Supersymmetric path integrals applied to the transmission of a mesoscopic ring

The average transmission probability of a one-dimensional mesoscopic ring is calculated for arbitrary disorder strength. With increasing disorder the dependence on the magnetic flux changes from ₀ to ₀/2 periodicity. A supersymmetric method is used for the calculation which leads to the quantum mechanics in super-space of an anharmonic oscillator with hyperbolic symmetry. As a byproduct the transmission of a mesoscopic wire is obtained. The inverse of the localization length is given by the lowest eigenvalue of the oscillator.Research performed within the program of Sonderforschungsbereich 341 Köln-Aachen-Jülich     

Holstein model and Datta model of dephasing in one-dimensional chains

We consider the recently proposed “momentum conserving” dephasing model of Golizadeh-Mojarad and Datta [R. Golizadeh-Mojarad, S. Datta, Phys. Rev. B 75 (2007) 081301(R)] versus the “momentum relaxing” Holstein dephasing model. For both models, a detailed analysis of the coherent and incoherent components of the transmission coefficient is presented. The comparison between the two models reveals significant differences in the scaling properties of the coherent and incoherent contributions as functions of the dephasing strength and the length of the region where it acts. We also provide an analytic model that describes the peculiar behavior of the coherent component of the transmission coefficient. Our simulations are based on the Keldysh Green’s function method in the tight-binding framework.     

The effect of optically-induced random anisotropic disorder on a two-dimensional electron system

We have studied the effect of optically-induced random, anisotropic disorder on the magnetoresistance of a Al_0.3Ga_0.7As/ GaAs two-dimensional electron system by exposing the heterojunction to an asymmetric laser speckle pattern. Changes in the amplitude of the Shubnikov-de Haas oscillations can be explained in terms of easy and hard conductivity paths parallel and perpendicular to the long axis of the oval speckle grains. We also observe corresponding changes in the electron scattering rates.     

Improved expression for calculating electron transport cross sections

We report a simplified correction for the electron transport cross sections (TCSs) for a number of selected atomic targets ranging from H to U and electron energies between 50 and 4000 eV. The correction has been made to the approximate analytical expression of transport cross sections derived by Jablonski [A. Jablonski, Phys. Rev. B 58 (1998) 16470] where an argued parameter is introduced. The latter is obtained from a polynomial fit. The energy dependence of the percentage deviation between TCSs from the corrected expression and those obtained from other sources is presented. The TCSs calculated in the present work showed better agreement with accurate values of TCSs than those reported in earlier publications. This may facilitate the evaluation of parameters needed for quantitative Auger-electron spectroscopy and X-ray photoelectron spectroscopy.     

Transmitter with two probes: Resistance as a functional of the current distribution

As in the Landauer-Büttiker approach to transport, a resistor is characterized by its reflection and transmission coefficients. Such a transmitter is coupled incoherently to two resistive quantum wires. These leads define an asymptotic current-distribution over the different channels and allow to adapt the current distribution to the transmitter. For carriers at fixed energy, the resistance is defined via asymptotic density differences of the diffusing carriers. This resistance, and the corresponding current distribution, can be found by minimizing a variational functional that is additively composed of lead and transmitter terms. Thus the resistance functional can be defined for the transmitter alone, coupled incoherently to leads with unspecified properties, especially to ideal leads and reservoirs. The said functional is additive for transmitters in series if interference effects between them are excluded. The contacts between ideal leads and reservoirs can be modeled as special transmitters, reproducing thus the standard results for the total resistance.     

