Single-channel Wire Research Articles

Mechanism and Applications of Electrical Stimulation Disturbance on Motoneuron Excitability Studied Using Flexible Intramuscular Electrode.

Wearable and implantable devices are irreplaceable components in the modern healthcare system. Electrical stimulation on the nervous and neuromuscular system, as a way of therapeutic interventions, has been widely applied to people with neurological disorders and neuromuscular disabilities. The conventional way to study electrical stimulation on the skeletal muscle employs single-channel wire electrodes, which have limited capability to explore the complicated motoneuron distribution in muscle tissue. Here, a microfabricated flexible multiple-channel intramuscular electrode is presented, which enables the study of electrical stimulation using electrode sites of different spatial arrangements with respect to the motoneuron distribution. Observations are reported on slow disturbance on motoneuron excitability induced by large-distance electrodes targeting at the end motor nerves, as well as fast disturbance induced by small-distance electrodes targeting at the main motor nerve trunk. The phenomena of slow and fast disturbance have different potential applications in the field of neuromodulation. In the case of slow disturbance, force output is predictable and shows gradual change, which is suitable for accurately controlled functional electrical stimulation (FES). For fast disturbance, the disappearance of force output opens the possibility for muscle conduction block applications, which can be used for treatment of muscle sparsity by blocking the involuntary motor intentions.

Excess equilibrium noise in a topological SNS junction between chiral Majorana liquids

We consider a Josephson contact mediated by 1D chiral modes on a surface of a 3D topological insulator with superimposed superconducting and magnetic layers. The system represents an interferometer in which 1D chiral Majorana modes on the boundaries of superconducting electrodes are linked by ballistic chiral Dirac channels. We investigate the noise of the Josephson current as a function of the dc phase bias and the Aharonov-Bohm flux. Starting from the scattering formalism, a Majorana representation of the Keldysh generating action for cumulants of the transmitted charge is found. At temperatures higher than the Thouless energy $E_{\rm Th}$, we obtain the usual Johnson-Nyquist noise, $4G_0 k_{\rm B}T$, characteristic for a single-channel wire with $G_0 \equiv e^2/(2\pi \hbar)$. At lower temperatures the behavior is much richer. In particular, the equilibrium noise is strongly enhanced to a temperature-independent value $\sim G_0 E_{\rm Th}$ if the Aharonov-Bohm and superconducting phases are both close to $2\pi n$, which are points of emergent degeneracy in the ground state of the junction. The equilibrium noise is related to the Josephson junction's impedance via the fluctuation-dissipation theorem. In a striking contrast to usual Josephson junctions (tunnel junctions between two $s$-wave superconductors), the real part of the impedance does not vanish, reflecting the gapless character of Majorana modes in the leads.

Cooper pair splitting in ballistic ferromagnetic SQUIDs

We consider ballistic SQUIDs with spin filtering inside half-metallic ferromagnetic arms. A singlet Cooper pair cannot pass through an arm in this case, so the Josephson current is entirely due to the Cooper pair splitting, with two electrons going to different interferometer arms. In order to elucidate the mechanisms of Josephson transport due to split Cooper pairs, we assume the arms to be single-channel wires in the short-junction limit. Different geometries of the system (determined by the length of the arms and the phases acquired by quasiparticles during splitting between the arms) lead to qualitatively different behavior of the SQUID characteristics (the Andreev levels, the current-phase relation, and the critical Josephson current) as a function of two control parameters, the external magnetic flux and misorientation of the two spin filters. The current-phase relation can change its amplitude and shape, in particular, turning to a pi-junction form or acquiring additional zero crossings. The critical current can become a nonmonotonic function of the misorientation of the spin filters and the magnetic flux (on half of period). Periodicity with respect to the magnetic flux is doubled, in comparison to conventional SQUIDs.

Effects of impurity scattering on the quantized conductance of a quasi-one-dimensional quantum wire

We report an experimental observation of how the presence of an impurity in a quasi-one-dimensional wire influences the quantized conductance. The impurity is a chemically etched nanohole, relative to which the walls of the wire can be tuned via external gate voltages. Depending on the positions of the sidewalls, resonance features are observed in the quantized conductance due to either the multiple scatterings between the impurity and the wall of wire or the channel interference. Meanwhile, the differential conductance exhibits the well-known half-plateau features in a single channel wire or saturates in a wire with coupled two channels.

Sagnac interference in carbon nanotubes

The Sagnac interference mode arises when two interfering counterpropogating beams traverse a loop, but with their velocities detuned by a small amount $2u$, with ${v}_{R/L}={v}_{F}\ifmmode\pm\else\textpm\fi{}u$. In this paper we perform a perturbative nonequilibrium calculation of Sagnac interference in single-channel wires as well as armchair nanotube loops. We study the dependence of the Sagnac conductance oscillations on temperature and interactions. We find that the Sagnac interference is not destroyed by strong interactions, but becomes weakly dependent on the velocity detuning $u$. In armchairs nanotubes with typical interaction strength, $0.25\ensuremath{\le}g\ensuremath{\le}0.5$, we find that the necessary temperature for observing the interference effect, ${T}_{\text{SAG}}$ is also only weakly dependent on the interaction, and is enhanced by a factor of 8 relative to the temperature necessary for observing Fabry-P\'erot interference in the same system, ${T}_{\text{FP}}$.

