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Three different methods of experimental mesoscopic physics, namely, weak antilocalization effects, universal conductance fluctuations, and Aharonov-Bohm oscillations, have been used to extract the electron phase-coherence scattering rate in two-dimensional gas of InGaAs/AlInAs heterostructures. The Aharonov-Bohm oscillations reveal strong beating effects, indicating the existence of two similar periodicities of the flux dependencies. As suggested by certain theoretical models, such a behavior might be expected from the so-called Berry phase acquired by electrons with different spin orientations in the presence of strong spin-orbit coupling and Zeeman splitting. In our paper we deduce the experimental values of the dephasing length and try to compare the observed beating pattern with possible scenarios for the appearance of the Berry phase.

Much effort has been devoted to the electronic properties of relatively thick $\mathrm{Zr}{\mathrm{Te}}_{5}$ crystals, focusing on their three-dimensional topological effects. Thin $\mathrm{Zr}{\mathrm{Te}}_{5}$ crystals, on the other hand, were much less explored experimentally. Here we present detailed magnetotransport studies of few-layer $\mathrm{Zr}{\mathrm{Te}}_{5}$ devices, in which electron-electron interactions and weak antilocalization are observed. The coexistence of the two effects manifests itself in corroborating evidence presented in the temperature and magnetic field dependence of the resistance. Notably, the temperature-dependent phase coherence length extracted from weak antilocalization agrees with strong electron-electron scattering in the sample. Meanwhile, universal conductance fluctuations have temperature and gate voltage dependence that is similar to that of the phase coherence length. Last, all the transport properties in thin $\mathrm{Zr}{\mathrm{Te}}_{5}$ crystals show strong two-dimensional characteristics. Our results provide insight into the highly intricate properties of topological material $\mathrm{Zr}{\mathrm{Te}}_{5}$.

We have measured weak antilocalization effects, universal conductance fluctuations, and Aharonov-Bohm oscillations in the two-dimensional electron gas formed in InGaAs/AlInAs heterostructures. This system possesses strong spin-orbit coupling and a high Land\'{e} factor. Phase-coherence lengths of 2$-$4 $\mu$m at 1.5$-$4.2 K are extracted from the magnetoconductance measurements. The analysis of the coherence-sensitive data reveals that the temperature dependence of the decoherence rate complies with the dephasing mechanism originating from electron-electron interactions in all three experiments. Distinct beating patterns superimposed on the Aharonov-Bohm oscillations are observed over a wide range of magnetic fields, up to 0.7 Tesla at the relatively high temperature of 1.5 K. The possibility that these beats are due to the interplay between the Aharonov-Bohm phase and the Berry one, different for electrons of opposite spins in the presence of strong spin-orbit and Zeeman interactions in ring geometries, is carefully investigated. It appears that our data are not explained by this mechanism; rather, a few geometrically-different electronic paths within the ring's width can account for the oscillations' modulations.

Gallium nitride nanowires (GaN NWs) with triangular cross-section exhibit universal conductance fluctuations (UCF) originating from the quantum interference of electron wave functions in the NWs. The amplitude of UCF is inversely proportional to the applied bias current. The bias dependence of UCF, combined with temperature dependence of the resistance suggests that phase coherent transport dominates over normal transport in GaN NWs. A unique temperature dependence of phase-coherent length and fluctuation amplitude is associated with inelastic electron–electron scattering in NWs. The phase-coherence length extracted from the UCF is as large as 400 nm at 1.8 K, and gradually decreases as temperature increases up to 60 K.

The transport properties of Ge-doped single GaN nanowires are investigated, which exhibit a weak localization effect as well as universal conductance fluctuations at low temperatures. By analyzing these quantum interference effects, the electron phase coherence length was determined. Its temperature dependence indicates that in the case of highly doped nanowires electron-electron scattering is the dominant dephasing mechanism, while for the slightly doped nanowires dephasing originates from Nyquist-scattering. The change of the dominant scattering mechanism is attributed to a modification of the carrier confinement caused by the Ge-doping. The results demonstrate that the phase coherence length can be tuned by the donor concentration making Ge-doped GaN nanowires an ideal model system for studying the influence of impurities on quantum-interference effects in mesoscopic and nanoscale systems.

