Field Dependence Of Coefficient Research Articles

On asymptotic perturbation theory for quantum mechanics: Almost invariant subspaces and gauge invariant magnetic perturbation theory

Singular perturbation theory for quantum mechanics is considered in a framework generalizing the spectral concentration theory. Under very general conditions, asymptotic estimations on the Rayleigh–Schrödinger expansions of the perturbed spectral projections are obtained. As a consequence almost invariant subspaces of exponential order are constructed. The results cover practically all singular perturbations considered in nonrelativistic quantum mechanics. In the magnetic field case, under the condition that the magnetic field does not increase at infinity, a gauge invariant perturbation theory leading to convergent series with field-dependent coefficients is developed.

Adiabatic and non-adiabatic hopping conduction in La–Pb–Mn–O type system

Magnetic field dependent resistivity ( ρ) and Seebeck coefficient (S) of La 1− x Pb x MnO 3 (x=0.0–0.5), prepared by sol–gel method, have been reported. One set (LaMnO 3+ δ ) sample, termed as LCON, was found to show insulator (semiconductor) to metal transition (MIT) around T p∼265 K and the other set (LaMnO 3), termed as LSEM, did not show such transition. T p shifts to the lower temperature under magnetic field. It has been shown, for the first time, that both in presence or in absence of magnetic field, two different transport mechanisms (adiabatic and non-adiabatic) are followed, respectively, by LSEM and LCON (for T> T p). Like resistivity, S of the sample was also found to depend on the applied magnetic field particularly around T p. The samples showing no MIT indicated larger Debye temperatures and electron–phonon interaction constants ( γ p) than those of the sample showing MIT. Both ρ and S of La 1− x Pb x MnO 3 (x=0.0–0.5) behave similarly to those of LCON.

THE DYNAMICS OF THE BLUME–EMERY–GRIFFITHS MODEL

The Fock-space method is presented to analyze the dynamics of the Blume–Emery–Griffiths (BEG) model including both spin-flip and spin-exchange processes with weighted transition rates. In this connection, the master equation on a lattice is formulated in a Quantum–Hamiltonian technique with second quantized operators. Introducing raising and lowering Para-Fermi operators for spin S = 1 the master equation is established for a three-state model where two states represent two different kinds of particles and the remaining state corresponds to an empty state. The coupled dynamical equations for the particle density and the local relative composition are derived including fluctuation corrections in lowest order of a gradient expansion. Although the underlying dynamics are subjected to the exclusion principle the resulting equations of motion may be classified according to the scheme due to Halperin and Hohenberg where the field-dependent kinetic coefficients are given explicitly. The homogeneous stationary solutions of these dynamical equations correspond to the mean-field approximation of the BEG Hamiltonian. For a special case, the diluted kinetic Ising model, phase separation is observed below a characteristic temperature. Furthermore, the crossover between thermal and non-thermal driven processes is discussed.

Avalanche multiplication in Al/sub x/Ga/sub 1-x/As (x=0 to 0.60)

Electron and hole multiplication characteristics, M/sub e/ and M/sub h/, have been measured in Al/sub x/Ga/sub 1-x/As (x=0-0.60) homojunction p/sup +/-i-n/sup +/ diodes with i-region thicknesses, w, from 1 /spl mu/m to 0.025 /spl mu/m and analyzed using a Monte Carlo model (MC). The effect of the composition on both the macroscopic multiplication characteristics and microscopic behavior is therefore shown for the first time. Increasing the alloy fraction causes the multiplication curves to be shifted to higher voltages such that the multiplication curves at any given thickness are practically parallel for different x. The M/sub e//M/sub h/ ratio also decreases as x increases, varying from /spl sim/2 to /spl sim/1 as x increases from 0 to 0.60 in a w=1 /spl mu/m p/sup +/-i-n/sup +/. The Monte-Carlo model is also used to extract ionization coefficients and dead-space distances from the measured results which cover electric field ranges from /spl sim/250 kV/cm-1200 kV/cm in each composition. These parameters can be used to calculate the nonlocal multiplication process by solving recurrence equations. Limitations to the applicability of field-dependent ionization coefficients are shown to arise however when the electric-field profile becomes highly nonuniform.

Near band-edge field-dependent absorption coefficient and refractive index determined by photocurrent and transmittance measurements

A technique is proposed for the extraction of precise values of field-dependent absorption coefficient alpha and refractive index n from photocurrent and transmittance measurements of optical modulator structures. The technique uses approximate results of alpha and n extracted from a simplified device as the initial input into an iterative procedure that utilizes the consistency between alpha and n to obtain successively better estimates of these parameters. The technique was applied to results that were measured experimentally, and we verified the accuracy by using synthetic data. Errors caused by measurement inaccuracy are also investigated. It is shown that the absorption coefficient has a modest sensitivity whereas the refractive index is insensitive to these errors.

