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A force field as accurate as quantum mechanics (QM) and as fast as molecular mechanics (MM), with which one can simulate a biomolecular system efficiently enough and meaningfully enough to get quantitative insights, is among the most ardent dreams of biophysicists -- a dream, nevertheless, not to be fulfilled any time soon. Machine learning force fields (MLFFs) represent a meaningful endeavor towards this direction, where differentiable neural functions are parametrized to fit ab initio energies, and furthermore forces through automatic differentiation. We argue that, as of now, the utility of the MLFF models is no longer bottlenecked by accuracy but primarily by their speed (as well as stability and generalizability), as many recent variants, on limited chemical spaces, have long surpassed the chemical accuracy of $1$ kcal/mol -- the empirical threshold beyond which realistic chemical predictions are possible -- though still magnitudes slower than MM. Hoping to kindle explorations and designs of faster, albeit perhaps slightly less accurate MLFFs, in this review, we focus our attention on the design space (the speed-accuracy tradeoff) between MM and ML force fields. After a brief review of the building blocks of force fields of either kind, we discuss the desired properties and challenges now faced by the force field development community, survey the efforts to make MM force fields more accurate and ML force fields faster, envision what the next generation of MLFF might look like.

Recently, machine learning potentials (MLP) largely enhances the reliability of molecular dynamics, but its accuracy is limited by the underlying $\textit{ab initio}$ methods. A viable approach to overcome this limitation is to refine the potential by learning from experimental data, which now can be done efficiently using modern automatic differentiation technique. However, potential refinement is mostly performed using thermodynamic properties, leaving the most accessible and informative dynamical data (like spectroscopy) unexploited. In this work, through a comprehensive application of adjoint and gradient truncation methods, we show that both memory and gradient explosion issues can be circumvented in many situations, so the dynamical property differentiation is well-behaved. Consequently, both transport coefficients and spectroscopic data can be used to improve the density functional theory based MLP towards higher accuracy. Essentially, this work contributes to the solution of the inverse problem of spectroscopy by extracting microscopic interactions from vibrational spectroscopic data.

Uncontrollable dendrites growth during electrochemical cycles leads to low Coulombic efficiency and critical safety issues in Li metal batteries. Hence, a comprehensive understanding of the dendrite formation mechanism is essential for further enhancing the performance of Li metal batteries. Machine learning accelerated molecular dynamics (MD) simulations can provide atomic-scale resolution for various key processes at an ab-initio level accuracy. However, traditional MD simulation tools hardly capture Li electrochemical depositions, due to lack of an electrochemical constant potential (ConstP) condition. In this work, we propose a ConstP approach that combines a machine learning force field with the charge equilibration method to reveal the dynamic process of dendrites nucleation at Li metal anode surfaces. Our simulations show that inhomogeneous Li depositions, following Li aggregations in amorphous inorganic components of solid electrolyte interphases, can initiate dendrites nucleation, accompanied by dead Li cluster formation. Our study provides microscopic insights for Li dendrites formations in Li metal anodes. More importantly, we present an efficient and accurate simulation method for modeling realistic ConstP conditions, which holds considerable potential for broader applications in modeling complex electrochemical interfaces.

We consider two-dimensional spin-polarized dipolar Fermi gases confined in a double-layer system and calculate the momentum transfer between the layers as a function of temperature to investigate the transport properties of the system. We use the Hubbard approximation to describe the correlation effects and the screening between the dipoles within a single layer. The effective interlayer interaction between the dipoles across the layers is obtained by the random-phase approximation. We calculate the interaction strength and the layer separation distance dependence of the drag rate, and we show that there is a critical distance below which the system is unstable. In addition, we calculate the typical behavior of the collective modes related to the density fluctuations.

