Electron-hole Correlation Effects Research Articles

Imaging electron dynamics with time- and angle-resolved photoelectron spectroscopy

We theoretically study how time- and angle-resolved photoemission spectroscopy can be applied for imaging coherent electron dynamics in molecules. We consider a process in which a pump pulse triggers coherent electronic dynamics in a molecule by creating a valence electron hole. An ultrashort extreme ultraviolet (XUV) probe pulse creates a second electron hole in the molecule. Information about the electron dynamics is accessed by analyzing angular distributions of photoemission probabilities at a fixed photoelectron energy. We demonstrate that a rigorous theoretical analysis, which takes into account the indistinguishability of transitions induced by the ultrashort, broadband probe pulse and electron hole correlation effects, is necessary for the interpretation of time- and angle-resolved photoelectron spectra. We show how a Fourier analysis of time- and angle-resolved photoelectron spectra from a molecule can be applied to follow its electron dynamics by considering photoelectron distributions from an indole molecular cation with coherent electron dynamics.

Optical spectrum of bottom-up graphene nanoribbons: towards efficient atom-thick excitonic solar cells

Recently, atomically well-defined cove-shaped graphene nanoribbons have been obtained using bottom-up synthesis. These nanoribbons have an optical gap in the visible range of the spectrum which make them candidates for donor materials in photovoltaic devices. From the atomistic point of view, their electronic and optical properties are not clearly understood. Therefore, in this work we carry out ab-initio density functional theory calculations combine with many-body perturbation formalism to study their electronic and optical properties. Through the comparison with experimental measurements, we show that an accurate description of the nanoribbon's optical properties requires the inclusion of electron-hole correlation effects. The energy, binding energy and the corresponding excitonic transitions involved are analyzed. We found that in contrast to zigzag graphene nanoribbons, the excitonic peaks in the absorption spectrum are a consequence of a group of transitions involving the first and second conduction and valence bands. Finally, we estimate some relevant optical properties that strengthen the potential of these nanoribbons for acting as a donor materials in photovoltaic.

Electron-hole correlation effects in core-level spectroscopy probed by the resonant inelastic soft x-ray scattering map of C60

We have employed a unique spectroscopic approach, a resonant inelastic soft x-ray scattering (RIXS) map, to identify and separate electron-hole correlation effects in core-level spectroscopy. With this approach, we are able to derive a comprehensive picture of the electronic structure, separating ground state properties (such as the HOMO-LUMO separation) from excited state properties (such as the C 1s core-exciton binding energy of C(60)). In particular, our approach allows us to determine the difference between core- and valence exciton binding energies in C(60) [0.5 (±0.2) eV]. Furthermore, the RIXS map gives detailed insight into the symmetries of the intermediate and final states of the RIXS process.

Simultaneous effects of electron-hole correlation, hydrostatic pressure, and temperature on the third harmonic generation in parabolic GaAs quantum dots

The combined effects of electron-hole correlation, hydrostatic pressure, and temperature on the third harmonic generation in disk-shaped para- bolic GaAs quantum dots are studied under the density matrix formalism and the effective mass approximation. Two well-defined regimes are dis- cussed: (1) the strong-confinement regime, where the Coulomb interaction between the electron and hole is neglected and (2) the weak-confinement regime where the parabolic confinement term is neglected and the system reaches the limit of a hydrogenic problem. The results show that the third harmonic- generation coefficient is strongly dependent on the localization of the electron-hole pair. Also, that by using external perturbations like hydrostatic pressure or by considering the temperature effects it is possible to induce a blue-shift and/or red-shift on the resonant peaks of the third harmonic generation coefficient.

Strong excitonic effects inCuAlO2delafossite transparent conductive oxides

The imaginary part of the dielectric function of ${\text{CuAlO}}_{2}$ has been calculated including the electron-hole correlation effects within Bethe-Salpeter formalism (BSE). In the initial step of the BSE solver the band structure was calculated within density-functional theory plus an orbital field $(\text{LDA}/\text{GGA}+U)$ acting on Cu atoms. We discuss the influence of the strength of the additional orbital field on the band structure, electric field gradients, and the dielectric function. The calculated dielectric function shows very strong electron-hole correlation effects manifested with large binding energies of the lowest excitons. The electron-hole pair for the lowest excitations are very strongly localized at a single Cu plane and confined within only a few neighboring shells.