Non-linear response and electron-electron interactions in mesoscopic metal rings

A time-dependent electric field gives rise to a stationary non-equilibrium current I ⁽²⁾ around a mesoscopic metal ring threaded by a magnetic flux. We show that this current, which is proportional to the intensity of the field, is closely related to the exchange part of the interaction contribution to the equilibrium persistent current, and that the corresponding non-linear conductivity directly measures the weak localization correction to the polarization. We explicitly calculate the disorder average of I ⁽²⁾ in the diffusive regime as function of the frequency of the electric field and the static flux piercing the ring, and suggest an experiment to test our theory. Received: 5 September 1997 / Accepted: 4 November 1997     

Interband quantum kinetics with static disorder scattering I: Direct CPA solution for a rectangular-pulse-excited semiconductor

For a two band model semiconductor alloy with the disorder potential concentrated to the conduction band, the photoexcitation by a long rectangular pulse represents a case soluble in the coherent potential approximation. Explicit analytic expressions for the transient electron distribution are derived using the nonequilibrium Green functions. The evanescent coherent component is gradually superseded by the incoherent distribution whose saturation value is obtained using the Ward identity.     

Validity of the transfer-matrix method for a two-dimensional electron waveguide

We solve the singularity problem of the transfer matrix method for a two-dimensional electron wave guide consisting of a sequence of cells. The method is reformulated in terms of intrinsic Bloch waves of a device cell. The resulting method is proved to be both stable and accurate. The transfer matrix is formulated without divergence problems in the open-mode approximation. The formula relating the N-cell transmission probability with that of one cell in the single-mode approximation takes exactly the same form as in the one-dimensional case.     

Electron-LO-phonon interaction in wurtzite GaN quantum wells under a magnetic field

We calculate the electron-LO-phonon relaxation rates in wurtzite GaN quantum wells in the presence of a magnetic field parallel to the growth direction. Using the dielectric continuum model (DCM), we are able to include contributions from both the interface and the quasi-confined phonon modes. The relaxation rate expression takes the phonon dispersion into account, and is applicable to all phonon modes. We find that the relaxation rates show strong oscillations as a function of the applied magnetic field. In relatively wide (8 nm) quantum wells, the inclusion of interface phonon mode decreases this oscillation amplitude. But in thin wells (5 nm), the interface phonon mode is of the same importance as the quasi-confined mode, and it strongly modifies the oscillation behavior.     

Magnetoconductivity of disordered two dimensional tight binding electrons in CPA

The magnetotransport properties of a tight binding model of electrons on a two dimensional square lattice with diagonal disorder and in a perpendicular magnetic field is investigated. The disorder is treated in the coherent potential approximation (CPA) and a quasiclassical solution of the Harper equation is used to calculate the one particle Green's function. Analytical expressions for the CPA vertex corrections to the magnetoconductivity are derived. Numerical results for the density of states and the diagonal magnetoconductivity are discussed. The vertex correction vanishes if the Harper band width is neglected.     

Conductance fluctuation and degeneracy in nanocontact between a conductive AFM tip and a granular surface under small-load conditions

Conductive-tip atomic force microscope (c-AFM) has been extensively used in measuring electrical properties of surface nanostructures, but the electrical conduction in c-AFM tip-sample contacts in nanometer scale is not well understood. In the present work, we experimentally investigated the electrical properties of the nanocontact between a W₂C-coated c-AFM tip and granular gold film under small-load (∼5 nN) at ambient air conditions. We found that under a constant bias voltage (10 V), the electrical current passing through the tip-sample junction at fixed location of sample surface dramatically fluctuated and degenerated. By quantitatively estimating the mechanical and electrical aspects of the nanocontact, we explained the observed phenomena as mechanical instabilities, electron tunneling transport and atomic rearrangements at the contact junction. We think that our results are important for the realistic application of c-AFM in nanoelectronic measurement.     

Bulk and surface localized modes on a magnetic nonlinear impurity

We study localized modes on a single magnetic impurity positioned in the bulk or at the surface of a one-dimensional chain, in the presence of a magnetic field B acting at the impurity site. The strong on-site nonlinear interaction U between two electrons of opposite spin at the impurity site, modelled here as a nonlinear local term, and the presence of the external field induce a strong correlation between parallel and antiparallel spin bound states. We find that, for an impurity in the bulk, a localized vector mode (with up and down spin components) is always possible for any given value of U and B, while for a surface impurity, a minimum value of both, U and B is needed to create a vector mode. In this case, up to two localized modes are possible, but only one of them is stable. The presence of the surface seems to destabilize the bulk mode in the parameter region U∼B, creating a “forbidden strip” region in parameter space, bounded by U=B+V and U=B−V, approximately.     

