Search

We report highly anisotropic appearance of the quantum Hall effect (QHE) in epitaxial single-layer graphene grown on a vicinal SiC(0001) substrate. Well-developed QHE with zero resistance manifests itself for the current along the steps, whereas the QHE is obscured by pronounced positive magnetoresistance with quadratic magnetic-field dependence for the current across the steps. The latter, as well as the small slope of the Hall resistance, implies the presence of parallel conduction due to remnant carriers in the SiC substrate, albeit with seeming inconsistency with the zero resistance observed for the former current direction. We interpret the anisotropic behavior by assuming that the parallel conduction is sizable along the steps but is virtually prohibited across the steps.

The temperature-dependent electronic transport on the superconducting and insulating sides of the superconductor-insulator transition in disordered quasi-two-dimensional textured FeSe thin films is reported. The transition is driven by a perpendicular magnetic field applied to a film with its thickness close to the critical thickness of the thickness-, i.e., disorder-induced transition. The resistance in the superconducting phase might be dominated by thermally assisted flux flow, and in the phase diagram a metallic phase might intervene between the superconducting and insulating state at very low temperatures. In the insulating phase, weak insulating behavior is observed that can be described by weak localization theory of bosons, thus supporting the bosonic description of the superconductor-insulator transition in FeSe thin films.

The magnetoresistance (MR) of a two-dimensional hole gas in a quantum well of compressively strained Si0.05Ge0.95 has been investigated as a function of temperature. The MR shows a maximum at intermediate magnetic fields between the regions of weak localization and the Shubnikov-de Haas oscillations, which is discussed in terms of a recent theoretical study of the electron-electron interaction effect by Sedrakyan and Raikh (SR). The magnetic field MR dependence is clearly observed to cross over from quadratic to linear at T=7.8 K and B≈0.3 T. It is shown that the SR theory provides a good description of both the measured quadratic and positive linear MR, but over estimates the field position of the MR maximum and does not account for the shift in position with temperature that is observed. Earlier theories of electron-electron interaction (by Altshuler and Aronov, Gornyi and Mirlin) show a better agreement with the experimentally observed behavior of the MR maximum, but fit the low field MR less accurately.

We focus in this article on high field magnetoconductance results obtained on SnO2 polycrystalline (cassiterite) films. The high field magnetoconductance is found to be in agreement with the magnetoconductance universal behavior due to weak localization in two dimensions as suggested by Zduniak et al. (Phys. Rev. B 56: 1996, 1997).

Electrical resistivities of two icosahedral (I) Al-Pd-Re alloys have been measured between room temperature and mK temperatures. One quasicrystalline (QC) polygrain Al-Pd-Re sample exhibited insulating behavior in its resistivities, increasing by a factor of r=R(4 K)/R(300 K)=7.76; its room temperature resistivity was 9,890 μΩ cm. A “phenomenological” expression fitted the conductivity data well between 300 K to 0.5 K. Below 0.4 K a crossover to an activated variable-range hopping law was observed. Low temperature magnetoresistance ratio data and fits using the wave function shrinkage theory are presented. A second QC Al-Pd-Re sample had a small resistance temperature ratio r=2.12. The room temperature resistivity was extremely large, ρ(300 K)≈40,980 μΩ cm. Its conductivity could be described well using a simple temperature power law between 300 K to 20 K. Below 20 K there was a crossover to a new behavior. Below 1 K, the conductivity could be fitted using a very weakly insulating power law where σ(T)≈11.37T 0.032 in (Ω cm)−1, suggesting that this sample is located just below the metal-insulator transition. The magnetoconductivity data could not be fitted successfully using the 3D weak localization (WL) theory and inserting into it physical and realistic fitting magnitudes for the inelastic magnetic field B in.

Electrical resistivities of several quasicrystalline (QC) icosahedral Al–Cu–Ru alloys have been measured between room temperature and mK temperatures. None of the Al–Cu–Ru samples exhibited “insulating” behaviors in their resistivities. One sample had a large resistivity of ≈49,000 μΩ cm at room temperature. Its resistivity increased with decreasing temperatures by a factor of 3.2 and exhibited a maximum around 2 K. Fitting discrepancies between the experimental magnetoconductivity data and the weak localization and electron–electron interaction theories suggest that the weak localization theory poorly describes the electronic conduction behavior of this highly resistive quasicrystalline sample.

