Coulomb Polarization Research Articles

Excitons in metal halide perovskite nanoplatelets: an effective mass description of polaronic, dielectric and quantum confinement effects.

A theoretical model for excitons confined in metal halide perovskite nanoplatelets is presented. The model accounts for quantum confinement, dielectric confinement, short and long range polaron interactions by means of effective mass theory, image charges and Haken potentials. We use it to describe the band edge exciton of MAPbI3 structures surrounded by organic ligands. It is shown that the quasi-2D quantum and dielectric confinement squeezes the exciton radius, and this in turn enhances short-range polaron effects as compared to 3D structures. Dielectric screening is then weaker than expected from the static dielectric constant. This boosts the binding energies and radiative recombination probabilities, which is a requisite to match experimental data in related systems. The thickness dependence of Coulomb polarization and self-energy potentials is in fair agreement with sophisticated atomistic models.

Breakup of the proton halo nucleus 8B near barrier energies

The dynamics of a nuclear open quantum system could be revealed in the correlations between the breakup fragments of halo nuclei. The breakup mechanism of a proton halo nuclear system is of particular interest as the Coulomb polarization may play an important role, which, however, remains an open question. Here we use a highly efficient silicon detector array and measure the correlations between the breakup fragments of 8B incident on 120Sn at near-barrier energies. The energy and angular correlations can be explained by a fully quantum mechanical method based on the state-of-the-art continuum discretized coupled channel calculations. The results indicate that, compared to the neutron halo nucleus 6He, 8B presents distinctive reaction dynamics: the dominance of the elastic breakup. This breakup occurs mainly via the short-lived continuum states, almost exhausts the 7Be yield, indicating the effect of Coulomb polarization on the proton halo state. The correlation information reveals that the prompt breakup mechanism dominates, occurring predominantly on the outgoing trajectory. We also show that, as a large environment, the continuum of 8B breakup may not significantly influence elastic scattering and complete fusion.

A simple variational quantum Monte Carlo-effective mass approach for excitons and trions in quantum dots

A computational model is presented to calculate the ground state energy of neutral and charged excitons confined in semiconductor quantum dots. The model is based on the variational Quantum Monte Carlo method and effective mass Hamiltonians. Through an iterative Newton–Raphson process, minimizing the local energy, and (optional) parallelization of random walkers, fast and accurate estimates of both confinement and Coulomb binding energies can be obtained in standard desktop computers. To illustrate the reach of the model, we provide Fortran programs and illustrative calculations for colloidal CdSe nanoplatelets with large lateral dimensions and dielectric confinement, where electronic correlations are strong. The results compare well with exact variational calculations and largely outperform configuration interaction calculations in computational efficiency. Program summaryProgram title: vqmc-emaCPC Library link to program files:https://doi.org/10.17632/dtpyc2pffg.1Licensing provisions: GPLv3Programming language: Fortran 90Nature of problem: Calculation of either exciton or trion ground state energy and wave function in a cuboidal semiconductor nanoplatelet. Hard wall quantum confinement coexists with dielectric confinement (self-energy and Coulomb polarization terms).Solution method: Variational quantum Monte Carlo with effective mass Hamiltonians, integrated into a Newton–Raphson solver. OMP parallelization library can be (optionally) linked.

Two-Photon Up-Conversion Photoluminescence Realized through Spatially Extended Gap States in Quasi-2D Perovskite Films.

A new approach to generate a two-photon up-conversion photoluminescence (PL) by directly exciting the gap states with continuous-wave (CW) infrared photoexcitation in solution-processing quasi-2D perovskite films [(PEA)2 (MA)4 Pb5 Br16 with n = 5] is reported. Specifically, a visible PL peaked at 520 nm is observed with the quadratic power dependence by exciting the gap states with CW 980 nm laser excitation, indicating a two-photon up-conversion PL occurring in quasi-2D perovskite films. Decreasing the gap states by reducing the n value leads to a dramatic decrease in the two-photon up-conversion PL signal. This confirms that the gap states are indeed responsible for generating the two-photon up-conversion PL in quasi-2D perovskites. Furthermore, mechanical scratching indicates that the different-n-value nanoplates are essentially uniformly formed in the quasi-2D perovskite films toward generating multi-photon up-conversion light emission. More importantly, the two-photon up-conversion PL is found to be sensitive to an external magnetic field, indicating that the gap states are essentially formed as spatially extended states ready for multi-photon excitation. Polarization-dependent up-conversion PL studies reveal that the gap states experience the orbit-orbit interaction through Coulomb polarization to form spatially extended states toward developing multi-photon up-conversion light emission in quasi-2D perovskites.

