Dynamic Screening Processes Research Articles

Slow Conductance Relaxation in Graphene–Ferroelectric Field-Effect Transistors

Tuning graphene conduction states with the remnant polarization of ferroelectric oxides holds much promise for a range of low-power transistor and memory applications. However, understanding how the ferroelectric polarization affects the electronic properties of graphene remains challenging because of a variety of intricate and dynamic screening processes that complicate the interaction. Here, we report on a range of slow electrical conductance relaxation behavior in graphene–ferroelectric field-effect transistors with the extreme case leading to the convergence of two polarization-induced states. Piezoresponse force microscopy through the graphene channel reveals that the ferroelectric polarization remains essentially unchanged during this conductance relaxation. When measured in vacuum, the conductance relaxation is significantly reduced, suggesting equilibration with adsorbates from the ambient atmosphere that can cause charge transfer to and from graphene to be the origin of the slow relaxation.

Plasmonic Modes in Thin Films: Quo Vadis?

Herein, we discuss the status and the prospect of plasmonic modes in thin films. Plasmons are collective longitudinal modes of charge fluctuation in metal samples excited by an external electric field. Surface plasmons (SPs) are waves that propagate along the surface of a conductor with applications in magneto-optic data storage, optics, microscopy, and catalysis. In thin films the electronic response is influenced by electron quantum confinement. Confined electrons modify the dynamical screening processes at the film/substrate interface by introducing novel properties with potential applications and, moreover, they affect both the dispersion relation of SP frequency and the damping processes of the SP. Recent calculations indicate the emergence of acoustic surface plasmons (ASP) in Ag thin films exhibiting quantum well states and in graphene films. The slope of the dispersion of ASP decreases with film thickness. We also discuss open issues in research on plasmonic modes in graphene/metal interfaes.

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.

Influence of Structural and Electronic Properties on the Collective Excitations of Ag/Cu(111)

Herein, we report on the interplay between structural and electronic properties in the dynamical screening processes and the excitation of collective modes in Ag/Cu(111). For the (9 × 9)-Ag/Cu(111) structure, the excitation of the two-dimensional plasmon, observed in Ag/Si(111), is forbidden by both the high corrugation of the Ag layer and the presence of the underlying metal substrate. The increase of the sp density of states at the Fermi energy induces the appearance of a broad peak at 7–8 eV assigned to a free-electron collective excitation.

Fermi surface and magnetism in the Kondo lattice: A continuum field theory approach

We consider the Fermi surface inside the antiferromagnetic ordered region of a Kondo lattice system in an arbitrary dimension higher than one. We establish the existence of AF S , an antiferromagnetic phase whose Fermi surface is “small,” in the sense that the local moments do not participate in the Fermi-surface formation. This is in contrast to the “large” Fermi surface that is typically assumed for heavy fermion metals. We extend our earlier work to the case that the Fermi surface of the conduction electrons intersects the antiferromagnetic Brillouin zone boundary. Our results provide a new perspective on local quantum criticality. In addition, our results imply that, for the AF S phase, it is important to keep track of the dynamical screening processes; we suggest that this effect is not captured in a recent variational Monte Carlo study of the Kondo lattice.

Dynamical screening effects on dark current in a quantum-well infrared photodetector

Dark currents are calculated in ${\mathrm{Al}}_{0.27}{\mathrm{Ga}}_{0.73}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ heterojunction based quantum well taking the thermionic field emission current into account. In our model the currents are calculated by taking emission of electrons from the bound state inside the well to the continuum scattered by the coupled plasmon-phonon modes. The calculated dark currents are compared with those obtained taking the sum of thermionic field emission currents arising individually from plasmon- and phonon-scattering processes and experiments. Compared to previous theoretical results the present dark currents agree reasonably well with experiments due to the inclusion of dynamical screening processes.

Charge-density waves and superconductivity as an alternative to phase separation in the infinite-U Hubbard-Holstein model.