Electroabsorption in ordered and disordered GaInP

We have investigated the electroabsorption due to the Franz–Keldysh effect in GaInP/AlGaInP p-i-n double heterostructures grown by metalorganic vapor phase epitaxy. The simultaneous evaluation of transmission and photocurrent measurements allowed an accurate determination of the field dependent absorption coefficient of ordered and disordered GaInP alloys. For ordered and disordered material, similar changes of the absorption coefficient as high as 4000 cm−1 have been observed for field changes of ΔE=250 kV/cm. Thermally disordered samples, however, showed a degradation of the electrical and optical properties.

Strain-compensated InAsP/GaInP multiple quantum wells for 1.3 μm waveguide modulators

We show that high-quality strain-compensated InAsxP1−x/GayIn1−yP multiple quantum wells (MQWs) can be grown by gas-source molecular beam epitaxy (GSMBE) on InP substrates. Very sharp satellite peaks in double-crystal x-ray diffraction are obtained from a p-i-n waveguide structure consisting of 21 periods of 93 Å/135 Å InAs0.4P0.6/Ga0.13In0.87P MQWs. The surface-normal electroabsorption exhibits a significant quantum-confined Stark effect (QCSE) near 1.3 μm wavelength with a field-dependent absorption coefficient change of 6000 cm−1 and a very small zero-bias residual absorption. Electroabsorption waveguide modulators fabricated using similar material exhibit a large optical saturation threshold in excess of 10 mW incident optical power at 32 meV detuning energy.

Resistivity, magnetization, and specific heat of YbAgCu4 in high magnetic fields.

We report measurements of the resistivity \ensuremath{\rho}, Sommerfeld coefficient of specific heat \ensuremath{\gamma}, and magnetization M of polycrystalline ${\mathrm{YbAgCu}}_{4}$ in high magnetic fields 0\ensuremath{\le}B\ensuremath{\le}18 T [in the case of M(B) to 50 T]. A comparison of the temperature-dependent susceptibility \ensuremath{\chi}(T) as well as field-dependent Sommerfeld coefficient \ensuremath{\gamma} and magnetization to Kondo theory for a J=7/2 impurity shows that theory correctly predicts the functional dependence of these quantities on T and B, but the characteristic temperatures determined from the various measurements (120, 98, 77, and 63 K) differ by nearly a factor of 2, which is difficult to understand within the context of Kondo theory even when other possible contributions are considered. In addition the normalized (Wilson) ratio of \ensuremath{\chi} to \ensuremath{\gamma} is 1.00 at zero field (compared to 1.14 in Coqblin-Schrieffer theory) and decreases with increasing magnetic field. The magnetoresistance is positive at all temperatures, reaching a value \ensuremath{\Delta}\ensuremath{\rho}(B)/\ensuremath{\rho}(B=0)=0.6 at 25 mK and 18 T. The low-temperature magnetoresistivity \ensuremath{\Delta}\ensuremath{\rho}(B) varies as ${\mathit{B}}^{1.5}$. We argue that this is dominated by an ordinary impurity effect. Kohler's rule is clearly violated as the temperature is raised; the scattering rate appears to increase with field below 40 K and decrease with field above 40 K. This behavior is expected for an Anderson lattice when a pseudogap is present. At low temperature the resistivity increases as ${\mathit{AT}}^{2}$. The coefficient A (corrected for cyclotron-orbit effects) increases with field such that the ratio A(B)/\ensuremath{\gamma}(B${)}^{2}$ is a constant. Doping with Lu onto the Yb site, or with Ni onto the Cu site, changes the magnitude of the low-temperature resistivity in a manner consistent with the predictions of the theory of ligand-induced disorder in an Anderson lattice.

Diffusion, Joule heating, and band broadening in capillary gel electrophoresis of DNA.

We calculated the longitudinal and transverse diffusion coefficients for a DNA molecule undergoing gel electrophoresis in the limit where it is reptating through a dense polymer matrix. Our results indicate that both diffusion coefficients increase with the electric field intensity. The transverse and longitudinal diffusion coefficients are roughly equal for regular field intensities, but the former dominates for the high field intensities normally used for capillary gel electrophoresis (CE). This has important implications for the optimization of CE. Our results clearly show that the naive use of the zero-field diffusion constant, the Einstein relation or the longitudinal diffusion constant when calculating the contribution of the parabolic temperature profile to band broadening may lead to large overestimates under typical CE conditions. Finally, we show that the field-dependent diffusion coefficients may be responsible for the existence of an optimal field intensity for CE, even if Joule heating is neglected.