Order-disorder transition for corrugated Au layers

Atomic-scale structure of the growth of a gold film on (1 1 2) plane of Mo single crystal was investigated by means of low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) up to two monolayers (ML) of gold coverage. Both LEED and STM results establish that Au grows on Mo(1 1 2) in a layer-by-layer mode, for at least the first two monolayers. A number of ordered structures are formed and both the first and second layers adopt the Mo(1 1 2) 1 × 1 surface structure upon completion. For some gold layers on Mo(1 1 2), notably the 1.66 monolayer 3 × 1 and 1.75 monolayer 4 × 1 gold overlayers, we find evidence of a phase transition associated with increasing disorder in gold layers with structural corrugation and anisotropic band structure. The signature of this phase transition, at temperatures in the range of 400-500 K, is a sharp decrease in the overlayer effective Debye temperature.     

Influence of negative pressure on thermoelectric properties of Sb2Te3

We investigated the influence of negative pressure on the electrical conductivity, the Seebeck coefficient, and the power factor of Sb₂Te₃. We performed first-principles calculations with the linearized-augmented plane-wave method considering negative hydrostatic pressure in the range from zero to −2 GPa and doping for electrons and holes up to 10²⁰ cm⁻³. Our results predict a significant increase of the Seebeck coefficient and the power factor under negative pressure for certain doping concentrations.     

Electron-electron interactions in the 2D electron system

In this paper, we report studies of the electron-electron interaction effects in 2D electron systems. The interaction manifests in renormalization of the effective spin susceptibility, effective mass, g^∗-factor, conductivity etc. By applying in-plane magnetic field, we tuned the effective interaction between the electrons and compared with theory the temperature dependence of the conductivity. We find a good agreement with interaction corrections calculated within the Fermi liquid theory. To address the question on the origin of the metal-insulator transition (MIT) in 2D, we explored transport and magnetotransport properties in the vicinity of the MIT and compared our data with solutions of two equations of the renormalization group (RG) theory, which describes temperature evolutions of the resistivity and interaction parameters for 2D electron system. We found a good agreement between the ρ(T,B_∥) data and the RG-theory in a wide range of the in-plane fields. These results support the Fermi liquid type origin of the metallic state and the interpretation of the observed 2D MIT as the true quantum phase transition.     

Simulations of the phonon Bragg switch in GaAs

It has recently been proposed that X-rays can be switched on sub-picosecond time-scales by using laser-generated coherent optical phonons: The so called phonon Bragg switch. We present here detailed simulations of the effficiency of such a switch by solving the time-dependent generalized Takagi-Taupin equations utilizing a perturbative approach. We explore the switching efficiency in diffraction from the (004) planes of GaAs as a function of both excited phonon wave vector and amplitude.     

Pressure and the quadratic temperature term in the electrical resistance of NbTi

We have measured the electrical resistance of an Nb_0.53Ti_0.47 alloy sample as a function of temperature T (4-300 K) and pressure (<20 GPa). At low temperatures, above the superconducting transition, we observe a T² term whose coefficient decreases with pressure. It is linearly dependent on the residual resistance, that also decreases with pressure, in strong agreement with a Koshino-Taylor origin, i.e. inelastic carrier scattering against impurities.     

Derivative of Electron Density in Non-Equilibrium Green's Function Technique and Its Application to Boost Performance of Convergence

The non-equilibrium Green＇s function （NEGF） technique provides a solid foundation for the development of quantum mechanical simulators. However, the convergence is always of great concern. We present a general analytical formalism to acquire the accurate derivative of electron density with respect to electrical potential in the framework of NEGF. This formalism not only provides physical insight on non-local quantum phenomena in device simulation, but also can be used to set up a new scheme in solving the Poisson equation to boost the performance of convergence when the NEGF and Poisson equations are solved self-consistently. This method is illustrated by a simple one-dimensional example of an N＋＋ N＋ N＋＋ resistor. The total simulation time and iteration number are largely reduced.