Electron-hole Correlation Research Articles

Development of polaron-transformed explicitly correlated full configuration interaction method for investigation of quantum-confined Stark effect in GaAs quantum dots

The effect of external electric field on electron-hole (eh) correlation in gallium arsenide quantum dots is investigated. The electron-hole Schrodinger equation in the presence of an external electric field is solved using explicitly correlated full configuration interaction method and accurate exciton binding energy and electron-hole recombination probability are obtained. The effect of the electric field was included in the 1-particle single component basis functions by performing variational polaron transformation. The quality of the wavefunction at small inter-particle distances was improved by using Gaussian-type geminal function that depended explicitly on the electron-hole separation distance. The parameters of the explicitly correlated function were determined variationally at each field strength. The scaling of total exciton energy, exciton binding energy, and electron-hole recombination probability with respect to the strength of the electric field was investigated. It was found that a 500 kV/cm change in field strength reduces the binding energy and recombination probability by a factor of 2.6 and 166, respectively. The results show that the eh-recombination probability is affected much more strongly by the electric field than the exciton binding energy. Analysis using the polaron-transformed basis indicates that the exciton binding should asymptotically vanish in the limit of large field strength.

Ultrafast Spectral Migration of Photoluminescence in Graphene Oxide

We use subpicosecond time-resolved photoluminescence measurements to study the nature of photoluminescence in graphene oxide and reduced graphene oxide. Our data indicate that, in contrast to prior suggestions, the photoluminescence spectra of graphene oxide and reduced graphene oxide are inhomogeneously broadened. We observe substantial energy redistribution and relaxation among the emitting states within the first few picoseconds, leading to a progressive red shift of the emission spectrum. Blue shifts that arise in time-integrated spectra upon photothermal reduction are easily understood within this dynamical context without invoking a modified distribution of dipole-coupled states. Rather, reduction increases the nonradiative electron-hole recombination rate and curtails the red-shifting process, which is consistent with an increase in quenching through the introduction of larger and/or more numerous sp(2) clusters. Polarization memory measurements show energetic signatures of electron-hole correlations, established on a subpicosecond time scale and developing little thereafter.

Photoluminescence studies on exciton photophysics in carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are extremely thin cylinders of hexagonal carbon networks with diameters on the order of 1 nm. Over the past decade, optical properties of SWNTs, dominated by correlated electron–hole bound states known as excitons, have been intensively studied. The exciton photophysics of SWNTs is relevant to nearly all optical phenomena observed in SWNTs, and their detailed understanding is critical for the development of future optoelectronic devices using SWNTs. Here we review some interesting photophysics related to excitons in SWNTs, including fundamental features in optical absorption and photoluminescence spectra, effects of exciton–phonon interactions, anisotropic optical absorption and emission, effects of surrounding materials, radiative and nonradiative lifetimes, and diffusional exciton migration along SWNTs, which have been studied by continuous-wave and time-resolved optical spectroscopy.

Quantum transport in strongly disordered crystals: Electrical conductivity with large negative vertex corrections

We propose a renormalization scheme of the Kubo formula for the electrical conductivity with multiple backscatterings contributing to the electron-hole irreducible vertex derived from the asymptotic limit to high spatial dimensions. We use this vertex to represent the two-particle Green function via a symmetrized Bethe-Salpeter equation in momentum space. We further utilize the dominance of a pole in the irreducible vertex to an approximate diagonalization of the Bethe-Salpeter equation and a non-perturbative representation of the electron-hole correlation function. The latter function is then used to derive a compact representation for the electrical conductivity at zero temperature without the necessity to evaluate separately the Drude term and vertex corrections. The electrical conductivity calculated in this way remains nonnegative also in the strongly disordered regime where the localization effects become significant and the negative vertex corrections in the standard Kubo formula overweight the Drude term.

Electron spin orientation under in-plane optical excitation in GaAs quantum wells

We study the optical orientation of electron spins in GaAs/AlGaAs quantum wells for excitation in the growth direction and for in-plane excitation. Time- and polarization-resolved photoluminescence excitation measurements show, for resonant excitation of the heavy-hole conduction band transition, a negligible degree of electron spin polarization for in-plane excitation and nearly 100% for excitation in the growth direction. For resonant excitation of the light-hole conduction band transition, the excited electron spin polarization has the same (opposite) direction for in-plane excitation (in the growth direction) as for excitation into the continuum. The experimental results are well explained by an accurate multiband theory of excitonic absorption taking fully into account electron-hole Coulomb correlations and heavy-hole light-hole coupling.

