Ab initio results for the plasmon dispersion and damping of the warm dense electron gas

Warm dense matter (WDM) is an exotic state on the border between condensed matter and dense plasmas. Important occurrences of WDM include dense astrophysical objects, matter in the core of our Earth, and matter produced in strong compression experiments. As of late, x-ray Thomson scattering has become an advanced tool to diagnose WDM. The interpretation of the data requires model input for the dynamic structure factor S(q, ω) and the plasmon dispersion ω(q) . Recently, the first ab initio results for S(q, ω) of the homogeneous warm dense electron gas were obtained from path integral Monte Carlo simulations (Dornheim et al., Phys. Rev. Lett., 121, 255001, 2018). Here, we analyse the effects of correlations and finite temperature on the dynamic dielectric function and the plasmon dispersion. Our results for the plasmon dispersion and damping differ significantly from the random-phase approximation and from earlier models of the correlated electron gas. Moreover, we show when commonly used weak damping approximations break down and how the method of complex zeroes of the dielectric function can solve this problem for WDM conditions.     

Density functional theory for dielectric properties of warm dense matter

ABSTRACT

Density functional theory (DFT) is applied for the calculation of the dielectric function (DF), reflectivity, conductivity, plasma frequency and effective free electron density of warm dense matter (WDM). Shock-compressed xenon plasma and warm dense hydrogen are considered. The longitudinal expression in the long wavelength limit is applied for the calculation of the imaginary part of the DF. The real part of the DF is calculated by means of the Kramers–Kronig transformation. Sum rule within the framework of the DFT is used for determination of the plasma frequency and effective free electron density. Corrections to the reflectivity are considered, which allow for the finite width of the transient layer (wave front) at the WDM border.     

Investigation of warm dense matter using time-resolved X-ray absorption spectroscopy with third- and fourth- generation light sources

The advent of high-power lasers has provided insights into laboratory high energy density (>10¹¹ J/m³) physics. In particular, the properties of warm dense matter (WDM) with temperatures of 10⁴–10⁶ K and near-solid densities is a research area that has garnered significant interest recently. However, owing to the high temperatures and pressures associated with WDM, the measurement of fundamental properties is difficult, and insufficient data has been a significant setback in WDM research. Herein, we review recent developments in time-resolved X-ray absorption spectroscopy with synchrotron and X-ray free electron lasers for WDM research. Various physical properties, such as atomic bonding, electronic structures, electron–phonon coupling, and thermal conductivity of various elements in WDM conditions are investigated via this noble X-ray technique at various time scales from 100 ps to 100 fs.     

强流重离子束驱动的高能量密度物理研究进展

强流高能离子束可以准等容加热任何高密度样品,制备出尺度大、状态均匀、内部无冲击波的高能量密度物质,为实验室研究高能量密度物理提供了一种独特的新手段。介绍了国内外典型的强流重离子加速器装置及其与高能量密度物理相关的关键参数设计和研究规划;展示了基于粒子和流体模拟的离子束驱动高能量密度物质产生和状态演化规律进展;介绍了一套兼具高时空分辨和高穿透力的高能电子成像诊断技术;分析了中低能区离子束与等离子体相互作用过程中的碰撞和电荷交换微观机制,以及激光加速超短超强离子束在等离子体中的非线性输运和欧姆能损机制。     

Effects of relativistic and ponderomotive nonlinearities on the interaction of high-power laser beam with an inhomogeneous warm plasma

The influence of relativistic-ponderomotive nonlinearities and the plasma inhomogeneity on the nonlinear interaction between a high-power laser beam and a warm underdense plasma are studied. It is clear that the relativistic ponderomotive force and the electron temperature modify the electron density distribution and consequently change the dielectric permittivity of the plasma. Therefore, by presenting the modified electron density and the nonlinear dielectric permittivity of the warm plasma, the electromagnetic wave equation for the propagation of intense laser beam through the plasma is derived. This nonlinear equation is numerically solved and the distributions of electromagnetic fields in the plasma, the variations of electron density, and plasma refractive index are investigated for two different background electron density profiles. The results show that the amplitude of the electric field and electron density oscillations gradually increase and decrease, during propagation in the inhomogeneous warm plasma with linear and exponential density profiles, respectively, and the distribution of electron density becomes extremely sharp in the presence of intense laser beam. It is also indicated that the electron temperature and initial electron density have an impact on the propagation of the laser beam in the plasma and change the plasma refractive index and the oscillations' amplitude and frequency. The obtained results indicate the importance of a proper choice of laser and plasma parameters on the electromagnetic field distributions, density steepening, and plasma refractive index variations in the interaction of an intense laser beam with an inhomogeneous warm plasma.     

Topological insulators induced by disorder and non-Hermiticity

Searching for topological states of matter in real materials or engineered systems has been a fundamental theme in condensed matter physics in the past decade[1].It is known that the topological insulators are robust against certain disorders,but they usually become trivial under strong disorders due to the Anderson localization.In the year 2009,it was theoretically found that a topological insulator can be surprisingly driven from a trivial phase by disorders,which is called the topological Anderson insulator[2].Although the topological Anderson insulator has been studied in various theoretical models[3-5],it was not until 2018 that its experimental observation was reported in two different artificial systems[6,7],i.e.,the one-dimensional cold atomic wires and two-dimensional photonic waveguide arrays.     