Magnetic field effects on spin-flip processes in semimagnetic quantum wells

We report results of calculations on the spin-flip scattering rate of heavy holes and excitons in semimagnetic quantum wells in an external magnetic field due to the $sp\text{\ensuremath{-}}d$ exchange interaction. Numerical estimates are given for the example of $\mathrm{CdMnTe}$ quantum wells with magnetic wells and nonmagnetic barriers. Due to a giant spin splitting induced by the magnetic field in a semimagnetic quantum well, the heavy hole at zero momentum contributes significantly to spin-flip scattering processes, in marked contrast to the zero-field case. For single (free) heavy holes, the spin-flip scattering rate shows a fast increase with magnetic field which turns to saturation at fields exceeding $1\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The larger the thickness of a quantum well, the stronger is such an acceleration of the heavy-hole spin relaxation. Still the maximum values of the scattering rate (on the order of ${10}^{\ensuremath{-}13}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$) are largest for narrow wells. For the exciton-bound carriers (electron and holes), the electron-hole correlation effects get important, particularly, in the presence of an external magnetic field. As a result, the scattering rate for the spin-flip transitions within the states of the (heavy-hole) bright exciton, shows a pronounced maximum at low fields $(\ensuremath{\lesssim}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{T})$. The scattering rate saturates at moderate fields of about $2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ at a magnitude on the order of a few tens of picoseconds.

Prediction of an Excitonic Ground State inInAs/InSbQuantum Dots

Using atomistic pseudopotential and configuration-interaction many-body calculations, we predict an excitonic ground state in the InAs/InSb quantum-dot system. For large dots, the conduction band minimum of the InAs dot lies below the valence band maximum of the InSb matrix. Due to quantum confinement, at a critical size calculated here for various shapes, the gap E(g) between InAs conduction states and InSb valence states vanishes. Strong electron-hole correlation effects are induced by the spatial proximity of the electron and hole wave functions, and by the lack of strong (exciton unbinding) screening, afforded by the existence of discrete 0D confined energy levels. These correlation effects overcome E(g), leading to the formation of a biexcitonic ground state (two electrons in InAs and two holes in InSb) being energetically more favorable (by approximately 15 meV) than the dot without excitons.

Spatial carrier-carrier correlations in strain-induced quantum dots

The electron-hole correlation effects on the energy levels and the wave functions of the electrons and holes in a strain-induced quantum dot containing one to ten carrier pairs have been studied using large-scale configuration-interaction calculations. The present calculations show the formation of excitons and biexcitons in the quantum dot. By increasing the number of carrier pairs, one observes a transition from a strongly correlated system to a quantum dot system for which the electron-electron and hole-hole correlations are dominated by exchange interaction and are relatively well described at the Hartree-Fock level, while for an accurate description of the electron-hole correlations configuration-interaction calculations are necessary. Ring-shaped carrier distributions emerge with increasing number of carrier pairs.

Full Configuration Interaction Calculations of Electron-Hole Correlation Effects in Strain-Induced Quantum Dots

The electron–hole correlation effects on the energy levels and the wave functions of a strain-induced quantum dot containing 1 to 10 electron–hole pairs have been studied using large scale configuration-interaction calculations. The results show the importance of including the correlations in order to reproduce the experimentally measured quantum dot spectra and to obtain bound multi-exciton complexes. Increasing the number of electron–hole pairs in the quantum dot a transition from a strongly correlated system to the one approximated by the Hartree-Fock theory is observed.

Full Configuration Interaction Calculations of Electron–Hole Correlation Effects in Strain-Induced Quantum Dots

Full Configuration Interaction Calculations of Electron–Hole Correlation Effects in Strain-Induced Quantum Dots

Ab-initio study of Coulomb-correlated optical properties in conjugated polymers

The spatial extension and binding energy of excitons in semiconducting conjugated polymers are still the subject of a great debate. We address this problem through first-principles calculations (within DFT-LDA, plane-waves and ab-initio pseudopotentials), which allow to include electron-hole correlation effects in a fully three-dimensional approach through the density-matrix formalism. We show results for the correlated optical spectrum and the exciton wavefunctions of single-chain poly(para)phenylene-vinylene (PPV), that support the picture of a strongly bound anisotropic exciton localized over ∼ 4-5 monomers.

Hole Dynamics in Noble Metals

Hole dynamics in noble metals (Cu and Au) is investigated by means of first-principles many-body calculations. While holes in a free-electron gas are known to live shorter than electrons with the same excitation energy, our results indicate that d holes in noble metals exhibit longer inelastic lifetimes than excited sp electrons, in agreement with experiment. The density of states available for d-hole decay is larger than that for the decay of excited electrons; however, the small overlap between d and sp states below the Fermi level increases the d-hole lifetime. The impact of d-hole dynamics on electron-hole correlation effects is also addressed.

Full configuration interaction calculations of electron-hole correlation effects in strain-induced quantum dots

The electron-hole correlation effects on the energy levels and the wave functions of a strain-induced quantum dot containing 1 to 10 electron-hole pairs have been studied using large scale configuration-interaction calculations. The results show the importance of including the correlations in order to reproduce the experimentally measured quantum dot spectra and to obtain bound multi-exciton complexes. Increasing the number of electron-hole pairs in the quantum dot a transition from a strongly correlated system to the one approximated by the Hartree-Fock theory is observed.

