Narrow Wires Research Articles

Interface Phonons and Polaron Effect in Quantum Wires.

The theory of large radius polaron in the quantum wire is developed. The interaction of charge particles with interface optical phonons as well as with optical phonons localized in the quantum wire is taken into account. The interface phonon contribution is shown to be dominant for narrow quantum wires. The wave functions and polaron binding energy are found. It is determined that polaron binding energy depends on the electron mass inside the wire and on the polarization properties of the barrier material.

Nanopatterned Metallic Films for Use As Transparent Conductive Electrodes in Optoelectronic Devices

We investigate the use of nanopatterned metallic films as transparent conductive electrodes in optoelectronic devices. We find that the physics of nanopatterned electrodes, which are often optically thin metallic films, differs from that of optically thick metallic films. We analyze the optical properties when performing a geometrical transformation that maintains the electrical properties. For one-dimensional patterns of metallic wires, the analysis favors tall and narrow wires. Our design principles remain valid for oblique incidence and readily carry over to two-dimensional patterns.

Asymmetric magnetic NOT gate and shift registers for high density data storage

We have developed an asymmetric ferromagnetic NOT gate and shift register optimized on a square grid. This gives rise to a two-dimensional storage scheme built up by tessellating an elementary data unit, which is scalable down to very narrow wire widths. The areal footprint of each storage unit is 15F2, where F is the minimum feature size. We experimentally demonstrate NOT operations across a chain of three gates made from Permalloy with F=60 nm, and present a functional 15-gate, multichannel shift register with electrical injection, and optical readout.

Design of temperature-independent arrayed waveguide gratings based on the combination of multiple types of waveguide

We develop a design theory for a temperature-independent arrayed waveguide grating (TI-AWG) based on the combination of multiple types of waveguide. Each type of waveguide has a path-length difference between adjacent arrayed waveguides, and the path-length difference ratio is introduced as tuning parameter. A TI-AWG with Si wire and slot waveguides is given as an example. The thermal spectra shift of the TI-AWG can be tuned from redshift to blueshift in an ultralarge range, and the modified interference order can be reduced or enhanced. The device size is about one-fifth that of the narrow-wide-wire design that uses a combination of narrow and wide Si wire waveguides. The results are verified by the simulation of prototype devices via a two-dimensional finite-difference time-domain program.

Polaronic excitons in an unstrained GaAs/AlGaAs quantum wire

Energy spectrum of single excitons in an unstrained GaAs/AlGaAs quantum wire is studied. We perform calculations within the single band effective mass approximation but including band non-parabolicity in the first order approximation. Exciton binding energy, with the inclusion of exciton–phonon interaction, is calculated as a function of wire radius for light-hole and heavy-hole states, and Wannier exciton. The numerical calculations reveal that the influences of phonon on exciton are considerable and should not be neglected especially for narrow wires. The results show that (i) the heavy-hole exciton in a cylindrical quantum wire is more strongly bound than the light-hole exciton, (ii) the corrections to the total mass and reduced mass owing to interaction with the phonons are significant for narrow thin wire and they depend not on only the size of the wire but also the types of the exciton and (iii) the variation of increase in binding energy is observed when non-parabolicity is included and this effect is predominant for the narrow well wires. These results are discussed with the available data in the literature.

Crystalline analysis of permalloy narrow wires subject to current pulses

We investigate crystalline structures of a permalloy nanowire subject to current pulses by using standard transmission and scanning electron microscope techniques, together with the analysis of magnetic states by means of Lorenz microscopy. We find that crystalline grains grow slightly larger in a current region around the Curie temperature, which does not affect significantly the uniformly-magnetized state in the wire because of strong uniaxial shape anisotropy given by the wire geometry. When crystalline grains grow large enough to additionally provide in-plane anisotropy comparable to the shape anisotropy by averaging crystalline anisotropy over the grains, the magnetic ripple will be induced in the wire. Such situations can be found in the case when very large current pulses are applied to the wire.

