Effects Of Coulomb Coupling Research Articles

Magnetic field asymmetry of transport dynamics in a Coulomb-coupled quantum point contact

We report phase-resolved microwave magnetotransport measurements of a Coulomb-coupled quantum point contact in the edge state regime. We show that the inductive admittance is manifestly asymmetric under reversal of magnetic field, whereas the field-symmetric conductance is observed. Furthermore, the asymmetry is found to exhibit pronounced fluctuations as a function of magnetic field and transmission. This constitutes direct evidence that kinetic effects associated with the nonequilibrium polarization charge contribute to dynamical magnetoasymmetry in phase-coherent mesoscopic conductors. In addition, we find a striking magnetoasymmetry in the electronic transit time, providing important information about Coulomb coupling effects on quantum dynamics of electrons.

Kinetic model for electron-ion transport in warm dense matter.

We present a model for electron-ion transport in warm dense matter that incorporates Coulomb coupling effects into the quantum Boltzmann equation of Uehling and Uhlenbeck through the use of a statistical potential of mean force. Although the model presented here can be derived rigorously in the classical limit [S. D. Baalrud and J. Daligault, Phys. Plasmas 26, 082106 (2019)PHPAEN1070-664X10.1063/1.5095655], its quantum generalization is complicated by the uncertainty principle. Here we apply an existing model for the potential of mean force based on the quantum Ornstein-Zernike equation coupled with an average-atom model [C. E. Starrett, High Energy Density Phys. 25, 8 (2017)1574-181810.1016/j.hedp.2017.09.003]. This potential contains correlations due to both Coulomb coupling and exchange, and the collision kernel of the kinetic theory enforces Pauli blocking while allowing for electron diffraction and large-angle collisions. We use the Uehling-Uhlenbeck equation to predict the momentum and temperature relaxation times and electrical conductivity of solid density aluminum plasma based on electron-ion collisions. We present results for density and temperature conditions that span the transition from classical weakly-coupled plasma to degenerate moderately-coupled plasma. Our findings agree well with recent quantum molecular dynamics simulations.

Effects of Coulomb coupling on friction in strongly magnetized plasmas

The friction force on a test particle traveling through a plasma that is both strongly coupled and strongly magnetized is studied using molecular dynamics simulations. In addition to the usual stopping power component aligned antiparallel to the velocity, a transverse component that is perpendicular to both the velocity and Lorentz force is observed. This component, which was previously only characterized in weakly coupled plasmas, is found to increase in both absolute and relative magnitude in the strongly coupled regime. Strong coupling is also observed to induce a third component of the friction force in the direction of the Lorentz force. These first-principles simulations reveal novel physics associated with collisions in strongly coupled, strongly magnetized plasmas that are not predicted by existing kinetic theories. The effect is expected to influence macroscopic transport in a number of laboratory experiments and astrophysical plasmas.

Effects of Coulomb coupling on stopping power and a link to macroscopic transport

Molecular dynamics simulations are used to assess the influence of Coulomb coupling on the energy evolution of charged projectiles in the classical one-component plasma. The average projectile kinetic energy is found to decrease linearly with time when να/ωp ≲ 10−2, where να is the Coulomb collision frequency between the projectile and the medium, and ωp is the plasma frequency. Stopping power is obtained from the slope of this curve. In comparison to the weakly coupled limit, strong Coulomb coupling causes the magnitude of the dimensionless stopping power, (a/kBT)dE/dx, to increase, the Bragg peak to shift to several times the plasma thermal speed, and for the stopping power curve to broaden substantially. The rate of change of the total projectile kinetic energy averaged over many independent simulations is shown to consist of two measurable components: a component associated with a one-dimensional friction force and a thermal energy exchange rate. In the limit of a slow and massive projectile, these can be related to the macroscopic transport rates of self-diffusion and temperature relaxation in the plasma. Simulation results are compared with available theoretical models for stopping power, self-diffusion, and temperature relaxation.

Two-Temperature Magnetohydrodynamics Simulations of Propagation of Semi-Relativistic Jets

In astrophysical jets observed in active galactic nuclei and in microquasars, the energy exchange rate by Coulomb collision is insufficient for thermal equilibrium between ions and electrons. Therefore, it is necessary to consider the difference between the ion temperature and the electron temperature. We present the results of two-temperature magnetohydrodynamics(MHD) simulations to demonstrate the effects of Coulomb coupling. It is assumed that the thermal dissipation heats only ions. We find that the ion and electron temperatures are separated through shocks. Since the ion entropy is increased by energy dissipation at shocks and the Coulomb collisions are inefficient, electron temperature becomes about 10 times lower than the ion temperature in the hotspot ahead of the jet terminal shock. In the cocoon, electron temperature decreases by gas mixing between high temperature cocoon gas and low temperature shocked-ambient gas even when we neglect radiative cooling, but electrons can be heated through collisions with ions. Radiation intensity maps are produced by post processing numerical results. Distributions of the thermal bremsstrahlung radiation computed from electron temperature have bright filament and cavity around the jet terminal shock.

