Direct Coulomb Research Articles

Investigation of exciton ground state in quantum dots via Hamiltonian diagonalization method

We analyze the electron-hole (exciton) ground state associated with the first peak in the optical absorption spectra of semiconductor quantum dots. We assume the effective mass approximation and a dot radius R on the order of the exciton Bohr radius aB. A Hamiltonian diagonalization method which accounts for the exciton’s kinetic, direct Coulomb, and surface polarization energies is used. We obtain a representation of the exciton ground-state wavefunction and a value for its energy using a basis set consisting of only three composite infinite spherical well wavefunctions. We discuss the precision obtained by this basis set by comparing with results from a much more extended basis set. Our results are used to predict the radius-dependent energy of the first peak in visible-light absorption spectra for CdSe quantum dots. Our analysis accurately describes the experimental data for dots with radii in the range aB<R<2aB. We discuss why our model breaks down for smaller radii.

Fine-structure splitting of exciton states in semiconductor quantum dot molecules

Exciton levels and fine-structure splitting in laterally coupled quantum dot molecules arestudied. The electron and hole tunnelling energies as well as the direct Coulomb interactionare essential for the exciton levels. It is found that fine-structure splittings of the two lowestexciton levels are contributed by the intra- and interdot exchange interactions which aregreatly influenced by the symmetry and tunnel-coupling between the two dots. As theinterdot separation is reduced, fine-structure splitting of the exciton ground state is largelyincreased while those of the second and fourth states are decreased. Moreover, thedependence of the fine-structure splitting in quantum dot molecules on the Coulombcorrelation is clearly clarified.

Accelerating molecular modeling applications with graphics processors

Molecular mechanics simulations offer a computational approach to study the behavior of biomolecules at atomic detail, but such simulations are limited in size and timescale by the available computing resources. State-of-the-art graphics processing units (GPUs) can perform over 500 billion arithmetic operations per second, a tremendous computational resource that can now be utilized for general purpose computing as a result of recent advances in GPU hardware and software architecture. In this article, an overview of recent advances in programmable GPUs is presented, with an emphasis on their application to molecular mechanics simulations and the programming techniques required to obtain optimal performance in these cases. We demonstrate the use of GPUs for the calculation of long-range electrostatics and nonbonded forces for molecular dynamics simulations, where GPU-based calculations are typically 10-100 times faster than heavily optimized CPU-based implementations. The application of GPU acceleration to biomolecular simulation is also demonstrated through the use of GPU-accelerated Coulomb-based ion placement and calculation of time-averaged potentials from molecular dynamics trajectories. A novel approximation to Coulomb potential calculation, the multilevel summation method, is introduced and compared with direct Coulomb summation. In light of the performance obtained for this set of calculations, future applications of graphics processors to molecular dynamics simulations are discussed.

Bipolaron in a quasi-0D quantum dot

Bipolaron states in a quasi-0D quantum dot with a spherical parabolic confinement potential are investigated by applying the Feynman variational principle . The bipolaron coupling energy and self-action potential energy are found to increase with an increase in the Fröhlich electron–phonon-coupling constant. There is also a non-monotonic dependence of the bipolaron coupling energy on the quantum dot radius. With decreasing structure radius the bipolaron coupling energy increases. However, from a critical radius it starts decreasing as the radius decreases, due to the dominance of the coulomb-to-phonon mediated interaction. When electrons in the bipolaron are forcefully neighboured, the polarization of the structure is intensified and consequently there is Coulomb repulsion. The possibility of bipolaron formation depends on the strength of the direct Coulomb repulsion which, in turn, depends on the quantum dot radius. The main contribution to the bipolaron coupling energy comes from the self-action potential. This self-action potential energy influences the energy state of the bipolaron considerably. The ratio of optical-to-static dielectric constants significantly affects the bipolaron coupling energy.

Quantum gates between capacitively coupled double quantum dot two-spin qubits

We study the two-qubit controlled-not gate operating on qubits encoded in the spin state of a pair of electrons in a double quantum dot. We assume that the electrons can tunnel between the two quantum dots encoding a single qubit, while tunneling between the quantum dots that belong to different qubits is forbidden. Therefore, the two qubits interact exclusively through the direct Coulomb repulsion of the electrons. We find that entangling two-qubit gates can be performed by the electrical biasing of quantum dots and/or tuning of the tunneling matrix elements between the quantum dots within the qubits. The entangling interaction can be controlled by tuning the bias through the resonance between the singly-occupied and doubly-occupied singlet ground states of a double quantum dot.

