab initio density functional theory implementation of the metadynamics method that predicts with sufficient compression, Sn will transform from the bcc structure into an hcp structure. The low-free-energy pathway associated with this phase transition is characterized as the Burgers transition mechanism. The superconducting properties of Sn under pressure are also investigated. Both bcc and hcp structures of Sn exhibit very weak electron-phonon coupling and therefore would not sustain superconductivity at high pressure.     

Spin-orbit coupling,antilocalization, and parallel magnetic fields in quantum dots

We investigate antilocalization due to spin-orbit coupling in ballistic GaAs quantum dots. Antilocalization that is prominent in large dots is suppressed in small dots, as anticipated theoretically. Parallel magnetic fields suppress both antilocalization and also, at larger fields, weak localization, consistent with random matrix theory results once orbital coupling of the parallel field is included. In situ control of spin-orbit coupling in dots is demonstrated as a gate-controlled crossover from weak localization to antilocalization.     

Non-linear response and electron-electron interactions in mesoscopic metal rings

A time-dependent electric field gives rise to a stationary non-equilibrium current I ⁽²⁾ around a mesoscopic metal ring threaded by a magnetic flux. We show that this current, which is proportional to the intensity of the field, is closely related to the exchange part of the interaction contribution to the equilibrium persistent current, and that the corresponding non-linear conductivity directly measures the weak localization correction to the polarization. We explicitly calculate the disorder average of I ⁽²⁾ in the diffusive regime as function of the frequency of the electric field and the static flux piercing the ring, and suggest an experiment to test our theory. Received: 5 September 1997 / Accepted: 4 November 1997