Quasi-bound Electronic States Research Articles

Electronic properties of type-II $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$/GaAs quantum rings for applications in intermediate band solar cells

We present a theoretical analysis of the electronic properties of type-II $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$ /GaAs quantum rings (QRs), from the perspective of applications in intermediate band solar cells (IBSCs). We outline the analytical solution of Schrodinger’s equation for a cylindrical QR of infinite potential depth, and describe the evolution of the QR ground state with QR morphology. Having used this analytical model to elucidate general aspects of the electronic properties of QRs, we undertake multi-band k·p calculations—including strain and piezoelectric effects—for realistic $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$ /GaAs QRs. Our k·p calculations confirm that the large type-II band offsets in $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$ /GaAs QRs provide strong confinement of holes, and further indicate the presence of resonant (quasi-bound) electron states which localise in the centre of the QR. From the perspective of IBSC design the calculated electronic properties demonstrate several benefits, including (i) large hole ionisation energies, mitigating thermionic emission from the intermediate band, and (ii) electron-hole spatial overlaps exceeding those in conventional $$\hbox {GaAs}_{1-x} \hbox {Sb}_{x}$$ /GaAs QDs, with the potential to engineer these overlaps via the QR morphology so as to manage the trade-off between optical absorption and radiative recombination. Overall, our analysis highlights the flexibility offered by the QR geometry from the perspective of band structure engineering, and identifies specific combinations of QR alloy composition and morphology which offer optimised sub-band gap energies for QR-based IBSCs.

Realization of a tunable artificial atom at a supercritically charged vacancy in graphene

The remarkable electronic properties of graphene have fueled the vision of a graphene-based platform for lighter, faster and smarter electronics and computing applications. One of the challenges is to devise ways to tailor its electronic properties and to control its charge carriers. Here we show that a single atom vacancy in graphene can stably host a local charge and that this charge can be gradually built up by applying voltage pulses with the tip of a scanning tunneling microscope (STM). The response of the conduction electrons in graphene to the local charge is monitored with scanning tunneling and Landau level spectroscopy, and compared to numerical simulations. As the charge is increased, its interaction with the conduction electrons undergoes a transition into a supercritical regime 6-11 where itinerant electrons are trapped in a sequence of quasi-bound states which resemble an artificial atom. The quasi-bound electron states are detected by a strong enhancement of the density of states (DOS) within a disc centered on the vacancy site which is surrounded by halo of hole states. We further show that the quasi-bound states at the vacancy site are gate tunable and that the trapping mechanism can be turned on and off, providing a new mechanism to control and guide electrons in graphene

Configuration interaction study on electronic structures and transitional properties of excited states of GeO molecule

GeO molecule, which plays an important role in fabricating integrated optics and semiconductor components, has received much attention. However, the electronic state density of the molecule is very large, and the electric structures and transitional properties of the molecule have not been well investigated. In this work, the 18 Λ -S states correlated to the lowest dissociation limit (Ge(3Pg)+O(3Pg)) are calculated by a complete active space self-consistent field (CASSCF) method, through using the previous Hatree-Fock molecular orbitals as the starting orbitals. Furthermore, we take all configurations in the configuration interaction expansions of the CASSCF wave functions as a reference configuration, and calculate the energies of the 18Λ-S states by a high-level multireference configuration interaction method. The core-valence correlation effect of the 3d orbit of Ge atom, the scalar relativistic effect, and the Davidson correction are taken into consideration in the calculations. On the basis of the calculated potential energy curves of the bound and quasibound electronic states, the spectroscopic constants (Re, Te, ωe, ωeχe, and Be), vibrational energy levels, vibrational wave functions, and Franck-Condon factors (FCFs) are obtained by solving the radical Schrödinger equation. The computed spectroscopic constants of these electronic states are well consistent with previously available experimental results. We calculate the electric dipole moments of electronic states with different bound lengths, and analyze the influences of the variation of electron configuration on the electric dipole moment. The calculated potential energy curves indicate that the adiabatic transition energies of A1Π, 11Σ-, D1Δ, a3Π, a’3Σ+, d3Δ, and e3Σ- sates are located in a range of 26000-37000 cm-1, and the spin-orbit coupling of the states can obviously affect the corresponding vibrational wave functions. With the help of calculated spin-orbit coupling matrix elements, the perturbations of the nearby states to a3Π and A1Π are discussed in detail. Our calculation results indicate that the spin-orbit coupling between A1Π and e3Σ- states has an evident perturbation on the v’> 4 vibrational levels of A1Π, and the v’≥ 0 vibrational levels of a3Π state are perturbed by the crossing states a’3Σ+, d3Δ, e3Σ-, 11Σ-, and D1Δ. On the basis of computed transition dipole moments and FCFs of A1Π-X1Σ+ and A’1Σ+-X1Σ+ transitions, the radiative lifetimes of the six lowest vibrational levels of the two singlet excited states are computed.

