Dynamical Screening Effects Research Articles

Phonon spectral functions of photo-generated hot carrier plasmas: effects of carrier screening and plasmon–phonon coupling

We investigate spectral behavior of phonon spectral functions in an interacting multi-component hot carrier plasma. Spectral analysis of various phonon spectral functions is performed considering carrier–phonon channels of polar and nonpolar optical phonons, acoustic deformation-potential, and piezoelectric Coulomb couplings. Effects of phonon self-energy corrections are examined at finite temperature within a random phase approximation extended to include the effects of dynamic screening, plasmon–phonon coupling, and local-field corrections of the plasma species. We provide numerical data for the case of a photo-generated electron–hole plasma formed in a wurtzite GaN. Our result shows the clear significance of the multiplicity of the plasma species in the phonon spectral functions of a multi-component plasma giving rise to a variety of spectral behaviors of carrier–phonon coupled collective modes. A useful sum rule on the plasma-species-resolved dielectric functions is also found.

Dynamical Excitonic Effects in Doped Two-Dimensional Semiconductors.

It is well-known that excitonic effects can dominate the optical properties of two-dimensional materials. These effects, however, can be substantially modified by doping free carriers. We investigate these doping effects by solving the first-principles Bethe-Salpeter equation. Dynamical screening effects, included via the sum-rule preserving generalized plasmon-pole model, are found to be important in the doped system. Using monolayer MoS2 as an example, we find that upon moderate doping, the exciton binding energy can be tuned by a few hundred millielectronvolts, while the exciton peak position stays nearly constant due to a cancellation with the quasiparticle band gap renormalization. At higher doping densities, the exciton peak position increases linearly in energy and gradually merges into a Fermi-edge singularity. Our results are crucial for the quantitative interpretation of optical properties of two-dimensional materials and the further development of ab initio theories of studying charged excitations such as trions.

Cohesive properties of noble metals by van der Waals–corrected density functional theory: Au, Ag, and Cu as case studies

The cohesive energy, equilibrium lattice constant, and bulk modulus of noble metals are computed by different van der Waals-corrected Density Functional Theory methods, including vdW-DF, vdW-DF2, vdW-DF-cx, rVV10 and PBE-D. Two specifically-designed methods are also developed in order to effectively include dynamical screening effects: the DFT/vdW-WF2p method, based on the generation of Maximally Localized Wannier Functions, and the RPAp scheme (in two variants), based on a single-oscillator model of the localized electron response. Comparison with results obtained without explicit inclusion of van der Waals effects, such as with the LDA, PBE, PBEsol, or the hybrid PBE0 functional, elucidates the importance of a suitable description of screened van der Waals interactions even in the case of strong metal bonding. Many-body effects are also quantitatively evaluated within the RPAp approach.

Classical scattering and stopping power in dense plasmas: the effect of diffraction and dynamic screening

AbstractIn the present work, classical electron–ion scattering, Coulomb logarithm, and stopping power are studied taking into account the quantum mechanical diffraction effect and the dynamic screening effect separately and together. The inclusion of the quantum diffraction effect is realized at the same level as the well-known first-order gradient correction in the extended Thomas–Fermi theory. In order to take the effect of dynamic screening into account, the model suggested by Grabowski et al. in 2013 is used. Scattering as well as stopping power of the external electron (ion) beam by plasma ions (electrons) and scattering of the plasma's own electrons (ions) by plasma ions (electrons) are considered differently. In the first case, it is found that in the limit of the non-ideal plasma with a plasma parameter Γ → 1, the effects of quantum diffraction and dynamic screening partially compensate each other. In the second case, the dynamic screening enlarges scattering cross-section, Coulomb logarithm, and stopping power, whereas the quantum diffraction reduces their values. Comparisons with the results of other theoretical methods and computer simulations indicate that the model used in this work gives a good description of the stopping power for projectile velocities $v\,{\rm \lesssim}\, 1.5 v_{{\rm th}}$, where vth is the thermal velocity of the plasma electrons.

