Quantum transport properties in quantum Hall wires in the presence of spatially correlated random potential are investigated numerically. It is found that the potential correlation reduces the localization length associated with the edge state, in contrast to the naive expectation that the potential correlation increases it. The effect appears as the sizable shift of quantized conductance plateaus in long wires, where the plateau transitions occur at energies much higher than the Landau band centers. The scale of the shift is of the order of the strength of the random potential and is insensitive to the strength of magnetic fields. Experimental implications are also discussed.
Rashba spin-orbit coupling appears in 2D systems lacking inversion symmetry, and causes the spin-splitting of otherwise degenerate energy bands into an upper and lower helicity band. In this paper, we explore how impurity scattering affects transport in the ultra-low-density regime where electrons are confined to the lower helicity band. A previous study has investigated the conductivity in this regime using a treatment in the first Born approximation. In this work, we use the full T-matrix to uncover new features of the conductivity. We first compute the conductivity within a semiclassical Bo…
We report on theoretical and experimental investigations of the integer quantized Hall effect in narrow channels at various mobilities. The Hall bars are defined electrostatically in two-dimensional electron systems by biasing metal gates on the surfaces of GaAs/AlGaAs heterostructures. In the low mobility regime the classical Hall resistance line is proportional to the magnetic field as measured in the high temperature limit and cuts through the center of each Hall plateau. For high mobility samples we observe in linear response measurements, that this symmetry is broken and the classical Hal…
We report on observation of an unconventional structure of the quantum Hall effect (QHE) in a $ p$-type HgTe/Cd$_x$Hg$_{1-x}$Te double quantum well (DQW) consisting of two HgTe layers of critical width. The observed QHE is a reentrant function of magnetic field between two $i=2$ states (plateaus at $ρ_{xy}=h/ie^2$) separated by an intermediate $i=1$ state, which looks like some anomalous peak on the extra-long $i=2$ plateau when weakly expressed. The anomalous peak apparently separates two different regimes: a traditional QHE at relatively weak fields for a small density of mobile holes $p_s$ …
In dirty superconducting wires, quasiparticle states at the Fermi level need not be exponentially localized if spin-rotation symmetry is broken [Brouwer {\em et al.}, Phys. Rev. Lett. {\bf 85}, 1064 (2000)]. Here we present evidence that not-localized states are generic in the thick-wire limit, while for wires of finite thickness delocalization requires fine tuning of the disorder, consistent with earlier results of Motrunich {\em et al.} [Phys. Rev. B {\bf 63}, 224204 (2001)]. The thick-wire limit is defined as the simultaneous limit where the length $L$ of the wire and the number $N$ of prop…
We report on magnetic field dependence of half-integer quantized conductance plateaus (HQPs) in InAs quantum wires. We observed HQPs at zero applied magnetic field in InAs quantum wires fabricated from a high-quality InAs quantum well. The application of in-plane magnetic field causes Zeeman splitting of the HQP features, indicating that the origin of the observed HQP is not spontaneous spin polarization. Additionally we observe that conductance of the split HQPs decreases gradually as the in-plane magnetic field increases. We finally assume electron-electron interaction as a possible mechanis…
In this work we uncover an interesting quantum plateau behavior for the Andreev reflection between a one-dimensional quantum wire and superconductor. The quantum plateau is achieved by properly tuning the interplay of the spin-orbit coupling within the quantum wire and its tunnel coupling to the superconductor. This plateau behavior is justified to be unique by excluding possible existences in the cases associated with multi-channel quantum wire, the Blonder-Tinkham-Klapwijk continuous model with a barrier, and lattice system with on-site impurity at the interface.
When a quantum wire is weakly confined, a conductance plateau appears at e^2/h with decreasing carrier density in zero magnetic field accompanied by a gradual suppression of the 2e^2/h plateau. Applying an in-plane magnetic field B|| does not alter the value of this quantization; however, the e^2/h plateau weakens with increasing B|| up to 9 T, and then strengthens on further increasing B||, which also restores the 2e^2/h plateau. Our results are consistent with spin-incoherent transport in a one-dimensional wire.
