Effect of interdiffusion and magnetic field on two-electron states in Gaussian-shaped double quantum rings

The effects of interdiffusion and electrons' Coulomb interaction on the energy spectrum in Gaussian-shaped single and double quantum rings in the presence of magnetic field has been considered in the framework of exact diagonalization method. The one-electron energies as functions of magnetic field for different values of diffusion parameter have been obtained. The two-electron energies and electron probability density distributions are obtained as well. It is shown that the energy oscillations which are more pronounced for a single quantum ring, smooth out due to the interdiffusion. The Coulomb interaction transforms the crossings of the two-electron levels to anticrossings and can lead to the appearance of an additional level between the anticrossing levels.     

Zeeman splitting,its specific features,and gyromagnetic ratios for configurations 1<Emphasis Type="Italic">snf</Emphasis> (<Emphasis Type="Italic">n</Emphasis> = 4–10) of the helium atom

The fine structure parameters of configurations 1snf (n = 4–10) with new refined energy values are calculated by the semiempirical method. The emphasis is on the study of the Zeeman structure in order to determine the gyromagnetic ratios of all four the levels of the configuration from the splitting. For this purpose, the matrices of an energy operator with allowance for the interaction between the atom and a magnetic field were diagonalized for all possible values of quantum number M. For each configuration, 17 values of crossing fields of the Zeeman sublevels with ΔМ = ±1, ±2 and the regions of anticrossings with ΔМ = 0 were determined. It is remarkable that, because the levels are closely spaced, anticrossings are observed for each pair of levels in these systems. The regions of linearity of a magnetic field, which are different for different configurations, and the levels in them are established. The g-factors are calculated from the coefficients of an intermediate coupling scheme in a magnetic field that is guaranteed to be linear. They are compared with the analogous values in the absence of a field.     

Anticrossing of spin-split subbands in quasi-one-dimensional wires

In quantum Hall systems, both anticrossings and magnetic phase transitions can occur when opposite-spin Landau levels coincide. Our results indicate that both processes are also possible in quasi-1D quantum wires in an in-plane B field, Bparallel. Crossings of opposite-spin 1D subbands resemble magnetic phase transitions at zero dc source-drain bias, but display anticrossings at high dc bias. Our data also imply that the well-known 0.7 structure may evolve into a spin-hybridized state in finite dc bias.     

Interaction of Wannier–Stark levels in a wide well superlattice

We have investigated the energy spectrum of a superlattice with wide quantum wells under the bias of an electric field perpendicular to the superlattice layers. By using photocurrent spectroscopy, transitions of Wannier–Stark levels for the various electron and hole states are observed, and at low fields, further structures corresponding to miniband edge transitions are found. Various anticrossings could be observed at higher and lower electric fields. The anticrossings at high electric fields are due to energy alignment of different electronic sublevels in adjacent wells. The anticrossing structures at low fields could be interpreted as resonances between intrawell and interwell excitonic Wannier–Stark states with equal sublevel states, where the anticrossing is caused by differences in exciton binding energy. Fitting of transitions and anticrossings was done by using a semi-empirical model and we have extracted relevant fitting parameters like the quantum-confined Stark coefficient, binding energies for the excitonic Wannier–Stark levels and the resonant coupling strength for states involved in the various anticrossing transitions. Finally, insight into the excitonic influences on the coupling of the WS states could be obtained by comparing the fitted parameters for the various transitions.     

Observation of level anticrossing effects in the laser-Stark spectra of POF3

Nonlinear level anticrossing effects have been observed in saturated absorption in transitions in the ν₄ band of POF₃ by the method of laser-Stark spectroscopy. The anticrossings occur as narrow additional absorptions superimposed on the Doppler-broadened transitions. From the resonant fields of the anticrossings, values of A₀-B₀ have been calculated which agree well with values obtained previously. The widths of the anticrossings were found to be independent of sample pressure in the 10- to 50-m Torr range. A procedure for calculating the lineshape for level crossing or level anticrossing that has a greater range of validity than the conventional perturbation theory is described.     

Charged excitons in quantum dots: novel magnetic behavior and Auger processes

We describe theoretically multiply-charged excitons interacting with a continuum of delocalized states. Such excitons exist in relatively shallow quantum dots and have been observed in recent optical experiments on InAs self-assembled dots. The interaction of an exciton and delocalized states occurs via Auger-like processes. To describe the optical spectra, we employ the Anderson-like Hamiltonian by including the interaction between the localized exciton and delocalized states of the wetting layer. In the absence of a magnetic field, the photoluminescence line shapes exhibit interference effects. When a magnetic field is applied, the photoluminescence spectrum demonstrates anticrossings with the Landau levels of the extended states. We show that the magnetic-field behavior of charged excitons is very different to that of diamagnetic excitons in three and two-dimensional systems.     