Coulomb coupling effects in the gigahertz complex admittance of a quantum R–L circuit

We report on the gigahertz admittance measurements of a quantum conductor, i.e. a quantum R–L circuit, to probe the intrinsic dynamic of the conductor. The magnetic field dependence of the admittance phase provides us with an effective way to study the role of Coulomb interaction between counterpropagating edge channels. In addition, there is a small jump in the admittance phase when the transmitted modes are changed. This is because the gate voltage leads to a static potential shift of the quantum channel, then a quantum capacitance related to the density of states of the edge channels are influenced. Our study has made new discoveries of the dynamic transport in a quantum conductor, finding evidence for the deviations from quantum chiral transport associated with Coulomb interactions.

Ab initio results for the static structure factor of the warm dense electron gas

The uniform electron gas at finite temperature is of high current interest for warm dense matter research. The complicated interplay of quantum degeneracy and Coulomb coupling effects is fully contained in the pair distribution function or, equivalently, the static structure factor. By combining exact quantum Monte Carlo results for large wave vectors with the long‐range behaviour from the Singwi‐Tosi‐Land‐Sjölander approximation, we are able to obtain highly accurate data for the static structure factor over the entire k‐range. This allows us to gauge the accuracy of previous approximations and discuss their respective shortcomings. Further, our new data will serve as valuable input for the computation of other quantities.

Correlational and thermodynamic properties of finite-temperature electron liquids in the hypernetted-chain approximation.

Correlational and thermodynamic properties of homogeneous electron liquids at finite temperatures are theoretically analyzed in terms of dielectric response formalism with the hypernetted-chain (HNC) approximation and its modified version. The static structure factor and the local-field correction to describe the strong Coulomb-coupling effects beyond the random-phase approximation are self-consistently calculated through solution to integral equations in the paramagnetic (spin unpolarized) and ferromagnetic (spin polarized) states. In the ground state with the normalized temperature θ=0, the present HNC scheme well reproduces the exchange-correlation energies obtained by quantum Monte Carlo (QMC) simulations over the whole fluid phase (the coupling constant rs≤100), i.e., within 1% and 2% deviations from putative best QMC values in the paramagnetic and ferromagnetic states, respectively. As compared with earlier studies based on the Singwi-Tosi-Land-Sjölander and modified convolution approximations, some improvements on the correlation energies and the correlation functions including the compressibility sum rule are found in the intermediate to strong coupling regimes. When applied to the electron fluids at intermediate Fermi degeneracies (θ≈1), the static structure factors calculated in the HNC scheme show good agreements with the results obtained by the path integral Monte Carlo (PIMC) simulation, while a small negative region in the radial distribution function is observed near the origin, which may be associated with a slight overestimation for the exchange-correlation hole in the HNC approximation. The interaction energies are calculated for various combinations of density and temperature parameters ranging from strong to weak degeneracy and from weak to strong coupling, and the HNC values are then parametrized as functions of rs and θ. The HNC exchange-correlation free energies obtained through the coupling-constant integration show reasonable agreements with earlier results including the PIMC-based fitting over the whole fluid region at finite degeneracies in the paramagnetic state. In contrast, a systematic difference between the HNC and PIMC results is observed in the ferromagnetic state, which suggests a necessity of further studies on the exchange-correlation free energies from both aspects of analytical theory and simulation.

Origins of improved carrier multiplication efficiency in elongated semiconductor nanostructures.

