Geometrical effects on the optical properties of quantum dots doped with a single magnetic atom

The emission spectra of individual self-assembled quantum dots containing a single magnetic Mn atom differ strongly from dot to dot. The differences are explained by the influence of the system geometry, specifically the in-plane asymmetry of the quantum dot and the position of the Mn atom. Depending on both these parameters, one has different characteristic emission features which either reveal or hide the spin state of the magnetic atom. The observed behavior in both zero field and under magnetic field can be explained quantitatively by the interplay between the exciton-manganese exchange interaction (dependent on the Mn position) and the anisotropic part of the electron-hole exchange interaction (related to the asymmetry of the quantum dot).Comment: 5 pages, 5 figures, to be published in Phys. Rev. Let

Theory of excitons in cubic III-V semiconductor GaAs, InAs and GaN quantum dots: fine structure and spin relaxation

Exciton fine structures in cubic III-V semiconductor GaAs, InAs and GaN quantum dots are investigated systematically and the exciton spin relaxation in GaN quantum dots is calculated by first setting up the effective exciton Hamiltonian. The electron-hole exchange interaction Hamiltonian, which consists of the long- and short-range parts, is derived within the effective-mass approximation by taking into account the conduction, heavy- and light-hole bands, and especially the split-off band. The scheme applied in this work allows the description of excitons in both the strong and weak confinement regimes. The importance of treating the direct electron-hole Coulomb interaction unperturbatively is demonstrated. We show in our calculation that the light-hole and split-off bands are negligible when considering the exciton fine structure, even for GaN quantum dots, and the short-range exchange interaction is irrelevant when considering the optically active doublet splitting. We point out that the long-range exchange interaction, which is neglected in many previous works, contributes to the energy splitting between the bright and dark states, together with the short-range exchange interaction. Strong dependence of the optically active doublet splitting on the anisotropy of dot shape is reported. Large doublet splittings up to 600 $\mu$eV, and even up to several meV for small dot size with large anisotropy, is shown in GaN quantum dots. The spin relaxation between the lowest two optically active exciton states in GaN quantum dots is calculated, showing a strong dependence on the dot anisotropy. Long exciton spin relaxation time is reported in GaN quantum dots. These findings are in good agreement with the experimental results.Comment: 22+ pages, 16 figures, several typos in the published paper are corrected in re

Photoemission study of poly(dA)-poly(dT) DNA : Experimental and theoretical approach to the electronic density of states

We present results of an ultraviolet photoemission spectroscopy study of artificially synthesized poly(dA)-poly(dT) DNA molecules on $p$-type Si substrates. For comparison, we also present the electronic density of states (DOS) calculated using an \emph{ab initio} tight-binding method based on density-functional theory (DFT). Good agreement was obtained between experiment and theory. The spectra of DNA networks on the Si substrate showed that the Fermi level of the substrate is located in the middle of the band gap of DNA. The spectra of thick ($\sim 70$ nm) DNA films showed a downward shift of $\sim 2$ eV compared to the network samples.Comment: 4 pages, 4 figure

The role of electron-electron scattering in spin transport

We investigate spin transport in quasi 2DEG formed by III-V semiconductor heterojunctions using the Monte Carlo method. The results obtained with and without electron-electron scattering are compared and appreciable difference between the two is found. The electron-electron scattering leads to suppression of Dyakonov-Perel mechanism (DP) and enhancement of Elliott-Yafet mechanism (EY). Finally, spin transport in InSb and GaAs heterostructures is investigated considering both DP and EY mechanisms. While DP mechanism dominates spin decoherence in GaAs, EY mechanism is found to dominate in high mobility InSb. Our simulations predict a lower spin relaxation/decoherence rate in wide gap semiconductors which is desirable for spin transport.Comment: to appear in Journal of Applied Physic