We calculate the linear and nonlinear conductance of spinless fermions in clean, long quantum wires where short-ranged interactions lead locally to equilibration. Close to the quantum phase transition where the conductance jumps from zero to one conductance quantum, the conductance obtains an universal form governed by the ratios of temperature, bias voltage and gate voltage. Asymptotic analytic results are compared to solutions of a Boltzmann equation which includes the effects of three-particle scattering. Surprisingly, we find that for long wires the voltage predominantly drops close to one…
There are known two distinct types of the integer quantum Hall effect. One is the conventional quantum Hall effect, characteristic of two-dimensional semiconductor systems, and the other is its relativistic counterpart recently observed in graphene, where charge carriers mimic Dirac fermions characterized by Berry's phase pi, which results in a shifted positions of Hall plateaus. Here we report a third type of the integer quantum Hall effect. Charge carriers in bilayer graphene have a parabolic energy spectrum but are chiral and exhibit Berry's phase 2pi affecting their quantum dynamics. The L…
Quantum transport properties in quantum Hall wires in the presence of spatially correlated disordered magnetic fields are investigated numerically. It is found that the correlation drastically changes the transport properties associated with the edge state, in contrast to the naive expectation that the correlation simply reduces the effect of disorder. In the presence of correlation, the separation between the successive conductance plateau transitions becomes larger than the bulk Landau level separation determined by the mean value of the disordered magnetic fields. The transition energies co…
We study theoretically the proximity effect of a one-dimensional metallic quantum wire (in the absence of spin-orbit interaction) lying on top of an unconventional superconductor. Three different material classes are considered as a substrate: (i) a chiral superconductor in class D with broken time-reversal symmetry; a class DIII superconductor (ii) with and (iii) without a nontrivial Z2 number. Interestingly, we find degenerate zero energy Majorana bound states at both ends of the wire for all three cases. They are unstable against spin-orbit interaction in case (i) while they are topological…
Coupled-wire descriptions of unconventional quantum states in twisted nanostructures
Institute of Physics, Academia Sinica（中央研究院物理研究所）；National Center for Theoretical Sciences（国家理论科学中心）；Department of Electronics and Electrical Engineering, National Yang Ming Chiao Tung University（国立阳明交通大学电子电气工程学系）
The unexpected "0.7" plateau of conductance quantisation is usually observed for ballistic one-dimensional devices. In this work we study a quasi-ballistic quantum wire, for which the disorder induced backscattering reduces the conductance quantisation steps. We find that the transmission probability resonances coexist with the anomalous plateau. The studies of these resonances as a function of the in-plane magnetic field and electron density point to the presence of spin polarisation at low carrier concentrations and constitute a method for the determination of the effective g-factor suitable…
The transport properties of the two-dimensional system in HgTe-based quantum wells containing simultaneously electrons and holes of low densities are examined. The Hall resistance, as a function of perpendicular magnetic field, reveals an unconventional behavior, different from the classical N-shaped dependence typical for bipolar systems with electron-hole asymmetry. The quantum features of magnetotransport are explained by means of numerical calculation of the Landau level spectrum based on the Kane Hamiltonian. The origin of the quantum Hall plateau σxy = 0 near the charge neutrality point …
We show that partially separated Andreev bound states (ps-ABSs), comprised of pairs of overlapping Majorana bound states (MBSs) emerging in quantum dot-semiconductor-superconductor heterostructures, produce robust zero bias conductance plateaus in end-of-wire charge tunneling experiments. These plateaus remain quantized at $2e^2/h$ over large ranges of experimental control parameters. In light of recent experiments reporting the observation of robust $2e^2/h$-quantized conductance plateaus in local charge tunneling experiments, we perform extensive numerical calculations to explicitly show tha…
We study the van der Waals interaction of a metallic or narrow-gap semiconducting nanowire with a surface, in the regime of intermediate wire-surface distances $(v_{F}/c)L \ll d \ll L $ or $L \ll d \ll (c/v_{F})L $, where $L$ is the nanowire length, $d$ is the distance to the surface, and $v_{F}$ is the characteristic velocity of nanowire electrons (for a metallic wire, it is the Fermi velocity). Our approach, based on the Luttinger liquid framework, allows one to analyze the dependence of the interaction on the interplay between the nanowire length, wire-surface distance, and characteristic l…
We study an unconventional quantum Hall effect for the surface states of ultrathin Floquet topological insulators in a perpendicular magnetic field. The resulting band structure is modified by photon dressing and the topological property is governed by the low-energy dynamics of a single surface. An exchange of symmetric and antisymmetric surface states occurs by reversing the light's polarization. We find a novel quantum Hall state in which the zeroth Landau level undergoes a phase transition from a trivial insulator state, with Hall conductivity $σ_{yx}=0$ at zero Fermi energy, to a Hall ins…