Excitonic instability of three-dimensional gapless semiconductors: Large-Ntheory

Three-dimensional gapless semiconductors with quadratic band touching, such as HgTe, $\alpha$-Sn, or Pr$_2$Ir$_2$O$_7$ are believed to display a non-Fermi-liquid ground state due to long-range electron-electron interaction. We argue that this state is inherently unstable towards spontaneous formation of a (topological) excitonic insulator. The instability can be parameterized by a critical fermion number $N_c$. For $N < N_c$ the rotational symmetry is spontaneously broken, the system develops a gap in the spectrum, and features a finite nematic order parameter. To leading order in the 1/N expansion and in the static approximation, the analogy with the problem of dynamical mass generation in (2+1)-dimensional quantum electrodynamics yields $N_c = 16/[3\pi(\pi-2)]$. Taking the important dynamical screening effects into account, we find that $N_c \geq 2.6(2)$ and therefore safely above the physical value of $N = 1$. Some experimental consequences of the nematic ground state are discussed.

Effect of vertex corrections on the possibility of chiral symmetry breaking induced by long-range Coulomb repulsion in graphene

In this paper, we consider the possibility of chiral (charge or spin density wave) symmetry breaking in graphene due to long-range Coulomb interaction by comparing the results of the Bethe-Salpeter and functional renormalization-group approaches. The former approach performs a summation of ladder diagrams in the particle-hole channel and reproduces the results of the Schwinger-Dyson approach for the critical interaction strength of the quantum phase transition. The renormalization-group approach combines the effect of different channels and allows to study the role of vertex corrections. The critical interaction strength, which is necessary to induce the symmetry breaking in the latter approach, is found in the static approximation to be ${\ensuremath{\alpha}}_{c}={e}^{2}/(\ensuremath{\epsilon}{v}_{F})\ensuremath{\approx}1.05$ without considering the Fermi velocity renormalization, and ${\ensuremath{\alpha}}_{c}=3.7$ with accounting the renormailzation of the Fermi velocity. The latter value is expected to be, however, reduced, when the dynamic screening effects are taken into account, yielding the critical interaction, which may be comparable to the one in freely suspended graphene. We show that the vertex corrections are crucially important to obtain the mentioned values of critical interactions.

Dynamics of screening in photodoped Mott insulators

We use a nonequilibrium implementation of extended dynamical mean field theory to study the effect of dynamical screening in photo-excited Mott insulators. The insertion of doublons and holes adds low-energy screening modes and leads to a reduction of the Mott gap. The coupling to low-energy bosonic modes further- more opens new relaxation channels and significantly speeds up the thermalization process. We also consider the effect of the energy distribution of the doped carriers on the screening.

Analysis of dielectric function and dynamic structure factor of a multi-component plasma in a wurtzite GaN

We investigated dielectric responses of a multi-component plasma at finite temperature and applied the results to spectral analysis of dynamic structure factor of a photo-generated high density plasma in a wurtzite GaN. The behavior of effective dielectric function and dynamic structure factor in energy loss measurement are examined focusing to the effects of dynamic screening of carriers in the plasma. Features of their spectral behavior are illustrated including the roles of various elementary excitations in the three-component solid state plasma.

Kinematic HAADF-STEM image simulation of small nanoparticles

The high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) has been widely used in nanoparticle characterization due to its relatively straightforward interpretability, although multislice simulation is often required in order to take into account the strong dynamical screening effect if quantitative structure information is needed. The multislice simulation is very time-consuming, which can be a hurdle in cases when one has to deal with a large set of images. In this paper, we introduce a simple computer program, based on kinematic-scattering method, which allows users to simulate HAADF-STEM images of small nanoparticles, in ‘real time’ on a standard desktop computer. By comparing with the sophisticated multislice simulation, we demonstrate that such an approach is adequate for nanoparticles of ∼3nm in diameter (assuming an approximately spherical shape), particularly away from strict zone axis conditions. As an application, we show that the efficient kinematic simulation allows quick identification of orientation of nanoparticles.

Equations of state, transport properties, and compositions of argon plasma: combination of self-consistent fluid variation theory and linear response theory.

A consistent theoretical model that can be applied in a wide range of densities and temperatures is necessary for understanding the variation of a material's properties during compression and heating. Taking argon as an example, we show that the combination of self-consistent fluid variational theory and linear response theory is a promising route for studying warm dense matter. Following this route, the compositions, equations of state, and transport properties of argon plasma are calculated in a wide range of densities (0.001-20 g/cm(3)) and temperatures (5-100 kK). The obtained equations of state and electrical conductivities are found in good agreement with available experimental data. The plasma phase transition of argon is observed at temperatures below 30 kK and density about 2-6g/cm(3). The minimum density for the metallization of argon is found to be about 5.8 g/cm(3), occurring at 30-40 kK. The effects of many-particle correlations and dynamic screening on the electrical conductivity are also discussed through the effective potentials.