Calculation of the fields of crossing for the 1s7h configuration of helium

The picture of magnetic level splitting for the 1s7h configuration of helium is calculated with the fine-structure parameters obtained by the semiempirical method. Only the terms linear in the magnetic field strength are taken into account in the matrix elements of the energy operator of interaction of the atom with the magnetic field. In the region of magnetic field strength up to 30 Oe, 4 anticrossings and 73 crossings of the magnetic sublevels were found. Based on the pure $j_1 j_2 m_{j_1 } m_{j_2 } The picture of magnetic level splitting for the 1s7h configuration of helium is calculated with the fine-structure parameters obtained by the semiempirical method. Only the terms linear in the magnetic field strength are taken into account in the matrix elements of the energy operator of interaction of the atom with the magnetic field. In the region of magnetic field strength up to 30 Oe, 4 anticrossings and 73 crossings of the magnetic sublevels were found. Based on the pure coupling between the electron angular momenta, the results obtained are compared to the results calculated using the jj and model representations. __________ Translated from Optika i Spektroskopiya, Vol. 93, No. 3, 2002, pp. 361–364. Original Russian Text Copyright ? 2002 by Anisimova, Zhikhareva, Semenov.     

Fine structure of excitons in InAs/GaAs coupled auantum dots: a sensitive test of electronic coupling

Exciton fine structure in InAs/GaAs coupled quantum dots has been studied by photoluminescence spectroscopy in magnetic fields up to 8 T. Pronounced anticrossings and mixings of optically bright and dark states as functions of magnetic field are seen. A theoretical treatment of the mixing of the excitonic states has been developed, and it traces observed features to structural asymmetries. These results provide direct evidence for coherent coupling of excitons in quantum dot molecules.     

The suppression of magnetic level coincidences in tilted magnetic fields in the HgTe quantum well as a consequence of electronic phase transitions under quantum hall effect conditions

Spin level coincidences in the HgTe quantum well observed in comparatively weak tilted magnetic fields were found to transform into anticrossings, and gaps formed in anticrossings nonmonotonically depended on the level filling factor. This behavior was indicative of phase transitions into a quantum Hall ferromagnet state.     

Excitonic influences on the anticrossing gap of Wannier–Stark levels in a wide well superlattice

In this paper we compare different theoretical calculations with experimental data of anticrossings of a wide well semiconductor superlattice in the Wannier–Stark regime. Calculations have been performed by using a variation method and a numerical method. For the variation method we used different approaches to minimize energy. With the numerical model, continuum states were calculated and anticrossing of 1s states with continuum states were found. Calculations have been performed for h1e1(0)–h1e2( − 1) and h1e1(1)–h1e2(0) anticrossings and are compared with data from photocurrent experiments.     

Crossings and Anticrossings of Unbound States

We investigate the characteristic crossings and anticrossings of energies and widths of a doublet of resonances, observed in the vicinity of, and at a degeneracy of unbound states, when the control parameters of the system are varied. This characteristic behavior is explained in terms of the local, topological structure of the surfaces that represent the complex energy eigenvalues in parameter space in the vicinity of a degeneracy point. In the simple but illustrative case of the scattering of a beam of particles by a double barrier potential well with two regions of trapping, we solved numerically the implicit, transcendental equation that defines the eigenwave numbers of a degenerate isolated doublet of resonances as functions of the real, control parameters of the system. We found that, at a degeneracy of unbound states, the surface representing the resonance eigenwave numbers as functions of the control parameters has an algebraic branch point of rank one. Unfolding the degeneracy point, crossings and anticrossings of energies and widths are obtained as projections of sections of the eigenwave number surfaces.     

Continuous-distribution puddle model for conduction in trilayer graphene

Influence of the weak electric field on the electronic structure of the Fibonacci superlattice is considered. The electric field produces a nonlinear dynamics of the energy spectrum of the aperiodic superlattice. Mechanism of the nonlinearity is explained in terms of energy levels anticrossings. The multifractal formalism is applied to investigate the effect of weak electric field on the statistical properties of electronic eigenfunctions. It is shown that the applied electric field does not remove the multifractal character of the electronic eigenfunctions, and that the singularity spectrum remains non-parabolic, however with a modified shape. Changes of the distances between energy levels of neighbouring eigenstates lead to the changes of the inverse participation ratio of the corresponding eigenfunctions in the weak electric field. It is demonstrated, that the local minima of the inverse participation ratio in the vicinity of the anticrossings correspond to discontinuity of the first derivative of the difference between marginal values of the singularity strength. Analysis of the generalized dimension as a function of the electric field shows that the electric field correlates spatial fluctuations of the neighbouring electronic eigenfunction amplitudes in the vicinity of anticrossings, and the nonlinear character of the scaling exponent confirms multifractality of the corresponding electronic eigenfunctions.     

Energy spectrum and quantum magnetotransport in type-II heterojunctions

Specific features of the energy spectrum of a separated type-II heterojunction in an external magnetic field are studied theoretically and experimentally. It is shown that, due to hybridization of the states of the valence band of one semiconductor and the conduction band of the other semiconductor at the heterointerface, there are level anticrossings, which produce quasigaps in the density of states in a nonzero magnetic field. The experimental results of magnetotransport studies for the GaInAsSb/p-InAs quaternary solid solutions with different doping levels are shown to agree well with the results of simulation, and specific features of the energy spectrum of separated type-II heterojunctions are established.     