We have investigated the dephasing processes in a weak localization (WL) experiment of a two-dimensional electron system on liquid helium. From low-field magnetoconductivity measurements we can separate the damping of WL on the dephasing of electrons due to electron-electron interaction and the motion of the helium vapour atoms. We observe an intermediate regime where both damping mechanisms are of comparable importance and determine the cross-over from one dominant regime to the other.

The transverse magnetoresistive effects in a nondegenerate quasi-one-dimensional (Q1D) electron system over superfluid helium have been investigated experimentally. The longitudinal magnetoresistance ρxx have been measured in magnetic fields B up to 2.5 T in the temperature range 0.48 – 2 K. The width of conducting channels was 90 nm and 35 μm, the mean electron density varied from ∼ 109 m−2 to 1.5 × 1012 m−2. It has been shown that the value of ρxx practically does not depend on B at low B and ρxx ∼ B in the quantum regime. The effective mobility of electrons in narrow channels increases under decreasing temperature and is determined by electron scattering by gas atoms, ripplons, and non-uniformities of the substrate. The mobility of electrons in wide channels increases with decreasing temperature and, below some temperature Tm, decreases. The negative magnetoresistance has been observed in the gas- and ripplon-scattering region. This effect has been explained by weak localization of carriers caused by the interaction of electrons with gas atoms in vapor at high temperatures and with ripplons or with non-uniformities of the substrate at low temperatures.

The magnetoresistance ρxx in a quasi-two-dimensional electron system over liquid helium has been measured for magnetic fields up to 2 T in the temperature range 0.5 – 1.7 K. It has been shown that the behavior of the value of ρxx as a function of a magnetic field B in the temperature region 1.2 – 1.7 K and at temperature T < 0.9 K is different. At high temperatures the negative magnetoresistance is observed due to weak localization of carriers, at T < 0.9 K the ρxx increases monotonically under increasing B. It is supposed that such a difference in the behavior of the ρxx is connected with the different character of the electron scattering: elastic at high temperatures, in the region of gas scattering, and inelastic at low temperatures. The last effect gives an additional evidence either for spatial ordering of electrons or for the formation of electron polarons in quasi-two- dimensional channels.

We derive a general expression for the conductivity of a disordered conductor with electron–electron interactions (treated within the standard model) and evaluate the weak localization correction δσ wl employing no approximations beyond the accuracy of the definition of δσ wl . Our analysis applies to all orders in the interaction and extends our previous calculation by explicitly taking into account quantum fluctuations around the classical paths for interacting electrons (pre-exponent). We specifically address the most interesting low temperature limit and demonstrate that such fluctuations can only be important in the perturbative regime of short times while they are practically irrelevant for the Cooperon dynamics at longer times. We fully confirm our conclusion about the existence of interaction-induced decoherence of electrons at zero temperature for the problem in question. We also demonstrate irrelevance of a perturbative calculation by Aleiner et al. (AAV) [J. Low Temp. Phys. 126, 1377 (2002)] and refute AAV's critique of our earlier analysis.

As-grown superconducting Bi-riched Bi2Sr2CuO6+δ single crystals have been grown by the traveling solvent floating zone technique. The superconducting transition temperature Tc was about 6 K and the room temperature resistivity was about 2×10−3 Ohm-cm. Transport properties, such as resistivity, magnetoresistance and Hall effect were measured from overdoped to underdoped samples annealed in inert atmosphere at 650°C. The transition temperature can be raised to 12 K after post annealing. The Hall measurement shows that the hole carrier density decrease after annealing. The temperature dependence of Hall angle is T1.5, not quadratic as observed for most high-Tc superconducting oxides such as YBa2Cu3O7. The variation of onset Tc with different external magnetic field is very different from high-Tc superconductors. The in-plane conductivity shows the dependence of ln T and can be explained by weak localization theory.

The quantum coherence of electrons can be probed by studying weak localizationcorrections to the conductivity. Interaction effects lead to dephasing, with electron-electron interactions being the important intrinsic mechanism. A controversy exists whether or not the dephasing rate, as measured in a weak localization experiment, vanishes at low temperatures. We review the non-perturbative analysis of this question and some of the arguments whichhave been raised against it. The compact form of the presentation shouldmake the derivation more transparent and accessible for discussions. Wealso compare with recent experiments.

We review work on the problem of disorder in the 2D d-wave superconducting state, and show that the symmetries of the normal state and the disorder distribution are vital for understanding the low-energy behavior. Most previous theoretical results for the density of states (DOS) are reconciled by a combination of exact numerical solutions of the Bogoliubov–de Gennes equations and weak localization calculations, which suggest that a novel diffusive mode with momentum (π, π) is responsible for a divergence of the DOS in the globally particle-hole symmetric case. We note briefly that the simple problem of a disordered tight-binding band of normal electrons displays some similar effects, which have been overlooked in the literature. Finally, in the physically realistic case of binary alloy disorder, no particle-hole symmetry, and an order parameter which is supressed around each impurity site, a power law with nonuniversal exponent is predicted.