Multi-messenger searches for BNS and NSBH mergers are currently one of the most exciting areas of astronomy. The search for joint electromagnetic and neutrino counterparts to GWs has resumed with O4. To support this effort, public semi-automated data products are sent in near real-time and include localization and source properties to guide complementary observations. In preparation for O4, we have conducted a study using a simulated population of compact binaries and a MDC in the form of a real-time replay to optimize and profile the software infrastructure and scientific deliverables. End-to-end performance was tested, including data ingestion, running online search pipelines, performing annotations, and issuing alerts to the astrophysics community. We present an overview of the low-latency infrastructure and the performance of the data products that are now being released during O4 based on the MDC. We report the expected median latency for the preliminary alert of full bandwidth searches (29.5s) and show consistency and accuracy of released data products using the MDC. For the first time, we report the expected median latency for triggers from early warning searches (-3.1s), which are new in O4 and target neutron star mergers during inspiral phase. This paper provides a performance overview for LVK low-latency alert infrastructure and data products using the MDC and serves as a useful reference for the interpretation of O4 detections.

Proton tunneling is believed to be non-local in ice but has never been shown experimentally. Here we measured thermal conductivity of ice under pressure up to 50 GPa and found it to increase with pressure until 20 GPa but decrease at higher pressures. We attribute this anomalous drop of thermal conductivity to the collective tunneling of protons at high pressures, supported by large-scale quantum molecular dynamics simulations. The collective tunneling loops span several picoseconds in time and are as large as nanometers in space, which match the phonon periods and wavelengths, leading to strong phonon scattering at high pressures. Our results show direct evidence of collective quantum motion existing in high-pressure ice and provide a new perspective to understanding the coupling between phonon propagation and atomic tunneling.

Diamond is a solid-state platform to develop quantum technologies, but it has been a long-standing problem that the current understanding of quantum states in diamond is mostly limited to single-electron pictures. Here, we combine the full configuration interaction quantum Monte Carlo method and the density-matrix functional embedding theory, to achieve unprecedented accuracy in describing the many-body quantum states of nitrogen vacancy (NV) centers in diamond. More than 30 electrons and 130 molecular orbitals are correlated, which reveals the multi-configurational wavefunction of the many-body quantum states in diamond. The multi-configurational description explains puzzling experimental measurements in intersystem crossing and charge state transition in NV centers in diamond. The calculations not only reproduce the available experimental measurements of the energy gaps between quantum states but also provide new benchmarks for states that are still subject to considerable uncertainty. This study highlights the importance of multi-configurational wavefunction in the many-body quantum states in solids.

We study the observation of polarized stochastic gravitational-wave background (SGWB) in pulsar-timing-array experiments. The time residual for an observed pulsar is formulated as a line-of-sight integral that incorporates the effects of the pulsar term, from which we construct the correlation function of the time residual between a pair of pulsars in terms of the overlap reduction functions (ORFs) for the SGWB intensity and polarization anisotropies. Our formulation provides a numerical scheme for computing the ORFs for high multipole moments and the lowest-moment ORFs for the SGWB linear polarization are worked out for the first time., Comment: 13 pages, 4 figures, Submitted to PRD. arXiv admin note: text overlap with arXiv:2002.01606

An accurate force field is the key to the success of all molecular mechanics simulations on organic polymers and biomolecules. Accuracy beyond density functional theory is often needed to describe the intermolecular interactions, while most correlated wavefunction (CW) methods are prohibitively expensive for large molecules. Therefore, it posts a great challenge to develop an extendible ab initio force field for large flexible organic molecules at CW level of accuracy. In this work, we face this challenge by combining the physics-driven nonbonding potential with a data-driven subgraph neural network bonding model (named sGNN). Tests on polyethylene glycol polymer chains show that our strategy is highly accurate and robust for molecules of different sizes. Therefore, we can develop the force field from small molecular fragments (with sizes easily accessible to CW methods) and safely transfer it to large polymers, thus opening a new path to the next-generation organic force fields.

We study the interferometric observation of intensity and polarization anisotropies of a stochastic gravitational wave background (SGWB). We show that the observed correlated data is defined in the group manifold of the three-dimensional rotation. Explicit correlation between two detectors in the interferometry experiments such as LIGO-Virgo and KAGRA is constructed in terms of the Wigner D-functions. Our results may provide a tool for constructing data pipelines to estimate the power spectra of the SGWB anisotropies.