Supersymmetric calculation of mixed Kähler-gauge and mixed Kähler-Lorentz anomalies

We present a manifestly supersymmetric procedure for calculating the contributions from matter loops to the mixed Kähler-gauge and to the mixed Kähler-Lorentz anomalies in N = 1, D = 4 supergravity-matter systems. We show how this procedure leads to the well-known result for the mixed Kähler-gauge anomaly. For general supergravity-matter systems the mixed Kähler-Lorentz anomaly is found to contain a term proportional to R 2 with a background field dependent coefficient as well as terms proportional to ( C mnpq ) 2 and to the Gauss-Bonnet topological density. We briefly comment on the relationship between the mixed Kähler-Lorentz anomaly and the moduli dependent threshold corrections to gravitational couplings in Z N orbifolds.

Optical switching in three-coupler configurations

We introduce an optical coupler configuration consisting of two nonlinear waveguides coupled to a linear one. Selftrapping of electric field amplitude is possible in each nonlinear waveguide arising from the nonlinear field-dependent dielectric coefficient. We show that this coupler system can act as an optical switching device and it can have switching properties superior to that of conventional two and three all nonlinear coupler configurations.

Magnetic-field dependence of the collisional disalignment of neon 2p53p-configuration atoms.

Laser-induced-fluorescence spectroscopy has been applied to neon and helium-neon discharge plasmas in magnetic fields lower than 10 T. Disalignment-rate coefficients of the neon 2{ital p}{sub 2} and 2{ital p}{sub 7} atoms (2{ital p}{sup 5}3{ital p} configuration, Paschen notation) have been determined for neon and helium collisions in a field strength that is lower than the critical field at which the collision time is equal to the inverse Larmor frequency. The 2{ital p}{sub 7} atoms have rate coefficients independent of the magnetic-field strength, whereas the 2{ital p}{sub 2} atoms have a field-dependent rate coefficient for neon collisions.

An evaluation of energy transport models for silicon device simulation

The reduction in feature size of silicon devices has resulted in operating conditions characterized by extremely large internal electric fields and field gradients. The rapid spatial variation of the electric field precludes a point-wise (or local) dependence of average carrier energy on the electric field. Because traditional drift-diffusion transport models and field-dependent hot-carrier models force a one-to-one correspondence between the local electric field and carrier energy, they fail to provide sufficient accuracy for contemporary device design. Impact ionization in submicron MOSFETs, for example, is poorly described by field-dependent ionization coefficients since impact ionization is an energy-dependent process. Energy transport models, which can account for the nonlocal energy-field relationship in submicron devices, have become especially attractive for design oriented silicon device simulation. The severe constraints that hot (high energy) carriers impose on current submicron MOSFET technology have motivated the development and application of energy transport models, such as the hydrodynamic model. In this work, the hydrodynamic model and a simple, efficient energy transport model are compared with a Monte Carlo model in order to evaluate their performance under submicron device conditions, and to better assess how they may be improved. The results indicate that energy transport models can predict peak average energy quite well, even under severely nonuniform electric field conditions. The hydrodynamic and simple energy transported models examined here, however, cannot accurately model carrier cooling associated with an abrupt decrease in the electric field. Discrepancies in cooling are shown to strongly influence drift velocity, and the implications of this for submicron device analysis are discussed.

Transport Properties of Ions in Ferroelectric Liquid Crystal Cells

The transient ionic current in a ferroelectric liquid crystal (FLC) cell was measured as a function of applied voltage at different temperatures. The measured current shows a delay peak which decays rapidly to a steady-state value. The steady-state current exhibits a thermal activated behavior with an activation energy of 0.94 eV, and depends exponentially on the square root of the field strength similar to the Schottky effect. To describe the transport behaviors of the ions, a model with ionization-recombination equations which include a field-dependent ionization coefficient, and a continuity equation for the accumulated surface charges has been introduced. According to the model, the current decay time and the peak position of the delay peak current can be used to determine the ion mobility. Two ionized species have been identified in the FLC medium: a weakly ionized species with a relatively constant mobility of 2.0×10-7 cm2/V·s, and a highly ionized species with a room temperature mobility of 4.1×10-7 cm2/V·s and a thermal activation energy of about 0.71 eV. Detailed calculation of the ionic current and comparison with experimental results are given.

