Magnetoconductivity and Quantum Interaction Mechanisms in 2D SiGe Heterostructures

Low temperature longitudinal magnetoresistance measurement in pulsed laser deposited undoped ZnO film has been investigated. The temperature and magnetic field dependence of the resistivity of the film is in good agreement with the theory of weak localization and electron-electron interactions in a disordered system. Negative magnetoresistance is a consequence of the suppression of quantum interference effect in the weak localization theory. The inelastic scattering time τi ~ T −3/2 and phase coherence length Lϕ(T) ~ T −3/4 indicates that the electron-electron interaction is the dominant inelastic scattering process.

Magnetoconductivity (MC) and zero field conductivity have been measured at low temperatures in metallic and barely insulating AlPdRe quasicrystalline films. For a metallic film, both the magnetoconductivity and conductivity can be fitted well using the 3D weak localization (WL) and electron-electron interaction (EEI) theories. The AlPdRe films exhibit very strong spin-orbit scattering with . Therefore, the inelastic scattering times can be directly extracted from the low field magnetoconductivity data taken at various temperatures, yielding . Using the fitting parameters extracted from the MC data, the zero field conductivity as a function of temperature can be described nicely using the WL and EEI theories. MC data and conductivity data obtained on barely insulating films below the metal-insulator transition cannot be explained in the framework of the WL theory.

The effects of perpendicular magnetic fieldsH on the temperature dependence of the resistanceR(T) (T 1 kOhm) indium films prepared by simultaneous condensation of the metal and hydrogen onto a substrate cooled to 5 K. AtT«T c the resistance slightly decreased with rising temperature, in agreement with the weak localization and electron-electron interaction (EEI) theories. Processing of theR(T, H) curves in terms of the theories permitted the electron phase relaxation time τϕ(T) to be determined and the time τ so of electron spin relaxation due to spin-orbital interaction (SOI) to be estimated. In the temperature rangeT−T c ≫ħ/kτϕ the effect of the fluctuation EEI on conductivity was investigated. The contribution of the fluctuation EEI is shown to be given by a sum of two corrections: the Cooper and the Maki-Thompson ones. In the rangeT−T c ≪ħ/kτϕ the influence of magnetic field was investigated on the transition from two-to zerodimensional fluctuations of the order parameter resulting from temperature increase in the rangeT>T c . Increasing field was found to cause enlargement of the zero-dimensional fluctuation critical region and lowering of the fluctuation dimensionality alteration temperature, leading eventually to complete two-dimensional fluctuation suppression. The experimental dependencesT c (H) experienced, in addition to orbital effects of the magnetic field, depairing effect of the field on electron spins. Thus the τ so magnitude could also be estimated and proved roughly equal to τ so determined from the analysis ofR(T, H)

Theory of weak localization in two-dimensional high-mobility semiconductor systems is developed with allowance for the spin–orbit interaction. The obtained expressions for anomalous magnetoresistance are valid in the whole range of classically weak magnetic fields and for arbitrary strengths of bulk and structural inversion asymmetry contributions to the spin splitting. The theory serves for both diffusive and ballistic regimes of electron propagation taking into account coherent backscattering and nonbackscattering processes. The transition between weak localization and antilocalization regimes is analyzed. The manifestation of the mutual compensation of Rashba and Dresselhaus spin splittings in magnetoresistance is discussed. A perfect description of experimental data on anomalous magnetoresistance in high-mobility heterostructures is demonstrated. The in-plane magnetic-field dependence of the conductivity caused by an interplay of the spin–orbit splittings and Zeeman effect is described theoretically.

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.

Weak localization and weak anti-localization are quantum interference effects in quantum transport in a disordered electron system. Weak anti-localization enhances the conductivity and weak localization suppresses the conductivity with decreasing temperature at very low temperatures. A magnetic field can destroy the quantum interference effect, giving rise to a cusp-like positive and negative magnetoconductivity as the signatures of weak localization and weak anti-localization, respectively. These effects have been widely observed in topological insulators. In this article, we review recent progresses in both theory and experiment of weak (anti-)localization in topological insulators, where the quasiparticles are described as Dirac fermions. We predicted a crossover from weak anti-localization to weak localization if the massless Dirac fermions (such as the surface states of topological insulator) acquire a Dirac mass, which was confirmed experimentally. The bulk states in a topological insulator thin film can exhibit the weak localization effect, quite different from other system with strong spin-orbit interaction. We compare the localization behaviors of Dirac fermions with conventional electron systems in the presence of disorders of different symmetries. Finally, we show that both the interaction and quantum interference are required to account for the experimentally observed temperature and magnetic field dependence of the conductivity at low temperatures.

The electrical transport properties in sample 1 of impurity concentration n=xx of the 70Ge: Ga system are studied in the absence of a magnetic field and at low temperature in the range 0.53 to 0.017 K. It is noted that the electrical conductivity of sample 1 exhibits a metallic behavior. We found that the exponent S is equal to 0.5 (σ=σ(T=0)+mTs). This result is in agreement with the theory of weak localization (WL) at 3D and the theory of electronelectron interactions (EEI). We also found that sample 1 is located near the metal-insulator transition (MIT) of the metallic side.