Beyond Wigner's isobaric multiplet mass equation: Effect of charge-symmetry-breaking interaction and Coulomb polarization

The quadratic form of the isobaric multiplet mass equation (IMME), which was originally suggested by Wigner and has been generally regarded as valid, is seriously questioned by recent high-precision nuclear mass measurements. The usual resolution to this problem is to add empirically the cubic and quartic $T_z$-terms to characterize the deviations from the IMME, but finding the origin of these terms remains an unsolved difficulty. Based on a strategy beyond the Wigner's first-order perturbation, we derive explicitly the cubic and quartic $T_z$-terms. These terms are shown to be generated by the effective charge-symmetry breaking and charge-independent breaking interactions in nuclear medium combined with the Coulomb polarization effect. Calculations for the $sd$- and lower $fp$-shells explore a systematical emergence of the cubic $T_z$-term, suggesting a general deviation from the original IMME. Intriguingly, the magnitude of the deviation exhibits an oscillation-like behavior with mass number, modulated by the shell effect.

Symmetry energy of hot nuclei in the relativistic Thomas-Fermi approximation

We develop a self-consistent description of hot nuclei within the relativistic Thomas-Fermi approximation using the relativistic mean-field model for nuclear interactions. The temperature dependence of the symmetry energy and other physical quantities of a nucleus are calculated by employing the subtraction procedure in order to isolate the nucleus from the surrounding nucleon gas. It is found that the symmetry energy coefficient of finite nuclei is significantly affected by the Coulomb polarization effect. We also examine the dependence of the results on nuclear interactions and make a comparison between the results obtained from relativistic and nonrelativistic Thomas-Fermi calculations.

Dynamic polarization potentials and dipole polarizabilities of11Li,6He and6Li compared

Elastic scattering data for ${}^{11}$Li, ${}^{6}$Li, and ${}^{6}$He $+$ ${}^{208}$Pb at incident energies of 29.8 MeV, 29 MeV, and 18 MeV, respectively, were analyzed by means of coupled discretized continuum channels (CDCC) calculations. Dynamic polarization potentials (DPPs) of the trivially equivalent local potential (TELP) type were derived from these calculations and compared. The dipole polarizability factor $\ensuremath{\alpha}$ for ${}^{11}$Li obtained by fitting a Coulomb polarization potential to the long-range part of the real DPP is consistent with the theoretical value of $5.7$ fm${}^{3}$. These results point to the dineutron model providing a very good description of ${}^{11}$Li breakup coupling effects. The much larger Coulomb dipole polarizability of ${}^{11}$Li suggests the persistence of large deviations from Rutherford scattering at sub-barrier energies for medium mass targets, a prediction confirmed by CDCC calculations.

Band Structures of Narrow Gap Semiconductors in Terms of Coulombic Polaron Effect

We consider the formation of a polaron in an interacting Fermi gas due to Coulomb polarization of electron gas by a single electron. We show that the polarization effect due to the Coulomb field in an interacting Fermi gas leads to the Coulomb polaron caused by the exchange-correlation hole and the electron spectrum shows an insulating behavior with the dispersion law of quasiparticles similar to the Kane kp-model of a band structure for narrow gap semiconductors. We apply the polaronic model to the case of inhomogeneous periodic and interacting electron Fermi gas of crystals. We show that the formation of the Coulomb polaron leads to the transition to the semiconductor with the energy bands similar to the well-known semiempirical Kane kp-model of the band structure of narrow gap semiconductors. We also discuss the impact of the results on band structure calculations.

Origin of the optical gap in half-doped manganites

We have analyzed the coexisting charge and orbital ordering in half-doped manganites using a model which includes Coulomb and Jahn-Teller orbital polarization interactions. Most surprisingly, the gap in the optical conductivity is reduced by both on-site and intersite Coulomb interactions, but increases and explains the experimental results when the Jahn-Teller terms with orbital polarization are considered. The origin of this behavior is explained within a molecular model which arises in the limit of extreme topological frustration, when single electrons are confined to molecular units consisting of three orbitals.