We have performed magneto transport measurements on a multi-layer graphene device fabricated by conventional mechanical exfoliation. Suppression of weak localization (WL) as evidenced by the negative magnetoresistance (NMR) centered at zero field, and reproducible universal conductance fluctuations (UCFs) are observed. Interestingly, it is found that the phase coherence lengths calculated by fitting the observed NMR to conventional WL theory are longer than those determined from fitting the amplitudes of the UCFs to theory in the low temperature regime ( T ≤ 8 K). In the high temperature regime ( T > 8 K), the phase coherence lengths calculated by fitting the observed NMR to conventional WL theory are shorter than those determined from fitting the amplitudes of the UCFs to theory. Our new results therefore indicate a difference in the electron phase-breaking process between the two models of WL and UCFs in graphene. We speculate that the presence of the capping and bottom graphene layers, which leads the enhancement of disorder in-between, improves the localization condition for WL effect during carrier transportation in the low temperature regime. With increasing temperature, the localization condition for WL in multi-layer graphene becomes much weaker due to strong thermal damping. Therefore, the phase coherence lengths calculated by fitting the observed NMR to conventional WL theory are shorter than those determined from fitting the amplitudes of the UCFs to theory at high temperatures.

We report the fabrication of nanopatterned conduction channels and electrical transport of a two-dimensional electron gas formed at a LaAlO3/SrTiO3 heterointerface. The nanoscale channels are formed by electron beam lithography and Ar-ion etching. The negative magnetoconductance data observed at low temperature are attributed to the weak antilocalization effect due to the strong spin-orbit interaction inherent in the electron gas at the heterointerface whereas the conductance fluctuations are caused by quantum interference between the electron wavefunctions. The gatedependent characteristic parameters, obtained using the Maekawa-Fukuyama model, reveal that the spin-orbit interaction in the nanopatterned metal-oxide heterostructure is gate-tunable, which will be useful for building spintronic devices.

Magnetotransport measurements at low temperatures have been performed on InAs nanowires grown by In-assisted molecular beam epitaxy. Information on the electron phase coherence is obtained from universal conductance fluctuations measured in a perpendicular magnetic field. By analysis of the universal conductance fluctuations pattern of a series of nanowires of different length, the phase-coherence length could be determined quantitatively. Furthermore, indications of a pronounced flux cancelation effect were found, which is attributed to the topology of the nanowire. Additionally, we present measurements in a parallel configuration between wire and magnetic field. In contrast to previous results on InN and InAs nanowires, we do not find periodic oscillations of the magnetoconductance in this configuration. An explanation of this behavior is suggested in terms of the high density of stacking faults present in our InAs wires.

Two-dimensional materials, such as graphene, topological insulators, and two-dimensional electron gases, represent a technological playground to develop coherent electronics. In these systems, quantum interference effects and in particular weak localization, are likely to occur. These coherence effects are usually characterized by well-defined features in dc electrical transport, such as a resistivity increase and negative magnetoresistance below a crossover temperature. Recently, it has been shown that in magnetic and superconducting compounds, undergoing a weak-localization transition, a specific low-frequency 1/f noise occurs. An interpretation in terms of nonequilibrium universal conductance fluctuations has been given. The universality of this unusual electric noise mechanism has been here verified by detailed voltage-spectral density investigations on ultrathin copper films. The reported experimental results validate the proposed theoretical framework and also provide an alternative methodology to detect weak-localization effects by using electric noise spectroscopy.

Recent low temperature conductance measurements on multiwall carbon nanotubes in perpendicular and parallel magnetic fields are reported. An efficient gating technique allows for considerable tuning of the nanotube doping level. This enables us to study extensively the effect of the nanotube bandstructure on electron quantum interference effects such as weak localization, universal conductance fluctuations and the Aharonov–Bohm effect. We show that the magnetoresistance in the perpendicular magnetic field is strongly suppressed at certain gate voltages Ugate which can be linked with the bottoms of one-dimensional electronic subbands. This assignment allows a detailed comparison of theoretical calculations with the experimental data. In agreement with the theory, a pronounced energy dependence of the elastic mean free path with a strong enhancement close to the charge neutrality point is observed. In the large parallel magnetic field, we observe a superposition of h/2e-periodic Altshuler–Aronov–Spivak oscillations and an additional h/e-periodic contribution to the conductivity. The latter contribution shows a diamond-like pattern in the B − Ugate-plane, which reflects the magnetic field dependence of the density of states of the nanotube's outermost shell.

Electron interference effects and spin transport are investigated in semiconductor nanowires. In InAs bulk nanowires, grown by molecular beam epitaxy, the phase coherence length is determined by analyzing universal conductance fluctuations. Information on spin-orbit scattering is obtained by employing the weak antilocalization effect. In GaAs/InAs core/shell nanowires magnetic flux periodic oscillations are observed in the conductance under application of an axially oriented magnetic field. These oscillations are explained by the presence of quantized closed-loop states in the InAs shell.

We review some general statistical properties of wave transport through surface disordered waveguides. These systems are shown to present both striking similarities and differences with respect to quasi-one-dimensional waveguides with volume disorder. The statistical properties are analysed using extensive numerical calculations and random matrix theory results. The transport properties are characterized by the statistical behaviour of different transport coefficients that can be defined for both classical (light, microwaves, sound, etc.) and quantum (electrons) waves. In analogy with bulk-disordered systems, the behaviour of the waveguide conductance/resistance (defined for both classical and quantum waves) as a function of the system length defines three different transport regimes: ballistic, diffusive and localization. However, the coupling between waveguide modes presents significant differences with respect to the coupling induced by volume defects. For any incoming mode, there is a strong preferenc...