Nanorod solar cells have been attracting a lot of attention recently, as they have been shown to exhibit a lower carrier multiplication onset and a higher quantum efficiency than quantum dots with similar bandgaps. The underpinning theory for this phenomenon is not yet completely understood, and is still the subject of ongoing study. Here we conduct a theoretical investigation into CM efficiency in elongated semiconductor nanostructures with square cross section made of different materials (GaAs, GaSb, InAs, InP, InSb, CdSe, Ge, Si and PbSe), using a single-band effective mass model. Following Luo, Franceschetti and Zunger we adopt the CM figure of merit (the ratio between biexciton and single-exciton density of states) as a measure of CM efficiency and investigate its dependence on the aspect ratio for both (a) constant cross section (i.e. varying the volume) and (b) constant volume (i.e., varying the cross section), by decoupling electronic structure effects from surface-related effects, increased absorption and Coulomb coupling effects. The results show that in both (a) and (b) cases elongation causes an increase in both single- and bi-exciton density of states, with the latter, however, growing much faster with increasing energy. This leads to the availability of more bi-exciton states than single-exciton states for photon energies just above the bi-exciton ground state and therefore suggests a higher probability of CM at these energies for elongated structures. Our results therefore show that the origin of the observed decrease of the CM threshold in elongated structures can be attributed purely to electronic structure effects, paving the way to the implementation of CM-efficiency-boosting strategies in nanostructures based on the lowering of the CM onset.

Optical excitations of hybrid metal-semiconductor nanoparticles

We theoretically investigate Coulomb coupling effects in hybrid metal-semiconductor nanostructures, whose optical response is governed by plasmonic and excitonic effects (plexcitons). The plasmonic response of the nanoparticle is modeled within the framework of Maxwell’s equations, using a suitable dielectric function for the metal, and the excitonic response is described through the Schrodinger equation and the semiconductor Bloch equations. Our approach accounts for the quantum confinement of carriers in the semiconductor, for static screening in the formation of the exciton, and for a dynamic coupling between plasmons and excitons in the optical absorption or scattering. We apply our model to a prototypical CdS-based matchstick structure and investigate the importance of the various Coulomb coupling effects.

Dispersion relations for the dust-acoustic wave under experimental conditions

The dust acoustic wave dispersion relation is tested to quantify its sensitivity to many physical processes that are important in laboratory dusty plasmas. It is found that inverse Landau damping and ion-neutral collisions contribute about equally to the growth rate ωi, pointing to the advantage of using a kinetic model for the instability. The growth rate ωi increases the most with an increase of dust number density, followed by an increase in ion-drift speed. The quantities that cause ωi to decrease the most when they are increased are the dust-neutral collision rate followed by the ion-neutral collision rate, ion collection current onto dust particles, and the ion thermal speed. In general, ωi is affected more than ωr by the choice of processes that are included. Strong Coulomb-coupling effects can be included in a compressibility term. The susceptibilities derived here can be combined in various ways in a dispersion relation to account for different combinations of physical processes.

Nonlinear wave propagation in strongly coupled dusty plasmas

The nonlinear propagation of low-frequency waves in a strongly coupled dusty plasma medium is studied theoretically in the framework of the phenomenological generalized hydrodynamic (GH) model. A set of simplified model nonlinear equations are derived from the original nonlinear integrodifferential form of the GH model by employing an appropriate physical ansatz. Using standard perturbation techniques characteristic evolution equations for finite small amplitude waves are then obtained in various propagation regimes. The influence of viscoelastic properties arising from dust correlation contributions on the nature of nonlinear solutions is discussed. The modulational stability of dust acoustic waves to parallel perturbation is also examined and it is shown that dust compressibility contributions influenced by the Coulomb coupling effects introduce significant modification in the threshold and range of the instability domain.

Theory of ultrafast nonlinear optics of Coulomb-coupled semiconductor quantum dots: Rabi oscillations and pump-probe spectra

We investigate the optical properties of a Coulomb-coupled double-quantum dot system excited by coherent light pulses. Basic effects of Coulomb coupling regarding linear and nonlinear optical processes are discussed. By numerically solving the Heisenberg equation of motion we are able to present the temporal evolution of the system’s density matrix for a wide range of coupling parameters. The two main coupling effects in dipole approximation, biexcitonic shift and Forster energy transfer, are investigated and their qualitative and quantitative influence on absorption spectra, Rabi oscillations, and single- and two-pulse excitation is discussed. We present simulated differential transmission spectra to allow for comparison with recent experimental studies.

The influence of coulomb coupling effects on solar-like stellar models

The equation of state (EOS) is one of the most important physical equations in stellar modeling theory. For research fields such as the helioseismology, which need high accuracy, an EOS including only ideal effects is not sufficient, and a more precise one. Involving non-ideal effects is needed. For a weakly coupled and weakly degenerate plasma system, the Coulomb interactions between the following charged particles are treated as follows: The ion-electron interactions, by the extended Debye-Hfickel theory with hard-core corrections, the ion-ion interactions, by many-body theory of classical point particles, and the electron-electron interactions, by quantum statistical methods. Their effects on the free energy are described by semi-analytic formulas. A group of solarlike star models are selected for comparing the results before and after the EOS improvement.