We show that the maximum penetration depth of the anomalous quasiclassical Green's function from a superconductor into an adjacent normal metal is directly related to the phase relaxation time τ ϕ. The latter describes normal-metal phase correlations between two electrons with opposite spins. In the presence of spin-dependent interactions this phase relaxation time differs from the one found in weak localization corrections to the conductivity. The Josephson current measurements in a SINIS-structure could be used to determine τ ϕ experimentally.

In a series of samples whose composition was systematically changed, we have studied the magnetoresistance (ΔR/R = [R(B)-R]/R) of RuO 2 -based thick film resistors (TFR's) in magnetic fields (B) up to 20 Tesla and in a temperature range (1.2 K < T < 40 K) in which their resistance exhibits insulating behavior at the boundary between the strong and the weak localization. At the higher temperatures, ΔR/R exhibits a positive bump that does not depend on the RuO 2 concentration but it changes with the concentration of magnetic Mn impurities diluted in the glassy matrix. For T ≤ 20 K magnetoresistance is entirely negative and it has a quadratic magnetic field dependence at low field. We use the weak localization theory to relate these features of the high temperature magnetoresistance to the composition of TFR's. At low temperature (T < 4.2 K) the negative magnetoresistance shows some peculiarities. The quadratic term shrinks within a vanishing magnetic field range and the magnetoresistance linearly increases in a wide range of B. At the strongest fields and the lowest temperature ΔR/R shows a tendency to saturation. Although these results do not enable to discriminate among different mechanisms, we note that the low temperature (T < 4.2 K) behavior is essentially consistent with recent theory of Nguen et al.

Weak localization by vapor-atom scattering is studied for electrons on a helium surface. Dephasing times are measured as a function of electron density. The electron-electron collision time is determined. The dephasing mechanism is due to the motion of helium atoms.

The magnetism of the 5d metals W and Re is investigated with the method of weak localization. We studied W on the surface of Au and Re on the surface of Ag. For both systems the impurity thicknesses are in the range between 1/100 of a monolayer (single impurities) and one monolayer. For single W and Re impurities the dephasing of weak localization is by two to three orders of magnitude smaller than that of a typical magnetic impurity. For the monolayers it is reduced by a factor of about five. A strong temperature dependence of the dephasing suggests that thermally activated fluctuating moments are present. An extrapolation of this temperature dependence towards T = 0 yields no dephasing and therefore no magnetic behavior at T = 0. Numerical calculations predicted that W and Re impurities possess a magnetic moment of about 3.5 μBon the surface of Ag. These large moments are not found in our experiments.

Temperature- and magnetic-field-dependences of the resistivity {rho} and Hall effect R{sub H}(j{parallel}ab, B{parallel}c) in Nd{sub 1.82}Ce{sub 0.18}CuO{sub 4{minus}{delta}} single crystal film (T{sub c} = 6 K) were investigated at temperatures 1.4 {le} T {le} 20K and magnetic fields 0 {le} B {le} 5.5 T. At the lowest temperature T = 1.4 K the resistive state and Hall effect appear at the same magnetic field B = 0.5 T. A transition to the normal state is completed at B{sub c2} {approx_equal} 3T when a Hall coefficient becomes almost invariable. The negative magnetoresistance (d{rho}/dB 3 T was observed also. The sign change of the temperature coefficient d{rho}/dT corresponds B = 1.06 T. This fact does not connect with transition in a new state but this is an accidental compensation of two factors: a decreasing of B{sub c2} with temperature and negative value d{rho}/dT in the normal state owing to weak localization effect.