We have investigated the conductance of long quantum wires formed in GaAs/AlGaAs heterostructures. Using realistic fluctuation potentials from donor layers we have simulated numerically the conductance of four different kinds of wires. While ideal wires show perfect quantization, potential fluctuations from random donors may give rise to strong conductance oscillations and degradation of the quantization plateaux. Statistically there is always the possibility of having large fluctuations in a sample that may effectively act as a microconstriction. We therefore introduce microconstrictions in t…
We report on the formation of critical states in disordered graphene, at the origin of variable and unconventional transport properties in the quantum Hall regime, such as a zero-energy Hall conductance plateau in the absence of an energy bandgap and Landau level degeneracy breaking. By using efficient real-space transport methodologies, we compute both the dissipative and Hall conductivities of large size graphene sheets with random distribution of model single and double vacancies. By analyzing the scaling of transport coefficients with defect density, system size and magnetic length, we elu…
We study the conductance threshold of clean nearly straight quantum wires in which an electron is bound. We show that such a system exhibits spin-dependent conductance structures on the rising edge to the first conductance plateau, one near 0.25(2e^2/h), related to a singlet resonance, and one near 0.75(2e^2/h), related to a triplet resonance. As a quantitative example we solve exactly the scattering problem for two-electrons in a wire with circular cross-section and a weak bulge. From the scattering matrix we determine conductance via the Landauer-Buettiker formalism. The conductance anomalie…
We numerically study the interplay of band structure, topological invariant and disorder effect in two-dimensional electron system of graphene in a magnetic field. Two \emph{distinct} quantum Hall effect (QHE) regimes exist in the energy band with the unconventional "half-integer" QHE appearing near the band center, consistent with the experimental observation. The latter is more robust against disorder scattering than the conventional QHE states near the band edges. The phase diagram for the unconventional QHE is obtained where the destruction of the Hall plateaus at strong disorder is throug…
We have investigated hole transport in one-dimensional quantum wires in strained germanium two-dimensional layers. The ballistic conductance characteristics show the regular quantised plateaux in units of n2e2/h, where n is an integer. Additionally, new quantised levels are formed which correspond to values of n = 1/4 reducing to 1/8 in the presence of a strong parallel magnetic field which lifts the spin degeneracy but does not quantise the wavefunction. A further plateau is observed corresponding to n = 1/32 which does not change in the presence of a parallel magnetic field. These values ind…
Magnetoresistance in long correlated nanowires of degenerate semiconductor InSb in asbestos matrix (wire diameter of around 5 nm, length 0.1 - 1 mm) is studied over temperature range 2.3 - 300 K. At zero magnetic field the electric conduction $G$ and the current-voltage characteristics of such wires obey the power laws $G\propto T^α$, $I\propto V^β$, expected for one-dimensional electron systems. The effect of magnetic field corresponds to a 20% growth of the exponents $α$, $β$ at H=10 T. The observed magnetoresistance is caused by the magnetic-field-induced breaking of the spin-charge separat…
We analyze the Landau level (LL) structure and spin Hall effect in a MoS2 trilayer. Due to orbital asymmetry, the low-energy Dirac fermions become heavily massive and the LL energies grow linearly with $B$, rather than with $\sqrt{B}$. Spin-orbital couplings break spin and valley degenerate LL's into two time reversal invariant groups, with LL crossing effects present in the valence bands. We find a field-dependent unconventional Hall plateau sequence $ν=...$ $-2M-6$, $-2M-4$, $-2M-2$, $-2M-1$, ..., -5, -3, -1, 0, 2, 4 .... In a p-n junction, spin-resolved fractionally quantized conductance ap…
We present systematic quantitative description of the magnetoconductance of the split-gate quantum wires. Accounting for the exchange and correlation interactions within the spin density function theory (DFT) leads to the lifting of the spin degeneracy and formation of the spin-resolved plateaus at odd values of $e^{2}/h$. We show that the width of the odd conductance steps in the spin DFT calculations is equal to the width of the transition intervals between the conductance steps for the spinless electrons in the Hartree approximation. A detailed analysis of the structure of compressible/inco…
For unpolarized electrons in a clean quantum wire, density functional theory reveals a thermally activated suppression of conductance. The activation temperature T_A grows slowly (roughly quadratically) with gate voltage due to pinning of the band edge at the chemical potential. Similar results were obtained experimentally by Kristensen et al. [Phys. Rev. B62, 10950 (2000)] for the temperature dependence of the anomalous conductance plateau near 0.7 (2e^2/h) in a quantum point contact.