Kondo screening cloud and charge staircase in one-dimensional mesoscopic devices

We propose that the finite size of the Kondo screening cloud, xi(K), can be probed by measuring the charge quantization in a one-dimensional system coupled to a small quantum dot. When the chemical potential mu in the system is varied at zero temperature, one should observe charge steps whose locations are at values of mu that are controlled by the Kondo effect when the system size L is comparable to xi(K). We show that, if the standard Kondo model is used, the ratio between the widths of the Coulomb blockade valleys with odd or even number of electrons is a universal scaling function of xi(K)/L. If we take into account electron-electron interactions in a single-channel wire, this ratio also depends on the parameters of the effective Luttinger model; in addition, the scaling is weakly violated by a marginal bulk interaction. For the geometry of a quantum dot embedded in a ring, we show that the dependence of the charge steps on a magnetic flux through the ring is controlled by the size of the Kondo screening cloud.

Pumping spin with electrical fields

Spin currents can be obtained through adiabatic pumping by means of electrical gating only. This is possible by making use of the tunability of the Rashba spin-orbit coupling in semiconductor heterostructures. We demonstrate the principles of this effect by considering a single-channel wire with a constriction. We also consider realistic structures, consisting of several open channels where subband spin-mixing and disorder are present, and we confirm our predictions. Two different ways to detect the spin-pumping effect, either using ferromagnetic leads or applying a magnetic field, are discussed.

Transmission throigh barriers and resonant tunneling in a Luttinger liquid

We present a review of the theory of the single channel Luttinger liquuid applied to transport in nanostructure devices. Our main focus will be on the effects of barriers, or constrictions in a single channel wire. In particular, we show that repulsively interacting electrons incident upon a single barrier are completely reflected at zero temperature ( T). At finite T, the conductance vanishes as a power of T, and at T = 0, power law current-voltage characteristics are predicted. We next discuss resonant tunneling through a double barrier structure and show that the resonance peaks have a width which vanishes as a power of T as T → 0. Moreover, the T dependence of the resonance line shapes are described by a universal scaling function, which has non-Lorentzian tails. Possible experimental realizations of this are discussed, including a connection to edge transport in the fractional quantum Hall regime.

Resonant tunneling in an interacting one-dimensional electron gas.

We study the resonant tunneling of a single-channel interacting electron gas through a double-barrier structure. In striking contrast to the noninteracting electron gas, which exhibits resonances with a temperature-independent Lorentzian line shape at low T, we find that with repulsive interactions present the resonances have a width which vanishes as T\ensuremath{\rightarrow}0. Moreover, at low T the resonance line shapes are determined by a universal scaling function, with power law but non-Lorentzian tails. These predictions should be accessible to experiments in single-channel wires in gated GaAs.

A velocity-modulation television system

The paper describes a complete television system operating on the velocity-modulation principle. The theory, advantages, and limitations, of the principle are discussed, and it is shown how these limitations are overcome by the use of a further principle, that of intensification.The practical development of the system is described in detail, particular attention being given to the transmitting end, where the most fundamental problems are encountered.The results obtained to date (with single-channel wire transmission) are specified.The complete system is shown to be characterized by the following features:—(a) Absence of synchronizing problem in the line-scanning direction.(b) Simple solution of synchronizing problem in the picturetraversing direction, including automatic framing.(c) Greatly relaxed modulation requirements on the receiving oscillograph.(d) Increased picture-brightness for a given receiving oscillograph as compared with the intensity-modulation system.(e) Favourable concentration of detail in the light portions of the picture.(f) Nearly constant percentage modulation of radio transmitter.

A velocity-modulation television system

The paper describes a complete television system operating on the velocity-modulation principle. The theory, advantages, and limitations, of the principle are discussed, and it is shown how these limitations are overcome by the use of a further principle, that of intensification. The practical development of the system is described in detail, particular attention being given to the transmitting end, where the most fundamental problems are encountered. The results obtained to date (with single-channel wire transmission) are specified. The complete system is shown to be characterized by the following features:? (a) Absence of synchronizing problem in the line-scanning direction. (b) Simple solution of synchronizing problem in the picturetraversing direction, including automatic framing. (c) Greatly relaxed modulation requirements on the receiving oscillograph. (d) Increased picture-brightness for a given receiving oscillograph as compared with the intensity-modulation system. (e) Favourable concentration of detail in the light portions of the picture. (f) Nearly constant percentage modulation of radio transmitter.