We report on measurements of the magnetoconductance of an open circular InGaAs quantum dot between 1.3K and 204K. We observe two types of magnetoconductance fluctuations: universal conductance fluctuations (UCFs), and 'focusing' fluctuations related to ballistic trajectories between openings. The electron phase coherence time extracted from UCFs amplitude is larger than in GaAs/AlGaAs quantum dots and follows a similar temperature dependence (between T^-1 and T^-2). Below 150K, the characteristic length associated with 'focusing' fluctuations shows a slightly different temperature dependence from that of the conductivity., 6 pages, 4 figures, proceedings of ICSNN2002, to appear in Physica E

Low-temperature transport in nanowires is accompanied by phase-coherent effects, which are observed as modulation of the conductance in an external magnetic field. In the GaAs/InAs core/shell nanowires investigated here, these are h/e flux periodic oscillations in a magnetic field aligned parallel to the nanowire axis and aperiodic universal conductance fluctuations in a field aligned perpendicularly to the nanowire axis. Both electron interference effects are used to analyse the phase coherence of the system. Temperature-dependent measurements are carried out, in order to derive the phase coherence lengths in the cross-sectional plane as well as along the nanowire sidewalls. It is found that these values show a strong anisotropy, which can be explained by the crystal structure of the GaAs/InAs core/shell nanowire. For nanowires with a radius as low as 45 nm, flux periodic oscillations were observed up to a temperature of 55 K.

We report experimental evidence of a remarkable spontaneous time reversal symmetry breaking in two dimensional electron systems formed by atomically confined doping of phosphorus (P) atoms inside bulk crystalline silicon (Si) and germanium (Ge). Weak localization corrections to the conductivity and the universal conductance fluctuations were both found to decrease rapidly with decreasing doping in the Si:P and Ge:P $\delta-$layers, suggesting an effect driven by Coulomb interactions. In-plane magnetotransport measurements indicate the presence of intrinsic local spin fluctuations at low doping, providing a microscopic mechanism for spontaneous lifting of the time reversal symmetry. Our experiments suggest the emergence of a new many-body quantum state when two dimensional electrons are confined to narrow half-filled impurity bands.

We present thermopower $S$ and resistance $R$ measurements on GaAs-based mesoscopic two-dimensional electron gases (2DEGs) as functions of the electron density $n_s$. At high $n_s$ we observe good agreement between the measured $S$ and $S_{\rm{MOTT}}$, the Mott prediction for a non-interacting metal. As $n_s$ is lowered, we observe a crossover from Mott-like behaviour to that where $S$ shows strong oscillations and even sign changes. Remarkably, there are absolutely no features in $R$ corresponding to those in $S$. In fact, $R$ is devoid of even any universal conductance fluctuations. A statistical analysis of the thermopower oscillations from two devices of dissimilar dimensions suggest a universal nature of the oscillations. We critically examine whether they can be mesoscopic fluctuations of the kind described by Lesovik and Khmelnitskii in Sov. Phys. JETP. \textbf{67}, 957 (1988)., Comment: New Journal of Physics (accepted)

We study low temperature magnetoconductance fluctuations in arrays of electron billiards in an effort to quantitatively measure hybridization between array elements. In contrast to previous research in the 'open' regime, which is mostly limited to the canonical AlxGa1-xAs/GaAs heterostructure, our devices are etched into the Ga0.25In0.75As/InP heterostructure. To chart hybridization (i.e. the extent of the coherent electron wavefunction) in the arrays, we use a Fourier analysis to examine the frequency content of the universal conductance fluctuations in one and two element arrays. This analysis reveals strong evidence of wavefunction hybridization over all measured coupling strengths between the array elements. (c) 2009 Elsevier B.V. All rights reserved.

The observation of quantum transport phenomena in metals is limited by the eventual loss of phase coherence of the conduction electrons on the length scale ${L}_{\ensuremath{\varphi}}.$ We address the question of whether ${L}_{\ensuremath{\varphi}}$ is the same in the context of different quantum transport phenomena. Specifically, we have measured ${L}_{\ensuremath{\varphi}}$ from two different experiments on the same sample. The experiments are magnetoresistance and $1/f$ noise versus magnetic field, and the samples are thin-Ag films in the quasi-two-dimensional regime. We determine ${L}_{\ensuremath{\varphi}}$ from fits of the magnetoresistance data to weak-localization theory (WL), and from fits of the noise versus magnetic field to universal conductance fluctuation theory (UCF). We find that the two values of ${L}_{\ensuremath{\varphi}}$ are the same at temperatures above about 10 K, but that ${L}_{\ensuremath{\varphi}}$ in the UCF experiment is shorter than that in the WL experiment at lower temperatures. This result is consistent with a recent theoretical discussion of quasielastic electron-electron scattering in disordered metals.