The conductance threshold of a clean nearly straight quantum wire in which a single electron is bound is studied. This exhibits spin-dependent conductance anomalies on the rising edge to the first conductance plateau, near G=0.25(2e^{2}/h) and G=0.7(2e^{2}/h), related to a singlet and triplet resonances respectively. We show that the problem may be mapped on to an Anderson-type of Hamiltonian and calculate the energy dependence of the energy parameters in the resulting model.
We predict the occurrence of Fano lineshapes in a semiconductor quantum wire with local spin-orbit (Rashba) coupling. We show that the Rashba interaction acts in a strictly one dimensional channel as an attractive impurity, leading to the formation of purely bound states. In a quasi-one dimensional system these bound states couple to the conduction ones through the Rashba intersubband mixing, giving rise to pronounced dips in the linear conductance plateaus. We give exact numerical results and propose an approximate model capturing the main ingredients of the effect.
We investigate a possibility of pair electron-electron e-e collisions in a ballistic wire with spin-orbit coupling and only one populated mode. Unlike in a spin-degenerate system, a combination of spin-splitting in momentum space with a momentum-dependent spin-precession opens up a finite phase space for pair e-e collisions around three distinct positions of the wire's chemical potential. For a short wire, we calculate corresponding resonant contributions to the conductance, which have different power-law temperature dependencies, and, in some cases, vanish if the wire's transverse confinement…
Ballistic spin resonance was experimentally observed in a quasi-one-dimensional wire by Frolov et al. [Nature (London) 458, 868 (2009)]. The spin resonance was generated by a combination of an external static magnetic field and the oscillating effective spin-orbit magnetic field due to periodic bouncings of the electrons off the boundaries of a narrow channel. An increase of the D'yakonov-Perel spin relaxation rate was observed when the frequency of the spin-orbit field matched that of the Larmor precession frequency around the external magnetic field. Here we develop a model to account for th…
Density, temperature and magnetic field dependences on electron transport in a quantum wire were studied. Decrease of carrier density gives a negative conductance correction on the first plateau at low temperatures. The prominent and mysterious "0.7 structure" is more clearly resolved at low densities. The thermal behavior of the conductance follows the predictions of the spin-incoherent transport. The 0.7 structure at a low density drops to $e^2/h$ in a smaller in-plane magnetic field. The features are qualitatively consistent with the theoretical predictions and provide evidence for the exis…
Quantum wire superlattices (1D) realized by controlled dislocation slipping in quantum well superlattices (2D) (atomic saw method) have already shown magnetophonon oscillations. This effect has been used to investigate the electronic properties of such systems and prove the quantum character of the physical properties of the wires. By cooling the temperature and using pulsed magnetic field up to 35 T, we have observed both quantum Hall effect (QHE) and Shubnikov de Haas (SdH) oscillations for various configurations of the magnetic field. The effective masses deduced from the values of the fund…
The conductance through a quantum wire of cylindrical cross section and a weak bulge is solved exactly for two electrons within the Landauer-Buettiker formalism. We show that this 'open' quantum dot exhibits spin-dependent Coulomb blockade resonances resulting in two anomalous structure on the rising edge to the first conductance plateau, one near 0.25(2e^2/h), related to a singlet resonance, and one near 0.7(2e^2/h), related to a triplet resonance. These resonances are generic and robust, occurring for other types of quantum wire and surviving to temperatures of a few degrees.
