Interwire Interaction Research Articles

Interaction effects in Permalloy nanowire systems

Two series of self-organized hexagonal arrays of uniaxial Permalloy nanowires are grown by electroplating, filling of nanopores in anodic alumina films. Nanowires with periodicities of 65 and 105nm and wire diameter of 25–60nm were investigated by magnetization and ferromagnetic resonance (FMR) measurements. The length of the wires is 2.5μm, the volume ratio 0.13⩽v⩽0.43. A crossover transition from a one-dimensional easy axis “wire” behavior of weakly interacting uniaxial nanowires to a two-dimensional behavior of strongly coupled “wire film” having an easy plane anisotropy is determined from FMR measurements. The crossover occurs at v=0.27 for 105nm periodicity, and v=0.43 for 65nm periodicity. The system of the thinnest nanowires corresponds to a statistical ensemble of weakly interacting uniaxial particles, characterized by the highest coercivity of 1276Oe, the highest switching field distribution, and highest FMR linewidth, resulting from the broad distribution of individual wires. The coercivity monotonously decreases to 440Oe with increasing nanowire radius and volume fraction, as the wire shape anisotropy is reduced and the array anisotropy takes over due to dipolar interwire interaction, smoothing out the individuality of the wires. The behavior of coercivity, switching field distribution, and FMR linewidth is in accordance with the increase of the coupling of the nanowires.

Angular dependence of coercivity and remanence of Ni nanowire arrays and its relevance to magnetoviscosity

Ni nanowire arrays with varying wire dimensions (diameter d, length l) and center-to-center distances d CC were synthesized by pulsed electrodeposition of Ni in porous Al templates. The magnetization-reversal behavior of the arrays was investigated by means of magnetometry for different angles θ between the wire axes and the applied magnetic field. The functional dependences of the characteristic parameters coercivity H C( θ) and reduced remanence m R /m S( θ) exhibit a strong dependence on the wire dimensions and the center-to-center distance. For instance, for nanowire arrays with d=40 nm, d CC=100 nm, and for θ=0°, the coercivity takes on a rather large value of μ 0 H C=85 mT and m R /m S≅94%; reducing d CC to 30 nm and d to 17 nm results in μ 0 H C=49 mT and m R /m S≅57%, an observation which suggests an increasing magnetostatic interwire interaction at increased ( d/ d CC)-ratio. The potential application of nanowires as the constituents of ferrofluids is discussed.

Luttinger liquids with curvature: Density correlations and Coulomb drag effect

We consider the effect of the curvature in fermionic dispersion on the observable properties of Luttinger liquid (LL). We use the bosonization technique where the curvature is irrelevant perturbation, describing the decay of LL bosons (plasmon modes). When possible, we establish the correspondence between the bosonization and the fermionic approach. We analyze modifications in density correlation functions due to curvature at finite temperatures, T. The most important application of our approach is the analysis of the Coulomb drag by small momentum transfer between two LL, which is only possible due to curvature. Analyzing the a.c. transconductivity in the one-dimensional drag setup, we confirm the results by Pustilnik et al. for T-dependence of drag resistivity, R_{12} ~ T^2 at high and R_{12} ~ T^5 at low temperatures. The bosonization allows for treating both intra- and inter-wire electron-electron interactions in all orders, and we calculate exact prefactors in low-T drag regime. The crossover temperature between the two regimes is T_1 ~ E_F \Delta, with \Delta relative difference in plasmon velocities. We show that \Delta \neq 0 even for identical wires, due to lifting of degeneracy by interwire interaction, U_{12}, leading to crossover from R_{12} ~ U_{12}^2 T^2 to R_{12} \~ T^5/U_{12} at T ~ U_{12}.

Exchange-Correlation Effects on Low-Dimensional Plasmons in an Array of Metallic Quantum Wires

We investigate low-dimensional plasmons (PL’s) in an array of metallic quantum wires. By developing a self-consistent local-field-correction (LFC) theory for treatment of the PL’s in the wire array, we take account of both the intrawire exchange-correlation (XC) effect and the interwire correlation effect. By comparing the results involving the LFC with those in the random-phase approximation, we examine the XC effect on the energy distribution and the energy-loss intensity of all the PL modes in an array of a finite number of wires. Our theoretical scheme is applied to an array of Au-atom wires on the Si(557) surface. The XC effect is found to play a significant role because of strong one-dimensional confinement, though a high effective density of the electron system is suggestive of the free-electron-gas character. With an increase in wave number q, the XC effect operates to lower the mode energy increasingly, and to start the decline of the energy-loss intensity at smaller q values. In a smaller q range of qd\\lesssim1, where d denotes the interwire separation, the interwire interaction has the mode energies separated from one another to form an energy distribution of considerable width. Our results of the XC effect and the effect of the interwire interaction yield general insights into the PL’s in a variety of wire arrays.

