Configuration Interaction Description Research Articles

Ultrafast Transfer and Transient Entrapment of Photoexcited Mg Electron in Mg@C_{60}.

Electron relaxation is studied in endofullerene Mg@C_{60} after an initial localized photoexcitation in Mg by nonadiabatic molecular dynamics simulations. Two approaches to the electronic structure of the excited electronic states are used: (i)an independent particle approximation based on a density-functional theory description of molecular orbitals and (ii)a configuration-interaction description of the many-body effects. Both methods exhibit similar relaxation times, leading to an ultrafast decay and charge transfer from Mg to C_{60} within tens of femtoseconds. Method (i)further elicits a transient trap of the transferred electron that can delay the electron-hole recombination. Results shall motivate experiments to probe these ultrafast processes by two-photon transient absorption or photoelectron spectroscopy in gas phase, in solution, or as thin films.

Understanding the nature of mean-field semiclassical light-matter dynamics: An investigation of energy transfer, electron-electron correlations, external driving, and long-time detailed balance

Semiclassical electrodynamics is an appealing approach for studying light-matter interactions, especially for realistic molecular systems. However, there is no unique semiclassical scheme. On the one hand, intermolecular interactions can be described instantaneously by static two-body interactions connecting different molecules plus a classical transverse E-field; we will call this Hamiltonian #I. On the other hand, intermolecular interactions can also be described as effects that are mediated exclusively through a classical one-body E-field without any quantum effects at all (assuming we ignore electronic exchange); we will call this Hamiltonian #II. Moreover, one can also mix these two Hamiltonians into a third, hybrid Hamiltonian, which preserves quantum electron-electron correlations for lower excitations but describes higher excitations in a mean-field way. To investigate which semiclassical scheme is most reliable for practical use, here we study the real-time dynamics of a pair of identical two-level systems (TLSs) undergoing either resonance energy transfer (RET) or collectively driven dynamics. While all approaches perform reasonably well when there is no strong external excitation, we find that no single approach is perfect for all conditions. Each method has its own distinct problems: Hamiltonian #I performs best for RET but behaves in a complicated manner for driven dynamics. Hamiltonian #II is always stable, but obviously fails for RET at short distances. One key finding is that, under externally driving, a full configuration interaction description of Hamiltonian #I strongly overestimates the long-time electronic energy, highlighting the not obvious fact that, if one plans to merge quantum molecules with classical light, a full, exact treatment of electron-electron correlations can actually lead to worse results than a simple mean-field treatment.

Atomistic effect of internal and external shell on physical behaviours of InP/GaP/ZnS core/shell/shell nanocrystals: Empirical tight-binding theory

By imposing the atomistic tight-binding theory in the conjunction with a configuration interaction description, the impact of the GaP internal and ZnS external growth shell on the greener InP/GaP core/shell and InP/GaP/ZnS core/shell/shell nanocrystals is theoretically carried out. Herein, the demonstration to control the structural and optical properties by engineering the thickness of the interior and exterior encapsulated shell is underlined. To theoretically analyze the influence of the terminated shell on the atomistic behaviours, I demonstrate single-particle spectra, optical band gaps, ground-state oscillation strengths, electron-hole interaction and excitonic splitting. The computations of InP/GaP core/shell and InP/GaP/ZnS core/shell/shell nanocrystals are sensitive with the internal and external growth shell. The reduction of the optical band gaps with the increasing internal and external growth shell thicknesses reflects the quantum confinement effect. The optical band gaps are tuned across the visible wave lengths from 515 nm to 775 nm by changing the internal and external growth shell thicknesses. Remarkably, termination of GaP internal and ZnS external shell on core and core/shell nanocrystals obviously improves the optical property, respectively. In addition, InP/GaP core/shell and InP/GaP/ZnS core/shell/shell nanocrystals with the large growth shell thickness can be probable to be a source of entangled photons. By means of the Stokes shift, InP/GaP core/shell and InP/GaP/ZnS core/shell/shell nanocrystals with a small growth shell thickness can be implemented as the optical filter. Based on the scientific research, this insight is important for the theoretical understanding and practical control by internal and external shell thicknesses, thus introducing a stepping-stone towards the commercialization of InP/GaP core/shell and InP/GaP/ZnS core/shell/shell nanocrystals.