Plasma Diagnostics Using K‐Line Emission Profiles of Silicon

Modifications of K‐line profiles due to a warm dense plasma environment are a suitable tool for plasma diagnostics. We focus on Si K_α emissions due to an electron transfer from 2P to 1S shell. Besides 2P fine structure effects we also consider the influence of excited and higher ionized emitters. Generally spoken, a plasma of medium temperature and high density (warm dense matter) is created from bulk Si the greater part of atoms is ionized. The high energy of K_α x‐rays is necessary to penetrate and investigate the Si sample. The plasma effect influences the many‐particle system resulting in an energy shift due to electron‐ion and electron‐electron interaction. In our work we focus on pure Si using LS coupling. Non‐perturbative wave functions are calculated as well as ionization energies, binding energies and relevant emission energies using the chemical ab initio code Gaussian 03. The plasma effect is considered within a perturbative approach to the Hamiltonian. Using Roothaan‐Hartree‐Fock wave functions we calculate the screening effect within an ion‐sphere model. The different excitation and ionization probabilities of the electronic L‐shell and M‐shell lead to a charge state distribution. Using this distribution and a Lorentz profile convolution with a Gaussian instrument function we calculate spectral line profiles depending on the plasma parameters. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comparisons of RZ-OOK and RZ-DPSK in dense OTDM-WDM systems using Q factor models

In view of the differential phase Q (DP-Q) and the traditional Q factor, we compared, using numerical simulations, the performances of the RZ-OOK and RZ-DPSK in dense OTDM-WDM systems. When signal pulse widths and optical filter bandwidths are optimized, there is no upper limit to the WDM channel bit rate (BR) in the purely linear back-to-back configuration. Here, RZ-DPSK performed increasingly better than RZ-OOK in a higher spectral density with Q gain increasing from 3 to 5 dB. In the nonlinear point-to-point configuration, a higher BR leads to increased performance penalties for both the RZ-DPSK and RZ-OOK, while the RZ-DPSK still outperforms RZ-OOK by up to 4 dB. The results obtained correlate with conventional results, indicating the potential of the DP-Q as a performance evaluation tool in numerical simulations.     

用于温密物质研究的强流电子束加载技术

能量20 MeV、流强2.5 kA的电子束脉冲可以在数十ns的时间内将靶材料加载至温密物质状态，进而可以开展材料状态方程、电导率以及不透明度等的实验研究工作。介绍了在神龙一号加速器上开展温密物质实验研究的束靶作用方式以及相应的测试技术。对电子束在直径0.3 mm、长1 mm的金属靶丝内的能量沉积和流体动力学响应进行了数值模拟。结果表明:靶丝的温度随着靶材料原子序数的增加而上升，而靶丝内温度分布的均匀性随着原子序数的增加而降低；在电子束加载后40 ns时刻Ta丝内的最高温度可以达到约1.6 eV。     

Nonlinear interactions between electromagnetic waves and electron plasma oscillations in quantum plasmas

We consider nonlinear interactions between intense circularly polarized electromagnetic (CPEM) waves and electron plasma oscillations (EPOs) in a dense quantum plasma, taking into account the electron density response in the presence of the relativistic ponderomotive force and mass increase in the CPEM wave fields. The dynamics of the CPEM waves and EPOs is governed by the two coupled nonlinear Schr?dinger equations and Poisson's equation. The nonlinear equations admit the modulational instability of an intense CPEM pump wave against EPOs, leading to the formation and trapping of localized CPEM wave pipes in the electron density hole that is associated with a positive potential distribution in our dense plasma. The relevance of our investigation to the next generation intense laser-solid density plasma interaction experiments is discussed.     

强粒子束产生温稠密物质热力学状态估计

基于能量平衡原理,结合SRIM统计方法,以铝靶为例,对重离子束和强电子束产生高温高密度物质所需的束流参数进行估计,分析各自产生温稠密物质的优缺点。结果显示,从电子辐射能损和束流利用观点来看,1~10MeV电子束产生温稠密物质具有较好的均匀性和较高的利用率;而重离子束加载可以获得较宽区域的温稠密物质。     

Localized plasmons in quantum plasmas

We consider the nonlinear interactions between finite amplitude electron and ion plasma oscillations in a fermionic quantum plasma. Accounting for the quantum statistical electron pressure and the quantum Bohm potential, we derive a set of coupled nonlinear equations that govern the dynamics of modulated electron plasma oscillations (EPOs) in the presence of the nonlinear ion oscillations (NLIOs). We numerically study stationary solutions of our coupled nonlinear equations. We find that the quantum parameter H (equal to the ratio between the plasmonic and electron Fermi energy densities) introduces new features to the electron density and electric potential humps of localized NLIOs in the absence of EPOs. Furthermore, the nonlinear coupling between the EPOs and NLIOs gives rise to a new class of envelope solitons composed of bell shaped electric field envelope of the EPOs, which are trapped in the electron density hole (and an associated negative oscillatory electric potential) that is produced by the ponderomotive force of the EPOs. The knowledge of the localized plasmonic structures is of immense value for interpreting experimental observations in dense quantum plasmas.     