Electron transport in parallel quantum wires with random potentials

We study an electron transport property in two parallel quantum wires with random potentials. Assuming the same microscopic parameters for both wires, we focus on the relationship between inter-wire interaction and electron backward scattering by random potentials at low energy regime. Our analytical and numerical calculations show that the Drude weight, a measure of the electron transport, is influenced by inter-wire interaction and random potential independently, and little coupling between those two is observed, which is in contrast to a deep relationship between up- and down-spin interactions and random potentials in a single wire. It leads to that inter-wire interactions do not have a great influence on the Anderson localization in each wire.

Plasmon excitations and one- to two-dimensional crossover in quantum crossbars

Spectrum of boson fields and two-point correlators are analyzed in quantum crossbars (QCBs, a superlattice formed by m crossed interacting arrays of quantum wires), with short range inter-wire capacitive interaction. Spectral and correlation properties of double (m=2) and triple (m-3) QCBs are studied. It is shown that the standard bosonization procedure is valid, and the system behaves as a sliding Luttinger liquid in the infrared limit, but the high frequency spectral and correlation characteristics have either 1D or 2D nature depending on the direction of the wave vector in the 2D elementary cell of reciprocal lattice. As a result, the crossover from 1D to 2D regime may be experimentally observed. It manifests itself as appearance of additional peaks of optical absorption, non-zero transverse space correlators and periodic energy transfer between arrays ("Rabi oscillations").

Electronic excitations and correlations in quantum bars

The spectrum of boson fields and two-point correlators are analyzed in a quantum bar system (a superlattice formed by two crossed interacting arrays of quantum wires), with a short-range interwire interaction. The standard bosonization procedure is shown to be valid, within the two-wave approximation. The system behaves as a sliding Luttinger liquid in the vicinity of the Γ point, but its spectral and correlation characteristics have either 1D or 2D nature depending on the direction of the wave vector in the rest of the Brillouin zone. Due to the interwire interaction, unperturbed states propagating along the two arrays of wires are always mixed, and the transverse components of the correlation functions do not vanish. This mixing is especially strong around the diagonals of the Brillouin zone, where the transverse correlators have the same order of magnitude as the longitudinal ones.

Collective and single-particle excitations in coupled quantum wires in magnetic fields

The full spectra of magnetoplasmons and single-particle excitations are obtained of coupled one-dimensional electron gases in parallel semiconductor quantum wires with tunneling. We show the effects of the interwire Coulomb interaction and the tunneling, as well as the magnetic-field-induced localization on the elementary excitations in symmetric and asymmetric coulped quantum wire structures. The interacton and resonance between the plasmon and the intersubband single-particle excitations are found in magnetic fields.

Radiative damping of collective excitations in periodic arrays of quantum wires and dots

The problem of electromagnetic response and collective excitations in arrays of quantum wires and dots is solved taking into account electrodynamic (radiative) effects and the interwire (interdot) interaction. The radiative contribution (Γ) to the total linewidth of collective modes is calculated. It is shown that the interwire (interdot) interaction increases the radiative damping of collective modes by several orders of magnitude, and the radiative contribution to the linewidth of plasma excitations, which has been always neglected so far, can exceed the collisional contribution (γ). The result allows us to remove the discrepancy between observed and calculated linewidths of the collective modes. The related problem of optical properties of small polarizable particles is discussed.

Energy-transfer rate in Coulomb coupled quantum wires

We study the energy transfer rate for electrons in two parallel quantum wires due to interwire Coulomb interactions. The energy transfer rate between the wires (similar to the Coulomb drag effect in which momentum transfer rate is measured) is calculated as a function of temperature for several wire separation distances. We employ the full wave vector and frequency dependent random-phase approximation at finite temperature to describe the effective interwire Coulomb interaction. We find that the energy transfer rate at intermediate temperatures (i.e., T∼0.3EF) is dominated by the collective modes (plasmons) of the system. Nonlinear effects on the energy transfer rate is also explored.