Determining whether diabolical singularities limit the accuracy of molecular property based diabatic representations: The 1,21A states of methylamine.

An efficient, easily implemented method for locating singularities attributable to the failure of the defining equations in a molecular property based diabatization, termed diabolical singular points, is reported. For two state diabatizations, the singular points form a seam of dimension N int - 2, where N int is the number of internal degrees of freedom. The dynamical outcomes of nuclear trajectories that reach the region of this seam are flawed. The algorithm easily identifies these otherwise hard to anticipate regions of fallaciously large derivative coupling. The fact that the algorithm is easily incorporated into a two state diabatization code based on molecular properties makes it a practical tool for determining whether the existence of diabolical singularities is relevant to the problem being considered. The algorithm is illustrated using a multireference single and double excitation configuration interaction description of the 1,21A states of CH3NH2.

Stokes shift and fine-structure splitting in CdSe / CdTe invert type-II and CdTe / CdSe type-II core / shell nanocrystals: Atomistic tight-binding theory

Using the atomistic tight-binding (TB) theory and configuration interaction (CI) description, it is showed that the Stokes shift and fine-structure splitting (FSS) in semiconductor core / shell nanocrystals are predominantly affected by the shell thickness and band profiles. CdSe / CdTe invert type-II and CdTe / CdSe type-II core / shell nanocrystals are used as the simulated candidates in order to obtain the different locations of the charge separation. This insight is important for the theoretical understanding and practical control by the type of the band alignments and sizes in growth shell for the entanglement of polarised photon pairs based on a biexciton cascade process. The manipulation of the Stokes shift and FSS by changing the band alignments and the thickness of the growth shell is studied here. The observations emphasise that there is a reduction of Stokes shift and FSS in CdSe / CdTe core / shell nanocrystals. In addition, it is found that the Stokes shift and FSS decrease with the increase in growth shell thickness in both CdSe / CdTe and CdTe / CdSe core / shell nanocrystals.

Atomistic tight-binding calculations of near infrared emitting CdxHg1−xTe nanocrystals

I present the structural and optical calculations for CdxHg1−xTe zinc-blende nanocrystals with the experimentally synthesized Cd compositions (x) in the framework of atomistic tight-binding model (TB) and configuration interaction description (CI). This atomistic tight-binding model incorporates the sp3s∗ orbital and the first neighbouring interaction, in addition to the non-linear dependence of compositions on the band gaps the bowing factor is involved into this model through the extended virtual crystal approximation (VCA). The information of DOS highlights that the contribution of the conduction and valence bands is subjected by cation and anion atoms, respectively. The improvement of the optical band gaps in CdxHg1−xTe nanocrystals is presented with the increasing Cd compositions. The optical band gaps can be used to tune their optical properties across a technologically useful range from 688nm to 2755nm which can be implemented for the near infrared emitting devices. In addition, the enhancement of the optical property is reported with the increasing Cd contents. With the increasing Cd compositions, the atomistic electron-hole Coulomb interaction is mainly increased, whereas the atomistic electron-hole exchange interaction is reduced. The Stokes shift and fine structure splitting are progressively reduced with the increasing compositions and diameters. The application of fine structure splitting is utilized to implement as a source of entangled photon pairs in the quantum information. Finally, the comprehensive computations of alloy CdxHg1−xTe nanocrystals effectively determine the composition- and size-dependent structural and optical properties which render these nanocrystals promising candidates as the near infrared emitting devices and optical amplifiers.

Atomistic tight-binding theory in 2D colloidal CdSe zinc-blende nanoplatelets

Using atomistic tight-binding theory in conjunction with the configuration interaction description, I investigate the structural and optical properties of colloidal CdSe zinc-blende nanoplatelets. I highlight that the new class of CdSe zinc-blende nanoplatelets has strong thickness and lateral size dependence on the natural properties. In an effort to theoretically demonstrate the dependent atomistic behaviors, the single-particle spectra, orbital occupation, optical band gaps, electron–hole wave function overlaps, oscillation strengths, ground-state Coulomb energies and Stokes shift are realized under different lateral $$(l_x)$$ and vertical $$(l_z)$$ sizes. The electronic structures and optical properties of CdSe zinc-blende nanoplatelets are monotonically dependent on the lateral sizes, while those of CdSe zinc-blende nanoplatelets are nonmonotonically sensitive to the vertical sizes. This atomistic prediction will contribute to the understanding of physical behaviors in colloidal CdSe zinc-blende nanoplatelets and will deliver some significant data for experimental study which can be produced by inexpensive means.