A theoretical study of spectroscopy and metastability of the CN 2+ dication

Potential energy curves of low-lying electronic states of the CN 2+ dication and of the electronic ground states of CN + and the neutral CN molecule were calculated using internally contracted multireference CI and the coupled cluster RCCSD(T) methods. Spectroscopic constants and adiabatic excitation energies of 13 quasibound electronic states of the dication were obtained and the energy of charge stripping of CN + and double ionization energy of CN were predicted. Tunneling and spin–orbit induced predissociation lifetimes for the vibrational levels in the low-lying electronic states are presented and the metastability of the dication is discussed.

Classical Molecular Dynamics Model for Coupled Two‐Component Plasmas – Ionization Balance and Time Considerations

AbstractThe dynamic properties of ion‐electron two‐component plasmas (TCP) are studied by using classical molecular dynamics (MD) simulations. There is a variety of time dependent and structural results that MD is able to provide in complement to other methods, e.g., useful micro‐field sequences can be generated. The method deals with some specific difficulties: the mass ratio between ions and electrons enforces very small time‐steps appropriate to follow electrons motion while, ions must move significantly in order to build, self consistently, their spatial structure. This results in expensive simulations. Electron trajectories are trapped and de‐trapped with multiple electron collisions around ions resulting in the occurrence of quasi metastable bound electron states. An analysis of micro‐fields at neutral in a hydrogen plasma reveals the need to consider a complete hierarchy of time scales extended typically over 7 order of magnitude, i.e., from a time‐step: ∼10‐19s, to a time required to obtain statistical averages, ∼10‐11s. In order to extend the MD capabilities in representing real coupled plasmas a classical ionization/recombination process has been implemented allowing to follow the evolution of plasmas involving several ion stages and model the ionization balance. Here again TCP simulations deal with extended time‐scale providing information about relaxation of non equilibrium plasma states (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quasi-bound Electronic States in Multiple Delta-doped Quantum Wells

Multiple delta-doped quantum wells of n-type (Si doped GaAs) and p-type (Be doped GaAs) are studied theoretically looking for quasi-bound electron and hole states. The existence of these states and their strong energy and spatial localizations are demonstrated. The line-width and the mean life time are calculated. The FWHM is less than 10 i11 eV (10 i9 eV) and the corresponding mean life time is greater than 10 i5 s (10 i7 s) for n- (p-) type wells respectively. A strong spatial localization is observed in both cases.

Quasi-bound electronic states in parabolic GaAs/AlGaAs quantum wells and barriers

A full quantum-mechanical description of the quasi-bound states in parabolic quantum wells and barriers is presented. The GaAs/AlGaAs heterostructures are considered within the framework of the semi-empirical sp 3 s * spin dependent tight-binding model and the surface Green function matching method (SGFM). Series of equidistant energy quasi-bound states are observed in the continuous spectrum and their FWHM and mean life time are calculated. Strong energy and spatial localizations are found in correlation with the large mean life times.