Optical properties of isolated and interacting silicon quantum wires

We have studied the effect of hydrogen passivation and inter-wire interaction on the electronic structure and optical properties of nanoscale Si wires through two first-principle techniques: linear muffin tin orbitals method in the atomic sphere approximation (LMTO-ASA) and norm-conserving pseudopotential. We have considered free, partially and totally H-passivated [001] Si quantum wires with various rectangular cross-sections; moreover we have investigated the inter-wire interaction, by varying the wire density. The optical properties have been computed by evaluating the imaginary part of the dielectric function and the absorption coefficient. We find that wires with diameters as small as 10–25 Å are active in the visible range. Inter-wire interaction leads to the presence of localized interface states which lower the bandgap energy. These results are important for the discussion about the dimensionality of confined Si quantum particles in porous Si and for the debate on quantum confinement models. © 1997 Elsevier Science S.A.

Coulomb drag effect in parallel cylindrical quantum wires

We study the Coulomb drag rate for electrons in two parallel quantum wires. The doublequantum wire structure is modeled for a GaAs material with cylindrical wires having infinite potential barriers. The momentum transfer rate between the wires (Coulomb drag effect) is calculated as a function of temperature for several wire separation distances. We employ the full wave vector and frequency dependent random-phase approximation (RPA) at finite temperature to describe the effective interwire Coulomb interaction. We find that the drag rate at high temperatures (i.e., T ≥ E F 2 ) is dominated by the collective modes (plasmons) of the system similar to the case in double-well structures. Including the local-field effects in an approximate way we estimate the importance of intrawire correlations to be significant.

Ab-Initio Calculation of the Optical Properties of Silicon Quantum Wires

AbstractWe studied the effect of H, O passivation and inter-wire interaction on the optical properties of nanoscale Si wires. We find that wires with diameters as small as 10–25 Å are active in the visible range. Inter-wire interaction leads to the presence of localized states which lower the band gap energy. The presence of dangling bonds generates broad features in the infrared region. O-Si bonds reduce the absorption threshold. These results are important for the discussions concerning absorption and luminescence in porous Si.

Inter-Wire Interaction in Ni80Fe20 Wire Arrays.

Ni80Fe20 wire arrays with widths and spacings in the range of 0.6-2.0μm were fabricated by using high-resolution electron-beam lithography and lift-off techniques. Both the coercive force and demagnetizing field for arrays with a constant wire width decreased with the inter-wire distance. The demagnetizing fields of the arrays with short spacings were smaller than those calculated for isolated wires. When a static field was applied normal to the stripes and parallel to the array plane, multiple magnetic resonances, which were strongly dependent on the artificial periodic structure, were observed at fields higher than the uniform precessional mode. These results indicate the presence of inter-wire interaction in Ni80Fe20 wire arrays with short spacings.

Magnetic wire and box arrays (invited)

Ferromagnetic wire and box arrays with widths and spacings in the range of 2–0.6 μm were successfully fabricated utilizing high-resolution electron-beam lithography and lift-off techniques. The box arrays possessed a unique magnetic switching mechanism. As the magnetic field decreased, the magnetic coherency between the boxes which are in a row first disappeared. Domain wall motion in each box occurred in the next step. The demagnetizing fields observed in the wire arrays with short spacings were different from those calculated by a model, in which the wires are isolated from each other. Ferromagnetic resonance measurement implied the appearance of interwire dipole-dipole interaction. Multilayered wire arrays were also prepared to study the magnetoresistive characteristics.

Nonparabolic confinement in quantum wire superlattices.

The high-frequency polarizability of quantum wires in a novel metal-insulator-semiconductor field-effect transistor heterojunction is studied in the far-infrared regime. Whereas on conventional narrow quantum wires only the fundamental mode of the dimensional resonance is observed, the quantum wires investigated here exhibit higher harmonics. Additionally, at high electron densities and intermediate magnetic fields we observe a completely unexpected splitting of the fundamental mode. The behavior is discussed in view of nonparabolic confinement as well as interwire interaction.