Ballistic Wires Research Articles

Onset of energy dissipation in ballistic atomic wires.

Electronic transport at finite voltages in free-standing gold atomic chains of up to seven atoms in length is studied at low temperatures using a scanning tunneling microscope. The conductance vs voltage curves show that transport in these single-mode ballistic atomic wires is nondissipative up to a finite voltage threshold of the order of several mV. The onset of dissipation and resistance within the wire corresponds to the excitation of the atomic vibrations by the electrons traversing the wire and is very sensitive to strain.

Phase coherent electronics: a molecular switch based on quantum interference.

The phenomenon of quantum mechanical interference may be used to control the conductivity of ballistic molecular wires. Using a simple model we demonstrate plausible effects and discuss its potential uses for constructing coherence-based molecular electronics.

Charge and low-frequency response of normal-superconducting heterostructures

Charge distribution is a basic aspect of electrical transport. In this work we investigate the self-consistent charge response of normal-superconducting heterostructures. Of interest is the variation of the charge density due to voltage changes at contacts and due to changes in the electrostatic potential. We present response functions in terms of functional derivatives of the scattering matrix. We use these results to find the dynamic conductance matrix to lowest order in frequency. We illustrate similarities and differences between normal systems and heterostructures for specific examples such as a ballistic wire and a quantum point contact.

CLASSICAL TO QUANTUM CROSSOVER IN HIGH-CURRENT NOISE OF ONE-DIMENSIONAL BALLISTIC WIRES

Microscopic current fluctuations and conductance are inseparable. We present a unified kinetic theory of both quantized conductance and nonequilibrium noise in one-dimensional ballistic wires. We show that high-current ballistic fluctuations depend, directly and robustly, on carrier statistics. This is dramatically evident in the large noise peaks predicted to coincide with the quantized steps in the two-probe conductance. The outstanding features of nonlinear ballistic fluctuations are: (i) their observability by standard experimental techniques; (ii) their strong suppression by electron degenercy; (iii) their divergence in any model that ignores the explicit dynamics of dissipative scattering in the macroscopic leads; (iv) their numerical dominance over the noise predictions of the Landauer-Büttiker model. The nonequilibrium noise of hot ballistic carriers is strikingly sensitive to their inelastic energy loss on entering the leads. Uniquely and quantitatively, it characterizes the observed nonideality of quantized conductance.

Effect of temperature and magnetic field on the 0.7 structure in a ballistic one-dimensional wire

Low temperature measurements of ballistic one-dimensional (1D) wires show an additional structure at 0.7(2 e 2/ h) in the conductance characteristics (Phys. Rev. Lett. 77 (1996) 135; Phys. Rev. B 62 (2000) 10 950). This 0.7 structure develops into a plateau at e 2/ h, if, either the in-plane magnetic field is increased (Phys. Rev. Lett. 77 (1996) 135) or the carrier density is decreased (Phys. Rev. B 61 (2000) 13 365). A suggested explanation for this effect is based on spin polarisation induced by many-body interactions. As temperature increases, the e 2/ h plateau in a magnetic field shifts to 0.6(2 e 2/ h), whereas other plateaus remain unchanged. We study the effect of temperature on the evolution of the 0.7 structure with increasing in-plane magnetic field.

Multisublevel magnetoquantum conductance in single and coupled double quantum wires

We study the ballistic and diffusive magnetoquantum transport using a typical quantum point contact geometry for single and tunnel-coupled double wires that are wide (less than or similar to1 mum) in one perpendicular direction with densely populated sublevels and extremely confined in the other perpendicular (i.e., growth) direction. A general analytic solution to the Boltzmann equation is presented for multisublevel elastic scattering at low temperatures. The solution is employed to study interesting magnetic-field dependent behavior of the conductance such as a large enhancement and quantum oscillations of the conductance for various structures and field orientations. These phenomena originate from the following field-induced properties: magnetic confinement, displacement of the initial- and final-state wave functions for scattering, variation of the Fermi velocities, mass enhancement, depopulation of the sublevels and anticrossing (in double quantum wires). The magnetoconductance is strikingly different in long diffusive (or rough. dirty) wires from the quantized conductance in short ballistic (or clean) wires. Numerical results obtained for the rectangular confinement potentials in the growth direction are satisfactorily interpreted in terms of the analytic solutions based on harmonic confinement potentials. Some of the predicted features of the field-dependent diffusive and quantized conductances are consistent with recent data from GaAs/AlxGa1-xAs doublemore » quantum wires.« less

IQHE and FQHE in a wire with incompressible and compressible strips

We report the study of the longitudinal (RL) and Hall (RH) resistances in a wide ballistic wire (LH⪡W⪡l, l is mean free path, W is the width of wire, LH is the magnetic length in QHE regime) in the IQHE and FQHE regimes. It was observed a partial suppression of the RL minima in the IQHE and their practically total disappearance in the FQHE while the Hall resistance plateaus corresponding to these minima remained well pronounced in both the IQHE and FQHE regimes. At lower filling factors (ν<1/3) we have observed a quantum Hall liquid–quantized Hall insulator transition that takes place at a certain critical magnetic field Bc and a critical resistance value close to h/e2. Finally, at ν<1/2 and at the lowest experimental temperatures T<0.3K there have been observed correlated fluctuations in the longitudinal and in the Hall resistance both as a function of magnetic field and gate voltage. It is supposed that the observed features are due to interplay between wide compressible and narrow incompressible strips in the wire.