An alternative elementary argument for the magnetic field outside a solenoid is described., Comment: Comment on article by Aritro Pathak, European Journal of Physics, Vol. 38, 015201 (2017)

We develop the diagrammatic formulation of the many-body theory for the coupled collective modes in interacting electron systems of different dimensions. The formalism is then applied in detail to a two-dimensional system coupled to a three-dimensional electron gas. We find two dimensionally-mixed plasmon modes which in the long wavelength limit are respectively 3D-like and acoustic in nature, but are strongly coupled at larger wave vectors. The same formalism can be applied to any dimensional combinations, and we also present the results for 1D-2D and 1D-3D coupled systems., Comment: 5 pages, 3 figures

Coulomb drag between parallel two-dimensional electronic layers is an excellent tool for the study of electron-electron interactions. In actual experiments, the layers display spatial charge density fluctuations due to imperfections such as external charged impurities. However, at present a systematic way of taking these inhomogeneities into account in drag calculations has been lacking, making the interpretation of experimental data problematic. On the other hand, there exists a highly successful and widely accepted formalism describing transport within single inhomogeneous layers known as effective medium theory. In this work, we generalize the standard effective medium theory to the case of Coulomb drag between two inhomogeneous sheets and demonstrate that inhomogeneity in the layers has a strong impact on drag transport. In the case of exciton condensation between the layers, we show that drag resistivity takes on a value determined by the amplitude of density fluctuation. Next we consider drag between graphene sheets, in which the existence of spatial charge density fluctuations is well-known. We show that these inhomogeneities play a crucial role in explaining existing experimental data. In particular, the temperature dependence of the experimentally observed peaks in drag resistivity can only be explained by taking the layer density fluctuations into account. We also propose a method of extracting information on the correlations between the inhomogeneities of the layers. The effective medium theory of Coulomb drag derived here is general and applies to all two-dimensional materials., Comment: 15 pages, 8 figures. Substantially rewritten to better reflect generality of the results. Numerical errors fixed

We study two-stream instabilities in a non-equilibrium system in which a stream of electrons is injected into doped graphene. As with equivalent non-equilibrium parabolic band systems, we find that the graphene systems can support unstable charge-density waves whose amplitudes grow with time. We determine the range of wavevector $\boldsymbol{q}$ that are unstable, and their growth rates. We find no instability for waves with wavevectors parallel or perpendicular to the direction of the injected carriers. We find that, within the small wavevector approximation, the angle between $\boldsymbol{q}$ and the direction of the injected electrons that maximizes the growth rate increases with increasing $\boldsymbol{|q|}$. We compare the range and strength of the instability in graphene to that of two and three dimensional parabolic band systems., Comment: 21 pages, 7 figures

Even though the sensitivity of direct dark matter search experiments reach the level about $10^{-45}~{\rm cm}^2$, there is no confident signal of dark matter been observed. We point out that, if dark matter is a vector boson, the null result in direct dark matter search experiments may due to the destructive effects in dark-matter-nucleon elastic scattering. We illustrate the scenario using a modified Higgs portal model that includes exotic quarks. The significant cancellation can occur for certain mass gap between heavy quark and dark matter. As a result, the spin-independent dark-matter-nucleon elastic scattering is so suppressed that the future direct search experiments can hardly observe the signal of dark matter., Comment: 11 pages, 5 figures

The total linear electromagnetic field momentum $\mathbf P_{\mathrm{em}}$ of a stationary electric dipole $\mathbf p$ in a static magnetic field $\mathbf B$ is considered. The expression $\mathbf P_{\mathrm{em}} = \frac12\mathbf B \times \mathbf p$, which has previously been implied to hold in all static magnetic field situations, is not valid in general. The contribution of the electromagnetic momentum of the fringing fields of the dipole is discussed. It is shown that when either the static magnetic field or the electric dipole moment is changed, the mechanical impulse on the system equals $-\Delta\mathbf P_{\mathrm{em}}$, and hidden momentum does not need to be invoked in order to conserve total momentum., Comment: 31 pages, 6 figures