Dynamical Screening and Wake Effects in Classical, Quantum, and Ultrarelativistic Plasmas

AbstractWakefields are of omnipresent nature in non‐equilibrium situations, but their appearance and parametrical scaling is very diverse. Therefore, in this work the topological structure and characteristics of the dynamically screened potential is outlined for three representative systems in completely different physical regimes: (i) a classical complex (dusty) plasma, (ii) a degenerate electron‐ion plasma at high densities, and (iii) an ultrarelativistic quark‐gluon plasma. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Screening-induced surface polar optical phonon scattering in dual-gated graphene field effect transistors

The effect of surface polar optical phonons (SOs) from the dielectric layers on electron mobility in dual-gated graphene field effect transistors (GFETs) is studied theoretically. By taking into account SO scattering of electron as a main scattering mechanism, the electron mobility is calculated by the iterative solution of Boltzmann transport equation. In treating scattering with the SO modes, the dynamic dielectric screening is included and compared to the static dielectric screening and the dielectric screening in the static limit. It is found that the dynamic dielectric screening effect plays an important role in the range of low net carrier density. More importantly, in-plane acoustic phonon scattering and charged impurity scattering are also included in the total mobility for SiO2-supported GFETs with various high-κ top-gate dielectric layers considered. The calculated total mobility results suggest both Al2O3 and AlN are the promising candidate dielectric layers for the enhancement in room temperature mobility of graphene in the future.

Dielectric response functions of multi-component hot carrier plasmas

Finite temperature dielectric responses of a multi-component hot carrier plasma are investigated. Spectral analyses of component-resolved electronic polarization functions and the effective dielectric function of the plasma are performed to explore their dynamic and nonlocal behaviors including the effects of finite-temperature dynamic screening of the carriers. We show that spectral behavior of dielectric response functions of a multi-component plasma are quite distinct from that of a conventional single component plasma and that, for a photo-generated hot carrier plasma of an ideal wurtzite GaN with high carrier densities, the well-defined optic plasmonic modes are clearly seen well outside the single particle excitation continuum of lighter mass electrons and that an additional low frequency acoustic plasmonic branch is located outside single particle excitation continuum of doubly degenerate heavier mass A-holes. The contribution of the heavier mass species to dressed polarization function is very similar to the case of bare polarization function. We also find that the dispersion relation of the anti-screening peaks follows that of the optical plasmon mode in a multi-component plasma.

Size Effects in the Interface Level Alignment of Dye-Sensitized TiO2 Clusters.

The efficiency of dye-sensitized solar cells (DSCs) depends critically on the electronic structure of the interfaces in the active region. We employ recently developed dispersion-inclusive density functional theory (DFT) and GW methods to study the electronic structure of TiO2 clusters sensitized with catechol molecules. We show that the energy level alignment at the dye-TiO2 interface is the result of an intricate interplay of quantum size effects and dynamic screening effects and that it may be manipulated by nanostructuring and functionalizing the TiO2. We demonstrate that the energy difference between the catechol LUMO and the TiO2 LUMO, which is associated with the injection loss in DSCs, may be reduced significantly by reducing the dimensions of nanostructured TiO2 and by functionalizing the TiO2 with wide-gap moieties, which contribute additional screening but do not interact strongly with the frontier orbitals of the TiO2 and the dye. Precise control of the electronic structure may be achieved via "interface engineering" in functional nanostructures.

Ab initiostudy of the downfolded self-energy for correlated systems: Momentum dependence and effects of dynamical screening

The electronic structure of strongly correlated systems is usually calculated by using an effective model Hamiltonian with a small number of states and an effective on-site interaction. The mode, however, neglects the frequency dependence of the interaction, which emerges as a result of dynamical screening processes not included in the model. The self-energy calculated in this kind of model within dynamical mean-field theory (DMFT) is usually assumed to contain on-site components only. To study the validity of model calculations for the simulation of realistic materials, we make a detailed comparison between the downfolded self-energy in a model Hamiltonian with static and dynamic on-site interaction and the full $ab$ initio self-energy for Fe and SrVO${}_{3}$ within the $GW$ approximation. We find that the model $GW$ self-energy shows weaker $\mathbf{k}$ (momentum) dependence than the ab initio $GW$ self-energy, which is attributed to the lack of the long-range interaction and of contributions from other electrons not included in the models. This weak $\mathbf{k}$ dependence is found to lead to an artificial narrowing of the quasiparticle band structure. Moreover, this band narrowing is stronger for the dynamic (frequency-dependent) interaction, due to a larger renormalization of the quasiparticle states. These findings indicate a crucial role of the $\mathbf{k}$ dependence of the self-energy and dynamical screening for the electronic structure of correlated systems. We also discuss the effects beyond the $GW$ approximation for correlated systems by comparing the $GW$ and DMFT results.