We study one dimensional wires with spin-orbit coupling. We show that in the presence of Zeeman field and strong electron-electron interaction a clean wire may form fractional helical liquid states with phenomenology similar to fractional quantum Hall liquids. Most notably, the wire's two terminal conductance is predicted to show fractional quantized conductance plateaus at low electron density. When the system is proximity-coupled to a superconductor, fractional Majorana bound states may be stabilized. We discuss how disorder destabilizes these fractional phases. Possible experimental realiza…
We study the conductance threshold of clean nearly straight quantum wires in the magnetic field. As a quantitative example we solve exactly the scattering problem for two-electrons in a wire with planar geometry and a weak bulge. From the scattering matrix we determine conductance via the Landauer-Buettiker formalism. The conductance anomalies found near 0.25(2e^2/h) and 0.75(2e^2/h) are related to a singlet resonance and a triplet resonance, respectively, and survive to temperatures of a few degrees. With increasing in-plane magnetic field the conductance exhibits a plateau at e^2/h, consiste…
The integer quantized conductance of one-dimensional electron systems is a well understood effect of quantum confinement. A number of fractionally quantized plateaus are also commonly observed. They are attributed to many-body effects, but their precise origin is still a matter of debate, having attracted considerable interest over the past 15 years. This review reports on experimental studies of fractionally quantized plateaus in semiconductor quantum point contacts and quantum wires, focusing on the 0.7 x 2e^2/h conductance anomaly, its analogs at higher conductances, and the zero bias peak …
We have studied quantum wires using the Green's function technique and the density-functional theory, calculating the electronic structure and the conductance. All the numerics are implemented using the finite-element method with a high-order polynomial basis. For short wires, i.e. quantum point contacts, the zero-bias conductance shows, as a function of the gate voltage and at a finite temperature, a plateau at around 0.7G_0. (G_0 = 2e^2/h is the quantum conductance). The behavior, which is caused in our mean-field model by spontaneous spin polarization in the constriction, is reminiscent of …
We study the anomalous conductance plateau around $G = 0.7(2e^{2}/h)$ and the zero-bias anomaly in ballistic hole quantum wires with respect to in-plane magnetic fields applied parallel $B_{\parallel}$ and perpendicular $B_{\perp}$ to the quantum wire. As seen in electron quantum wires, the magnetic fields shift the 0.7 structure down to $G = 0.5(2e^{2}/h)$ and simultaneously quench the zero bias anomaly. However, these effects are strongly dependent on the orientation of the magnetic field, owing to the highly anisotropic effective Landé \emph{g}-factor $g^{*}$ in hole quantum wires. Our resu…
We model one-dimensional transport through each open channel of a quantum wire by a Luttinger liquid with three different interaction parameters for the leads, the contact regions and the wire, and with two barriers at the contacts. We show that this model explains several features of recent experiments, such as the flat conductance plateaux observed even at finite temperatures and for different lengths, and universal conductance corrections in different channels. We discuss the possibility of seeing resonance-like structures of a fully open channel at very low temperatures.