Equation of motion for the solvent polarization apparent charges in the polarizable continuum model: Application to time-dependent CI.

The dynamics of the electrons for a molecule in solution is coupled to the dynamics of its polarizable environment, i.e., the solvent. To theoretically investigate such electronic dynamics, we have recently developed equations of motion (EOM) for the apparent solvent polarization charges that generate the reaction field in the Polarizable Continuum Model (PCM) for solvation and we have coupled them to a real-time time-dependent density functional theory (RT TDDFT) description of the solute [S. Corni et al., J. Phys. Chem. A 119, 5405 (2014)]. Here we present an extension of the EOM-PCM approach to a Time-Dependent Configuration Interaction (TD CI) description of the solute dynamics, which is free from the qualitative artifacts of RT TDDFT in the adiabatic approximation. As tests of the developed approach, we investigate the solvent Debye relaxation after an electronic excitation of the solute obtained either by a π pulse of light or by assuming the idealized sudden promotion to the excited state. Moreover, we present EOM for the Onsager solvation model and we compare the results with PCM. The developed approach provides qualitatively correct real-time evolutions and is promising as a general tool to investigate the electron dynamics elicited by external electromagnetic fields for molecules in solution.

Structural and optical manipulation of colloidal Ge1-xSnx nanocrystals with experimentally synthesized sizes: Atomistic tight-binding theory

Nontoxic, maintainable and cost-effective group IV semiconductors are gorgeous for an expansive range of electronic and optoelectronic applications, even though the presence of the indirect band gap obstructs the optical performance. However, band structures can be modified from indirect to direct band gaps by constructing the nanostructures or by alloying with tin (Sn) material. In the study presented here, I investigate the impact of ion-centred types, Sn compositions and dimensions on the electronic structures and optical properties in Ge1-xSnx diamond cubic nanocrystals of the experimentally synthesized Sn contents and diameters using the atomistic tight-binding theory (TB) in the conjunction with the configuration interaction description (CI). The analysis of the mechanism suggests that the physical properties are mainly sensitive with ion-centred types (anion (a) and cation (c)), Sn compositions and dimensions of Ge1-xSnx diamond cubic nanocrystals. The reduction of optical band gaps is reported with the increasing diameters and Sn alloying contents. The visible spectral range is obtained allowing for the applications in bio imaging and chemical sensing. The optical band gaps based on tight-binding calculations are in close agreement with the experimental data for Ge1-xSnx nanocrystals with diameter of 2.1 nm, while for Ge1-xSnx nanocrystals with diameter of 2.7 nm there is a discrepancy of 0.4 eV with experimental results and first-principles calculations. An improvement in the luminescence properties of such Ge1-xSnx nanocrystals becomes possible in the presence of the Sn contents. The electron-hole coulomb interaction is reduced with the increasing Sn components, while the electron-hole exchange interaction is increased with the increasing Sn contents. In addition, I have to point out an astonishing phenomenon, stokes shift and fine structure splitting, with the aim for the realization of the entangled source. The stokes shift and fine structure splitting are enhanced with the increasing Sn contents and decreasing diameters as can be elucidated by the trend of ground electron-hole wave function overlaps. Ge1-xSnx nanocrystal with Sn-free content and large size is the best candidate to be a source of entangled photon pairs. Finally, the combinations of direct band gap character and broad tunable visible spectra advise the promise for use in optoelectronic devices as well as solar cells.