In a recent papers, Turner and Turner (2010 {\em Am. J. Phys.} {\bf 78} 905-7) and Jensen (2011 {\em Eur. J. Phys.} {\bf 32} 389-397) analysed the motion of asymmetric rolling rigid bodies on a horizontal plane. These papers addressed the common misconception that the instantaneous point of contact of the rolling body with the plane can be used to evaluate the angular momentum $\mathbf L$ and the torque $\boldsymbol\tau$ in the equation of motion $d\mathbf L/dt = \boldsymbol\tau$. To obtain the correct equation of motion, the "phantom torque" or various rules that depend on the motion of the point about which $\mathbf L$ and $\boldsymbol\tau$ are evaluated were discussed. In this paper, I consider asymmetric disks rolling down an inclined plane and describe the most basic way of obtaining the correct equation of motion; that is, to choose the point about which $\mathbf L$ and $\boldsymbol\tau$ are evaluated that is stationary in an inertial frame.

We present derivations of the expressions for the spherical volume averages of static electric and magnetic fields that are virtually identical. These derivations utilize the Coulomb and Biot-Savart laws, and make no use of vector calculus identities or potentials., Comment: 9 pages, 3 figures

We calculate the average single particle density of states in graphene with disorder due to impurity potentials. For unscreened short-ranged impurities, we use the non-self-consistent and self-consistent Born and $T$-matrix approximations to obtain the self-energy. Among these, only the self-consistent $T$-matrix approximation gives a non-zero density of states at the Dirac point. The density of states at the Dirac point is non-analytic in the impurity potential. For screened short-ranged and charged long-range impurity potentials, the density of states near the Dirac point typically increases in the presence of impurities, compared to that of the pure system., Comment: 8 pages, 5 figures

We develop a theory for the renormalization of the phonon energy dispersion in graphene due to the combined effects of both Coulomb and electron-phonon (e-ph) interactions. We obtain the renormalized phonon energy spectrum by an exact analytic derivation of the phonon self-energy, finding three distinct Kohn anomalies (KAs) at the phonon wavevector $q = \omega/v, 2k_F\pm\omega/v$ for LO phonons and one at $q = \omega/v$ for TO phonons. The presence of these new KAs in graphene, in contrast to the usual KA $q = 2k_F$ in ordinary metals, originates from the dynamical screening of e-ph interaction (with a concomitant breakdown of the Born-Oppenheimer approximation) and the peculiar chirality of the graphene e-ph coupling., Comment: Accepted by PRL; updated version to appear in Physical Review Letters

We discuss the validity (or not) of the ring-diagram approximation (i.e. RPA) in the calculation of graphene self-energy in the weak-coupling ($r_s \ll 1$) limit, showing that RPA is a controlled and valid approximation for \textit{extrinsic} graphene where the Fermi level is away from the Dirac point., Comment: updated with additional paragraph and reference

We calculate, within the leading-order dynamical-screening approximation, the electron self-energy and spectral function at zero temperature for extrinsic (or gated/doped) graphene. We also calculate hot carrier inelastic scattering due to electron-electron interactions in graphene. We obtain the inelastic quasiparticle lifetimes and associated mean free paths from the calculated self-energy. The linear dispersion and chiral property of graphene gives energy dependent lifetimes that are qualitatively different from those of parabolic-band semiconductors., Comment: Submitted on July 8, 2007 to EP2DS-17, Genova, Italy

We study the Coulomb drag between two single graphene sheets in intrinsic and extrinsic graphene systems with no interlayer tunneling. The general expression for the nonlinear susceptibility appropriate for single-layer graphene systems is derived using the diagrammatic perturbation theory, and the corresponding exact zero-temperature expression is obtained analytically. We find that, despite the existence of a non-zero conductivity in an intrinsic graphene layer, the Coulomb drag between intrinsic graphene layers vanishes at all temperatures. In extrinsic systems, we obtain numerical results and an approximate analytical result for the drag resistivity $\rho_{\textrm{D}}$, and find that $\rho_{\textrm{D}}$ goes as $T^2$ at low temperature $T$, as $1/d^4$ for large bilayer separation $d$ and $1/n^3$ for high carrier density $n$. We also discuss qualitatively the effect of plasmon-induced enhancement on the Coulomb drag, which should occur at a temperature of the order of or higher than the Fermi temperature.