Dimensional dependence of weak localization corrections and spin relaxation in quantum wires with Rashba spin-orbit coupling

The quantum correction to the conductivity in disordered quantum wires with linear Rashba spin-orbit coupling is obtained. For quantum wires with spin-conserving boundary conditions, we find a crossover from weak anti- to weak localization as the wire width W is reduced using exact diagonalization of the Cooperon equation. This crossover is due to the dimensional dependence of the spin relaxation rate of conduction electrons, which becomes diminished, when the wire width is smaller than the bulk spin precession length $L_{SO}$. We thus confirm previous results for small wire width, $W/L_{SO}<= 1$ [PRL98,176808(2007)], where only the transverse 0-modes of the Cooperon equation had been taken into account. We find that spin helix solutions become stable for arbitrary ratios of linear Rashba and Dresselhaus coupling in narrow wires. For wider wires, the spin relaxation rate is found to be not monotonous as function of wire width: it becomes first enhanced for W on the order of the bulk $L_{SO}$ before it becomes diminished for smaller wire widths. In addition, we find that the spin relaxation is smallest at the edge of the wire for wide wires. The effect of the Zeeman coupling to the magnetic field perpendicular to the 2D electron system is studied and found that it shifts the crossover from weak anti- to weak localization to larger wire widths $W_c$. When the transverse confinement potential of the quantum wire is smooth (adiabatic), the spin relaxation rate is found to be enhanced as W is reduced. We find that only a spin polarized state retains a finite spin relaxation rate in such narrow wires. Thus, we conclude that the injection of polarized spins into nonmagnetic quantum wires should be favorable in wires with smooth confinement potential. Finally, in wires with tubular shape, corresponding to transverse periodic boundary conditions, we find no reduction of the spin relaxation rate.

Spin and phase coherence lengths inn-InSbquasi-one-dimensional wires

We present measurements of the magnetoconductance of quasi-one-dimensional wires fabricated on a two-dimensional electron system in an InSb/InAlSb heterostructure. The width and temperature dependence of the spin and phase coherence lengths in the narrow wires are examined by analyzing the magnetoconductance in antilocalization theory, modified to account for ballistic transport. The experiments indicate that the confined geometry can enhance spin coherence lengths in systems not in the motional narrowing regime and in the presence of strong cubic Dresselhaus spin-orbit interaction. Experimentally, the spin coherence lengths are found to be inversely proportional to wire width and to display a weak temperature dependence. For all wire widths the phase coherence length, after correction for finite length effects, shows a temperature dependence indicative of phase decoherence via the one-dimensional Nyquist mechanism.

Crossover from Fermi Liquid to Multichannel Luttinger Liquid in High-Mobility Quantum Wires

We investigate the electrical conductance of long, high-mobility quantum wires formed by the split-gate technique, which allows for adjustment of the wire width and the number of one-dimensional electron subbands, n. In wires with 3<or=n<or=8, a logarithmic temperature dependence of the conductance is observed for 1<T<10 K, which reaches as much as 30% of the Drude conductance. In even narrower wires, the logarithmic dependence changes to a power-law variation. Our observations are shown to be in good agreement with recent theoretical studies, which attribute the logarithmic term to interaction effects in a weakly disordered quasi-one-dimensional conductor. This interaction correction is associated with the emergence of a crossover from a quasi-one-dimensional weakly disordered Fermi liquid to a multichannel Luttinger liquid.

The binding energy of excitons in a cylindrical quantum wire under the influence of laser field intensity

We study the effects of laser field intensity over the ground state binding energy of light and heavy hole excitons confined in GaAs/Ga 1− x Al x As cylindrical quantum wire. We have applied the variational method using 1s-hydrogenic wavefunctions, in the framework of the single band effective mass approximation and computed the exciton binding energy as a function of the wire radius for different field of laser intensities. The valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. The results show that (i) the binding energy is found to increase with decrease in the wire radius, and decrease with increase in the value of laser field amplitude, (ii) the heavy-hole exciton in a cylindrical quantum wire is more strongly bound than the light-hole exciton and (iii) the binding energy of the impurity for the narrow wire is more sensitive to the laser field amplitude.