Direction-resolved transport and possible many-body effects in one-dimensional thermopower

A single-particle theory due to Mott predicts a proportionality between the diffusion thermopower and the energy derivative of the logarithm of the conductance. Measurements of a ballistic 1D wire show that the Mott theory remains valid in the presence of a finite current, and that it leads to a direction-sensitive probe of electron transport. We observe an apparent violation of the Mott model at low electron densities, when there is a nonquantized plateau in the conductance at ${0.7(2e}^{2}/h).$ There is as yet no successful theoretical explanation of this so called 0.7 structure, but the distinctive thermopower signature, which deviates from single-particle predictions, may provide the key to a better understanding.

Bonding and antibonding states in strongly coupled ballistic one-dimensional wires

Low-temperature conductance measurements of strongly coupled ballistic one-dimensional (1D) wires show the formation of bonding and antibonding 1D states. Two parallel 1D wires are defined by a split-gate structure deposited on a GaAs/AlGaAs double quantum well with a 25 Å AlGaAs barrier. A midline gate positioned at the centre of the split-gate allows both the widths and electron densities of the two wires to be tuned. At resonance when the wires are matched, symmetric and antisymmetric 1D energy levels are formed, and are separated by an energy gap Δ SAS. The overlap integral that describes the mixing of the 1D wave functions of different index in the two wires can be altered with a small in-plane magnetic field B. We also present preliminary measurements of the mixing of edge states in the quantum Hall regime.

Focused multi-peaks in gated ballistic wires

We have observed several symmetrical paired peaks against the zero field in the magneto-resistance of the double corrugation gated wires. These symmetrical peaks can be explained by each commensurate trajectory due to an internal magnetic focusing in the ballistic cavity formed in the quantum dot of the corrugation unit.

Multi-terminal scattering approach to conductance and noise at scanning tunnelling microscope tips

Experiments that measure the average current and current fluctuations at one or two local tunnelling contacts on a mesoscopic multiprobe conductor are proposed and theoretically investigated. The average current and the current fluctuations at a single tunnelling tip are determined by an effective local distribution function , which is given as the product of local partial densities of states (injectivities) and the Fermi distribution functions in the electron reservoirs. The current correlations at two tips connected to a phase-coherent conductor are determined by spatially non-diagonal density of states elements and can give information about the phase and correlations of wavefunctions. All results are illustrated for measurements on a ballistic wire and on a metallic diffusive wire. Copyright © 1999 John Wiley & Sons, Ltd.

Charge Relaxation Resistances and Charge Fluctuations in Mesoscopic Conductors

A brief overview is presented of recent work which investigates the time-dependent relaxation of charge and its spontaneous fluctuations on mesoscopic conductors in the proximity of gates. The leading terms of the low frequency conductance are determined by a capacitive or inductive emittance and a dissipative charge relaxation resistance. The charge relaxation resistance is determined by the ratio of the mean square dwell time of the carriers in the conductor and the square of the mean dwell time. The contribution of each scattering channel is proportional to half a resistance quantum. We discuss the charge relaxation resistance for mesoscopic capacitors, quantum point contacts, chaotic cavities, ballistic wires and for transport along edge channels in the quantized Hall regime. At equilibrium the charge relaxation resistance also determines via the fluctuation-dissipation theorem the spontaneous fluctuations of charge on the conductor. Of particular interest are the charge fluctuations in the presence of transport in a regime where the conductor exhibits shot noise. At low frequencies and voltages charge relaxation is determined by a nonequilibrium charge relaxation resistance.

Interacting electrons with spin in a one-dimensional dirty wire connected to leads