Atomistic tight-binding theory of excitonic splitting energies in CdX(X = Se, S and Te)/ZnS core/shell nanocrystals

Using the atomistic tight-binding theory (TB) and a configuration interaction description (CI), we numerically compute the excitonic splitting of CdX(X = Se, S and Te)/ZnS core/shell nanocrystals with the objective to explain how types of the core materials and growth shell thickness can provide the detailed manipulation of the dark-dark (DD), dark-bright (DB) and bright-bright (BB) excitonic splitting, beneficial for the active application of quantum information. To analyze the splitting of the excitonic states, the optical band gaps, ground-state wave function overlaps and atomistic electron-hole interactions tend to be numerically demonstrated. Based on the atomistic computations, the single-particle and excitonic gaps are mainly reduced with the increasing ZnS shell thickness owing to the quantum confinement. In the range of the higher to lower energies, the order of the single-particle gaps is CdSe/ZnS, CdS/ZnS and CdTe/ZnS core/shell nanocrystals, while one of the excitonic gaps is CdS/ZnS, CdSe/ZnS and CdTe/ZnS core/shell nanocrystals because of the atomistic electron-hole interaction. The strongest electron-hole interactions are mainly observed in CdSe/ZnS core/shell nanocrystals. In addition, the computational results underline that the energies of the dark-dark (DD), dark-bright (DB) and bright-bright (BB) excitonic splitting are generally reduced with the increasing ZnS growth shell thickness as described by the trend of the electron-hole exchange interaction. The high-to-low splitting of the excitonic states is demonstrated in CdSe/ZnS, CdTe/ZnS and CdS/ZnS core/shell nanocrystals because of the fashion in the electron-hole exchange interaction and overlaps of the electron-hole wave functions. As the resulting calculations, it is expected that CdS/ZnS core/shell nanocrystals are the best candidates to be the source of entangled photons. Finally, the comprehensive information on the excitonic splitting can enable the use of suitable core/shell nanocrystals for the entangled photons in the application of quantum information.

Atomistic tight-binding computations in the new class of CdSe/AlP II–VI core/III–V shell nanocrystals

Using the atomistic tight-binding theory in conjunction with a configuration interaction description, I investigate the new class of CdSe/AlP II---VI core/III---V shell nanocrystals. In an effort to theoretically analyze the atomistic behaviors, I calculate single-particle spectra, atomistic character, optical band gaps, ground-state wave function overlaps, ground-state oscillation strengths, ground-state Coulomb energies, ground-state exchange energies, Stokes shift and fine structure splitting under different numbers of the growth shell monolayers (MLs). I highlight that CdSe/AlP II---VI core/III---V shell nanocrystals have strong thickness dependence on the structural and optical properties. The reduction of the optical band gaps is recognized with the increasing coated shell thickness because of quantum confinement. The improvement of wave function overlaps, oscillation strengths, Stokes shift, and fine structure splitting is demonstrated at the growth shell thickness of 1.0 ML. This atomistic prediction will contribute to the understanding of natural properties in the new class of colloidal CdSe/AlP II---VI core/III---V shell nanocrystals and will deliver some significant guidelines for further experimental investigations.

Large-scale configuration interaction description of the structure of nuclei around 100Sn and 208Pb

In this contribution I would like to discuss briefly the recent developments of the nuclear configuration interaction shell model approach. As examples, we apply the model to calculate the structure and decay properties of low-lying states in neutron-deficient nuclei around 100Sn and 208Pb that are of great experimental and theoretical interests.

Microwave transitions from pairs of Rbnd5/2nd5/2atoms

We have observed resonant microwave transitions between pairs of atoms. Specifically, we have observed the processes $n{d}_{5/2}n{d}_{5/2}\ensuremath{\rightarrow}(n+1){d}_{j}(n\ensuremath{-}2){f}_{7/2}$ and $n{d}_{5/2}n{d}_{5/2}\ensuremath{\rightarrow}(n+2){p}_{3/2}(n\ensuremath{-}1){d}_{5/2}$ for $35\ensuremath{\le}n\ensuremath{\le}44$. These transitions are allowed due to the dipole-dipole-induced configuration interaction between the $n{d}_{5/2}n{d}_{5/2}$ state and the energetically nearby $(n+2){p}_{3/2}(n\ensuremath{-}2){f}_{7/2}$ state, which admixes some of the latter into the former. The resulting microwave transitions are analogous to two-photon transitions in which one of the photons has been replaced by the dipole-dipole interaction. We have developed a configuration interaction description of the transitions, which gives a good description of the interaction over the range $35\ensuremath{\le}n\ensuremath{\le}42$. Over this range of $n$ the detuning varies from 0.095 to 1.482 GHz, the microwave frequencies vary from 26.970 to 45.916 GHz, and the requisite microwave powers vary by a factor of over 1000.