We investigate fluctuations in the magnetoconductance (measured at T o 50 mK) of noble-metal nanowires in a mesoscopic two-lead configuration, with lengths 500 and 1000 nm and widths w between 45 and 360 nm. We determine the dependence of the correlation field Bc on the angle u between the direction of the magnetic field and the long axis of the wires. u is changed at low T continuously from 90± (usual geometry) to 0±. We present a calculation taking into account the 3D diffusive motion of the conduction electrons describing the angular dependence of Bc. We compare this Bcsud dependence with conductance fluctuations observed while sweeping u at constant magnetic field. [S0031-9007(97)03042-1]

Mesoscopic phenomena in quantum structures which incorporate magnetic impurities with localized spins may exhibit a number of novel features driven by spin-disorder scattering, exchange spin-splitting of electron bands, and the formation of bound magnetic polarons. After brief information on these effects, their influence on universal conductance fluctuations as well as on low frequency noise and quantum localization is presented. Millikelvin investigations of diffusive charge transport, which have been carried out for submicron wires of n+-Cdi_ xMnx Te epilayers, are reviewed in some details. These studies have provided information on the significance of spin-disorder scattering in semiconductors and put into the evidence a new driving mechanism of the magnetoconductance fluctuations — the redistribution of the electrons between energy levels of the system, induced by the giant s-d exchange spin-splitting. Important implications of these findings for previous interpretations of spin effects in semiconductor and metal nanostructures are discussed. PACS numbers: 72.15.Rn, 73.20.Fz, 73.61.Ga, 75.50.Pp

Periodic Aharonov–Bohm and Altshuler–Aronov–Spivak oscillations have traditionally been observed in lateral transport through patterned mesoscopic loops of diffusive conductors. However, our studies of perpendicular-to-plane magnetotransport in straight-channel, diffusive devices of epitaxial Bi2Se3 surprisingly reveal signatures of Aharonov–Bohm orbits, periodic conductance fluctuation magneto-fingerprints, even though the devices are not explicitly patterned into loops. We show that the length scale of these orbits corresponds to the typical perimeter of triangular terraces found on the surface of these thin film devices, strongly suggesting that the periodic magneto-fingerprint arises from coherent scattering of electron waves from the step-edges. Our interpretation is bolstered by control measurements in devices without such surface morphology that only show a conventional, aperiodic magneto-fingerprint. These results show that lithographically patterned Bi2Se3 devices provide a novel class of mesoscopic physical systems for systematic studies of coherent surface sensitive transport.

Mesoscopic phenomena in nanostructures that incorporate diluted magnetic semiconductors may exhibit a number of novel features driven by spin interactions between mobile electrons and localized spins. Millikelvin studies of linear and nonlinear diffusive charge transport, which have been carried out for submicron wires of epilayers as well as for microstructures of bicrystals, are reviewed. These studies have provided information on the significance of spin-disorder scattering and evidence of a new driving mechanism of the magnetoconductance fluctuations - the redistribution of the electrons between energy levels of the system, induced by the giant s - d exchange spin-splitting. Important implications of these findings for previous interpretations of spin effects in semiconductor and metal nanostructures are discussed.

We have studied the dynamics of single bistable defects in submicron disordered Bi wires at temperatures 0.1--2 K. The wires are sufficiently small so that the motion of a single defect can be detected as a random telegraph signal in the resistance time trace. The amplitude of the resistance jumps increases as the temperature is lowered due to the physics of universal conductance fluctuations. The defect transitions obey Poisson statistics and detailed balance, indicating that the defect is a two-state system. We interpret the signal as arising from incoherent tunneling of an atom or group of atoms between the two wells of a double-well potential. The temperature dependence of the tunneling rates agrees quantitatively with the predictions of dissipative quantum tunneling theory, which describes tunneling of a defect in the presence of strong dissipation from the electron bath. Our measurements exploit the previous discovery that the energy asymmetry \ensuremath{\varepsilon} of the defect varies with magnetic field. By varying both the temperature and magnetic field, we can probe the behavior of the tunneling rates spanning two very different regimes, from kT\ensuremath{\ll}\ensuremath{\varepsilon} to kT\ensuremath{\gg}\ensuremath{\varepsilon}. Data from a single defect at several values of magnetic field suggest that the defect-bath coupling constant \ensuremath{\alpha} and the renormalized tunneling matrix element ${\mathrm{\ensuremath{\Delta}}}_{\mathit{r}}$ are nearly independent of magnetic field. \textcopyright{} 1996 The American Physical Society.