A ionization theory based on united and separated atom model

The direct Coulomb ionization process can be generally well described by the ECPSSR theory, which bases on the perturbed-stationary-state(PSS) and accounts for the energy-loss, Coulomb-deflection, and relativistic effects. But the ECPSSR calculation has significant deviations for heavy projectile at low impinging energies. In this paper we propose a new modified ECPSSR theory, i.e. MECUSAR, in which PSS is replaced by an united and separated atom model, and molecule-orbit effect is considered. The MECUSAR calculations give better agreement with the experimental data at lower impinging energies, and agree with the ECPSSR calculations at high energies. By using OBKN (Oppenheimer-Brinkman-Kramers formulas of Nikolaev) theory to describe the contribution of the electron capture, we further modified the proposed MECUSAR theory, and calculated the target ionization cross sections for different charge states of the projectile.

Atomistic simulation of sorption in model pores with reduced spatial periodicity

Grand Canonical Monte Carlo (GCMC) was used to study the sorption thermodynamics of carbon dioxide in model graphite slit-like pores and nanotubes by means of the Ewald technique, suitably adapted for the computation of long range (electrostatic) interactions of sorbates confined in pore systems exhibiting reduced periodicity in space. The computed thereby micropore size distributions extracted via mathematical elaboration of the simulation results with respect to real graphitic materials, reproduced successfully the experimentally measured isotherms of carbon dioxide in these materials at various temperatures. In the case of nanotubes, a direct Coulomb summation over a large number of periodic images proved to be a sufficient approximation leading to excellent agreement between the simulated and measured isotherms.

A rapid boundary integral equation technique for protein electrostatics

A new boundary integral formulation is proposed for the solution of electrostatic field problems involving piecewise uniform dielectric continua. Direct Coulomb contributions to the total potential are treated exactly and Green’s theorem is applied only to the residual reaction field generated by surface polarisation charge induced at dielectric boundaries. The implementation shows significantly improved numerical stability over alternative schemes involving the total field or its surface normal derivatives. Although strictly respecting the electrostatic boundary conditions, the partitioned scheme does introduce a jump artefact at the interface. Comparison against analytic results in canonical geometries, however, demonstrates that simple interpolation near the boundary is a cheap and effective way to circumvent this characteristic in typical applications. The new scheme is tested in a naive model to successfully predict the ground state orientation of biomolecular aggregates comprising the soybean storage protein, glycinin.

Decoherence and fidelity of single-electron spin states in quantum dots: effects of nuclearhyperfine coupling and double occupancy

The decoherence and fidelity of spin states in a localized single-electron quantum dot in thepresence of a dc magnetic field, arising either from the nuclear hyperfine interaction withinthe dot or due to its coupling with another localized quantum dot, are examined in detail.A general framework for determining the time evolution of the reduced density matrixρ for a single dot is presented, which is exact up to the second order in interaction with anyreservoir. In particular, it is applied to the problem of nuclear hyperfine coupling, andapproximate estimates of coherence decay time are made when the nuclear spins are eitherpolarized or unpolarized and the internal dynamics of nuclear spins is determinedmainly by the nuclear magnetic dipole–dipole interaction. The fidelity of a pureunperturbed electronic one-qubit spin state is obtained as a function of time,which is exact even on a very short timescale of logic gate operations. The timevariation of the fidelity of the same one-qubit state on the localized dot as a part ofthe direct product with another one-qubit state on another localized dot arisingbecause of coupling between these quantum dots is also calculated in this paper.In this case, we include both the single-particle tunnelling between the dots aswell as the direct and exchange Coulomb interactions, including on-site Coulombrepulsion. This allows for the double occupation of a single dot. It is found that theloss of fidelity of such two-qubit states due to double occupancy and additionalphase errors in the presence of appreciable dot–dot coupling can become a moresevere limiting factor than that due to the hyperfine interaction in individual dots.

Renormalization of Coulomb interactions in [formula omitted]-wave superconductor [formula omitted

We study the renormalized Coulomb interactions due to retardation effect in Na x CoO 2 . Although the Morel Anderson's pseudo-potential for a 1 g orbital μ a 1 g * is relatively large because the direct Coulomb repulsion U is large, that for interband transition between a 1 g and e g ′ orbitals μ a 1 g , e g ′ * is very small since the renormalization factor for pair hopping J is square of that for U. Therefore, the s-wave superconductivity due to valence-band Suhl–Kondo mechanism will survive against strong Coulomb interactions. The interband hopping of Cooper pairs due to shear phonons is essential to understand the superconductivity in Na x CoO 2 .