Influence on the Electro-Migration Resistance by Line Width and Average Grain Size along the Longitudinal Direction of Very Narrow Cu Wires

The influence of microstructures of Cu wires on electromigration (EM) resistance has been investigated using Cu wires with various line widths from 50 to 280 nm and line heights of 300 nm or 500 nm. A strong line width dependence of median time to failure was observed as the EM resistance decreases substantially with narrowing of line widths from 280 nm to 50 nm. The activation energies for 50, 80, 100 and 140 nm widths were 0.61, 0.64, 0.68 and 0.71 eV. The EM resistance and activation energy of Cu wires could be represented as a function of the ratio of line length to average grain size, which corresponds to the number of grains along the longitudinal direction, and they both increased as this ratio decreased. The influence of this ratio was particularly significant when the line width was below 100 nm. These results indicate that coarsening of grain sizes in the current flow direction is mandatory to enhance EM resistance and lower resistivity for the very narrow Cu wires.

Experimental demonstration of resonant spin-orbit interaction effect

We investigated the spin lifetime in gate-fitted InGaAs narrow wires by magnetotransport measurement. By applying positive gate bias voltage, the spin lifetime in narrow wires became more than one order longer than those obtained from a Hall bar sample with two-dimensional electron gas. By comparison with a theoretical model of the quasi one-dimensional transport, it is found that this enhancement of spin lifetime in gated wires is due to dimensional confinement and resonant spin-orbit interaction effect by gate bias modulation of the Rashba spin-orbit interaction. Spin relaxation due to the cubic Dresselhaus term is negligible in the present InGaAs wires.

Application of Characteristic Modes and Non-Foster Multiport Loading to the Design of Broadband Antennas

A conceptual framework is proposed to systematically design antennas with broadband impedance and pattern characteristics using multiple reactive loads. Antennas of arbitrary geometry can have their bandwidths expanded using this technique. The technique is applied to a narrow band thin wire dipole antenna to demonstrate its main features. It is shown that the loaded antenna resonates a desired current over a wide frequency band. The loads are shown to require non-Foster elements when realized. Simulations demonstrate the broadband characteristics of both the dipole input impedance and radiation pattern.

Variability of electrical resistivity in epitaxial grown La0.5Ba0.5MnO3 patterned micro-wires

We investigated the mesoscopic electrical transport properties of micro-patterned perovskite manganite (La0.5Ba0.5MnO3; LBMO) films that show electronic phase separation between ferromagnetic and A-type antiferromagnetic phases. The patterned-wire LBMO film of 10 μm width showed an insulator–metal (IM) transition associated with a ferromagnetic transition, followed by re-emergence of insulator characteristics below 200 K that did not appear in the wide wire film. Through continuous multiple measurements of ρ-T curve, we observed the variability of temperature dependent resistivity behaviour. The metal-insulator transition temperature was very scattered every measurements, indicating the probing randomly emerging antiferromagnetic micro-domains in the patterned narrow wire with a small channel.

Width and temperature dependence of lithography-induced magnetic anisotropy in (Ga,Mn)As wires

In-plane magnetic anisotropy of 40-μm-long (Ga,Mn)As wires with different widths (0.4, 1.0, and 20 μm) has been investigated between 5 and 75 K by measuring anisotropic magneto-resistance (AMR). The wires show in-plane 〈1 0 0〉 cubic and [−1 1 0] uniaxial anisotropies, and an additional lithography-induced anisotropy along the wire direction in narrow wires with width of 0.4 and 1.0 μm. We derive the temperature dependence of the cubic, uniaxial, and lithography-induced anisotropy constants from the results of AMR, and find that a sizable anisotropy can be provided by lithographic means, which allows us to control and detect the magnetization reversal process by choosing the direction of the external magnetic fields.

