Search results for "Exciton"
showing 10 items of 317 documents
Dephasing of orbital and spin degrees of freedom in semiconductor quantum dots due to phonons and magnons
2006
Phonon-induced decoherence of orbital degrees of freedom in quantum dots (QDs) (GaAs/InAs) is studied and the relevant time-scales are estimated versus dot dimension. Dephasing of excitons due to acoustic phonons and optical phonons, including enhancement of the effective Frohlich constant caused by localization, is assessed for the state-of-art QDs. Temporal inefficiency of Pauli blocking in QDs due to lattice inertia is additionally predicted. For QD placed in a diluted magnetic semiconductor medium a magnon induced dephasing of spin is estimated in accordance with experimental results for Zn(Mn)Se/CdSe. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Coherent and incoherent phonon processes in artificial atoms
2003
Carrier-phonon interaction in semiconductor quantum dots leads to three classes of phenomena: coherent effects (spectrum reconstruction) due to the nearly-dispersionless LO phonons, incoherent effects (transitions) induced by acoustical phonons and dressing phenomena, related to non-adiabatic, sub-picosecond excitation. Polaron spectra, relaxation times and dressing-related decoherence rates are calculated, in accordance with experiment.
Single scatterings in single artificial atoms: Quantum coherence and entanglement
2003
We employ the quantum-jump approach to study single scatterings in single semiconductor quantum dots. Two prototypical situations are investigated. First, we analyze two-photon emissions from the cascade biexciton decay of a dot where the single-exciton states exhibit a fine-structure splitting. We show that this splitting results for appropriately chosen polarization filters in an oscillatory behavior of two-photon correlations, and carefully examine the proper theoretical description of the underlying scattering processes. Secondly, we analyze the decay of a single-electron charged exciton in a quantum dot embedded in a field effect structure. We show how the quantum properties of the cha…
Singlet ground state magnetism: III. magnetic excitons in antiferromagnetic TbP
1981
The dispersion of the lowest magnetic excitations of the singlet ground state system TbP has been studied in the antiferromagnetic phase by inelastic neutron scattering. The magnetic exchange interaction and the magnetic and the rhombohedral molecular fields have been determined.
Lévy distributions and disorder in excitonic spectra.
2020
We study analytically the spectrum of excitons in disordered semiconductors like transition metal dichalcogenides, which are important for photovoltaic and spintronic applications. We show that ambient disorder exerts a strong influence on the exciton spectra. For example, in such a case, the wellknown degeneracy of the hydrogenic problem (related to Runge–Lenz vector conservation) is lifted so that the exciton energy starts to depend on both the principal quantum number n and orbital l. We model the disorder phenomenologically substituting the ordinary Laplacian in the corresponding Schro¨dinger equation by the fractional one with Le´vy index m, characterizing the degree of disorder. Our v…
Review on up/down conversion materials for solar cell application
2012
The present paper reviews the methods of photon up- and down conversion strategies for improving the efficiency of solar cells. Photons with a lower energy than the band gap will be lost in a normal solar cell. The principle of the up conversion technique is that two or more photons are converted into a photon with energy higher than the band gap energy. High energy photons will lose the energy above the band gap energy limit. Down conversion is a process where a high energy photon is converted into several lower energy photons with energies above the band gap. A description is given of the most common methods and materials for these conversions resulting in higher solar cell efficiencies.
Intravalley spin-flip relaxation dynamics in single-layer WS2
2019
Two-dimensional Transition Metal Dichalcogenides (TMDs) have been widely studied because of the peculiar electronic band structure and the strong excitonic effects [1]. In these materials the large spin-orbit coupling lifts the spin degeneracy of the valence (VB) and the conduction band (CB) giving rise to the A and B interband excitonic transitions. In monolayer WS2, the spins of electrons in the lowest CB and in the highest VB at K/K' point of the Brillouin zone are antiparallel resulting in an intravalley dark exciton state at a lower energy than the bright exciton, see left panel of Fig.1. On the one hand, the presence of dark excitons has been revealed indirectly from the observation o…
THE LIMITING EFFICIENCY OF FOUR-BAND CELLS REVISITED
2014
The limiting theoretical efficiency of four-band solar cells is revisited. In previous work, researchers have looked at the theoretical efficiency of four band cells where the smallest of the three sub-band gaps is closest to the valence band and the largest closest to the conduction band. In this work, limits are calculated also for other possible band configurations. In multi-band cells, photon selectivity can be assured by adjusting the band widths. The present work shows that previous authors have put too rigid constraints on the band structure to achieve spectral selectivity. Relieving these constraints gives a considerably higher limiting efficiency for cells with band width restricti…
Spinorial formulation of the GW-BSE equations and spin properties of excitons in two-dimensional transition metal dichalcogenides
2021
In many paradigmatic materials, such as transition metal dichalcogenides, the role played by the spin degrees of freedom is as important as the one played by the electron-electron interaction. Thus an accurate treatment of the two effects and of their interaction is necessary for an accurate and predictive study of the optical and electronic properties of these materials. Despite the fact that the GW-BSE approach correctly accounts for electronic correlations, the spin-orbit coupling effect is often neglected or treated perturbatively. Recently, spinorial formulations of GW-BSE have become available in different flavors in material-science codes. However, an accurate validation and comparis…
A Novel Method Of Measuring Light Absorption On A Self-Assembled Single Quantum Dot
2005
Abstract. We present a novel method by wich excitonic interband optical transitions within single InAs self-assembled quantum dots can be directly observed in a transmission experiment. Due to the extremely high resolution of the tecnique, individual peaks associated to single exciton absorption resonances in single quantum dots can be spectrally resolved. Using this technique we investigate the oscillator strength, homogeneous linewidth and fine structure splitting in a collection of such individual resonances.