Double photoionization of N 2O molecules in the 28–40 eV energy range

The double photoionization of N 2O molecules, in the 28–40 eV energy range, has been studied by synchrotron radiation. In the whole energy range, dissociative ionization producing N + + NO + or N 2 + + O + has been observed. Below ∼35.5 eV, these two processes involve the formation of some autoionizing states. In the range between ∼35.5 and ∼38.5 eV the two processes occur instead by a direct coulomb explosion of the N 2O 2+ dication. Above ∼38.5 eV the dissociation leading to NO + + N + is also promoted by the formation of a dication metastable state which decays by fluorescence to the ground state and then dissociates.

Density Functional Theory for Polyelectrolytes near Oppositely Charged Surfaces

We report a nonlocal density functional theory of polyelectrolyte solutions that faithfully accounts for both short- and long-range correlations neglected in a typical mean-field method. It is shown that for systems with strong electrostatic interactions, the long-range correlations are subdued by direct Coulomb attractions, thereby manifesting strong local excluded-volume effects. The theory has also been used to describe the influence of the polyion chain length and small ion valence on charge inversion due to the adsorption of polyelectrolytes at an oppositely charged surface.

Pressure effects on neutral and charged excitons in self-assembled (In,Ga)As∕GaAsquantum dots

By combining an atomistic pseudopotential method with the configuration interaction approach, we predict the pressure dependence of the binding energies of neutral and charged excitons: $X^0$ (neutral monoexciton), $X^{-}$ and $X^{+}$ (charged trions), and $XX^0$ (biexciton) in lens-shaped, self-assembled In$_{0.6}$Ga$_{0.4}$As/GaAs quantum dots. We predict that (i) with applied pressure the binding energy of $X^0$ and $X^+$ increases and that of $X^-$ decreases, whereas the binding energy of $XX^0$ is nearly pressure independent. (ii) Correlations have a small effect in the binding energy of $X^0$, whereas they largely determine the binding energy of $X^-$, $X^+$ and $XX^0$. (iii) Correlations depend weakly on pressure; thus, the pressure dependence of the binding energies can be understood within the Hartree-Fock approximation and it is controlled by the pressure dependence of the direct Coulomb integrals $J$. Our results in (i) can thus be explained by noting that holes are more localized than electrons, so the Coulomb energies obey $J^{(hh)}>J^{(eh)}>J^{(ee)}$.

Calculation of orbital polarization effects in small Co clusters

The effects of orbital polarizations on the magnetic properties of small Co clusters are investigated in the framework of a self-consistent electronic model Hamiltonian including local Coulomb interactions at three different levels of approximation: (i) full orbital dependence of the direct and exchange Coulomb interactions Umm′ and Jmm′ as given by atomic symmetry, (ii) orbital-independent interactions U=Umm′¯andJ=Jmm′¯, and (iii) orbital polarization (OP) approximation of the form HOP=-(B/2)∑iLˆiδ2, where Lˆiδ refers to the orbital momentum operator at atom i and B to the Racah coefficient. Results are given for the orbital magnetic moments 〈Lδ〉 along high-symmetry magnetization directions δ and for the corresponding magnetic anisotropy energy ΔE. A compact five-fold symmetric Co7 cluster is considered as representative example. It is shown that with an appropriate choice of B the OP approximation yields a very good agreement with the rigorous orbital-dependent calculations. Moreover, one observes that the orbital-independent approach accounts qualitatively for the main general trends in 〈Lδ〉 and MAE. Advantages and limitations of the various approaches are discussed.

Secondary ion emission from CO 2–H 2O ice irradiated by energetic heavy ions : Part II: Analysis–search for organic ions

Secondary ion mass spectrometry is used to investigate ion emission from a frozen-gas mixture of CO 2 and H 2O ( T = 80–90 K) bombarded by MeV nitrogen ions and by 252Cf fission fragments. The aim of the experiment is to detect organic molecules, produced in the highly excited material around the nuclear track, which appear as ions in the flux of sputtered particles. Part I of the present work [L.S. Farenzena, V.M. Collado, C.R Ponciano, E.F. da Silveira, K. Wien. Int. J. Mass Spectrom. 243 (2005) 85–93] described the method and presented the time-of-flight mass spectra; a list of the CO 2 specific and H 2O specific reaction products was provided. In Part II, the peaks of the TOF mass spectra are analyzed. Projectile-ice direct coulomb interaction leads to the production in the track of the H +, C +, O +, O 2 +, CO + and CO 2 + primarily ions, which react in the highly energized nuclear track plasma mainly with CO 2 and H 2O or H 2CO 3. The positive molecular hybrid ions formed are identified as organic species like COH +, COOH +, CH n = 1–3 +, H n = 1,2 COOH + and cluster ions. Similarly, the negative primarily ions O − and OH − formed by electron capture produce negative hybrid ions such as (CO 2) n OH −, the four ions (CO 4H m = 0–3 ) − and the corresponding cluster ions. By far, most of the molecular ions have been formed by one-step reactions; exceptions are eventually the CO 4H m − ions created by a reaction between CO 3 − and water molecules. An intense mass line corresponding to HCO 3 + has been observed, but not the one due to formaldehyde ion. Weak signals of ionic ketene, hydrogen peroxide and carbonic acid were seen.

