Behavior of excitonic levels in symmetric and asymmetric double quantum wells in a magnetic field

We studied theoretically the excitonic energy levels and the optical absorption spectra for double quantum wells, both symmetric and asymmetric, in the presece of an homogeneous magnetic field. Within the effective mass approach, we expanded the excitonic wave function, in an orthogonal basis formed by products of electron and hole wave functions in the growth direction z, and one particle solutions of the magnetic Hamiltonian in the x-y plane. We applied our method to the case of Al_xGa_{1-x}As, for which we showed how the exciton wave function vary, and how the basis functions are mixed in a non trivial way by the effect of the Coulomb potential. By taking into account all the mixing between the elements in our base, we get anti-crossing between excited excitonic states not reported previously.Comment: 6 pages, 6 figures, revtex, submitted to Phys Rev

Sudjelovanje potomaka hrvatskih iseljenika u političkom životu Općinski izbori u čileanskoj regiji Magallanes u periodu od 1992. do 2012. godine

The paper studies the participation of descendants of Croatian immigrants in the Region of Magallanes and Chilean Antarctica, after more than a century from the arrival of these to the region. The study focuses on the descendants as candidates for mayor and councillor positions in the ten municipalities of the Region, in the period between 1992 and 2012, within six elections. The participation is analyzed in relation to other candidates, by municipality, their election results, the belonging of the candidates to different parties and electoral pacts, and the participation of women descendants.Članak analizira sudjelovanje potomaka hrvatskih iseljenika u političkom životu čileanske regije Magallanes i čileanske Antarktike više od stoljeća nakon njihovog dolaska u Regiju. U fokusu su istraživanja potomci iseljenika kao kandidati za položaje načelnika i članova općinskih vijeća u deset općina Regije na izborima u periodu od 1992. do 2012. godine. Analiza obuhvaća učešće Hrvata u odnosu na ostale kandidate po općinama, njihove izborne rezultate, pripadnost različitim političkim strankama i izbornim paktovima kao i participaciju žena

Magnetoabsorption spectra of intraexcitonic transitions in GaAs-(Ga,Al)As semiconductor quantum wells

ABSTRACT: We present a theoretical study, within the effective-mass approximation, of the magnetoabsorption spectra of intraexcitonic terahertz transitions of light-hole and heavy-hole confined magnetoexcitons in GaAs-~Ga,Al!As quantum wells. The semiconductor quantum wells are studied under magnetic fields applied in the growth direction of the semiconductor heterostructure. The various magnetoexciton states are obtained in the effective-mass approximation by an expansion of the exciton-envelope wave functions in terms of products of hole and electron quantum-well states with appropriate Gaussian functions for the various excitonic states. Intramagnetoexciton transitions are theoretically studied by exciting the allowed excitonic transitions with s1 ~or s2! far-infrared radiation circularly polarized in the plane of the GaAs-~Ga,Al!As quantum well. Theoretical results are obtained for the intramagnetoexciton transition energies and magneto-absorption spectra associated with excitations from 1s-like to 2p6 , and 3p6-like magnetoexciton states, and found in overall agreement with optically detected resonance measurements

Magnetic-field effects on transport in carbon nanotube junctions

Here we address a theoretical study on the behaviour of electronic states of heterojunctions and quantum dots based on carbon nanotubes under magnetic fields. Emphasis is put on the analysis of the local density of states, the conductance, and on the characteristic curves of current versus voltage. The heterostructures are modeled by joining zigzag tubes through single pentagon-heptagon pair defects, and described within a simple tight binding calculation. The conductance is calculated using the Landauer formula in the Green functions formalism. The used theoretical approach incorporates the atomic details of the topological defects by performing an energy relaxation via Monte Carlo calculation. The effect of a magnetic field on the conductance gap of the system is investigated and compared to those of isolated constituent tubes. It is found that the conductance gap of the studied CNHs exhibits oscillations as a function of the magnetic flux. However, unlike the pristine tubes case, they are not Aharonov-Bohm periodic oscillations

Coulomb interaction effects on the electronic structure of radial polarized excitons in nanorings

The electronic structure of radially polarized excitons in structured nanorings is analyzed, with emphasis in the ground-state properties and their dependence under applied magnetic fields perpendicular to the ring plane. The electron-hole Coulomb attraction has been treated rigorously, through numerical diagonalization of the full exciton Hamiltonian in the non-interacting electron-hole pairs basis. Depending on the relative weight of the kinetic energy and Coulomb contributions, the ground-state of polarized excitons has "extended" or "localized" features. In the first case, corresponding to small rings dominated by the kinetic energy, the ground-state shows Aharonov-Bohm (AB) oscillations due to the individual orbits of the building particles of the exciton. In the localized regime, corresponding to large rings dominated by the Coulomb interaction, the only remaining AB oscillations are due to the magnetic flux trapped between the electron and hole orbits. This dependence of the exciton, a neutral excitation, on the flux difference confirms this feature as a signature of Coulomb dominated polarized excitons. Analytical approximations are provided in both regimens, which accurate reproduce the numerical results.Comment: 9 pages, including 6 figure

Persistent Currents in Small, Imperfect Hubbard Rings

We have done a study with small, imperfect Hubbard rings with exact diagonalization. The results for few-electron rings show, that the imperfection, whether localized or not, nearly always decrease, but can also \emph{increase} the persistent current, depending on the character of the imperfection and the on-site interaction. The calculations are generally in agreement with more specialized studies. In most cases the electron spin plays an important role.Comment: 6 pages, 4 figure

Finite Size Effects on the Optical Transitions in Quantum Rings under a Magnetic Field

We present a theoretical study of the energy spectrum of single electron and hole states in quantum dots of annular geometry under a high magnetic field along the ring axis in the frame of uncorrelated electron-hole theory. We predict the periodic disappearance of the optical emission of the electron-hole pair as the magnetic field increases, as a consequence of the finite height of the barriers. The model has been applied to semiconductor rings of various internal and external radii, giving as limiting cases the disk and antidot.Comment: 22 pages, 7 figures, LaTeX, published in Eur. Phys. J. B 53, 99-108 (2006

Conductance and persistent current of a quantum ring coupled to a quantum wire under external fields

The electronic transport of a noninteracting quantum ring side-coupled to a quantum wire is studied via a single-band tunneling tight-binding Hamiltonian. We found that the system develops an oscillating band with antiresonances and resonances arising from the hybridization of the quasibound levels of the ring and the coupling to the quantum wire. The positions of the antiresonances correspond exactly to the electronic spectrum of the isolated ring. Moreover, for a uniform quantum ring the conductance and the persistent current density were found to exhibit a particular odd-even parity related with the ring-order. The effects of an in-plane electric field was also studied. This field shifts the electronic spectrum and damps the amplitude of the persistent current density. These features may be used to control externally the energy spectra and the amplitude of the persistent current.Comment: Revised version, 7 pages and 9 figures. To appear in Phys. Rev.