We calculate $\partial\mu/\partial n$ in extrinsic graphene as a function of carrier density $n$ at zero temperature by obtaining the electronic self-energy within the Hartree-Fock approximation. The exchange-driven Dirac-point logarithmic singularity in the quasiparticle velocity of intrinsic graphene disappears in the extrinsic case. The calculated renormalized $\partial\mu/\partial n$ in extrinsic graphene has the same qualitative $n^{-\frac12}$ density dependence as the inverse bare density of states with a 20% enhancement from the corresponding bare value, a relatively weak effect compared to the corresponding parabolic-band case., Comment: 4 pages, 3 figures

Based on relativistic velocity addition and the conservation of momentum and energy, I present derivations of the expressions for the relativistic momentum and kinetic energy, and $E=mc^2$., Comment: 9 pages, 2 Figures, submitted to American Journal of Physics

Mermin [Am. J. Phys. {\bf 51}, 1130--1131 (1983)] derived the relativistic addition of the parallel components of velocity using the constancy of the speed of light. In this note, the derivation is extended to the perpendicular components of velocity.

We consider hot carrier inelastic scattering due to electron--electron interactions in graphene, as functions of carrier energy and density. We calculate the imaginary part of the zero-temperature quasiparticle self-energy for doped graphene, utlizing the $G_0W$ and random phases approximations. Using the full dynamically screened Coulomb interaction, we obtain the inelastic quasiparticle lifetimes and associated mean free paths. The linear dispersion of graphene gives lifetime energy dependences that are qualitatively different from those of parabolic-band semiconductors. We also get good agreement with data from angle-resolved photoemission spectroscopy experiments., Comment: 6 pages, 5 figures

We develop a theory for describing frictional drag in bilayer systems with in-plane periodic potential modulations, and use it to investigate the drag between bilayer systems in which one of the layers is modulated in one direction. At low temperatures, as the density of carriers in the modulated layer is changed, we show that the transresistivity component in the direction of modulation can change its sign. We also give a physical explanation for this behavior., Comment: 4 pages, 4 figures

Submitted to "Notes and Discussions" in American Journal of Physics., Comment: 4 pages

I investigate the possibility of using Coulomb drag to detect a precursor of the predicted (but as yet not definitively observed) superfluid transition in electron-hole coupled quantum wells. The drag transresisitivity $\rho_{21}$ is shown to be significantly enhanced above the transition temperature $T_c$ and to diverge logarithmically as $T\to T_c^+$, due to electron-hole pairing fluctuations which are somewhat analogous to the Maki-Thompson contribution to conductivity in metals above the superconducting $T_c$. The enhancement in $\rho_{21}$ is estimated to be detectable at temperatures much higher than $T_c$., Comment: 14 pages, 1 figure

We derive a theory for transmission through disordered finite superlattices in which the interface roughness scattering is treated by disorder averaging. This procedure permits efficient calculation of the transmission thr ough samples with large cross-sections. These calculations can be performed utilizing either the Keldysh or the Landauer-B\"uttiker transmission formalisms, both of which yield identical equations. For energies close to the lowest miniband, we demonstrate the accuracy of the computationally efficient Wannier-function approximation. Our calculations indicate that the transmission is strongly affected by interface roughness and that information about scale and size of the imperfections can be obtained from transmission data., Comment: 12 pages, 6 Figures included into the text. Final version with minor changes. Accepted by Physical Review B

A first principles theory of inelastic tunneling between a model probe tip and an atom adsorbed on a surface is presented, extending the elastic tunneling theory of Tersoff and Hamann. The inelastic current is proportional to the change in the local density of states at the center of the tip due to the addition of the adsorbate. We use the theory to investigate the vibrational heating of an adsorbate below an STM tip. We calculate the desorption rate of H from Si(100)-H(2$\times$1) as function of the sample bias and tunnel current, and find excellent agreement with recent experimental data., Comment: 5 pages, RevTeX, epsf files