Recombination of Correlated Electron–Hole Pairs With Account of Hot Capture With Emission of Optical Phonons

Electron thermalization and electron-hole recombination in scintillating crystals is simulated with and without account for Coulomb field created by a hole using both analytical estimations and Monte-Carlo approach. The Monte-Carlo simulation is performed both for crystals with one and two branches of longitudinal optical phonons to check the role of additional branches of these phonons. The results of numerical simulation show that the account for Coulomb field at all stages of the thermalization and capture significantly increases the probability of the geminate electron-hole binding in case of high values of optical phonon energies.

Luminescence of impurity-bound excitons in Li6GdB3O9:Ce3+ single crystals

The anomalous (τ < 10 ns) luminescence of wide bandgap crystals of lithium–gadolinium orthoborate Li6GdB3O9 doped with trivalent cerium ions, has been revealed for the first time and investigated through the low-temperature time-resolved vacuum ultraviolet synchrotron spectroscopy. It was shown that the optical transitions at 6.2 eV are due to electron transfer from the ground 4f1 states of Ce3+ ion onto the autoionized states near the conduction band bottom of a crystal. These transitions lead to the formation of impurity-bound excitons in the form of correlated electron–hole pair, in which the hole component is localized at 4f-level of the cerium ion and an electron component is located at the conduction band bottom in the attractive potential of this hole. It is established that the direct radiative recombination of the cerium impurity-bound exciton leads to a fast broadband emission at 4.25 eV. The energy threshold for creation of the impurity-bound excitons was determined on the basis of the obtained spectroscopic data. We calculated the H(k) functions of distribution of the elementary relaxations over the reaction rate constants and explained on this basis the decay kinetics and quenching processes, not only for the anomalous emission at 4.25 eV, but for the ordinary 5d–4f luminescence at 3.0 eV in Ce3+ ions. The paper discusses the decay channels for the impurity-bound excitons and their influence on the decay kinetics and spectra of luminescence in Li6GdB3O9 crystals.

Adsorbate-substrate charge transfer and electron-hole correlation at adsorbate/metal interfaces

Adsorbate-substrate charge transfer and electron-hole correlation at adsorbate/metal interfaces

Correlated electron—hole transitions in wurtzite GaN quantum dots: the effects of strain and hydrostatic pressure

Within the effective-mass and finite-height potential barrier approximation, a theoretical study of the effects of strain and hydrostatic pressure on the exciton emission wavelength and electron—hole recombination rate in wurtzite cylindrical GaN/AlxGa1−xN quantum dots (QDs) is performed using a variational approach. Numerical results show that the emission wavelength with strain effect is higher than that without strain effect when the QD height is large (> 3.8 nm), but the status is opposite when the QD height is small (< 3.8 nm). The height of GaN QDs must be less than 5.5 nm for an efficient electron—hole recombination process due to the strain effect. The emission wavelength decreases linearly and the electron—hole recombination rate increases almost linearly with applied hydrostatic pressure. The hydrostatic pressure has a remarkable influence on the emission wavelength for large QDs, and has a significant influence on the electron—hole recombination rate for small QDs. Furthermore, the present numerical outcomes are in qualitative agreement with previous experimental findings under zero pressure.

The electronic structure and optical response of rutile, anatase and brookite TiO2

In this study, we present a combined density functional theory and many-body perturbation theory study on the electronic and optical properties of TiO2 brookite as well as the tetragonal phases rutile and anatase. The electronic structure and linear optical response have been calculated from the Kohn–Sham band structure applying (semi)local as well as nonlocal screened hybrid exchange–correlation density functionals. Single-particle excitations are treated within the GW approximation for independent quasiparticles. For optical response calculations, two-particle excitations have been included by solving the Bethe–Salpeter equation for Coulomb correlated electron–hole pairs. On this methodological basis, gap data and optical spectra for the three major phases of TiO2 are provided. The common characteristics of brookite with the rutile and anatase phases, which have been discussed more comprehensively in the literature, are highlighted. Furthermore, the comparison of the present calculations with measured optical response data of rutile indicate that discrepancies discussed in numerous earlier studies are due to the measurements rather than related to an insufficient theoretical description.