Weak-field magnetoresistance and universal conductance fluctuations have been studied at millikelvin temperatures in submicron wires of In-doped n + -CdTe and n + -Cd 0.99 Mn 0.01 Te. The fluctuations contain a periodic component of unknown origin. The giant spin-splitting of the electron states, caused by the s-d exchange interaction with the Mn spins, results in the positive magnetoresistance and the temperature dependence of the fluctuation pattern.

We have studied the negative magnetoresistance and universal conductance fluctuations ~UCF’s ! in a twodimensional electron gas ~2DEG! grown on substrates with prepatterned, submicrometer dimples. Since the 2DEG is sensitive only to the normal component of B, electrons move in a spatially inhomogeneous ( B perpendicular to the substrate: B’) or sign-alternating random magnetic field (B parallel to the substrate: Bi). Quantum interference effects in a random magnetic field due to the magnetic flux appear only due to second-order effects. The UCF’s are found to be 1.5‐2 times smaller for Bi than for B’ . @S01631829~96!09419-2#

We study the magnetic field dependence of the correlation field ΔBcand amplitude δgof the conductance fluctuations, observed in the low temperature magnetoresistance of GaAs/AlGaAs split-gate wires. Near zero field, universality of quantum interference is retained and the magnetoresistance shows universal conductance fluctuations. At high magnetic fields, although the discrete Landau level quantization becomes resolved. ΔBcand δgare found to increase linearly with magnetic field, with a slope which depends upon the nature of electron scatterings in the wire.

We report magnetoresistance measurements on a two-dimensional electron gas at low magnetic field and near Landau filling factor v = 1 2 in mesoscopic samples. Aperiodic reproducible resistance fluctuations have been found in both cases. In order to show that the fluctuations in a strong field are due to the quantum interference of the composite Fermions, we have compared coherence properties for the electrons and composite particles.

Single, sub-micrometer wide quantum wires have been fabricated using molecular beam epitaxy on mesa-etched GaAs substrates, where the GaAs wire is embedded in AlGaAs. By using a Hall bar pattern, potential probes were directly attached to the wires while grown. To avoid problems associated with anisotropic etching and regrowth, the wire structures were oriented along the 100 crystallographic directions. From this fabrication technique, described in detail by Shitara et al., Appl. Phys. Lett. 66, 2385 (1995), one can expect the formation of high quality �self-aligned� quantum wires with a confinement potential determined by the conduction band discontinuity of AlGaAs and GaAs. Here we study the four-point magnetoresistance of 50 im long single quantum wires with widths between 250 and 700 nm from 1.3 to 21 K. A distinct weak localization peak and universal conductance fluctuations dominate the low magnetic field regime and are used to estimate the phase- coherence length of the electrons. Pronounced 1/B periodic quantum oscillations at magnetic fields above 1 T are consistent with the picture of wires with a square-well shaped confining potential.

Single-crystalline iridium dioxide nanowires show the time-dependent universal conductance fluctuations (TUCFs) at cryogenic temperatures. The conductance fluctuations persist up to temperature T as high as nearly 10 K. The root-mean-square TUCF magnitudes increase with decreasing T, reaching approximately 0.1 e2 / h at 1.7 K. We ascribe these conductance fluctuations to originating from the conduction electrons scattering upon mobile defects (moving scattering centers). Our measured TUCF characteristics are satisfactorily explained in terms of the existing TUCF theory in its three-dimensional form. The extracted electron dephasing length Lφ(1.7 K) ≃90 nm is smaller than the diameter (≈ 180 nm) of our nanowires.

Magnetic field dependent universal conductance fluctuations (UCF's) are observed in weakly disordered indium tin oxide nanowires from 0.26 K up to $\sim 25$ K. The fluctuation magnitudes increase with decreasing temperature, reaching a fraction of $e^2/h$ at $T \lesssim 1$ K. The shape of the UCF patterns is found to be very sensitive to thermal cycling of the sample to room temperatures, which induces irreversible impurity reconfigurations. On the other hand, the UCF magnitudes are insensitive to thermal cycling. Our measured temperature dependence of the root-mean-square UCF magnitudes are compared with the existing theory [C. W. J. Beenakker and H. van Houten, Phys. Rev. B \textbf{37}, 6544 (1988)]. A notable discrepancy is found, which seems to imply that the experimental UCF's are not cut off by the thermal diffusion length $L_T$, as would be expected by the theoretical prediction when $L_T < L_\varphi$, where $L_\varphi$ is the electron dephasing length. The approximate electron dephasing length is inferred from the UCF magnitudes and compared with that extracted from the weak-localization magnetoresistance studies. A reasonable semiquantitative agreement is observed., Comment: 11 pages, 9 figures

We have investigated mesoscopic conductance fluctuations in a GaAs quantum well with five electrically quantized two-dimensional (2D) subbands occupied. In contrast to the case of a single occupied subband, where the fluctuations depend only on the component of magnetic field perpendicular to the plane of the electrons, in our multi-subband structures the conductance fluctuations occur even in a magnetic field parallel to the plane. We attribute the fluctuations to rapid electron-impurity scattering between different 2D subbands which leads to quasi-3D motion of electrons within the quantum well. We are able to confirm the 3D character of the fluctuations by correlations between the magnetoresistances observed when the magnetic field is applied at various angles relative to the plane of the quantum well.