We report STM-induced desorption of H from Si(100)-H(2$\times1$) at negative sample bias. The desorption rate exhibits a power-law dependence on current and a maximum desorption rate at -7 V. The desorption is explained by vibrational heating of H due to inelastic scattering of tunneling holes with the Si-H 5$\sigma$ hole resonance. The dependence of desorption rate on current and bias is analyzed using a novel approach for calculating inelastic scattering, which includes the effect of the electric field between tip and sample. We show that the maximum desorption rate at -7 V is due to a maximum fraction of inelastically scattered electrons at the onset of the field emission regime., Comment: 4 pages, 4 figures. To appear in Phys. Rev. Lett

The role of optical-phonons in frictional drag between two adjacent but electrically isolated two-dimensional electron gases is investigated. Since the optical-phonons in III-V materials have a considerably larger coupling to electrons than acoustic phonons (which are the dominant drag mechanism at low T and large separations) it might be expected that the optical phonons will give a large effect high temperatures. The two key differences between optical and acoustic phonon mediated drag are: (1) the optical phonon mediated interlayer interaction is short-ranged due to their negligible group velocity at the Brillouin zone center; and (2) the typical momentum transfer for an optical phonon mediated scattering is relatively large. These considerations make optical phonon mediated drag difficult to see in single subband GaAs systems, but it may be possible to see the effect in double subband GaAs systems or single subband quantum wells in a material with a lower effective mass and lower optical phonon energy such as InSb., Comment: Revtex, 14 pages, 2 postscript figures

We use the Kubo formalism to evaluate the contribution of acoustic phonon exchange to the frictional drag between nearby two-dimensional electron systems. In the case of free phonons, we find a divergent drag rate ($\tau_{D}^{-1}$). However, $\tau_{D}^{-1}$ becomes finite when phonon scattering from either lattice imperfections or electronic excitations is accounted for. In the case of GaAs quantum wells, we find that for a phonon mean free path $\ell_{ph}$ smaller than a critical value, imperfection scattering dominates and the drag rate varies as $ln (\ell_{ph}/d)$ over many orders of magnitude of the layer separation $d$. When $\ell_{ph}$ exceeds the critical value, the drag rate is dominated by coupling through an electron-phonon collective mode localized in the vicinity of the electron layers. We argue that the coupled electron-phonon mode may be observable for realistic parameters. Our theory is in good agreement with experimental results for the temperature, density, and $d$-dependence of the drag rate., Comment: 45 pages, LaTeX, 8 postscript file figures

At temperatures comparable to the Fermi temperature, we have measured a plasmon enhanced Coulomb drag in a GaAs/AlGaAs double quantum well electron system. This measurement provides a probe of the many-body corrections to the coupled plasmon modes, and we present a detailed comparison between experiment and theory testing the validity of local field theories. Using a perpendicular magnetic field to raise the magnetoplasmon energy we can induce a crossover to single-particle Coulomb scattering., Comment: plain tex, 4 pages, 3 figures, to be published in PRL

We study the transresistivity $\tensor\rho_{21}$ (or equivalently, the drag rate) of two Coulomb-coupled quantum wells in the presence of a perpendicular magnetic field, using semi-classical transport theory. Elementary arguments seem to preclude any possibility of observation of ``Hall drag'' (i.e., a non-zero off-diagonal component in $\tensor\rho_{21}$). We show that these arguments are specious, and in fact Hall drag can be observed at sufficiently high temperatures when the {\sl intra}layer transport time $\tau$ has significant energy-dependence around the Fermi energy $\varepsilon_F$. The ratio of the Hall to longitudinal transresistivities goes as $T^2 B s$, where $T$ is the temperature, $B$ is the magnetic field, and $s = [\partial\tau/ \partial\varepsilon] (\varepsilon_F)$., Comment: LaTeX, 13 pages, 2 figures (to be published in Physica Scripta, Proc. of the 17th Nordic Semiconductor Conference)