Calculation of electron-hole recombination probability using explicitly correlated Hartree-Fock method

The electron-hole explicitly correlated Hartree-Fock method (eh-XCHF) is presented as a general strategy for investigation of electron-hole correlation and computation of electron-hole recombination probability. The eh-XCHF method is a variational method which uses explicitly correlated wavefunction that depends on the electron-hole inter-particle distances. It is shown that the explicitly correlated ansatz provides a systematic route to variationally minimize the total energy. The parabolic quantum dot is used as the benchmark system and the eh-XCHF method is used for computation of the ground state energy and electron-hole recombination probability. The results are compared to Hartree-Fock and explicitly correlated full configuration interaction (R12-FCI) calculations. The results indicate that an accurate description of the electron-hole wavefunction at short electron-hole inter-particle distances is crucial for qualitative description of the electron-hole recombination probability. The eh-XCHF method successfully addresses this issue and comparison of eh-XCHF calculations with R12-FCI shows good agreement. The quality of the mean field approximation for electron-hole system is also investigated by comparing HF and R12-FCI energies for electron-electron and electron-hole systems. It was found that performance of the mean field approximation is worse for the electron-hole system as compared to the corresponding electron-electron system.

Investigation of electron–hole correlation using explicitly correlated configuration interaction method

The explicitly correlated configuration interaction (XCCI) method is presented as a general technique for accurate description of electron–hole interaction. The XCCI method is a variational method where an R12-explicitly correlated wavefunction that depends on the electron–hole separation is used as the zeroth order reference wavefunction for performing the CI expansion. This is the main feature that differentiates the XCCI from the conventional CI method. Benchmark calculations on parabolic quantum dot using XCCI, FCI, and R12-FCI methods are presented and the results demonstrate that XCCI wavefunction is variationally superior to the FCI wavefunction for the ground and the excited states.

Abnormal broadening of the optical transitions in (Ga,As)N/GaAs quantum wells

We have measured the near band-gap absorption of structurally well characterized GaAs${}_{1\ensuremath{-}x}$N${}_{x}$ quantum wells grown on GaAs(001) with $x&lt;0.014$. The spectra were reproduced by a model that includes electron-hole correlations. We find that the width of the excitonic and band-to-band optical transitions are more than twice larger than what is found in conventional III-V alloy heterostructures. This confirms the presence of strong nitrogen-configuration induced band-gap fluctuations reported previously by Bentoumi et al. [Phys. Rev. B 70, 035315 (2004)] for bulk dilute GaAsN alloys.

Ultrafast relaxation of charge-transfer excitons in low-bandgap conjugated copolymers

A photophysical study, employing linear optical and femtosecond two-dimensional electronic spectroscopies, of two conjugated copolymers in solution is reported. The study focuses on the effect of backbone conformation on the properties and dynamics of excitons. For the predominantly planar copolymer, poly((4,8-diethylhexyloxyl) benzo([1,2-b:4,5-b′]dithiophene)-2,6-diyl)-alt-((5-octylthieno[3,4-c]pyrrole-4,6-dione)-1,3-diyl) (PBDTTPD), the dipole moment of excitons was found to be unchanged upon photoexcitation, and no excitonic relaxation was detected within 1 ps. For poly[N-11′′-henicosanyl-2,7-carbazole-alt-5,5-(4′,7′-di-thienyl-2′.1′.3′-benzothiadiazole)] (PCDTBT), however, bathochromic shifts of the absorption and fluorescence spectra as a function of solvent polarity indicate a change in dipole moment of approximately 3 Debye upon photoexcitation at the red-edge of the absorption spectrum. Excitation at the PCDTBT absorption maximum, where an ensemble of polymer chains have a predominantly twisted backbone, causes relaxation in the excited state to excitons with stronger charge-transfer character on a timescale of ∼200 fs. This dynamic Stokes shift was not observed when exciting on the PCDTBT absorption red edge, which photo-selects more planar conformations. Ultrafast relaxation in the excited state involving coupled torsional and solvent coordinates increases the charge-transfer character of the exciton. Although there is a concomitant loss of energy, electron-hole correlation (binding) is weakened, which may contribute to the efficiency of organic solar cells based on PCDTBT as an electron donor.

BerkeleyGW: A massively parallel computer package for the calculation of the quasiparticle and optical properties of materials and nanostructures