We have identified two new features related to the coherent transport in the mesoscopic loop structure of aluminum wire, including the autocorrelation of the conductance fluctuations beyond and fine structure in the low-field magnetoresistance curve in the superconducting transition regime, which, to the best of our knowledge, have not been reported in the literature. Since the electrons in Al have a phase coherence length larger than at or below T = 3K, which is comparable to the dimensions of the structure, the wave nature of the electronic transport has been clearly observed: the universal conductance fluctuations, the Aharonov-Bohm oscillations, and the Altshuler-Aronov-Spivak oscillations. Due to the transition of Al to a superconducting state at T = 1.3 K, the coherent phenomena of Cooper pairs, i.e., the Little-Parks oscillations, have also been observed.

We are studying quantum interference effects in narrow GaAs/AlGaAs wires of the quasi-ballistic regime, where the wire dimensions are comparable to or less than the phase coherence length of electron transport. By analysing the auto-correlation function of the universal conductance fluctuations in low field magnetoresistances, we obtained the wire-width and the magnetic field dependences of the correlation field and observed phase-coherence behaviors of electron waves. Estimating the boundary correlation field as ω c τ = 1, we can discuss a mean free path in the split-gated wire. Such a boundary field analysis indicates a possibility of determination of the microscopic mean free path of quantum narrow wires.

We study the universal conductance fluctuations (UCF) in two-dimensional mesoscopic systems in the presence of a strong magnetic field when the Hall conductance σ xy is not negligible compared to σ xx . We find that the Hall component of the diffusion current modifies the boundary conditions for the diffuson and gives rise to a qualitatively new behavior of the UCF. In particular, when σ xy is larger than σ xx , the magnetic field correlation length B c is proportional to σ xx instead of the usual inverse proportionality leading to oscillations in B c in phase with the Shubnikov-de Haas oscillations of σ xx

We investigated the phase coherence length, l/sub /spl phi//, in large Si-MOSFET's fabricated using current process technology, with a particular emphasis on highly doped silicon substrates, and then studied the effects of quantum conductance fluctuations in deep sub-/spl mu/m MOSFET's, with channel length comparable to l/sub /spl phi//. We identified, in a 0.2 /spl mu/m MOSFET, universal conductance fluctuations in the strong inversion regime and conductance fluctuations due to variable range hopping in the weak inversion regime. The drain bias dependence of these fluctuations indicates clearly that they become a serious concern only at drain voltages lower than 10 mV. Therefore, even if the wave nature of electrons results in quantum conductance fluctuations, it will not lead to a limitation on device miniaturization in future Si-ULSI's. >

We report low-frequency 1/f noise measurements of degenerately doped Si:P \delta-layers at 4.2K. The noise was found to be over six orders of magnitude lower than that of bulk Si:P systems in the metallic regime and is one of the lowest values reported for doped semiconductors. The noise was found to be nearly independent of magnetic field at low fields, indicating negligible contribution from universal conductance fluctuations. Instead interaction of electrons with very few active structural two-level systems may explain the observed noise magnitude, Comment: 4 pages, 4 figures

Temporal universal conductance fluctuations (TUCF) are observed in RuO$_2$ nanowires at cryogenic temperatures. The fluctuations persist up to very high $T \sim 10$ K. Their root-mean-square magnitudes increase with decreasing $T$, reaching $\sim 0.2 e^2/h$ at $T \lesssim 2$ K. These fluctuations are shown to originate from scattering of conduction electrons with rich amounts of mobile defects in artificially synthesized metal oxide nanowires. TUCF characteristics in both one-dimensional saturated and unsaturated regimes are identified and explained in terms of current theories. Furthermore, the TUCF as a probe for the characteristic time scales of the mobile defects (two-level systems) are discussed., Comment: 5 pages, 3 figures

The conductance of a two-dimensional system subject to a random magnetic field is calculated within the Landauer-Buttiker formalism. Although there is no potential disorder the random fluctuating magnetic field introduces a random vector potential. Electrons in bulk states are found to be strongly scattered by the fluctuating field. In contrast, edge states are found to be quite stable even if the fluctuation of the field is of the order of a uniform background field. In a fully random field, the U(1) model, clear universal conductance fluctuations (UCF) are observed. The magnitude of the fluctuations is compared with that in the time reversal symmetric Z 2 model.