Mean lifetimes of quasi-bound electronic states in rectangular GaAs/AlGaAs barriers

In our previous work [S.J. Vlaev, V.M. González Robles, Phys. Stat. Sol.(c) 2 (2005) 3653.] we have developed a numerical scheme to calculate the mean lifetimes of virtual energy levels. In the present study, this scheme is applied to the quasi-bound electronic states in rectangular GaAs/AlGaAs quantum barriers. We have considered rectangular barriers of different widths and heights. The numerical calculations of the energy spectrum, the line shape and the mean lifetime are performed in a semi-empirical sp 3s* tight-binding model including the spin. The SGFM method is applied to treat the interface problem [S. Vlaev, V.R. Velasco, F. García-Moliner, Phys. Rev. B 49 (1994) 11222]. We found that the mean lifetime increases significantly for thin barriers reaching values of nanoseconds and even more in some cases. The strong spatial localizations of the quasi-bound states correspond to their strong energy localizations and large mean lifetimes. The relation between the de Broglie wavelength of the barrier material and the barrier width determines the characteristics of the quasi-bound states for electrons and holes. A qualitative interpretation in terms of electron wave diffraction is suggested.

Mechanism of electron localization in a quantum wire

We show that quasi-bound electron states are formed in a quantum wire as a result ofelectron backscattering in the transition regions between the wire and the electronreservoirs, to which the wire is coupled. The backscattering mechanism is caused byelectron density oscillations arising even in smooth transitions due to the reflection ofelectrons not transmitting through the wire. The quasi-bound states reveal themselves inresonances of the electron transmission probability through the wire. The calculations werecarried out within the Hartree–Fock approximation using quasi-classic wavefunctions.

Mean lifetimes of quasi‐bound electronic states in single rectangular quantum wells

A detailed theoretical study of quasi-bound electronic states in single AlGaAs/GaAs quantum wells is presented. The dependence of the full width at half maxima (FWHM) of the virtual states and their mean lifetime on the well width is revealed for quantum well widths from 10 to 500 monolayers (MLs). Series of strongly localized electronic states are found in the continuum spectra of wide quantum wells and the energy and spatial localizations of these states are investigated. The numerical calculations are performed within the frame-work of a semi-empirical sp3s* tight-binding model including spin. The Surface Green-Function Matching (SGFM) method is applied and a new numerical scheme is proposed to determine the energy position and the line shape of the quasi-bound states. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin-dependent resonant tunneling in semiconductor nanostructures

The spin-dependent quantum transport of electrons in non magnetic III-V semiconductor nanos-tructures is studied theoretically within the envelope function approximation and the Kane model for the bulk. It is shown that an unpolarized beam of conducting electrons can be strongly polarized in zero magnetic field by resonant tunneling across asymmetric double-barrier structures, as an effect of the spin-orbit interaction. The electron transmission probability is calculated as a function of energy and angle of incidence. Specific results for tunneling across lattice matched politype Ga0.47In0.53As / InP/Ga0.47In0.53As / GaAs0.5Sb0.5 / Ga0.47In0.53 As double barrier heterostructures show sharp spin split resonances, corresponding to resonant tunneling through spin-orbit split quasi-bound electron states. The polarization of the transmitted beam is also calculated and is shown to be over 50%.

Three-dimensional scattering matrix simulation of resonant tunnelling via quasi-bound states in vertical quantum dots

This paper presents a numerical simulation technique for quantum-dot-based electronic devices based on the three-dimensional (3D) scattering matrix (S-matrix) theory. Starting from the 3D time-independent Schrödinger equation with scattering boundary conditions, the multi-mode S-matrix and transmission rates are derived and the tunnelling current is calculated based on the global coherent tunnelling model. The present simulation technique is applied for zero-dimensional (0D) resonant tunnelling diodes (RTDs). The effects of a complex mixture of lateral mode conserving and non-conserving tunnelling processes on the I– V characteristics are investigated in terms of multi-mode transmission rates and quasi-bound electronic states at resonance. The simulation is also used for analysing resonant tunnelling (RT) assisted by ionised impurities in a quantum dot. By introducing ionised impurities in the quantum dot region, a new type of RT via single-impurity-induced quasi-bound states is investigated.