Nonlinear theory for a quantum diode in a dense Fermi magnetoplasma

We present a simple analytical nonlinear theory for quantum diodes in a dense Fermi magnetoplasma. By using the steady-state quantum hydrodynamical equations for a dense Fermi magnetoplasma, we derive coupled nonlinear Schr?dinger and Poisson equations. The latter are numerically solved to show the effects of the quantum statistical pressure, the quantum tunneling (or the quantum diffraction), and the external magnetic field strength on the potential and electron density profiles in a quantum diode at nanometer scales. It is found that the quantum statistical pressure introduces a lower bound on the steady electron flow in the quantum diode, while the quantum diffraction effect allows the electron tunneling at low flow speeds. The magnetic field acts as a barrier, and larger potentials are needed to drive currents through the quantum diode.     

Permutation blocking path integral Monte Carlo simulations of degenerate electrons at finite temperature

We analyse the simulation of strongly degenerate electrons at finite temperature using the recently introduced permutation blocking path integral Monte Carlo (PB‐PIMC) method [T. Dornheim et al., New J. Phys. 17 , 073017 (2015)]. As a representative example, we consider electrons in a harmonic confinement and carry out simulations for up to P = 2000 so‐called imaginary‐time propagators – an important convergence parameter within the PIMC formalism. This allows us to study the P‐dependence of different observables of the configuration space in the Monte Carlo simulations and of the fermion sign problem. We find a surprisingly persisting effect of the permutation blocking for large P, which is explained by comparing different length scales. Finally, we touch upon the uniform electron gas in the warm dense matter regime.     

On the Nature of Bose-Einstein Condensation in Disordered Systems

We study the perfect Bose gas in random external potentials and show that there is generalized Bose-Einstein condensation in the random eigenstates if and only if the same occurs in the one-particle kinetic-energy eigenstates, which corresponds to the generalized condensation of the free Bose gas. Moreover, we prove that the amounts of both condensate densities are equal. Our method is based on the derivation of an explicit formula for the occupation measure in the one-body kinetic-energy eigenstates which describes the repartition of particles among these non-random states. This technique can be adapted to re-examine the properties of the perfect Bose gas in the presence of weak (scaled) non-random potentials, for which we establish similar results. In addition some of our results can be applied to models with diagonal interactions, that is, models which conserve the occupation density in each single particle eigenstate.     

Inhomogeneous polarizable medium as a model for an electron gas in a nonconducting crystal

The response of the electron system to an external perturbation in an insulating medium is considered in the context of the electron density functional method. The energy functional minimization relies on the proper choice of the trial function considering different spatial scales of the electron polarization. An efficient scheme for the density functional parametrization is suggested. The electron response to atomic displacements and external electric fields has been calculated for a simple model of electron polarization centers embedded into the homogeneous dielectric background. Good agreement with the experiment is achieved for the phonon spectra ofSi, Ge, and α-Sn. The electron density induced in the external homogeneous electric field is in good agreement with that calculated by the well-known ab initio methods. V. D. Kuznetsov Siberian Physical-Technical Institute at Tomsk State University. Translated for Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6 pp. 30–37, June, 1999.     

Intense ion beams as a tool for opacity measurements in warm dense matter

Opacity measurements in warm dense matter (WDM) provide a valuable benchmark for the diverging theoretical models in this regime. Heating of thin foils with intense heavy-ion beams allows one to create isolated samples of warm dense matter suitable for experimental determination of frequency-dependent opacities. A prerequisite for the measurements is the isothermal expansion of the heated foil. Hydrodynamic simulations predict that this condition is fulfilled. The analysis shows that existing ion-beam accelerators are capable to contribute to this field of research.     

Imaging of electron potential landscapes on Au(111)

The Hohenberg-Kohn theorem states that the ground state electron density completely determines the external potential acting on an electron system. Inspired by this fundamental theorem, we developed a novel approach to map directly the electron potential in surface systems: linear response theory applied to the total electron density as measured with scanning tunneling microscopy determines the external potential. Potential imaging is demonstrated for the s-p derived surface state on Au(111), where the "herringbone" reconstruction induces a periodic potential modulation, the details of which are revealed by our technique.     

外粒子源注入下气体放电过程的模拟研究

利用Geant4程序建立外源注入式、低气压气体开关物理模型,通过模拟计算电子增益与极板间电场强度、电子增益与极板间隙距离的函数关系验证了模型的正确性。计算了气体种类、气体压强对电子增益的影响,分析得到形成自持放电所需最小入射电子数,计算结果表明:在相同的气压及电场条件下,氮气的电子增益远大于氦气,这与氦气的高电离能性质相吻合; 随气压增大,电子增益呈非线性增长; 为实现自持放电,外源注入电子数面密度为1×105~2×105 /cm2。