We investigate the density instabilities present in the infinite-U Hubbard-Holstein model both at zero and finite momenta as well as the occurrence of Cooper instabilities with a specific emphasis on the role of long-range Coulomb forces. In carrying out this analysis a special attention is devoted to the effects of the strong local $e$-$e$ interaction on the $e$-$ph$ coupling and particularly to both the static and dynamic screening processes dressing this coupling. We also clarify under which conditions in strongly correlated electron systems a weak additional interaction, e.g. a phonon-mediated attraction, can give rise to a charge instability. In the presence of long-range Coulomb forces, the frustrated phase separation leads to the formation of incommensurate charge density waves. These instabilities, in turn, lead to strong residual scattering processes between quasiparticles and to superconductivity, thus providing an interesting clue to the interpretation of the physics of the copper oxides.

Chemical effects in the carbon K V V Auger line shapes

We review the results of a consistent quantitative interpretation of the C KVV Auger line shapes of five different gas phase hydrocarbons (methane, ethane, cyclohexane, benzene, and ethylene), three solids (polyethylene, diamond, and graphite), and a molecularly chemisorbed system (ethylene/Ni). The normal kvv line shape accounts for only about one-half of the total experimental intensity for most of the Auger line shapes. The remaining part of the experimental line shape can be attributed to satellites resulting from resonant excitation or dynamic screening processes. The normal kvv Auger line shapes are seen to reflect delocalized holes; however, correlation effects are very evident. Although these screening and correlation effects complicate the interpretation of the line shapes, they provided a means to obtain unique chemical, electronic, and bonding information, and indeed cause the principal differences seen among the experimental line shapes.

Dynamic dielectric response to electron-hole and electron-electron interactions

The interaction potential between two charged particles in a dielectric medium is shown to have the form $U(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}},{\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}}_{\mathrm{c}.\mathrm{m}.})=\frac{V(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})}{\ensuremath{\epsilon}(q,\ensuremath{\omega}=\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}},{\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}}_{\mathrm{c}.\mathrm{m}.})}$, where $V(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})$ is the bare Coulomb potential, ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}}_{\mathrm{c}.\mathrm{m}.}$ is the center-of-mass velocity, and $\ensuremath{\epsilon}(q,\ensuremath{\omega})$ is the dielectric constant of the medium. An analysis of the dielectric screening is performed, incorporating core electron, lattice ion, and free-carrier contributions. The random-phase-approximation dielectric constant for arbitrary degeneracy is employed in calculating the free-carrier component of $\ensuremath{\epsilon}(q,\ensuremath{\omega})$. It is found that although screening of the bare potential by lattice ions may sometimes be evaluated in the static or high-frequency limits, it is virtually always a poor approximation to treat the free-carrier screening in either of these limits. Dynamic screening processes have been incorporated into a calculation of relaxation times for electron-hole scattering in semiconductors for which ${m}_{h}\ensuremath{\gg}{m}_{e}$. Sample calculations for GaAs at a wide variety of temperatures and photoexcited carrier densities show that dynamic screening has a significant effect on the relaxation time in most regimes. It is equally important to treat the screening of electron-electron interactions dynamically when calculating transport properties which are sensitive to electron-electron scattering.

Comment on "Negative shake-up energy in core ionization"

We point out that a recently claimed new interpretation of the origin of the dynamical screening processes in adsorbates does not differ qualitatively from out interpretation given previously.

Negative Shake-Up Energy in Core Ionization

It is shown that the dynamical screening phenomena, which have recently been found to affect the shape of adsorbate core-hole spectra, occur also in the core ionization of certain molecules. For $K$-shell ionization of paranitroaniline we find shake-up transitions of negative energy which cause a multipeak structure of the corresponding line in the ESCA (electron spectroscopy for chemical analysis) spectrum. The present first-principles results lead to a new interpretation of the origin of the dynamical screening processes in adsorbates.

Atomic and extra-atomic relaxation and infrared catastrophe in the core spectra of chemisorbed atoms

The pronounced asymmetric broadening in the core level photoemission spectra of chemisorbed atoms is explained in terms of intra- and extra-atomic relaxation and a qualitative comparison is made with similar effect due to the dynamic screening processes.