Effects of dynamical screening on single ionization of potassium by electron impact in doubly symmetric geometry

The dynamically screened three-Coulomb-wave (DS3C) method is applied to study the single ionization of potassium by electron impact. Triple differential cross-sections (TDCS) are calculated in doubly symmetric geometry at excess energies of 6, 10, 15, 20, 30, 40, 50 and 60 eV. Comparisons are made with recent experimental data and theoretical predictions from a three-Coulomb-wave (3C) and distorted-wave Born approximation (DWBA). The DS3C method is able to reproduce most of the trend of experimental data and in good agreement with DWBA results. It is shown that the DS3C calculation provides much better shape and relative magnitude agreement with experiment.

Static and dynamic screening effects in the electrostatic self-assembly of nano-particles.

In the description of charge screening in the electrostatic self-assembly of nanoparticles (molecules) embedded into a polar solvent, the static screening effects (a contribution associated with the rapid spatial redistribution of small and highly mobile ions of a solvent) are traditionally treated phenomenologically, using the Yukawa short-range potential for describing the interaction between these particles. However, this model has a limited range of applicability being valid only for infinitely diluted systems and high salt concentrations. During a slow self-assembling process with nanoparticle formation, very dense structural elements (aggregates) are formed, in which the distances between the nanoparticles could become comparable to the Debye radius in the Yukawa potential. For such structural elements dynamic screening effects (the contribution of nanoparticles themselves to the screening potential) become important. In this paper, using a novel integrated approach (nonlinear integro-differential kinetic equations for the correlation functions of particles), we have obtained the self-consistent solution in the 3d case and compared roles of both static (equilibrium) and dynamic (nonequilibrium) charge screening effects in different situations. This paper is a continuation of our recent study [Phys. Chem. Chem. Phys., 2014, 16, 13974], where the polar solvent effects were now taken into account.

Light scattering by a 2D electron system with spin-orbit interaction in a strong magnetic field

We theoretically investigate light scattering by electrons of a 2D system in a strong magnetic field perpendicular to the plane of the system (the filling factor is smaller than two, i.e., only the states of the zeroth Landau level are filled). Analysis is carried out in the resonance approximation, in which the frequencies of the incident and scattered light are close to the effective separation between the conduction band and one of the branches of the valence band of the semiconductor. Spin splitting of the Landau level in the conduction band associated with Zeeman and the spin-orbit interactions is taken into account. The dynamic screening effects are considered in the random phase approximation (RPA).

The Influence of Dynamical Screening on the Transport Properties of Dense Plasmas

AbstractWe investigate the effects of dynamical screening on the static transport properties. We review the low density expansion for the electron‐electron and electron‐ion correlation functions and give results for a 6‐moment approach within linear response theory. The expansion coefficients are given. The convergence of thermoelectric transport coefficients is examined in dependence of the rank of the collision term determinant. The results are compared with previous calculations and experiments. Effective Debye screening radii derived from the Gould‐DeWitt scheme for the inclusion of strong collisions as well as dynamical effects are discussed. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Influence of dynamic screening effect on coherent terahertz radiation from biased GaAs/AlGaAs quantum wells

We report results of investigations of process of generation of coherent terahertz (THz) radiation from 29 nm thick GaAs/Al0.37Ga0.63As quantum wells with transverse electric bias under interband femtoseconds laser photoexcitation at room temperature. The properties of the detected THz radiation allow us to attribute it to the excitation of time-varying dipole moment induced by electric field polarization of nonequilibrium electron–hole pairs in quantum wells. Noticeable sub-linearity in the dependence of THz amplitude on excitation density is observed. A theoretical model, which accounts for the dynamic screening of the electric field in wide GaAs quantum wells by nonequilibrium carriers, has been developed. The model describes well the properties of the observed THz signal.