Function Of Wire Width Research Articles

Spin wave eigenmodes in transversely magnetized thin film ferromagnetic wires

We report experimental and theoretical studies of spin wave eigenmodes in transversely magnetized thin film Permalloy wires. Using broadband ferromagnetic resonance technique, we measure the spectrum of spin wave eigenmodes in individual wires as a function of magnetic field and wire width. Comparison of the experimental data to our analytical model and micromagnetic simulations shows that the intrinsic dipolar edge pinning of spin waves is negligible in transversely magnetized wires. Our data also quantify the degree of extrinsic edge pinning in Permalloy wires. This work establishes the boundary conditions for dynamic magnetization in transversely magnetized thin film wires for the range of wire widths and thicknesses studied, and provides a quantitative description of the spin wave eigenmode frequencies and spatial profiles in this system as a function of the wire width.

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 in quasi-1D InSb wires under strong spin–orbit interaction

We investigate the low temperature spin and phase coherence lengths in quasi-one dimensional wires fabricated from a high mobility InSb/InAlSb two dimensional electron system. Spin and phase coherence lengths as a function of wire width and temperature are obtained by fitting the magnetoconductance to an antilocalization theory modified to account for ballistic transport through the wires. Extracted spin coherence lengths are found to be inversely proportional to wire width and display a weak dependence on temperature. Results for the phase coherence length suggest that mechanisms originating in the two dimensional electron system are responsible for phase decoherence.

Micromagnetic calculation of spin wave propagation for magnetologic devices

The propagation of magnetic wave packets in magnetic nanowires was calculated as a function of wire width, field strength, field ramp time, field area size, and geometry of a magnetic nanowire. Spin waves are excited locally by applying a small perturbation in the magnetization in a 20nm wide region. A wave packet is emitted from the input region and travels along the wire with a velocity of 740m∕s. The finite element micromagnetic simulations show that wave packets can be guided along a bent nanostructure without losses due to geometry; amplitude and frequency are exactly the same as in a straight wire with equal distance between excitation point and probe. The wave amplitude was found to decrease with increasing rise time of the excitation field with an upper limit of 100ps. For a Permalloy wire with a thickness of 10nm, the frequency peak changes from 10GHz in a wire with 60nm width to 6GHz in a wire with 140nm width.

Dimensional Control of Antilocalization and Spin Relaxation in Quantum Wires

The spin relaxation rate $1/\tau_s (W)$ in disordered quantum wires with Rashba and Dresselhaus spin-orbit coupling is derived analytically as a function of wire width $W$. It is found to be diminished when $W$ is smaller than the bulk spin-orbit length $L_{\rm SO}$. Only a small spin relaxation rate due to cubic Dresselhaus coupling $\gamma$ is found to remain in this limit. As a result, when reducing the wire width $W$ the quantum conductivity correction changes from weak anti- to weak localization and from negative to positive magnetoconductivity.

Nonchiral edge states at the chiral metal-insulator transition in disordered quantum Hall wires

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 conclude that these states are characteristic for the CMIT, have the appearance of nonchiral edges states, and are thereby distinguishable from other states in the quantum Hall wire, namely, extended edge states, two-dimensionally (2D) localized, quasi-1D localized, and 2D critical states.

Width and magnetic-field dependence of the transition temperature in ultranarrow superconducting wires

We calculate the transition temperature in ultranarrow superconducting wires as a function of wire width, resistance and applied magnetic field. We compare the results of first-order perturbation theory and the non-perturbative resummation technique developed by Oreg and Finkel'stein. The latter technique is found to be superior as it is valid even in the strong disorder limit. In both cases the predicted additional suppression of the transition temperature due to the reduced dimensionality is strongly dependent upon the boundary conditions used. When we use the correct (zero-gradient) boundary conditions, we find that theory and experiment are consistent, although more experimental data is required to verify this systematically. We calculate the magnetic field dependence of the transition temperature for different wire widths and resistances in the hope that this will be measured in future experiments. The predicted results have a rich structure - in particular we find a dimensional crossover which can be tuned by varying either the width of the wire or its resistance per square.

Free-induction decay in quantum wires

Using the time-dependent perturbation treatment, we report the occurrence of first-order free-induction decay in GaAs/GaAlAs quantum wire structures. The results of the theoretical investigation reveal oscillations in the decay profile of the polarization. The transient transmitted intensity has been found to decay in a time comparable to the inhomogeneous broadening time, which is much shorter than the dephasing time. We have also studied the transmitted intensity as a function of wire width and found the results to be in good qualitative agreement with that available in the literature.

Magneto-optics of quantum wires grown on V-grooved substrates

AlGaAs/GaAs undoped quantum wires (QWRs) grown by organometallic chemical vapour deposition on V-grooved substrates have been studied by photoluminescence (PL) and photoluminescence excitation (PLE) in zero and finite magnetic fields. From the study of the diamagnetic shift of the peaks as a function of wire width and orientation with respect to the magnetic field, we have deduced one-dimensional effective masses for the lowest subband transitions.

Size dependence of the Kondo resistivity in AuFe wires

We have measured the temperature and magnetic field dependence of the low temperature resistivity of Au wires with dilute (∼50 ppm) Fe impurities as a function of wire width. In contrast to previous experiments, we find an increase in slope of the temperature dependent resistivity Δϱ(T) as the wire width is decreased. A systematic trend is also evident in the field dependent resistivity Δϱ(H). We also find an unusual scaling relation between Δϱ(T) and Δϱ(H) in our wider samples.

Path-integral Monte Carlo calculation of free and confined carrier real self-energies at non-zero temperatures

A path-integral Monte Carlo method for calculation of real self-energies of free and confined carriers at non-zero temperatures is presented. An efficient numerical method for calculating the polar-optical phonon influence functional has been developed and incorporated into a previously developed PIMC for calculating equilibrium properties of uncoupled carriers. As illustrative examples, calculations of carrier real self-energies are performed for bulk GaAs as a function of temperature, and for AlGaAsGaAs-like square quantum wires at 77K and 300K as a function of wire width. The self-energies obtained here, in part, represent new results for these systems. Where previous results are available, the agreement is good.

Theory of Auger recombination in a quantum well wire

Calculations are presented for the rate of Auger recombination in a semiconductor quantum well wire. An expression is given for the rate of the CHSH Auger process in which the carriers remain in their ground subbands, and the rates of other processes such as CHCC may be obtained by simple algebraic substitution. Numerical calculations that consider all possible bound-to-bound intra- and inter-subband transitions are reported. Results are given for the rate of the CHCC process as a function of wire width and wire axis direction for an InGaAsP/InP quantum well wire.