Topological insulators with SU(2) Landau levels

We construct continuum models of 3D and 4D topological insulators by coupling spin-1/2 fermions to an SU(2) background gauge field, which is equivalent to a spatially dependent spin-orbit coupling. Higher dimensional generalizations of flat Landau levels are obtained in the Landau-like gauge. The 2D helical Dirac modes with opposite helicities and 3D Weyl modes with opposite chiralities are spatially separated along the third and fourth dimensions, respectively. Stable 2D helical Fermi surfaces and 3D chiral Fermi surfaces appear on open boundaries, respectively. The charge pumping in 4D Landau level systems shows quantized 4D quantum Hall effect.Comment: Accepted by Phys. Rev. Let

Piezo-Magneto-Electric Effects in p-Doped Semiconductors

We predict the appearance of a uniform magnetization in strained three dimensional p-doped semiconductors with inversion symmetry breaking subject to an external electric field. We compute the magnetization response to the electric field as a function of the direction and magnitude of the applied strain. This effect could be used to manipulate the collective magnetic moment of hole mediated ferromagnetism of magnetically doped semiconductors.Comment: 4 pages, 3 figure

Two-electron state in a disordered 2D island: pairing caused by the Coulomb repulsion

We show the existence of bound two-electron states in an almost depleted two-dimensional island. These two-electron states are carried by special compact configurations of four single-electron levels. The existence of these states does not require phonon mediation, and is facilitated by the disorder-induced potential relief and by the electron-electron repulsion only. The density of two-electron states is estimated and their evolution with the magnetic field is discussed.Comment: 9 pages, 1 fi

D'yakonov-Perel' spin relaxation for degenerate electrons in the electron-hole liquid

We present an analytical study of the D'yakonov-Perel' spin relaxation time for degenerate electrons in a photo-excited electron-hole liquid in intrinsic semiconductors exhibiting a spin-split band structure. The D'yakonov-Perel' spin relaxation of electrons in these materials is controlled by electron-hole scattering, with small corrections from electron-electron scattering and virtually none from electron-impurity scattering. We derive simple expressions (one-dimensional and two-dimensional integrals respectively) for the effective electron-hole and electron-electron scattering rates which enter the spin relaxation time calculation. The electron-hole scattering rate is found to be comparable to the scattering rates from impurities in the electron liquid - a common model for n-type doped semiconductors. As the density of electron-hole pairs decreases (within the degenerate regime), a strong enhancement of the scattering rates and a corresponding slowing down of spin relaxation is predicted due to exchange and correlation effects in the electron-hole liquid. In the opposite limit of high density, the original D'yakonov-Perel' model fails due to decreasing scattering rates and is eventually superseded by free precession of individual quasiparticle spins.Comment: 16 pages, 5 figure

Spin- and energy relaxation of hot electrons at GaAs surfaces

The mechanisms for spin relaxation in semiconductors are reviewed, and the mechanism prevalent in p-doped semiconductors, namely spin relaxation due to the electron-hole exchange interaction, is presented in some depth. It is shown that the solution of Boltzmann-type kinetic equations allows one to obtain quantitative results for spin relaxation in semiconductors that go beyond the original Bir-Aronov-Pikus relaxation-rate approximation. Experimental results using surface sensitive two-photon photoemission techniques show that the spin relaxation-time of electrons in p-doped GaAs at a semiconductor/metal surface is several times longer than the corresponding bulk spin relaxation-times. A theoretical explanation of these results in terms of the reduced density of holes in the band-bending region at the surface is presented.Comment: 33 pages, 12 figures; earlier submission replaced by corrected and expanded version; eps figures now included in the tex

Energy-resolved electron-spin dynamics at surfaces of p-doped GaAs

Electron-spin relaxation at different surfaces of p-doped GaAs is investigated by means of spin, time and energy resolved 2-photon photoemission. These results are contrasted with bulk results obtained by time-resolved Faraday rotation measurements as well as calculations of the Bir-Aronov-Pikus spin-flip mechanism. Due to the reduced hole density in the band bending region at the (100) surface the spin-relaxation time increases over two orders of magnitude towards lower energies. At the flat-band (011) surface a constant spin relaxation time in agreement with our measurements and calculations for bulk GaAs is obtained.Comment: 6 pages, 4 figure