Non-adiabatic pumping of single electrons affected by magnetic fields

Non-adiabatic pumping of discrete charges, realized by a dynamical quantum dot in an AlGaAs/GaAs heterostructure, is studied under influence of a perpendicular magnetic field. Application of an oscillating voltage to one of two top gates, crossing a narrow wire and confining a quantum dot, leads to quantized pumped current plateaus in the gate characteristics. The regime of pumping one single electron is traced back to the diverse tunneling processes into and out-of the dot. Extending the theory to multiple electrons allows to investigate conveniently the pumping characteristics in an applied magnetic field. In this way, a qualitatively different behavior between pumping even or odd numbers of electrons is extracted.

Theoretical investigation of modulated currents in open nanostructures

We investigate theoretically the transport properties of a mesoscopic system driven by a sequence of rectangular pulses applied at the contact to the input (left) lead. The characteristics of the current which would be measured in the output (right) lead are discussed in relation with the spectral properties of the sample. The time-dependent currents are calculated via a generalized non-Markovian master equation scheme. We study the transient response of a quantum dot and of a narrow quantum wire. We show that the output response depends not only on the lead-sample coupling and on the length of the pulse but also on the states that propagate the input signal. We find that by increasing the bias window the new states available for transport induce additional structure in the relaxation current due to different dynamical tunneling processes. The delay of the output signal with respect to the input current in the case of the narrow quantum wire is associated to the transient time through the wire.

Three-Dimensional Real-Space Simulation of Surface Roughness in Silicon Nanowire FETs

We address the transport properties of narrow gate-all-around silicon nanowires in the presence of surface-roughness (SR) scattering at the Si/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface, considering nanowire transistors with a cross section of 3 times 3 nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and gate length of 15 nm. We present transfer characteristics and effective-mobility calculations based on a full 3-D real-space self-consistent Poisson-Schrodinger solver within the nonequilibrium Green's function formalism. The effect of SR is included via a geometrical method consisting in a random realization of potential fluctuations described via an exponential autocorrelation law. The influence on transfer characteristics and on low-field mobility is evaluated by comparison with the clean case and for different values of the root mean square of potential fluctuations. The method allows us to exactly account for mode-mixing and subband fluctuations and to evaluate the effect of SR up to all orders of the interaction. We find that SR scattering is mainly responsible for positive threshold-voltage shift in the low-field regime, whereas SR-limited mobility slowly depends on the linear charge density, showing the inefficiency of mode-mixing scattering mechanism for very narrow wires.

Spin-orbit coupling and phase-coherent transport in InN nanowires

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.

Reducing thermal conductivity of thermoelectric materials by using a narrow wire geometry

The dependence of the thermal conductivity of narrow wires made from bismuth and covalently bonded materials on wire diameter was numerically calculated by considering contributions of mean free paths of carriers and phonons. The results suggest that a reduction in the thermal conductivity should be observable in a bismuth wire having a diameter of less than 1 μm sample. A reduction of nearly 20% in the temperature range of 150–300 K is expected due to the use of a narrow wire geometry. Such a geometry reduces the mobility and the thermal conductivity of the carriers, which is the dominant component, while the thermal conductivity due to phonons was dramatically reduced by using narrow wires at temperatures under 50 K due to the longer mean free path phonons. The thermal conductivity of materials with covalent bonding such as silicon was also estimated, and it is expected that the thermal conductivity of a silicon wire could be reduced due to the mean free path of phonons being longer than that of the carriers. The results suggest that it should be possible to enhance the figure of merit by reducing the thermal conductivity through using wire geometries having diameters of less than 100 nm in materials having low mobilities, high thermal conductivities, and high Debye temperatures.