Influence of spin–orbit interaction,Zeeman effect and light polarisation on the electronic and optical properties of pseudo-elliptic quantum rings under magnetic field

ABSTRACT

We theoretically investigated the influences of the magnetic field and light polarisation on the electronic and optical properties of a GaAs/GaAlAs pseudo-elliptic quantum ring, modelled by an outer ellipsis and an inner circle, in the presence of the Rashba and Dresselhaus spin–orbit interactions and Zeeman effect. We show that Aharonov-Bohm oscillations of the energy spectrum are not affected by the presence of the Zeeman effect alone but, in the presence of Rashba and Dresselhaus spin–orbit couplings, the periodicity of certain levels becomes hardly definite. The Zeeman effect generally enhances/diminishes the separation levels produced by Rashba/Dresselhaus interactions (SOI) and when both types of SOI are considered, the effect depends on their relative strength. The magnetic field can trigger spin-flip for each type of spin–orbit interaction and Zeeman effect or their combination through anticrossings in the energy spectra. Our results reveal that the absorption spectra are very sensitive to the magnetic field and light polarisation. For all polarisations considered, the magnetic field increment leads to the redshift or blueshift of some particular peaks (an effect of this ring geometry) and a better separation of the peaks. The x-polarised light determines spectra with many small, but separated peaks while the circular polarised light leads to spectra with large peaks of high amplitude.     

Zeeman structure of the 1s5g and 1s6g configurations of helium

The splitting of levels of the 1s5g and 1s6g configurations of the helium atom in an external magnetic field (0–160 Oe) is obtained by using the fine-structure parameters calculated semiempirically. For each configuration, more than 50 crossings and three anticrossings of Zeeman sublevels are determined. The calculated splittings are compared with those obtained earlier for the 1snp and 1snd configurations of a neutral helium atom.     

Temporal behavior of an unstable optical cavity

The relaxation of a geometrically unstable Fabry-Perot cavity is theoretically and experimentally investigated. It is observed that the usual sum of the exponential decays of the intensities in the different transverse modes of the cavity is replaced by a more complex behavior. In particular, light couplings into the fundamental mode of the cavity with initial wave excitation factors larger than unity, i.e., larger than in the case of mode-matched injection, are directly observed. The influence of the cavity Fresnel number and of transverse mode crossings and anticrossings on these cavity decays is isolated.     

Effect of spin–orbit coupling on spectral and transport properties of tubular electron gas in InAs nanowires

We constructed the Hamiltonian of spin–orbit splitting for carriers of a tubular electron gas in InAs nanowires. The spectral problem is solved using an exact numerical diagonalization. It is shown that the contribution of k-linear Dresselhaus-like spin–orbit (SO) coupling leads to renormalization of the so-called SO-gaps and appearance of anticrossings in subband spectrum. These features can be detected in ballistic transport.     

Internal rotation in methyl silane by avoided-crossing molecular-beam spectroscopy

The avoided-crossing molecular-beam electric-resonance technique was applied to methyl silane in the ground torsional state. A new type of anticrossing is introduced which breaks the torsional symmetry and obeys the selection rules ΔJ = 0, K = +1 /a3 ?1. For these “barrier” anticrossings, the values of the crossing fields E_c yield directly the internal rotation splittings; the E_c are independent of the difference (A-B) in the rotational constants. Such anticrossings were observed for J from 1 to 6. Studies were also conducted of several “rotational” anticrossings (J, K) = (1, ±1) /a3 (2, 0) for which E_c does depend on (A-B). The normal rotational transition (J, K) = (1, 0) ← (0, 0) was observed in the ground torsional state using the molecular beam spectrometer. The present data on CH₃²⁸SiH₃ were combined with Hirota's microwave spectra and analyzed with the torsion-rotation Hamiltonian including all quartic centrifugal distortion terms. In addition to evaluating B and several distortion constants, determinations were made of the moment of inertia of the methyl top I_α = 3.165(5) amu-Ų, the effective rotational constant A^eff = 56 189.449(32) MHz, and the effective height of the threefold barrier to internal rotation V₃^eff = 592.3359(73) cm^?1. The correlations leading to these two effective constants are discussed and the true values of A and V₃ are determined within certain approximations. For the isotopic species CH₃³⁰SiH₃, barrier and rotational anticrossings were observed. The isotopic changes in A and V₃ were determined, as well as an upper limit to the corresponding change in I_α.     

Floquet spectrum and transport through an irradiated graphene ribbon

Graphene subject to a spatially uniform, circularly polarized electric field supports a Floquet spectrum with properties akin to those of a topological insulator. The transport properties of this system, however, are complicated by the nonequilibrium occupations of the Floquet states. We address this by considering transport in a two-terminal ribbon geometry for which the leads have well-defined chemical potentials, with an irradiated central scattering region. We demonstrate the presence of edge states, which for infinite mass boundary conditions may be associated with only one of the two valleys. At low frequencies, the bulk dc conductivity near zero energy is shown to be dominated by a series of states with very narrow anticrossings, leading to superdiffusive behavior. For very long ribbons, a ballistic regime emerges in which edge state transport dominates.