Effects of electric field on the quasi-bound electronic surface states confined to the boundary of periodic superlattice structures

The effect of an applied electric field on the electronic surface states confined to the boundary of a novel type of quantum well structure composed of a semi-infinite, periodic superlattice bounded by a homogeneous barrier has been studied. It has been found that the surface states are very much sensitive to the applied electric field. Possible applications of such structure as opto-electronic devices are also discussed.

Consequences of electronic-state mixing on the dynamics of photodissociation of H2 by barrier tunneling.

The effect of electronic state mixing on tunneling dynamics is studied. To this aim the theoretical description of the differential photodissociation cross section is extended to include rotational and vibrational couplings. The mixing of bound and quasibound electronic states is reflected in resonance widths. The anisotropy of the photofragments reflects mixing of dissociative continua. It is shown that inclusion of these nonadiabatic couplings is required to reproduce the recently reported [Siebbeles, Schins, Los, and Glass-Maujean, Phys. Rev. A 44, 343 (1991)] differential photodissociation cross section of ${\mathrm{H}}_{2}$ for excitation around the ${\mathit{scri}}^{3}$${\mathrm{\ensuremath{\Pi}}}_{\mathit{g}}$ (v'=5, N'=1,2) resonances. Nonclassical photofragment anisotropies are found for coherent excitation of different rotational resonances (v'=5, N'=1, and N'=2) and of different rovibrational resonances (v'=5, N'=1 and v'=4, N'=2). The latter is strongly connected to the molecular excitation mechanism and has therefore no parallel in collision experiments.

Ab initio characterization of NF2+

Eight electronic states of the NF2+ ion have been calculated by a second-order CI (SOCI) method employing complete active space (CASSCF) reference wavefunctions and large triple zeta plus polarization basis sets. In analogy to O2+2, there are two low-lying quasibound electronic states. The calculated position of the 3 Pi state at 5.7 eV above the X 1 Sigma + ground state agrees with the recent experimental observation of Rogers et al. (1990). The spectroscopic data of the ground state agree reasonably well with the calculations of Wong et al (1989) but are believed to be more accurate.

Quasibound electronic states of CO2+

Quasibound electronic states of the CO2+ ion are investigated by means of photoion-photon-of-fluorescence coincidence (PIFCO) and ion kinetic energy spectroscopy (IKES) methods. Energies of the lowest four electronic states (3 Pi , 1 Sigma +), 3 Pi , 3 Sigma + and 1 Pi are accurately measured at 41.25+or-0.05, 41.3+or-0.1, 41.8+or-0.1 and 43.8+or-0.1 eV respectively. The latter two states dissociate in less than 1 mu s, while the 3 Pi and 1 Sigma 1 states can survive for a few mu s. Furthermore, the lack of detected fluorescence is discussed and an upper bound is derived for the fluorescence efficiency of CO2+.

Erratum: Variational Methods in the Wave Operator Formalism. A Unified Treatment for Bound and Quasibound Electronic and Molecular States

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Variational Methods in the Wave Operator Formalism. A Unified Treatment for Bound and Quasibound Electronic and Molecular States

The wave operator formalism of Löwdin, heretofore used to describe states belonging to the discrete energy spectrum, has been extended to unify the treatment of bound and quasibound (or decaying) states. The approach makes use of an arbitrary reference function that may be chosen to approximate the physical state at short distances. Real and complex eigenvalues are obtained, respectively, for bound and quasibound states from an implicit equation, valid for all coupling strengths. Resonance positions and linewidths are explicitly independent of energy. Variational principles of the Lippmann—Schwinger type are presented which apply to states with either bound-state or decay boundary conditions. Particular cases leading to minimization or maximization principles for real energies are discussed. The formalism is considered in connection with decaying electronic states of atoms and decaying molecular states.