BerkeleyGW is a massively parallel computational package for electron excited-state properties that is based on the many-body perturbation theory employing the ab initio GW and GW plus Bethe–Salpeter equation methodology. It can be used in conjunction with many density-functional theory codes for ground-state properties, including PARATEC, PARSEC, Quantum ESPRESSO, SIESTA, and Octopus. The package can be used to compute the electronic and optical properties of a wide variety of material systems from bulk semiconductors and metals to nanostructured materials and molecules. The package scales to 10 000s of CPUs and can be used to study systems containing up to 100s of atoms. Program summaryProgram title: BerkeleyGWCatalogue identifier: AELG_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AELG_v1_0.htmlProgram obtainable from: CPC Program Library, Queenʼs University, Belfast, N. IrelandLicensing provisions: Open source BSD License. See code for licensing details.No. of lines in distributed program, including test data, etc.: 576 540No. of bytes in distributed program, including test data, etc.: 110 608 809Distribution format: tar.gzProgramming language: Fortran 90, C, C++, Python, Perl, BASHComputer: Linux/UNIX workstations or clustersOperating system: Tested on a variety of Linux distributions in parallel and serial as well as AIX and Mac OSXRAM: (50–2000) MB per CPU (Highly dependent on system size)Classification: 7.2, 7.3, 16.2, 18External routines: BLAS, LAPACK, FFTW, ScaLAPACK (optional), MPI (optional). All available under open-source licenses.Nature of problem: The excited state properties of materials involve the addition or subtraction of electrons as well as the optical excitations of electron–hole pairs. The excited particles interact strongly with other electrons in a material system. This interaction affects the electronic energies, wavefunctions and lifetimes. It is well known that ground-state theories, such as standard methods based on density-functional theory, fail to correctly capture this physics.Solution method: We construct and solve the Dysonʼs equation for the quasiparticle energies and wavefunctions within the GW approximation for the electron self-energy. We additionally construct and solve the Bethe–Salpeter equation for the correlated electron–hole (exciton) wavefunctions and excitation energies.Restrictions: The material size is limited in practice by the computational resources available. Materials with up to 500 atoms per periodic cell can be studied on large HPCs.Additional comments: The distribution file for this program is approximately 110 Mbytes and therefore is not delivered directly when download or E-mail is requested. Instead a html file giving details of how the program can be obtained is sent.Running time: 1–1000 minutes (depending greatly on system size and processor number).

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.

Optical Aharonov-Bohm Effect in Type-II (ZnMn)Te/ZnSe Quantum Dots

Photoluminescence for type-II, II―VI Zn(SeTe) quantum dots (QDs) in ZnTe/ZnSe superlattice demonstrate large and persistent oscillations in the energy and intensity indicating the presence of coherently rotating correlated electron―hole states, and the existence of the optical Aharonov-Bohm effect. The magnitude of the effects observed is attributed to the unique geometry of the columnar type-II structures investigated, which creates a natural ring-like geometry. The potential of this geometry to favor the optical AB effect is further demonstrated in similar (ZnMn)Te QD structures. Interestingly, in these magnetic dots the strength of the Aharonov-Bohm coherence is extremely sensitive to the magnetic properties of the dots, which can be enhanced or destroyed according to the spin disorder in the system. This effect is related to the strong exchange interaction between the rotating carrier and the Mn-ions in the dots. In addition, the magnetic nature and type-II band alignment of these QDs results in the formation of robust magnetic polarons, evidence of which persists until > 150 K.

Excitonic Fano Resonance in Free-Standing Graphene

We investigate the role of electron-hole correlations in the absorption of free-standing monolayer and bilayer graphene using optical transmission spectroscopy from 1.5 to 5.5 eV. Line shape analysis demonstrates that the ultraviolet region is dominated by an asymmetric Fano resonance. We attribute this to an excitonic resonance that forms near the van Hove singularity at the saddle point of the band structure and couples to the Dirac continuum. The Fano model quantitatively describes the experimental data all the way down to the infrared. In contrast, the common noninteracting particle picture cannot describe our data. These results suggest a profound connection between the absorption properties and the topology of the graphene band structure.

Electroluminescence overshoot effect in single layer pulsed organic light emitting diodes

After switching off the applied voltage pulse to a single layer organic light emitting diode (OLED) such as ITO/MEH-PPV/Al, initially the electroluminescence (EL) decays rapidly corresponding to the discharging of the parallel plate capacitor of the device, however, after a delay of 1–2 μs, the EL intensity rises again, attains a peak value, and then decays slowly without the application of an electric field. The EL intensity and the position of the peak in this overshoot depend on temperature. The intensity of EL overshoot increases with increasing pulse duration but the position of spike is independent of the pulse duration. A phenomenological theory is explored for the EL overshoot produced during the turn off of single layer OLEDs. The EL transient after the turn off of the external bias can be divided into the following three regions: (i) fast decay region, (ii) delayed peak region, and (iii) slow decay region. The initial fast decay is attributed to the recombination of injected charge carriers; the rise and exponential decay just after the peak in the delayed EL peak are due to the recombination luminescence owing to the movement of detrapped holes with noncorrelated electrons; and the slowly decaying EL following power law t − z ( z lying between 1.5 and 2) is due to geminate recombination of the correlated electron–hole pairs. The release time τ r of holes trapped at Al 2O 3/polymer interface, the activation energy E a for the detrapping of holes from the interface and the time–constant of the OLED circuit, can be determined from the measurement of the time dependence of EL overshoot. A good agreement is found between the theoretical and experimental results.