The quantum phase diagram of disordered wires in a strong magnetic field is studied as a function of wire width and energy. The two-terminal conductance shows zero-temperature discontinuous transitions between exactly integer plateau values and zero. In the vicinity of this transition, the chiral metal-insulator transition (CMIT), states are identified that are superpositions of edge states with opposite chirality. The bulk contribution of such states is found to decrease with increasing wire width. Based on exact diagonalization results for the eigenstates and their participation ratios, we c…
We present Fokker-Planck equations that describe transport of heat and spin in dirty unconventional superconducting quantum wires. Four symmetry classes are distinguished, depending on the presence or absence of time-reversal and spin rotation invariance. In the absence of spin-rotation symmetry, heat transport is anomalous in that the mean conductance decays like $1/\sqrt{L}$ instead of exponentially fast for large enough length $L$ of the wire. The Fokker-Planck equations in the presence of time-reversal symmetry are solved exactly and the mean conductance for quasiparticle transport is calc…
Thermoelectric transport coefficients are determined for semiconductor quantum wires with weak thickness fluctuations. Such systems exhibit anomalies in conductance near 1/4 and 3/4 of 2e^2/h on the rising edge to the first conductance plateau, explained by singlet and triplet resonances of conducting electrons with a single weakly bound electron in the wire [T. Rejec, A. Ramsak, and J.H. Jefferson, Phys. Rev. B 62, 12985 (2000)]. We extend this work to study the Seebeck thermopower coefficient and linear thermal conductance within the framework of the Landauer-Buettiker formalism, which also …
We study the low-temperature transport properties of 1D quantum wires as the confinement strength V_conf and the carrier density n_1D are varied using a combination of split gates and a top gate in GaAs/AlGaAs heterostructures. At intermediate V_conf and n_1D, we observe a jump in conductance to 4e^2/h, suggesting a double wire. On further reducing n_1D, plateau at 2e^2/h returns. Our results show beginnings of the formation of an electron lattice in an interacting quasi-1D quantum wire. In the presence of an in-plane magnetic field, mixing of spin-aligned levels of the two wires gives rise to…
We theoretically investigate a quasi-one-dimensional quantum wire, where the lowest two subbands are populated, in the presence of a helical magnetic field. We uncover a backscattering mechanism involving the helical magnetic field and Coulomb interaction between the electrons. The combination of these ingredients results in scattering resonances and partial gaps which give rise to non-standard plateaus and conductance dips at certain electron densities. The positions and values of these dips are independent of material parameters, serving as direct transport signatures of this mechanism. Our …
We have studied the Fano effect in a few-electron quantum dot side-coupled to a quantum wire. The conductance of the wire, which shows an ordinal staircase-like quantization without the dot, is modified through the interference (the Fano effect) and the charging effects. These effects are utilized to verify the exhaustion of electrons in the dot. The "addition energy spectrum" of the dot shows a shell structure, indicating that the electron confinement potential is fairly circular. A rapid sign inversion of the Fano parameter on the first conductance plateau with the change of the wire gate vo…
We calculate the conductance of a ballistic point contact to a superconducting wire, produced by the s-wave proximity effect in a semiconductor with spin-orbit coupling in a parallel magnetic field. The conductance G as a function of contact width or Fermi energy shows plateaus at half-integer multiples of 4e^2/h if the superconductor is in a topologically nontrivial phase. In contrast, the plateaus are at the usual integer multiples in the topologically trivial phase. Disorder destroys all plateaus except the first, which remains precisely quantized, consistent with previous results for a tun…
We study ballistic hole transport through Ge/Si core/shell nanowires at low temperatures. We observe Fabry-P$\acute{e}$rot interference patterns as well as conductance plateaus at integer multiples of 2e$^2$/h at zero magnetic field. Magnetic field evolution of these plateaus reveals large effective Land$\acute{e}$ g-factors. Ballistic effects are observed in nanowires with silicon shell thicknesses of 1 - 3 nm, but not in bare germanium wires. These findings inform the future development of spin and topological quantum devices which rely on ballistic subband-resolved transport.
We demonstrate that, at the onset of conduction, an electrostatically defined quantum wire in bilayer graphene (BLG) with an interlayer asymmetry gap may act as a 1D semimetal, due to the multiple minivalley dispersion of its lowest subband. Formation of a non-monotonic subband coincides with a near-degeneracy between the bottom edges of the lowest two subbands in the wire spectrum, suggesting an $8e^2/h$ step at the conduction threshold, and the semimetallic behaviour of the lowest subband in the wire would be manifest as resonance transmission peaks on an $8e^2/h$ conductance plateau.