We study leading-order many-body effects of longitudinal optical (LO) phonons on electronic properties of one-dimensional quantum wire systems. We calculate the quasiparticle properties of a weakly polar one dimensional electron gas in the presence of both electron-phonon and electron-electron interactions. The leading-order dynamical screening approximation (GW approximation) is used to obtain the electron self-energy, the quasiparticle spectral function, and the quasiparticle damping rate in our calculation by treating electrons and phonons on an equal footing. Our theory includes effects (within the random phase approximation) of Fermi statistics, Landau damping, plasmon-phonon mode coupling, phonon renormalization, dynamical screening, and impurity scattering. In general, electron-electron and electron-phonon many-body renormalization effects are found to be nonmultiplicative and nonadditive in our theoretical results for quasiparticle properties., Comment: 21 pages, Revtex, 12 figures enclosed

We describe a method for numerically incorporating electron--electron scattering in quantum wells for small deviations of the distribution function from equilibrium, within the framework of the Boltzmann equation. For a given temperature $T$ and density $n$, a symmetric matrix needs to be evaluated only once, and henceforth it can be used to describe electron--electron scattering in any Boltzmann equation linear-response calculation for that particular $T$ and $n$. Using this method, we calculate the distribution function and mobility for electrons in a quantum-well, including full finite-temperature dynamic screening effects. We find that at some parameters which we investigated, electron--electron scattering reduces mobility by approximately 40\%., Comment: 12 pages, 2 figures (uuencoded)

We derive an expression for the drag rate (i.e., interlayer momentum transfer rate) for carriers in two coupled two-dimensional gases to lowest nonvanishing order in the screened interlayer electron--electron interaction, valid for {\sl arbitrary} intralayer scattering mechanisms, using the Boltzmann transport equation. We calculate the drag rate for experimentally relevant parameters, and show that for moderately high temperatures ($T\gtrsim 0.2 T_F$, where $T_F$ is the Fermi temperature) the dynamical screening of the interlayer results in a large enhancement of the drag rate due to the presence of coupled plasmon modes. This plasmon enhancement causes the scaled drag rate to have a peak (i) as a function of temperature at $T \approx 0.5 T_F$, and (ii) as a function of the ratio of densities of the carriers in the two layers when their Fermi velocities are equal. We also show that the drag rate can be significantly affected by the {\sl intralayer} scattering mechanisms; in particular, the drag rate changes approximately by a factor of 2 when the dopant layer modulation doped structures are moved in from 400~\AA to 100~\AA., Comment: RevTex, 21 pages, 7 postscript figures

We report a fully microscopic theory for transconductivity, or, equivalently, momentum transfer rate, of Coulomb coupled electron systems. We use the Kubo linear response formalism, and our main formal result expresses the transconductivity in terms of two fluctuation diagrams, which are topologically related, but not equivalent to, the Aslamazov-Larkin diagrams known for superconductivity. Previously reported results are shown to be special cases of our general expression; specifically, for constant impurity scattering rates, we recover the Boltzmann equation results in the semiclassical clean limit, and the memory function results in dirty systems. Furthermore, we show that for energy dependent relaxation times, the final result is not expressible in terms of standard density-response functions. Other new results include: (i) at T=0, the frequency dependence of the transfer rate is found to be proportional to $\Omega$ and $\Omega^2$ for frequencies below and above the impurity scattering rate, respectively and (ii) the weak localization correction to the transconductivity is given by $\delta\sigma^{WL}_{21} \propto \delta\sigma^{WL}_{11} +\delta\sigma^{WL}_{22}$., Comment: Revtex, 32 pages + 4 Figures (encoded with uufiles)

We show theoretically that the Coulomb drag rate between two parallel quasi-two-dimensional electron gases is substantially enhanced by the coupled acoustic and optic plasmon modes of the system at temperatures $T \gtrsim 0.2T_F$ (where $T_F$ is the Fermi temperature) for experimentally relevant parameters. The acoustic mode causes a sharp upturn in the scaled drag rate as a function of temperature at $T \approx 0.2 T_F$. Other experimental signatures of plasmon-dominated drag are a $d^{-3}$ dependence on the well separation $d$, and a peak in the drag rate as a function of relative carrier densities at matched Fermi velocities., Comment: 10 pages, RevTeX 3.0, MIC-TH-2