Ferromagnetic and freezing transitions in electron liquids: Exchange and Coulomb effects at finite temperatures.

Free energies of the paramagnetic and ferromagnetic electron liquids are evaluated through explicit calculations of the spin-dependent local-field corrections associated with the second-order, regular and anomalous, exchange diagrams. Their analytic expressions have been obtained as functions of temperature. Exchange- and Coulomb-correlation effects beyond the random-phase approximation are accounted for in reference to the known thermodynamic properties of the paramagnetic and ferromagnetic electron liquids in the ground state as well as at arbitrary degeneracy. The resultant density-dependent, Coulomb coupling effects act to reduce the exchange effects. A magnetic phase diagram for the ferromagnetic transition is thus obtained together with conditions for the freezing transition into the Coulomb solids.

Dynamic polarization potential induced by the Coulomb excitation of deformed heavy ions: Geometric scattering approach.

The geometric scattering theory and the Wentzel-Kramers-Brillouin (WKB) approximation are used to derive explicit expressions for the dynamic polarization potential induced by Coulomb excitation in heavy-ion collisions involving strongly deformed targets. We build two phase-equivalent potentials with very different radial and angular-momentum behaviors, which are discussed in the context of polarization potentials obtained previously within different frameworks. This correspondence provides a recipe for approximately correcting the deficiencies of the geometric scattering approach, which tends to overemphasize the effects of Coulomb coupling.

Secondary nuclear fusion reactions as evidence of electron degeneracy in highly compressed fusion fuel

Recent direct-drive implosion experiments on the GEKKO XII laser with plastic hollow shell targets demonstrated compressed densities of ∼600 g/cm3 (∼600 times liquid density) at a temperature of ∼0.3 keV. The highly compressed core plasmas are indicated to be strongly coupled (average Coulomb energy/thermal energy ≈5) and partially degenerate (thermal energy/Fermi energy ≈0.3). The diagnostic method based on the secondary nuclear fusion reactions is presented to prove the electron degeneracy in the highly compressed core plasma. The yield ratio of the secondary DT neutrons to the primary DD neutrons in such highly compressed core plasmas was calculated with inclusion of the strong Coulomb-coupling effects, the varied degrees of the electron degeneracy, and the electronic shielding effects. It was found in our calculations that there is a significant dependence of the yield ratio on the compressed core density. For the plastic targets at the electron temperature of ∼0.3 keV, the yield ratio increases from 9 × 10−4 to 8 × 10−3 for densities from 10 to 1000 g/cm3. The preliminary experiments using deuterated plastic hollow shell targets suggested that the enhancement of the yield ratio provided evidence of the electron degeneracy.

Theory of interparticle correlations in dense, high-temperature plasmas. VI. Probability densities of the electric microfields.

The radial distribution functions between the charged radiator atom and the partially degenerate electrons as well as the ions are calculated on the basis of the hypernetted-chain scheme developed in the earlier papers of this series; strong Coulomb coupling effects of the charged particles and the quantum-diffraction effects of the electrons are appropriately taken into account. The probability densities of the total electric microfields and of the low-frequency component perceived at the radiator are then evaluated for various cases of dense plasma in the potential-of-mean-force approximation, which exactly satisfies the second-moment sum rule without use of adjustable parameters.

Theory of interparticle correlations in dense, high-temperature plasmas. IV. Stopping power.

The stopping power of a dense, two-component plasma is calculated in the dielectric formulation, where the static and dynamic local-field corrections (LFC's) describing strong Coulomb-coupling effects beyond the random-phase approximation are explicitly taken into account. We thus clarify the extent to which the LFC's and the presence of ions act to modify the rate of inelastic scattering. It is found that when the velocity v of the injected particle is smaller than the Fermi velocity ${v}_{F}$ of the electrons, LFC's enhance the stopping power; when v\ensuremath{\ll}${v}_{F}$, the ionic contribution may predominate over the electronic contribution, which takes on a much reduced magnitude.

Theory of interparticle correlations in dense, high-temperature plasmas. I. General formalism.

This is the first in a series of papers in which we carry out a systematic study of multiparticle correlation effects in the dense, high-temperature plasmas, appropriate to the inertially-confined-fusion experiments and the interior of the main-sequence stars. In this paper (paper I), we develop a general density-response formalism with inclusion of varied degrees of the electron degeneracy and the local-field corrections (LFC's) describing the strong Coulomb-coupling effects. An explicit theoretical scheme of calculating the static LFC's is developed on the basis of the hypernetted-chain approximation; useful expressions for the dynamic LFC's are proposed.