We investigate a one--dimensional wire of interacting electrons connected to one--dimensional noninteracting leads in the absence and in the presence of a backscattering potential. The ballistic wire separates the charge and spin parts of an incident electron even in the noninteracting leads. The Fourier transform of nonlocal correlation functions are computed for $T\gg \omega$. In particular, this allows us to study the proximity effect, related to the Andreev reflection. A new type of proximity effect emerges when the wire has normally a tendency towards Wigner crystal formation. The latter is suppressed by the leads below a space--dependent crossover temperature; it gets dominated everywhere by the $2k_F$ CDW at $T<L^{3/2 (K-1)}$ for short range interactions with parameter $K<1/3$. The lowest--order renormalization equations of a weak backscattering potential are derived explicitly at finite temperature. A perturbative expression for the conductance in the presence of a potential with arbitrary spatial extension is given. It depends on the interactions, but is also affected by the noninteracting leads, especially for very repulsive interactions, $K<1/3$. This leads to various regimes, depending on temperature and on $K$. For randomly distributed weak impurities, we compute the conductance fluctuations, equal to that of $R=g-2e^2 /h$. While the behavior of $Var(R)$ depends on the interaction parameters, and is different for electrons with or without spin, and for $K<1/3$ or $K>1/3$, the ratio $Var(R)/R^2$ stays always of the same order: it is equal to $L_T/L\ll 1$ in the high temperature limit, then saturates at 1/2 in the low temperature limit, indicating that the relative fluctuations of $R$ increase as one lowers the temperature.

Quantum transmission in narrow ballistic wires containing a superconductor island

We investigate the transmission properties in quantum wires with a superconducting island placed in the center of the channel. The conductance quantization is destroyed, unless the Fermi energies in the quantum wire and the superconductor are identical as a consequence of the single-particle reflection from the normal-conductor–superconductor interface. The conductance exhibits fluctuations due to the quantum interference effect in the island. The fluctuation amplitude is found to depend strongly on the pair potential amplitude in the superconductor.

Closely separated one-dimensional wires:: coupled ballistic conduction, wave function hybridization and compressibility investigations

We report the experimental study of a system comprising two parallel quasi-one-dimensional (1D) ballistic wires formed in a GaAs-Al x Ga 1− x As double quantum well structure. A unique surface gate geometry allows the device to be tuned smoothly from a configuration comprising a pair of vertically aligned 1D ballistic wires, to a single 1D ballistic wire in either the upper or lower 2DES. We investigate coupled ballistic conduction in two samples with contrasting wire separations (35 and 300 Å). The strongly coupled sample exhibits a cross-over from double to single wire conduction as the number of occupied 1D subbands is reduced. The weakly coupled sample is used to observe directly variations in the compressibility of a single ballistic channel arising from the discrete 1D density of states.

Finite size corrections to the conductance of ballistic wires.

The dependence of the conductance of ballistic wires on the work function of different metals is analyzed. We show that quantum finite-size effects can be taken into account by a simple correction to the Sharvin formula. Although the conductance is suppressed relative to the classical Sharvin limit, the correction is much smaller than that obtained from calculations based on ``hard-wall'' boundary conditions. Different sources of errors in the determination of actual cross sections in atomic-scale contacts from conductance measurements are discussed. \textcopyright{} 1996 The American Physical Society.

Disorder-induced enhancement of the quantum size effects in quantum wires with a tunnel barrier.

In this paper, we report results of a theoretical study that demonstrates an unexpected effect of disorder on the conductance of quantum wires. It is known that disorder suppresses quantum size effects arising from the lateral confinements in the structure of the conductance. The presence of a tunnel barrier in ballistic wires also destroys the quantum size effects. However, in wires with a weak barrier, introduction of disorder can recover the suppressed quantum size effects. We present a qualitative explanation for this interesting result. \textcopyright{} 1996 The American Physical Society.

Scaling properties of universal conductance fluctuations in quasi ballistic narrow wires

We study the scaling properties of universal conductance fluctuations, observed in the magneto-resistance of quasi ballistic, split gate wires. In such devices the correlation field analysis is not simply determined by theoretical predictions for corresponding diffusive systems, and we discuss this breakdown in terms of a spreading of phase coherent electron interference, into the ungated regions of the devices. In order to characterise this effect we develop a simple model, in which the degree of spreading is represented by a single parameter γ. Performing this analysis we find that γ scales as a simple function of the wire length and width, enabling us to characterise the interplay of interference from the different regions of the split gate wire.

Conductance of quantum waveguides with a rough boundary

A quantum mechanical calculation of the conductance of quantum wires in the presence of boundary scattering is presented. The scattering-matrix method is used to evaluate the transmission coefficients through electron waveguides with a random fluctuation in the local width of the channel. The conductance quantization of ballistic wires breaks down for strong disorder. The authors find that the conductance has a dip when a new propagating mode opens. The conductance is suppressed exponentially as the length of the wire is increased, demonstrating localization of electron states in the quasi-one-dimensional system. The dependences of the localization length on the parameters of the wires are examined. The probability distribution of the conductance due to coherent scattering from the rough boundary is found to possess a long tail.

Quantum conductance fluctuations in 3d ballistic adiabatic wires

We predict quantum conductance fluctuations in three-dimensional (3D) adiabatic wires rising from fluctuations in the energy-spectrum of size-quantized electron motion across the channel. These fluctuations could be observed as random oscillations in the voltage dependence of the differential conductancedI/dV or the magnetic field dependence. They obey Poisson or Wigner-Dyson statistics depending on the cross sectional shape of the wire and can have an amplitude which exceeds the universale2/h value.