Theoretical study of the binding of the chloride anion to water and alcohols

The binding energies of chloride adducts to water, methanol, and isopropanol have been calculated by the molecular orbital method at the self-consistent field (SCF) and Möller-Plesset [MP2, and (partially) MP4] levels. Extended Gaussian basis sets enlarged with both standard valence polarization orbitals and semidiffuse Coulomb polarization orbitals have been used. The best theoretical values obtained by correcting the energy differences between the supermolecule and its fragments for basis-set superposition errors and zero-point vibration energies are in satisfactory agreement with the available reaction enthalpies (i.e., 12.4 kcal/mol computed for water, versus 13.1 experimentally). Equally in agreement with the experimental trend, an increase in the binding energy of Cl− is predicted in going from water to alcohols. The proper introduction of correlation in a sufficiently extended and polarized basis set is essential to the reproduction of this trend. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 567–574, 1997

Theoretical study of the binding of the chloride anion to water and alcohols

The binding energies of chloride adducts to water, methanol, and isopropanol have been calculated by the molecular orbital method at the self-consistent field (SCF) and Möller-Plesset [MP2, and (partially) MP4] levels. Extended Gaussian basis sets enlarged with both standard valence polarization orbitals and semidiffuse Coulomb polarization orbitals have been used. The best theoretical values obtained by correcting the energy differences between the supermolecule and its fragments for basis-set superposition errors and zero-point vibration energies are in satisfactory agreement with the available reaction enthalpies (i.e., 12.4 kcal/mol computed for water, versus 13.1 experimentally). Equally in agreement with the experimental trend, an increase in the binding energy of Cl− is predicted in going from water to alcohols. The proper introduction of correlation in a sufficiently extended and polarized basis set is essential to the reproduction of this trend. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 567–574, 1997

Coulomb and nuclear excitation in intermediate-energy heavy-ion collisions.

Following previous works where the field of application of Glauber methods were extended to heavy-ion scattering in the energy range of 30--400 MeV/nucleon, we develop here a formalism which includes both Coulomb and nuclear excitation processes and explicitly accounts for the effects arising from the energy difference in different channels. First order inelastic excitations are described in the framework of a distorted wave eikonal approximation; in this connection nuclear phase shifts and form factors are described in terms of microscopic nucleon-nucleon interactions while the Coulomb excitation is described through a proper manipulation of phenomenological form factors. Several specific examples are discussed. The interest is then focused on second-order processes, more specifically on the effects introduced in the elastic channel by the coupling to the inelastic channels, and the corresponding Coulomb and nuclear polarization potentials are derived and discussed. On the ground of these achievements a general formalism is built up, capable of describing coupled-channel problems at any order of scattering. Due to the eikonal propagation, the multichannel multistep series leads to simple algebraic forms for each partial-wave scattering matrix, which involve powers of a channel matrix whose elements only depend on the impact parameter.

Coulomb polarizability of the deuteron and the E1 capture mechanism in the 4He(d, gamma )6Li reaction.

The $^{4}\mathrm{He}$ (d,\ensuremath{\gamma}${)}^{6}$Li reaction, which proceeds through the overwhelming E2 transition, but with a small admixture of the E1 transition, is a possible mechanism for the $^{6}\mathrm{Li}$ nucleosynthesis in the Big Bang. The E1 radiation is, however, strongly suppressed in the radiative deuteron capture reaction in self-conjugate nuclei. Usually, the observed small, but not negligible, E1 radiation has been attributed to the spin-dependent electric dipole operator. In this study it is shown how the Coulomb polarization of the deuteron $^{4}\mathrm{He}$(d,\ensuremath{\gamma}${)}^{6}$Li reaction. Expressly we investigate the E1 capture process which proceeds with a small change of the deuteron internal structure in the entrance channel state. The structural change of the deuteron is engendered by the Coulomb field in the system of target and deuteron and such a deviant deuteron state is surveyed by calculating the expectation value of the relative coordinate between the proton and neutron in the deuteron. We then show how the electrically polarized deuteron state releases the usual E1 selection rule, and subsequently we calculate the relative intensity of the E1 versus E2 radiation in the $^{4}\mathrm{He}$(d,\ensuremath{\gamma}${)}^{6}$Li reaction.