Atomistic tight-binding computations of excitonic fine structure splitting in CdSe/ZnSe type-I and ZnSe/CdSe invert type-I core/shell nanocrystals

The excitonic fine structure splitting (FSS) in semiconductor core/shell nanocrystals is intrinsically caused by electron-hole exchange interactions. Here, I present the model based on the combination of the atomistic tight-binding theory (TB) and configuration interaction description (CI) that allows determining the band-edge excitonic fine structure. Using this atomistic model, I highlight the operation of the excitonic fine structure splitting by engineering the type of the band alignments and the thickness of the growth shell in CdSe/ZnSe type-I and ZnSe/CdSe invert type-I core/shell nanocrystals, indicating to control the location of the carrier confinement in these nanostructures. To theoretically examine the atomistic behaviors of excitonic fine structure splitting, the single-particle spectra, optical band gaps, ground-state wave function overlaps, ground-state coulomb energies, ground-state exchange energies, dark-bright (DB) excitonic splitting and bright-bright (BB) excitonic splitting are computed. I discover that ZnSe/CdSe invert type-I core/shell nanocrystals provide a sturdily reduced energies of DB and DB excitonic splitting as described by a diminished electron-hole exchange interaction. Besides, the energies of DB and DB excitonic splitting are decreased with the increasing dimensions of the growth shell. This deeper insight is much important for the theoretical understanding and practical control by type of the band alignments and sizes in growth shell with the aim to generate the entanglement of the polarized photon source in the application of the quantum information processing.

Atomistic Tight-Binding Theory of Electron-Hole Exchange Interaction in Morphological Evolution of CdSe/ZnS Core/Shell Nanodisk to CdSe/ZnS Core/Shell Nanorod

Based on the atomistic tight-binding theory (TB) and a configuration interaction (CI) description, the electron-hole exchange interaction in the morphological transformation of CdSe/ZnS core/shell nanodisk to CdSe/ZnS core/shell nanorod is described with the aim of understanding the impact of the structural shapes on the change of the electron-hole exchange interaction. Normally, the ground hole states confined in typical CdSe/ZnS core/shell nanocrystals are of heavy hole-like character. However, the atomistic tight-binding theory shows that a transition of the ground hole states from heavy hole-like to light hole-like contribution with the increasing aspect ratios of the CdSe/ZnS core/shell nanostructures is recognized. According to the change in the ground-state hole characters, the electron-hole exchange interaction is also significantly altered. To do so, optical band gaps, ground-state electron character, ground-state hole character, oscillation strengths, ground-state coulomb energies, ground-state exchange energies, and dark-bright (DB) excitonic splitting (stoke shift) are numerically demonstrated. These atomistic computations obviously show the sensitivity with the aspect ratios. Finally, the alteration in the hole character has a prominent effect on dark-bright (DB) excitonic splitting.

Atomistic tight-binding computations in electronic structures and optical properties of type-II CdTe/CdSe core/shell nanocrystals

I perform a detailed theoretical study of the characteristic electronic structures and optical properties of type-II CdTe/CdSe core/shell nanocrystals of the experimentally relevant dimensions by means of atomistic sp3s∗ empirical tight-binding theory (TB) and configuration interaction description (CI) with the aim to evidently understand the significance of the CdSe growth shell. Based on the theoretical results, the single-particle spectra, density of states (DOS), charge densities, overlaps of electron and hole wave function, oscillation strengths and excitonic energies are mainly analyzed as a function of CdSe shell thicknesses. The detailed analysis elucidates and quantifies the sensitivity of shell thickness in determining the electronic structures and optical properties of CdTe/CdSe core/shell nanocrystals. In addition, I emphasize that the atomistic tight-binding calculations show a reasonable congruence with the experimental results. As can be seen from the atomistic computations, I summarize that the efficient manipulation of structural and optical properties of CdTe/CdSe core/shell nanocrystals is theoretically achieved by introducing and changing the CdSe growth shell thickness. Finally, the comprehensive knowledge successfully exposes the important physical factors that influence the atomistic behaviours of type-II CdTe/CdSe core/shell nanocrystals for a given growth shell thickness which can aid in the understanding of the physical properties of a wider class of type-II nanoscale heterostructures, thus leading to a guideline for the design of their electronic and optical properties before implementing to the novel electronic nanodevices.