On van Kampen's objections against linear response theory

A discussion is given of the derivation of the Kubo-Green formulas. A reformulation of the derivation is presented, which leads to an exact result, involving field-dependent transport coefficients. Kubo's result is obtained as the first-order term in a resolvent expansion. For the general Liouville operator case, in accord with van Kampen's objections, it cannot be argued that the other terms are negligible. However, for a large class of systems, this can be justified. This is shown for systems where dissipation is due to weak interactions, amenable to the Van Hove limit, and having sufficiently short relaxation times.

On slip coefficients of polyatomic gases

A variational scheme is presented to evaluate field-dependent slip coefficients of polyatomic gases in a plane-parallel geometry. With the assumption of perfect accomodation at the surface and with bulk test functions only, fair agreement is obtained between theory and experiment. Also, it is shown that the present method is equivalent to a previously used variational technique, if bulk test functions are used in both cases. Finally, a comparison is made with Maxwell's and Eddington's methods. The equivalence of the latter method with the variational scheme is demonstrated.

Onsager symmetries in field-dependent flows of rarefied molecular gases

Investigations of field effects upon transport phenomena in molecular gases have recently been extended from the hydrodynamic to trasition and Knudsen regimes. In the steady state of a binary mixture between parallel plates the average flow velocity, diffusive flux aand heat flux linearly depend upon small gradients of pressure, concentration and temperature. The relation is specified by a matrix of field- and pressure-dependent coefficients Lαβ, each of which may be a tensor. The Onsager reciprocity, Lαβ(B)=Lβα†(-B, is verified by a derivation from the linearized Boltzmann equation, by taking account of the symmetries of intermolecular and gas-surface collision operators. A more detailed analysis is possible at moderate rarefaction, when the fluxes can be split into bulk and boundary-layer parts, the latter involving field-dependent slip coefficients.

Field dependence of the galvanomagnetic coefficients in lead single crystals

The authors have measured the transverse magnetoresistance and the Hall coefficient of lead single crystals for the field along the (100) and the (110) axes. Emphasis has been placed on their field dependence and on their temperature dependence in the intermediate-field range. The field-dependent galvanomagnetic coefficients have also been computed by path integration based on the 4 OPW Fermi surface model. The agreement between experiment and the computation is very good, though some discrepancies remain for the field along the (110) axis in the high-field range. At lower temperatures, some deviation from Kohler's rule is found in the low-field Hall coefficient.

Erasing characteristics of a thin-film electroluminescent ZnS:Mn faceplate

Experimental results show that it is not possible to erase completely electroluminescent ZnS:Mn memory device biased near the threshold voltage, for erase time pause of the order of tens of milliseconds, by either the erase or the electron beam erase method. The selective erase requires a time delay before reimposition of the normal sustain voltage to allow a significant decay of persistent beam-induced conductivity. The process of bulk erasure can be explained satisfactorily by a theoretical model which treats the macroscopic electroluminescent region as an ensemble of bistable microscopic filaments, with each filament having a field-dependent coefficient for the recombination of the deep hole traps with the electron flux. The simple assumption of a Gaussian distribution of extinction voltages for the ensemble of microscopic filaments is sufficient to explain the experimental results, except near the threshold voltage where the measured curves show oscillatory features which remain unexplained.

Comparative studies of tunnel injection and irradiation on metal oxide semiconductor structures

Tunnel injection and irradiation experiments on metal oxide semiconductor (MOS) structures are performed in order to compare the results of both experiments and to check the feasibility of radiation hardness prediction of MOS devices. The comparison is based on the fact that in both hot electron and ionizing irradiation experiments electron-hole pairs are generated in the SiO2. Due to an applied electrical field, these pairs are separated. The fraction of holes, trapped by neutral centers and the number of subsequently captured electrons by these now positively charged traps depend on the amount of available carriers, the magnitude of the respective capture cross sections for electrons and holes, and the number of hole traps. In the case of the tunnel injection experiment the number of the available carriers is a strong function of the field dependent ionization coefficient α. Up to now, its magnitude has not been accurately known. For this reason, a new method is presented which yields additional and reliable figures of α. When comparing the charge accumulation in the SiO2 by means of the flatband voltage shift as a consequence of the tunnel injection and of the irradiation, we determine α for various fields, α=3.3×10−6 exp [78/E(MV/cm)] (cm−1). For this determination we show that it is mandatory to correct for the steady state compensation of positively charged centers by electrons. From the saturation of the flatband voltage shift, caused by these trapped electrons, the field dependent capture cross sections for electrons are deduced. Their values are 2.3×10−14 and 3.5×10−15 cm2 for electrical fields of 7.8 and 8.6 MV/cm, respectively. The feasibility and limitations to predict the radiation hardness of MOS devices are discussed.