We calculate the average persistent current in a mesoscopic metal ring threaded by a magnetic flux in the diffusive regime. It is shown that the classical electromagnetic energy leads to a {\it{universal}} average current of the order of $ \alpha e c / C_{0}$, where $\alpha$ is the fine structure constant, $-e$ is the charge of the electron, $c$ is the velocity of light, and $C_{0}$ is the classical capacitance of the ring. Striking similarities between persistent currents and universal conductance fluctuations are discovered. We suggest a simple experiment to test our theory., Comment: 11 pages, Revtex 3.0, 2 Figures, will be mailed on request

Universal conductance fluctuations (UCF) arise in mesoscopic systems as a direct consequence of the quantum interference of electron waves. The universal feature of the fluctuations is that, at finite temperature, their behaviour can be expressed in terms of one parameter, the phase breaking length of electrons Lϕ. Though the mesoscopic fluctuations have also been observed in high magnetic fields, ω c τ > 1, where ω c is the cyclotron frequency and τ the electron scattering time, it was widely believed that the magnetic field had to suppress the fluctuations. However, experiments on an orthodox mesoscopic system have found out no signs of the suppression. The fluctuation amplitude remains unchanged in high magnetic fields while the period of fluctuations changes dramatically. This indicates the breakdown of universal scaling at high magnetic fields.

The understanding of the nature of quantum interference in disordered conductors gave the strong impulse for investigation of disordered systems. There were explained a lot of old puzzles in physics of conductors (e.g. anomalous temperature dependence on resistivity and different kinetic effects, the negative magnetoresistance etc.) and discovered new attractive phenomena. Such effects as oscillating effects in multiconnected systems, universal conductance fluctuations, local fluctuating currents in conductors have been converted in new branches of physics of disordered metals. The development of this new physics during the last years will be discussed. It is a great pleasure for me to talk today on behalf of my friends and colleagues. I would like to show the schematic picture where I tried to sketch the basis and development of electron coherence and interference phenomena in disordered conductors. The main basis of all this field of metal physics is the Anderson–Mott transition in disordered systems. This transition separates all substances on two groups – metals and dielectrics. Three trees grow on this basis now on the metal side: the weak localization, interaction effects and mesoscopics.

Nonlinear electrical properties of graphene-based three-terminal nanojunctions are presented. Intrinsic rectification of voltage is observed up to room temperature. The sign and the efficiency of the rectification can be tuned by a gate. Changing the charge carrier type from holes to electrons results in a change of the rectification sign. At a bias < 20mV and at a temperature below 4.2K the sign and the efficiency of the rectification are governed by universal conductance fluctuations.

Graphene provides a fascinating testbed for new physics and exciting opportunities for future applications based on quantum phenomena. To understand the coherent flow of electrons through a graphene device, we employ a nanoscale probe that can access the relevant length scales--the tip of a liquid-He-cooled scanning probe microscope (SPM) capacitively couples to the graphene device below, creating a movable scatterer for electron waves. At sufficiently low temperatures and small size scales, the diffusive transport of electrons through graphene becomes coherent, leading to universal conductance fluctuations (UCF). By scanning the tip over a device, we map these conductance fluctuations versus scatterer position. We find that the conductance is highly sensitive to the tip position, producing delta G approximately e(2)/h fluctuations when the tip is displaced by a distance comparable to half the Fermi wavelength. These measurements are in good agreement with detailed quantum simulations of the imaging experiment and demonstrate the value of a cooled SPM for probing coherent transport in graphene.

Graphene has opened new avenues of research in quantum transport, with potential applications for coherent electronics. Coherent transport depends sensitively on scattering from microscopic disorder present in graphene samples: electron waves traveling along different paths interfere, changing the total conductance. Weak localization is produced by the coherent backscattering of waves, while universal conductance fluctuations are created by summing over all paths. In this work, we obtain conductance images of weak localization with a liquid-He-cooled scanning probe microscope, by using the tip to create a movable scatterer in a graphene device. This technique allows us to investigate coherent transport with a probe of size comparable to the electron wavelength. Images of magnetoconductance \textit{vs.} tip position map the effects of disorder by moving a single scatterer, revealing how electron interference is modified by the tip perturbation. The weak localization dip in conductivity at B=0 is obtained by averaging magnetoconductance traces at different positions of the tip-created scatterer. The width $\Delta B_{WL}$ of the dip yields an estimate of the electron coherence length $L_\phi$ at fixed charge density. This "scanning scatterer" method provides a new way of investigating coherent transport in graphene by directly perturbing the disorder configuration that creates these interferometric effects., Comment: 18 pages, 7 figures