We report here the results of an extensive study of localization and electron-electron interaction effects in thin cadmium films with thickness ranging from 80 A to 350 A. Measurements of the resistance as a function of both temperature and magnetic field have allowed us to separate the contributions of localization and electron-electron interaction. The low resistive films of thicknesses 300 A and 350 A do not show any localization. The resistance of these samples decreases logarithmically with decreasing temperature below 10 K, while a ln(T) increases in resistance is observed for the high resistive films of thickness in the range of 80–120 A. Magnetoresistance of low resistive samples obeys the expression ΔR(H)/R(O)=AT n. But, both the resistance and magnetoconductance of the high resistive films are well explained by weak localization and electron-electron interaction effect. From the magnetoconductance measurement, we have calculated the inelastic scattering time (τ i ) and the spin-orbit scattering time (τ so ). The magnitude of spin-orbit scattering time is smaller than the inelastic scattering time. The inelastic scattering has been shown to arise due to the electron-electron scattering and the absolute magnitude of this scattering rate agrees reasonably well with the theory within the temperature range 1.8 K≤T≤5 K. At higher temperature (5 K

We present a calculation of a correction to the thermal conductivity due to the inclusion of electron-electron interaction in a disordered metallic system. We find that, to the first order in electron-electron interaction, the Wiedemann-Franz law is not satisfied.

Niobium nitride superconducting films have been deposited by reactive rf magnetron sputtering. Polyepitaxial growth on MgO-coated substrates greatly improves the superconducting properties. The overall film resistivity is found to be widely dependent on the total pressure of the gas mixture. Samples with a negative temperature coefficient of resistance exhibit 3D weak localization effects as characterized by magnetoresistance measurements. The inelastic scattering time is found to vary as T−p with p≃2, in the temperature range Tc-3Tc.

Weak localization magnetoresistance is used to investigate spin-orbit and the phase relaxation rates. The spin-orbit rate is independent of temperature. By plotting the spin-orbit rate as a function of the inverse thickness of the films, we separate the spin-orbit relaxation into two parts: one from the scattering off the bulk imperfections, and one from the scattering against the two surfaces. The ratio between the spin-orbit and the impurity relaxation rates in the bulk was found to be close to 2 × 10−5 for all the different samples. The influence of implanted heavy ions on the spin-orbit relaxation was also investigated. The dependence of the phase relaxation rate on resistivity, film thickness, and temperature has been studied. Theoretical results for electron-electron and electron-phonon scattering are compared to our data. We consider two novel temperature-independent phase relaxation mechanisms which may explain the residual rate we observe. The influence of a high-frequency electromagnetic field on the phase relaxation rate was investigated. For small microwave power levels the phase relaxation rate was found to increase linearly with microwave power. In the absence of a magnetic field and for samples having a dominating spin-orbit interaction (antilocalization) the resistance increases with microwave power, but turns into a decrease at high microwave power levels. For those samples having very weak spin-orbit interaction the resistance decreases continuously as we apply microwaves. This is roughly the expected behavior, but the observed change in resistance was larger than that calculated at small microwave power levels.

The weak localization contribution to the conductance depends sensitively on the applied magnetic field. This anomalous magnetoconductance provides a unique tool for the identification of weak localization and other phenomena associated with it. The standard theory of the anomalous magnetoconductance is based on the diffusion approximation, which applies to the case where the number of elastic scattering events is very large during a time of phase coherence in the electronic wave. For a strictly two-dimensional film, an arbitrary number of scattering events is considered and marked deviations are found from the standard theory if this number is not large.

Electrical and superconducting properties of indium films condensed in a H2 atmosphere (pressurep H 2=6×10−6 to 1.4×10−4 Torr) onto a substrate cooled with liquid helium are investigated. As hydrogen content is increased, a continuous increase in residual resistivity ρ* is observed, permitting systematic study of the resistance vs. temperature dependenceR(T) and the superconducting transition temperatureT c on approaching the metal-insulator transition (MIT). With regard to ρ*, four regimes of conductivity can be observed: (1) conductivity with a positive temperature resistance coefficient (TRC), (2) conductivity with a small, constant, negative TRC, (3) conductivity under weak localization with ΔR (T) ∼ln T or $$ \sim \sqrt T $$ type corrections, (4) hopping conductivity.T c rises continuously with ρ* and reaches its peak (∼5.2K) in the second regime. A further increase of ρ* leads to a decrease ofT c and complete suppression of superconductivity. The experimental dependenceR(T) is compared with theory. TheT c variation on approaching the MIT and the relation between Mooij's rule and the superconducting properties are discussed.

A consistent description of the effects of inelastic electron-electron scattering processes on weak localization in thin metal films is presented. Different techniques are used to treat strongly inelastic processes and quasielastic collisions, with special attention to the treatment of the latter. The calculations give an upper bound for the decay rate of the particle-particle propagator. This rate is proportional to the temperature and depends rather strongly on the conductance of the film. The calculation of the magnetoconductance connects this rate with measurable quantities and also reveals a dependence on the magnetic field. Finally, a description that avoids the splitting into two energy regions is presented.