Long- versus short-range Si, Al ordering in zeolites X and Y

A Monte Carlo method is employed to study the Si, Al distribution in zeolites X and Y. The simulation includes long-range Coulomb interactions and oxygen polarization effects. Both contributions to the lattice energy are found to be necessary for a realistic description of the tetrahedral cation ordering. The equilibrium Si, Al distributions show short-ange order for all analyzed compositions (1<Si/Al<3), in good agreement with 29Si NMR results. Long-range ordering is only found in the X region (Si/Al<1.4). Two order—disorder transitions are found at framework compositions for which earlier X-ray diffraction analyzes detected discontinuities in the lattice parameter as a function of Al loading.

Coulomb-dominated low-energy deuteron stripping.

Analysis of a three-body model shows that Coulomb polarization of the deuteron has very little influence on the branching ratio A(d,p)/A(d,n) for transfer reactions on target nucleus A at very low deuteron energies (the Oppenheimer-Phillips effect). We see that polarization effects in transfer reactions are not related to the long range of the Coulomb field, but are caused by the more intense fields near the target nucleus. However, even in that region the induced dipole moment is limited by the deuteron binding, and it is small for low Z targets. We see in addition that the transfer amplitudes tend to be insensitive to any polarization admixtures in the entrance channel. On the other hand, the branching ratio can be affected by the Coulomb barrier for the bound final-state wave function of the proton, especially for very weakly bound final states. Brief remarks about the relation of stripping theory to special properties of the d+d system are included.

Coulomb polarization and nuclear reactions at astrophysically low energies

Abstract The effects of electric polarizability of particles are shown to play an insignificant role in the nucleosynthesis reactions at astrophysically low energies. The polarization corrections to the reaction S -factors are defined by the value of the polarization potential at the boundary of the nuclear force range and do not exceed a quantity of the order of 0.1%. It is shown that the conclusion of papers claiming the very strong influence of the particle polarizability on pp-, pd- and dt-reaction cross sections is a consequence of an incorrect approximation of the initial-channel wave function.

Coulomb polarization corrections to the cross sections of nucleosynthesis at low energies

The effects of electric polarizability of particles are shown to play an insignificant role in nucleosynthesis reactions at astrophysical low energies. The polarization corrections to S-factors of the reactions under consideration are defined by the value of the polarization potential at the boundary of the nuclear force range and do not exceed the quantity of the order of 0.1%.

A simple theoretical description of coulomb polarization effects on the coulomb modified scattering length

A simple formula is derived for the Coulomb modified s-wave scattering length function in the presence of long-range polarization potentials. When applied to a model of low-energy p-d scattering, the formula reproduces behavior previously described by Kvitsinskii and Merkuriev [Sov. J. Nucl. Phys. 41 (1985) 412].

Coulomb polarization effects in low-energy p-d elastic scattering

As a model of p-d elastic scattering, s-wave scattering is studied for a two-body potential that consist of a repulsive Coulomb potential and a central potential that decreases at large radius asthe inverse fourth power of that radius. It is proved that the usual Coulomb modified s-wave scattering length does not exist for such potentials. We define a different modified scattering length and conjecture that it is the quantity that is to be compared with published calculated and experimental values.

Polarization potentials in heavy-ion scattering

A polarization potential is defined in terms of the Feshbach projection operator formalism to represent the effect upon the elastic channel of the coupling to non-elastic channels in heavy-ion scattering. The polarization potential represents coupling to specific surface degrees of freedom of the particular reaction considered and it is contrasted to the complementary global approaches for the volume potential such as the folding model and the proximity potential. The coupled channels method is used both as a source of exact model solutions for comparison with the various approximate potential forms and also as a numerical means of constructing trivially equivalent local potentials. The imaginary Coulomb polarization potential is due in lowest order to quadrupole coupling to the lowest collective 2 + state of a nucleus. It is considered in detail since it provides the insight of closed analytical forms in various approximations. Multistep coupling to higher states, energy loss and off-energy shell effects are also considered analytically. The real Coulomb polarization potential due to the virtual excitation of multipole giant resonances, and the polarization potential arising from relativistic corrections, are investigated in detail. Polarization potential components due to nuclear coupling are investigated numerically. Analytical cross section approaches are contrasted with the polarization potential approach and with coupled channels.