The low-temperature quantum transport of InN nanowires grown by plasma-assisted molecular beam epitaxy is investigated. Two sets of nanowires with diameters of 100 and 45 nm originating from two different growth runs are studied. Magnetic-field-dependent as well as gate-dependent measurements of universal conductance fluctuations are performed to gain information on the phase-coherence in the electron transport. By analyzing the correlation field and the average fluctuation amplitude a phase-coherence length of several hundred nanometers is extracted for both sets of nanowires at temperatures below 1 K. Conductance fluctuations are also observed when the Fermi wavelength is varied by applying a bias voltage to a back-gate. The results on the electron phase-coherence obtained from the gate-dependent measurements are consistent with the findings from the magnetic field dependent measurements. A considerable damping of the fluctuation amplitude by ensemble averaging is achieved by connecting nanowires in parallel. The suppression of the fluctuation amplitude is studied systematically by measuring samples with different numbers of nanowires. By utilizing the damping of the conductance fluctuations by connecting nanowires in parallel in combination with an averaging over the gate voltage, weak localization effects are resolved. For both sets of nanowires a clear evidence of the weak antilocalization is found, which indicates the presence of spin-orbit coupling. For the spin-orbit scattering length l(so) values in the order of 100 nm are extracted. (c) 2010 American Institute of Physics. [doi: 10.1063/1.3516216]

The low-temperature quantum transport properties of gated InN nanowires were investigated. Magnetic-field-dependent as well as gate-dependent measurements of universal conductance fluctuations were performed to gain information on the phase coherence in the electron transport. We found a pronounced decrease in the variance of the conductance by about a factor of 2 in gate-dependent fluctuation measurements if a magnetic field is applied. This effect is explained by the suppression of the Cooperon channel of the electron correlation contributing to the conductance fluctuations. Despite the fact that the diameter of the nanowire is less than 100 nm a clear weak antilocalization effect is found in the averaged magnetoconductance being in strong contrast to the suppression of weak antilocalization for narrow quantum wires based on planar two-dimensional electron gases. The unexpected robustness of the weak antilocalization effect observed here is attributed to the tubular topology of the surface electron gas in InN nanowires.

We investigate the transport properties of narrow quantum wires realized in disordered two-dimensional electron gases in the presence of k-linear Rashba and Dresselhaus spin-orbit interaction (SOI), and an applied in-plane magnetic field. Building on previous work [Scheid, et al., PRL 101, 266401 (2008)], we find that in addition to the conductance, the universal conductance fluctuations also feature anisotropy with respect to the magnetic field direction. This anisotropy can be explained solely from the symmetries exhibited by the Hamiltonian as well as the relative strengths of the Rashba and Dresselhaus spin orbit interaction and thus can be utilized to detect this ratio from purely electrical measurements., Comment: 10 pages, 4 figures, 1 table

The authors use the 2D quantum percolation model to simulate the mesoscopic transport properties of disordered metallic films. The conductance of a mesoscopic sample is calculated by the multichannel Landauer formula, with the transmission and reflection coefficients evaluated numerically with a recursive Green-function approach. The distribution of the conductance is found to exhibit three distinct types: normal, log-normal and between normal and log-normal, depending on the sample size L and metal fraction p. The variation of the mean with L yields the localization length and the root-mean-square conductance fluctuation Delta g is shown to have different regimes, including the universal-conductance fluctuation regime where Delta g is constant as a function of L and of the order of e2/h. Based on these results, they propose the use of mesoscopic samples of disordered metallic films for the direct experimental observation of Anderson localization for electrons. Two possible sample systems are discussed.

Experimental and theoretical results concerning the statistical properties of the conductance of electron systems at very low temperatures are reported. In the metallic regime the conductance fluctuations are of the order of e 2/h, independent of the size, and other parameters that characterize the system. The magnitude of these universal conductance fluctuations are determined by symmetry properties. In the insulating regime the configurational average of the conductance decreases much faster with the size of the system than its fluctuations such that the latter dominate transport in the thermodynamic limit at sufficiently low temperatures. The relation between the most probable value of the conductance, its configurational average, and the experimentally measured value are discussed. Conditions for the experimental observability of the fluctuations are briefly reviewed.

After a short presentation on the preparation and structural properties of Multi-Wall carbon NanoTubes (MWNTs), their outstanding electronic, magnetic, mechanical and field emitting properties are reviewed. The manifestation of mesoscopic transport properties in MWNTs is illustrated through the Aharonov-Bohm effect, universal conductance fluctuations, the weak localization effect and its power-law temperature/field dependences. Measurements of the Young’s modulus of individual nanotubes show the high strength of tubes having well-graphitized walls. Electron Spin Resonance (ESR) measurements indicate the low-dimensional character of the electronic states even for relatively large diameter tubes. The conducting nature of the tubes, together with their large curvature tip structure, make them excellent electron and light emitters suitable for applications.