Using a finite-frequency recursive Green's function technique, we calculate the dynamic magneto-conductance fluctuations and oscillations in disordered mesoscopic normal metal systems, incorporating inter-particle Coulomb interactions within a self-consistent potential method. In a disordered metal wire, we observe ergodic behavior in the dynamic conductance fluctuations. At low $\omega$, the real part of the conductance fluctuations is essentially given by the dc universal conductance fluctuations while the imaginary part increases linearly from zero, but for $\omega$ greater than the Thouless energy and temperature, the fluctuations decrease as $\omega^{-1/2}$. Similar frequency-dependent behavior is found for the Aharonov-Bohm oscillations in a metal ring. However, the Al'tshuler-Aronov-Spivak oscillations, which predominate at high temperatures or in rings with many channels, are strongly suppressed at high frequencies, leading to interesting crossover effects in the $\omega$-dependence of the magneto-conductance oscillations., Comment: 4 pages, REVTeX 3.0, 5 figures(ps file available upon request), #phd09

We calculate the electron elastic mean free path due to ionized impurity scattering in semiconductor quantum wires, using a scheme in which the screened ionized impurity potential and the electron screening self-consistently determine each other. By using a short-range scattering potential model, we obtain an {\sl exact} solution of the self-energy within the self-consistent Born and ``noncrossing" approximations. We find that, compared to the mean free path for the bare unscreened potential $\ell_{\rm bare}$, the calculated mean free path including self-consistent screening $\ell_{\rm sc}$ is {\sl substantially larger}, going as $\ell_{\rm sc} \sim \ell_{\rm bare}[\ln(\ell_{\rm bare})]^2$ for large $\ell_{\rm bare}$., Comment: 11 pages, 4 Postscript files (appended), Revtex Version 3.0, UMCP-CMT-Hu-9301

We show that, for some semiconductor devices and physical experiments, processes which scatter electrons {\em into} a state $|{\bf k}\rangle$ can contribute strongly to the decay of a nonequilibrium electron occupation of $|{\bf k}\rangle$. For electrons, the decay rate $\gamma({\bf k})$ is given by the sum of the total scattering-out {\em and scattering-in} rates of state $|{\bf k}\rangle$. The scattering-in term, which is often neglected in calculations, increases $\gamma({\bf k})$ of low energy electrons injected into semidegenerate systems, which includes many doped semiconductor structures at nonzero temperatures, particularly those of reduced dimensions., Comment: 4 pages, figure available on request, RevTex 2.0. (NIST-BAS-93-01)

We consider theoretically the electron--electron interaction induced exchange-correlation effects in the lowest subband of a quasi-one-dimensional GaAs quantum wire structures. We calculate, within the leading order dynamical screening approximation ({\em i.e.} the so-called $GW$ approximation of the electron gas theory), the electron self-energy, spectral function, momentum distribution function, inelastic scattering rate, band gap renormalization, and, the many-body renormalization factor both at zero and finite temperatures, and, both with and without impurity scattering effects. We also calculate the effects of finite temperatures and finite impurity scattering on the many-body properties. We propose the possibility of a hot-electron transistor device with a large negative differential resistance which is based on the sudden onset of plasmon emission by energetic ballistic electrons in one dimension. The issue of the existence or non-existence of the Fermi surface among the interacting one-dimensional quantum wire electrons is critically discussed based on our numerical results., Comment: 93 pages, REVTEX (LaTex) version 2, 16 Figures (not included, available upon request), UM-CM#0813

The development and optimization of flight control systems for modem fixed- and rotary-. wing aircraft consume a significant portion of the overall time and cost of aircraft development. Substantial savings can be achieved if the time required to develop and flight test the control system, and the cost, is reduced. To bring about such reductions, software tools such as Matlab/Simulink are being used to readily implement block diagrams and rapidly evaluate the expected responses of the completed system. Moreover, tools such as CONDUIT (CONtrol Designer's Unified InTerface) have been developed that enable the controls engineers to optimize their control laws and ensure that all the relevant quantitative criteria are satisfied, all within a fully interactive, user friendly, unified software environment.