Multiple excited state modification in InAs/InGaAs quantum dot structures at high excitation power

This paper presents the investigation of photoluminescence (PL), connected with ground (GS) and four excited states (1–4-ES) in highly uniform self-assembled InAs quantum dots (QDs) embedded into the In0.15Ga0.8As layers, using variable pumping power (P) in the range 10–1000W/cm2 at the temperature 12K. The peak positions of GS and ES emission bands depend on an excitation light power. The energy differences between GS and 1–4ES optical transitions are not equidistant and equal to 48.8, 46.5, 40.3 and 33.4meV, respectively, at highest power level. It was shown that many-particle effects in such high populated QDs is essential and the exchange/correlation and direct Coulomb contributions do not vanish in the investigated strong confined QDs.

Intramolecular energy transfer through charge transfer state in lanthanide compounds: A theoretical approach

A theoretical approach for the intramolecular energy transfer process involving the ligand-to-metal charge transfer (LMCT) state in lanthanide compounds is developed. Considering a two-electron interaction, both the direct Coulomb and exchange interactions are taken into account, leading to expressions from which selection rules may be derived and transfer rates may be calculated. These selection rules show that the direct Coulomb and exchange mechanisms are complementary, in the same way as obtained in previous works for the case of ligand-lanthanide ion energy transfer processes. An important result from numerical estimates is that the channel ligand-LMCT state is by far the dominant case, leading to transfer rates higher than for the channel lanthanide ion-LMCT state by several orders of magnitude. The analysis of the emission quantum yield as a function of the relative energy position of the LMCT state in a typical Eu(3+) compound allows the identification of two quenching regions, the most pronounced one occurring close to the lower ligand triplet level.

The apparent attraction between like-charged particles next to an oppositely charged planar surface

The analytic solution of the Poisson equation for a system of two identical charges near a uniformly charged plane is used to understand the phenomenon of an attraction between a pair of charged particles next to an oppositely charged surface that has been observed recently. We found that there is no direct attractive interaction between like charges. We have shown that what is observed experimentally could be interpreted as the ‘apparent attraction’ resulting from the extra force experienced by each charge being in the non-zero plane parallel electric field due to the charged surface. This surface-induced electric field and force are directed to the center of the surface and can oppose the direct Coulomb repulsion between like charges. Both the field and force can be obtained naturally within the framework of classical electrodynamics if the effects of the finite size of a charged surface are taken into account.

Critical currents of ideal quantum Hall superfluids

Filling factor $\nu=1$ bilayer electron systems in the quantum Hall regime have an excitonic-condensate superfluid ground state when the layer separation $d$ is less than a critical value $d_c$. On a quantum Hall plateau current injected and removed through one of the two layers drives a dissipationless edge current that carries parallel currents, and a dissipationless bulk supercurrent that carries opposing currents in the two layers. In this paper we discuss the theory of finite supercurrent bilayer states, both in the presence and in the absence of symmetry breaking inter-layer hybridization. Solutions to the microscopic mean-field equations exist at all condensate phase winding rates for zero and sufficiently weak hybridization strengths. We find, however, that collective instabilities occur when the supercurrent exceeds a critical value determined primarily by a competition between direct and exchange inter-layer Coulomb interactions. The critical current is estimated using a local stability criterion and varies as $(d_c-d)^{1/2}$ when $d$ approaches $d_c$ from below. For large inter-layer hybridization, we find that the critical current is limited by a soliton instability of microscopic origin.

Coulomb excitation and structure of the rotational bands in 231Pa

The level structure of 231Pa was investigated by Coulomb excitation using 148 MeV 32S and 255, 260 and 261 MeV 58Ni projectiles and the NORDBALL detector array. The 1/2[530] ground-state rotational band was excited up to spin 39/2 −. The 3/2[651] band, established up to spin 37/2 +, was observed to be populated apart from direct Coulomb excitation by the de-excitation of the 1/2[530] band. The 1/2[400] band could be followed up to spin 23/2 +. Weak excitation of the bands 5/2[642], 5/2[512], 3/2[532] and 3/2[521] was seen up to spins 11/2 +, 15/2 −, 9/2 − and 9/2 −, respectively. Matrix elements were extracted from the γ-ray yields with the help of the semi-classical Coulomb-excitation least-squares search code GOSIA. The level scheme and transition intensities were analyzed in the framework of the quasiparticle-plus-phonon model including Coriolis-coupling (QPMC).