Structural properties of SiC zinc-blende and wurtzite nanostructures: Atomistic tight-binding theory

The electronic structures of SiC nanostructures consisting of nanocrystals and nanorods are theoretically investigated by atomistic tight-binding model including sp3s⁎ orbitals, spin–orbit coupling and the first-nearest neighboring interaction. The excitonic energies and states are numerically computed by the configuration interaction (CI) description of the coulomb and exchange effect. Based on this atomistic model, single-particle gaps, excitonic gaps, coulomb energies and exchange energies of SiC nanocrystals and nanorods are analyzed as a function of diameters and aspect ratios, respectively. The comparison of the results obtained from different structures (zinc-blende and wurtzite) is theoretically reported with the aim to address the importance of the geometric structures on these detailed computations. The simulation emphasizes that the calculations of the electronic structures are mainly sensitive with the shapes, dimensions and geometric structures. The numerical results summarize that the efficient manipulation of structural properties for SiC nanostructures is theoretically accomplished by changing shapes, sizes and geometric structures. Ultimately, the atomistic information of shapes, sizes and structures in the structural properties of SiC nanostructures will be considerably implemented to SiC-based applications.

Insights into the structure of many-electron wave functions of Mott-insulating antiferromagnets: The three-band Hubbard model in full configuration interaction quantum Monte Carlo

We investigate the ground-state wave function of a prototypical strongly correlated system, a three-band $(p\ensuremath{-}d)$ Hubbard model of cuprates, using full configuration interaction quantum Monte Carlo. We show that the configuration interaction description of the exact ground state wave function is profoundly affected by the choice of single-particle representation, in a counterintuitive manner. Thus a broken-symmetry unrestricted Hartree-Fock basis, which at a single configuration level produces a qualitatively correct description of the antiferromagnet, results in a highly entangled exact solution consisting of high particle-hole excitations of the reference. This wave function is found to be very difficult to approximate using subspace diagonalizations. Conversely, a restricted Hartree-Fock basis, which yields at a single configuration level a qualitatively incorrect paramagnetic metal, results in a relatively rapidly converging configuration interaction expansion. Convergence can be further accelerated by adopting a natural orbital representation. Our results suggest that with the correct single-particle basis, such strongly correlated systems may be described by relatively compact wave functions.

From Quantum-Dot Nanorings to Polyacetylene via Small Annulenes: A Full Configuration Interaction Description Based on an Extended Hubbard–Su-Schrieffer-Heeger Model

From Quantum-Dot Nanorings to Polyacetylene via Small Annulenes: A Full Configuration Interaction Description Based on an Extended Hubbard–Su-Schrieffer-Heeger Model Quantum chemical approaches to molecules — like the annulenes (cyclic polyenes CNHN, where N is an integer) considered here — usually examine the impact of electron-electron interaction at frozen molecular geometries within approaches based on a single (or a few) Slater determinant(s). In the solid-state community, polyacetylene, their asymptotic limit [(CH)∞ = limN→∞CNHN] is oftentreated as a statically dimerized chain within a single-electron picture (static Su-Schrieffer-Heeger (SSH) model). More recent studies on quantum-dot nanorings employed SSH models extended to embody quantum phonon dynamics and Hubbardtype electron-electron interaction terms, which are treated exactly within a full configuration (CI) description. In the present paper we show that this full CI extended Hubbard-SSH framework provides a unitary description of important properties both of small annulenes (cyclobutadiene, benzene, and octatetraene) and of polyacetylene based on a few N-independent model parameters. These results emphasize the importance of the full CI description; provided that a few important terms are retained, a schematic description of the interaction does not preclude a reasonable description.

Mechanism for controlling the exciton fine structure in quantum dots using electric fields: Manipulation of exciton orientation and exchange splitting at the atomic scale

Using atomistic tight-binding theory with a configuration interaction description of Coulomb and exchange effects, we describe excitons in symmetric quantum dots in a vertical electric field to explain how field-induced manipulation of exciton orientation and phase can eliminate the anisotropic exchange, leading to a drastic reduction of the fine-structure splitting and a 90 degree rotation of polarization, similar to experiment. This reorientation and rephasing of the exciton is done plane-by-plane inside the dot without significant squeezing of the exciton.