Electron-hole Screening Research Articles

Electronic and optical properties of Janus-like hexagonal monolayer materials of group IV-VI

We report $ab\phantom{\rule{4pt}{0ex}}initio$ calculations of the electronic and optical response properties of Janus-like hexagonal monolayers materials of group IV-VI. By combining phonons spectra calculations and ab initio molecular dynamics simulations, we verify that some two-dimensional (2D) group IV-VI hexagonal materials are dynamically unstable. Our results of electronic band structure based on both density functional theory (DFT) with Heyd-Scuseria-Ernzerhof hybrid functional (HSE) and ${\mathrm{G}}_{0}{\mathrm{W}}_{0}$ formalisms indicate that the dynamically stable materials exhibit indirect band gaps. The calculations clearly reveal that the hexagonal crystal structure of the group IV-VI monolayers plays a crucial role not only in determining the width of the band gap but also on the electron-hole screening effect, and consequently the strength of the exciton binding energy. Despite their strong binding, exciton binding energies in these nanomaterials are found to be in agreement with a 2D Mott-Wannier model. We also show that most of the stable 2D materials exhibit strong optical absorbance in the visible range, being promising materials for ultra-thin-film photovoltaic applications. Thus, our systematic analysis on electronic and optical properties of hexagonal monolayer materials of group IV-VI will complement forthcoming experimental measurements and can pave the way for technological applications of those materials.

Dynamical correction to the Bethe-Salpeter equation beyond the plasmon-pole approximation.

The Bethe-Salpeter equation (BSE) formalism is a computationally affordable method for the calculation of accurate optical excitation energies in molecular systems. Similar to the ubiquitous adiabatic approximation of time-dependent density-functional theory, the static approximation, which substitutes a dynamical (i.e., frequency-dependent) kernel by its static limit, is usually enforced in most implementations of the BSE formalism. Here, going beyond the static approximation, we compute the dynamical correction of the electron-hole screening for molecular excitation energies, thanks to a renormalized first-order perturbative correction to the static BSE excitation energies. The present dynamical correction goes beyond the plasmon-pole approximation as the dynamical screening of the Coulomb interaction is computed exactly within the random-phase approximation. Our calculations are benchmarked against high-level (coupled-cluster) calculations, allowing one to assess the clear improvement brought by the dynamical correction for both singlet and triplet optical transitions.

Ion and mixed electron-ion screening of charged domain walls in ferroelectrics

We consider theoretically the properties of charged domain walls (CDWs) in proper ferroelectrics for the ion and mixed electron-ion charge screening mechanisms. This includes an analysis of the spatial profiles of the polarization and electrostatic potential in the one-dimensional case, determination of the characteristic spatial widths, estimates of the CDW formation energies, and comparison with the CDW characteristics relevant to the electron-hole screening regime. Also we consider restrictions on the ion-related screening scenarios caused by slowness of the ion migration at ambient conditions.

Excited states of biological chromophores studied using many-body perturbation theory: Effects of resonant-antiresonant coupling and dynamical screening

The excited states of model chromophores of the photoactive yellow protein and of rhodopsin are studied using ab initio many-body perturbation theory (within the GW approximation and Bethe-Salpeter equation). Calculations beyond the Tamm-Dancoff approximation, i.e., consideration of the resonant-antiresonant transition coupling, are needed for an accurate description of the lowest $\ensuremath{\pi}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{\ast}}$ excitations due to the large exchange interaction between the electron and hole localized in the low-dimension systems. The inclusion of dynamical effect in the electron-hole screening is important for an accurate description of the lowest $n\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{\ast}}$ excitations.

Double excitations in finite systems

Time-dependent density-functional theory (TDDFT) is widely used in the study of linear response properties of finite systems. However, there are difficulties in properly describing excited states, which have double- and higher-excitation characters, which are particularly important in molecules with an open-shell ground state. These states would be described if the exact TDDFT kernel were used; however, within the adiabatic approximation to the exchange-correlation (xc) kernel, the calculated excitation energies have a strict single-excitation character and are fewer than the real ones. A frequency-dependent xc kernel could create extra poles in the response function, which would describe states with a multiple-excitation character. We introduce a frequency-dependent xc kernel, which can reproduce, within TDDFT, double excitations in finite systems. In order to achieve this, we use the Bethe-Salpeter equation with a dynamically screened Coulomb interaction W(omega), which can describe these excitations, and from this we obtain the xc kernel. Using a two-electron model system, we show that the frequency dependence of W does indeed introduce the double excitations that are instead absent in any static approximation of the electron-hole screening.

Magnetic-field-induced charged exciton studies in aGaAs/Al0.3Ga0.7Assingle heterojunction

The magnetophotoluminescence (MPL) behavior of a ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ single heterojunction has been investigated to 60 T. We observed negatively charged singlet ${(X}_{s}^{\ensuremath{-}})$ and triplet ${(X}_{t}^{\ensuremath{-}})$ exciton states that are formed at high magnetic fields beyond the \ensuremath{\nu}=1 quantum Hall state. The variation of the charged exciton binding energies are in good agreement with theoretical predictions. The MPL transition intensities for the ${X}_{s}^{\ensuremath{-}}$ and ${X}_{t}^{\ensuremath{-}}$ states showed variations (maxima and minima) at the \ensuremath{\nu}=1/3 and 1/5 fractional quantum Hall states as a consequence of a large reduction of electron-hole screening at these filling factors.

Models of Electron-Hole Screening and Exciton Binding in Conjugated Polymers

ABSTRACTThe use of semi-empirical quantum chemistry to calculate the exciton binding energy of conjugated polymers is discussed. Both the Pariser-Parr-Pople (PPP) model with Ohno parameterization and the models present in the MOPAC program overestimate the exciton binding energy relative to that observed in solid-state materials. Inclusion of Coulomb screening from adjacent chains may correct this overestimation. The solvation energy of a point charge in polyacetylene is calculated as 0.9eV, using Hartree-Fock theory to describe the polarization induced in the solvent chains. It is argued that including screening by modifying the electron-electron interaction energy of the PPP model introduces physically unreasonable side effects and is not consistent with the 0.9eV solvation energy of a point charge. Electron-hole screening models are then discussed along with the need to consider the relative time scales of the electron-hole motion and the dielectric response.

X-ray photoemission of small platinum and palladium clusters

X-ray photoemission of the Pt(4f) and Pd(3d) core-levels of small Pt and Pd metal clusters have been investigated. As cluster size decreases, there is a reduction in the lineshape asymmetry, a broadening in the linewidth, and a shift of the core-level toward the higher binding energies. The changes in the linewidth and line asymmetry can be interpreted in terms of the final state effects — the changes in the response of the valence electrons to the core-hole as a function of the cluster size. The decrease in the lineshape asymmetry is due to the reduction in the low-energy electron-hole screening and the linewidth broadening is due to the damping of the plasmons by the boundary surface of the cluster.

Valence excitons and inner-shell excitons in gaseous and solid argon

Valence excitons from the $3p$ shell and core excitons from the $3s$ and $2p$ shells in gaseous and solid argon are studied within the envelope-function formalism. First, semiempirical model potentials for atomic excitons are considered; then solid-state effects, which include structure and electron-hole screening, are taken into account. The problem of a sufficiently accurate account of the band kinetic operator is solved with first-order perturbation theory. A new interpretation of the excitonic spectrum originating from the $2p$ shell is given in terms of first-class exciton ($1s$) and third-class excitons ($\mathrm{nd}$, $n\ensuremath{\ge}3$) associated with a unique edge.