Electron-Hole Correlation Effects in the Emission of Light from Quantum Wires

We present a self-consistent treatment of the electron-hole correlations in optically excited quantum wires within the ladder approximation and using a contact potential interaction. The limitation of the ladder approximation to the excitonic low-density region is largely overcome by the introduction of higher-order correlations through self-consistency. We show the relevance of these correlations in the low-temperature emission, even for high density, when a large gain replaces the excitonic absorption.

Interband magnetoabsorption in strained epitaxially grown ZnTe and ZnSe

Interband absorption in ultrathin epilayers of ZnTe and ZnSe has been measured in magnetic fields up to 6 T with photon energies below and above the fundamental gap. The spectra clearly show the evolution of the $1s$ and $2s$ exciton states and of the rich structure arising from the formation of Landau levels in the continuum. The observed spectral features are analyzed using the $8\ifmmode\times\else\texttimes\fi{}8 \mathbf{k}\mathbf{\ensuremath{\cdot}}\mathbf{p}$ model, which takes into account the residual strain introduced during sample preparation. From this analysis we extract highly accurate values for the energy gaps and the exciton binding energies. Effects of electron-hole correlation and resonant magnetopolaron coupling are also clearly detected in the continuum absorption. We discuss the dependence of these effects on Landau quantum number and the magnetic field by comparing the experimental transition energies with calculated ones.

Electron-hole system revisited: A variational quantum Monte Carlo study.

Properties of a model electron-hole system are studied with a variational Monte Carlo method. Energies of both the two-component plasma phase and the excitonic insulating phase are calculated. The excitonic insulating phase is stable for all densities studied, exhibiting a strong pairing state even near ${\mathit{r}}_{\mathit{s}}$=2.0, the density where the two-component plasma phase has its minimum energy. Analysis of the pair correlation functions reveals that the stabilization of the excitonic insulator traces to enhanced short range electron-hole correlations which are partially offset by reduced electron-electron correlations. In the two-component plasma phase, significant electron-hole correlation effects remain up to high density, e.g., ${\mathit{r}}_{\mathit{s}}$=0.5. \textcopyright{} 1996 The American Physical Society.

Exciton-phonon interaction in CdSe and CuCl polar semiconductor nanospheres.

We present a theoretical study of the effect of longitudinal-optical phonons and surface-optical phonons on the electronic states of conduction electrons and donorlike excitons in a spherical semiconductor quantum dot. The effect of the quantum confinement is described by an infinitely deep potential well in the framework of the envelope-function approximation associated with two nondegenerate bands. The charge-carrier--phonon coupling is treated within the adiabatic approximation. A variational calculation of the ground-state energy of the donorlike exciton is performed using a one-parameter trial envelope function, which includes electron-hole correlation effects. The results show that in the case of a donorlike exciton located at the center of the microsphere, the effect of the lattice polarization gives rise to a lowering of the absolute value of the energy for all values of the microsphere radius R. The Huang-Rhys factor S, which is a measure of the charge-carrier--phonon interaction and the extent of the charge-carrier density, has been determined as a function of R. For the donorlike exciton, S reaches a minimum value, respectively, equal to ${\mathit{S}}_{0}$=0.2 for ${\mathit{R}}_{0}$=12.7 nm and ${\mathit{S}}_{0}$=3.1 for ${\mathit{R}}_{0}$=2.3 nm in the case of CdSe and CuCl.

Red-shift of the excitonic level in two beam absorption spectra of GaSe

Optical absorption spectra of GaSe samples immersed in superfluid helium have been measured as a function of the optical excitation intensity, by using a two beam technique. As the electron-hole density increases a broadening and a red-shift of the fundamental excitonic absorption line occur. We compare our observation with the results of a simplified first order theory which accounts for electron-hole correlation effects in static screening approximation. The optical spectra, both calculated theoretically and observed experimentally, suggest that the observed Mott transition is, at finite temperatures, smooth and continuous. Our simple model explains surprisingly well the observed optical spectra.

Hard gaps in localized interacting systems

We study the one-particle density of excitations and the density of states of a strongly Anderson-localized system in the presence of Coulomb interactions. Using a pair correlation function obtained from computer simulation, we obtained, analytically, a density of excitations, including the effects of electron-hole correlations. These results are used to calculate hard gaps in the density of states by the techniques of Efros and Davies, and also in a new calculations where stabilization of the ground state is aided by polaron transitions.

Analysis of the excitonic mott transition in GaSe

Detailed photoluminescence spectra of GaSe at a bath temperature of 2K were measured at electron-hole densities ranging from below to above the screening limit for the free direct excitons. Special care was used to reduce the effects of spatial carrier inhomogeneity. We observe a continuous transition from exciton recombination processes to an e-h plasna emission. We compare our observations with the results of a simplified first order theory which accounts for electron-hole correlation effects. The observed red shift of the fundamental direct exciton level is well reproduced by our theoretical calculations.