We report conductance measurements of a ballistic one-dimensional (1D) wire defined in the lower two-dimensional electron gas of a GaAs/AlGaAs double quantum well. At low temperatures there is an additional structure at $0.7(2e^2/h)$ in the conductance, which tends to $e^2/h$ as the electron density is decreased. We find evidence for complete spin polarization in a weakly disorderd 1D wire at zero magnetic field through the observation of a conductance plateau at $e^2/h$, which strengthens in an in-plane magnetic field and disappears with increasing electron density. In all cases studied, with…
We consider the relaxation of a spin qubit in a quantum dot propagating as a whole in a one-dimensional semiconductor with hyperfine coupling. We show that this motion leads to qualitatively new features in this process compared to static quantum dots. For a fast straightforward motion, the initial spin density decreases to zero with the relaxation rate independent of the spatial spread of the electron wave function and inversely proportional to the electron speed. However, for the oscillatory motion, the qubit acquires memory, and the dephasing becomes Gaussian rather than exponential. After …
We study electron transport properties of some molecular wires and a unconventional disordered thin film within the tight-binding framework using Green's function technique. We show that electron transport is significantly affected by quantum interference of electronic wave functions, molecule-to-electrode coupling strengths, length of the molecular wire and disorder strength. Our model calculations provide a physical insight to the behavior of electron conduction across a bridge system.
Structural disorder is present in almost all experimental measurements of electronic transport through single molecules or molecular wires. To assess its influence on the conductance is computationally demanding, because a large number of conformations must be considered. Here we analyze an approximate recursive layer Green function approach for the ballistic transport through quasi one-dimensional nano-junctions. We find a rapid convergence of the method with its control parameter, the layer thickness, and good agreement with existing experimental and theoretical data. Because the computation…
We investigate nuclear spin effects in a two-dimensional electron gas in the quantum Hall regime modeled by a weakly coupled array of interacting quantum wires. We show that the presence of hyperfine interaction between electron and nuclear spins in such wires can induce a phase transition, ordering electrons and nuclear spins into a helix in each wire. Electron-electron interaction effects, pronounced within the one-dimensional stripes, boost the transition temperature up to tens to hundreds of millikelvins in GaAs. We predict specific experimental signatures of the existence of nuclear spin …
We study the temperature dependence of the electrical conductance of a clean strongly interacting quantum wire in the presence of a helical nuclear spin order. The nuclear spin helix opens a temperature-dependent partial gap in the electron spectrum. Using a bosonization framework we describe the gapped electron modes by sine-Gordon-like kinks. We predict an internal resistivity caused by an Ohmic-like friction these kinks experience via interacting with gapless excitations. As a result, the conductance rises from $G=e^2/h$ at temperatures below the critical temperature when nuclear spins are …
Sharp localization transitions of chiral edge states in disordered quantum wires, subject to strong magnetic field, are shown to be driven by crossovers from two- to one-dimensional localization of bulk states. As a result, the two-terminal conductance is found to exhibit at zero temperature discontinuous transitions between {\it exactly} integer plateau values and zero, reminiscent of first order phase transitions. We discuss the corresponding phase diagram. The spin of the electrons is shown to result in a multitude of phases, when the spin degeneracy is lifted by the Zeeman energy. The widt…
In one-dimensional electronic systems with strong repulsive interactions, charge excitations propagate much faster than spin excitations. Such systems therefore have an intermediate temperature range [termed the "spin-incoherent Luttinger liquid'" (SILL) regime] where charge excitations are "cold" (i.e., have low entropy) whereas spin excitations are "hot." We explore the effects of charge-sector disorder in the SILL regime in the absence of external sources of equilibration. We argue that the disorder localizes all charge-sector excitations; however, spin excitations are protected against ful…
We report the experimental observation of quantized conductance by Rashba bands at the surface of 50-nm Bi nanowires. With increasing magnetic fields along the wire axis, the wires exhibit a stepwise increase of conductance with as many as four distinct plateaus together with oscillatory thermopower. The observations can be accounted for by the increase of the number of propagating surface modes. The modes have very high mobility and are associated with Aharonov-Bohm interference around the perimeter consistently with theory of quasiballistic one-dimensional nanowires considering that Lorentz …