Theory of weak localization is developed for electrons in semiconductor quantum wells grown along [110] and [111] crystallographic axes. Anomalous conductivity correction caused by weak localization is calculated for symmetrically doped quantum wells. The theory is valid for both ballistic and diffusion regimes of weak localization in the whole range of classically weak magnetic fields. We demonstrate that in the presence of bulk inversion asymmetry the magnetoresistance is negative: The linear in the electron momentum spin-orbit interaction has no effect on the conductivity while the cubic in momentum coupling suppresses weak localization without a change of the correction sign. Random positions of impurities in the doping layers in symmetrically doped quantum wells produce electric fields which result in position-dependent Rashba coupling. This random spin-orbit interaction leads to spin relaxation which changes the sign of the anomalous magnetoconductivity. The obtained expressions allow determination of electron spin relaxation times in (110) and (111) quantum wells from transport measurements.

Theory of weak localization is developed for two-dimensional holes in the presence of in-plane magnetic field. The Zeeman splitting even in the hole momentum results in the spin-dependent phase changing the quantum interference. The negative correction to the conductivity is shown to decrease by a factor of two by the in-plane magnetic field. The positive magnetoconductivity in a classically weak perpendicular field caused by the weak localization is calculated for both quadratic and quartic in momentum Zeeman hole splittings. Calculations show that the conductivity corrections are very close to each other in these two cases of low and high hole density.

Weak localization and electron–electron interaction in disordered V 80Al 20− xFe x alloys at low temperature

The binary icosahedral Zr80Pt20 system has been synthesized during the crystallization of an initially amorphous alloy fabricated by melt quenching on the surface of a rotating copper wheel. The temperature and field dependences of the electrical resistivity and magnetoresistivity of the icosahedral and amorphous phases are studied and compared in a temperature range of 1.5–300 K and magnetic fields up to 8 T. Superconductivity has been detected for the first time in the icosahedral and amorphous phases of the Zr80Pt20 system. For both phases, the magnetoresistivity is positive and depends anomalously on the magnetic field. The anomalous behavior of magnetoresistivity is satisfactorily described by the theory of weak localization and electron-electron interaction in three-dimensional disordered systems, which takes into account electron scattering by superconducting fluctuations. The absolute values and temperature dependences of the electron-electron interaction constant and the times of inelastic scattering of conduction electrons are estimated for the icosahedral and amorphous phases of this binary system.

Electrical conductivity and Hall effect of AlCuRu and AlCuFe quasi-crystals

High-field magnetoresistance experiments performed on thick amorphous alloys offer a simple way to study three-dimensional (3D) weak localization of conduction electrons. After the precursor work of Fert et al. on nonmagnetic amorphous alloys, we found it interesting to study how these effects would disappear under substitutions of magnetic impurities (1% to 10% of Dy) in a nonmagnetic amorphous alloy (YNi). The experiments, performed between 1.5 to 50 K and in magnetic fields up to 20 T, showed (i) in YNi, characteristic features of the magnetoresistance due to weak localization under strong spin-orbit scattering and (ii) in DyxY1−xNi, a coexistence of weak localization effects with the classical contribution of spin alignment by the applied magnetic field, saturating at negative values. This last contribution dominates the behavior of Dy-richer samples whereas weak localization is clearly observed for x≤3%. In all the samples a dramatic increase of the initial magnetoresistance slope Δρ/ρH2 (where ρ=resistivity and H=applied field) is observed when magnetic impurity concentrations increase. We explain this increase of weak localization effects, in the framework of the available weak localization theory, by an enhancement of the Zeeman spin splitting due to interactions between localized (4f ) and delocalized (d, s) electronic states.

Weak localization and electron–electron scattering in fluorine-doped SnO2 random nanobelt thin films

The interaction between the itinerant carriers, lattice dynamics, and defects is a problem of long-standing fundamental interest for developing quantum theory of transport. Here, we study this interaction in the compositionally and strain-graded AlGaN heterostructures grown on AlN substrates. The results provide direct evidence that a dimensional crossover (2D–3D) occurs with increasing temperature as the dephasing scattering events reduce the coherence length. These heterostructures show a robust polarization-induced 3D electron gas and a metallic-like behavior down to liquid helium temperature. Using magnetoresistance measurements, we analyze the evolution of the interaction between charge carriers, lattice dynamics, and defects as a function of temperature. A negative longitudinal magnetoresistance emerges at low temperatures, in line with the theory of weak localization. A weak localization fit to near zero-field magneto-conductance indicates a coherence length that is larger than the elastic mean free path and film thickness (lφ>t>lel), suggesting a 2D weak localization in a three-dimensional electron gas. Our observations allow for a clear and detailed picture of two distinct localization mechanisms that affect carrier transport at low temperature.