Phonon-assisted relaxation between hole states in quantum dot molecules

We study theoretically phonon-assisted relaxation and inelastic tunneling of holes in a double quantum dot. We derive hole states and relaxation rates from kp Hamiltonians and show that there is a finite distance between the dots where lifetimes of hole states are very long which is related to vanishing tunnel coupling. We show also that the light hole admixture to hole states can considerably affect the hole relaxation rates even though its magnitude is very small

Origin of giant bulk Rashba splitting: Application to BiTeI

We theoretically propose the necessary conditions for realization of giant Rashba splitting in bulk systems. In addition to (i) the large atomic spin-orbit interaction in an inversion-asymmetric system, the following two conditions are further required; (ii) a narrow band gap, and (iii) the presence of top valence and bottom conduction bands of symmetrically the same character. As a representative example, using the first principles calculations, the recently discovered giant bulk Rashba splitting system BiTeI is shown to fully fulfill all these three conditions. Of particular importance, by predicting the correct crystal structure of BiTeI, different from what has been believed thus far, the third criterion is demonstrated to be met by a negative crystal field splitting of the top valence bands.Comment: 3 figure

Spin-phonon coupling in single Mn doped CdTe quantum dot

The spin dynamics of a single Mn atom in a laser driven CdTe quantum dot is addressed theoretically. Recent experimental results\cite{Le-Gall_PRL_2009,Goryca_PRL_2009,Le-Gall_PRB_2010}show that it is possible to induce Mn spin polarization by means of circularly polarized optical pumping. Pumping is made possible by the faster Mn spin relaxation in the presence of the exciton. Here we discuss different Mn spin relaxation mechanisms. First, Mn-phonon coupling, which is enhanced in the presence of the exciton. Second, phonon-induced hole spin relaxation combined with carrier-Mn spin flip coupling and photon emission results in Mn spin relaxation. We model the Mn spin dynamics under the influence of a pumping laser that injects excitons into the dot, taking into account exciton-Mn exchange and phonon induced spin relaxation of both Mn and holes. Our simulations account for the optically induced Mn spin pumping.Comment: 17 pages, 11 figures, submitted to PR

Dynamics of a nanowire superlattice in an ac electric field

With a one-band envelope function theory, we investigate the dynamics of a finite nanowire superlattice driven by an ac electric field by solving numerically the time-dependent Schroedinger equation. We find that for an ac electric field resonant with two energy levels located in two different minibands, the coherent dynamics in nanowire superlattices is much more complex as compared to the standard two-level description. Depending on the energy levels involved in the transitions, the coherent oscillations exhibit different patterns. A signature of barrier-well inversion phenomenon in nanowire superlattices is also obtained.Comment: 14 pages, 4 figure

Optoelectronics of Inverted Type-I CdS/CdSe Core/Crown Quantum Ring

Inverted type-I heterostructure core/crown quantum rings (QRs) are quantum-efficient luminophores, whose spectral characteristics are highly tunable. Here, we study the optoelectronic properties of type-I core/crown CdS/CdSe QRs in the zincblende phase - over contrasting lateral size and crown width. For this we inspect their strain profiles, transition energies, transition matrix elements, spatial charge densities, electronic bandstructure, band-mixing probabilities, optical gain spectra, maximum optical gains and differential optical gains. Our framework uses an effective-mass envelope function theory based on the 8-band k$\cdot$p method employing the valence force field model for calculating the atomic strain distributions. The gain calculations are based on the density-matrix equation and take into consideration the excitonic effects with intraband scattering. Variations in the QR lateral size and relative widths of core and crown (ergo the composition) affect their energy levels, band-mixing probabilities, optical transition matrix elements, emission wavelengths/intensity, etc. The optical gain of QRs is also strongly dimension and composition dependent with further dependency on the injection carrier density causing band-filling effect. They also affect the maximum and differential gain at varying dimensions and compositions.Comment: Published in AIP Journal of Applied Physics (11 pages, 7 figures

Electronic structure of silicon-based nanostructures

We have developed an unifying tight-binding Hamiltonian that can account for the electronic properties of recently proposed Si-based nanostructures, namely, Si graphene-like sheets and Si nanotubes. We considered the $sp^3s^*$ and $sp^{3}$ models up to first- and second-nearest neighbors, respectively. Our results show that the Si graphene-like sheets considered here are metals or zero-gap semiconductors, and that the corresponding Si nanotubes follow the so-called Hamada's rule [Phys. Rev. Lett. {\bf 68}, 1579 1992]. Comparison to a recent {\it ab initio} calculation is made.Comment: 12 pages, 6 Figure

Barriers to Workforce Re-Entry among Single Mothers: Insights from Urban Areas in Sarawak, Malaysia

Life can be challenging for single mothers unable to secure employment. However, little is known about the barriers that hinder single mothers’ re-entry into the workforce, more so in the context of urban areas in developing countries, the landscape of which differs from that of rural areas and developed countries. To address this gap, a study was conducted to investigate the barriers that single mothers encounter when re-entering the workforce in urban areas in developing countries, using a survey of 373 single mothers in urban Sarawak, Malaysia, as a case. The findings suggest that single mothers in urban areas considered family reasons as the main barrier to workforce re-entry, followed by work-life balance, personal, and workplace reasons, which remained true regardless of whether single mothers intend or do not intend to return to the workforce. Implications for theory and practice, limitations, and future research directions conclude the article