Comment on "Froehlich Mass in GaAs-Based Structures"

The results of recent measurements of the cyclotron resonance (CR) spectra for a GaAs quantum well are interpreted in terms of the resonant magnetopolaron effect. Owing to this effect, the CR peaks split near the TO-phonon frequency and also change their positions with respect to those obtained without electron-phonon interaction. The theoretical peak positions of the CR spectra calculated within the many-polaron approach compare well with experimental data, as distinct from the CR energies calculated without electron-phonon interaction, which show no particular features in the region of the optical-phonon frequencies. We conclude that the Froehlich polaron concept is valid and even necessary to interpret the CR spectra of quantum wells.Comment: 1 page, 1 figure, E-mail addresses: [email protected], [email protected]

Suppression of the D'yakonov-Perel' spin relaxation mechanism for all spin components in [111] zincblende quantum wells

We apply the D'yakonov-Perel' (DP) formalism to [111]-grown zincblende quantum wells (QWs) to compute the spin lifetimes of electrons in the two-dimensional electron gas. We account for both bulk and structural inversion asymmetry (Rashba) effects. We see that, under certain conditions, the spin splitting vanishes to first order in k, which effectively suppresses the DP spin relaxation mechanism for all spin components. We predict extended spin lifetimes as a result, giving rise to the possibility of enhanced spin storage. We also study [110]-grown QWs, where the effect of structural inversion asymmetry is to augment the spin relaxation rate of the component perpendicular to the well. We derive analytical expressions for the spin lifetime tensor and its proper axes, and see that they are dependent on the relative magnitude of the BIA- and SIA-induced splittings.Comment: v1: 5 pages, 2 figures, submitted to PRL v2: added 1 figure and supporting content, PRB forma

Anomalous Suppression of Valley Splittings in Lead Salt Nanocrystals without Inversion Center

Atomistic sp3d5s* tight-binding theory of PbSe and PbS nanocrystals is developed. It is demonstrated, that the valley splittings of confined electrons and holes strongly and peculiarly depend on the geometry of a nanocrystal. When the nanocrystal lacks a microscopic center of inversion and has T_d symmetry, the splitting is strongly suppressed as compared to the more symmetric nanocrystals with O_h symmetry, having an inversion center.Comment: 5 pages, 4 figures, 1 tabl

Theory of laser-induced demagnetization at high temperatures

Laser-induced demagnetization is theoretically studied by explicitly taking into account interactions among electrons, spins and lattice. Assuming that the demagnetization processes take place during the thermalization of the sub-systems, the temperature dynamics is given by the energy transfer between the thermalized interacting baths. These energy transfers are accounted for explicitly through electron-magnons and electron-phonons interaction, which govern the demagnetization time scale. By properly treating the spin system in a self-consistent random phase approximation, we derive magnetization dynamic equations for a broad range of temperature. The dependence of demagnetization on the temperature and pumping laser intensity is calculated in detail. In particular, we show several salient features for understanding magnetization dynamics near the Curie temperature. While the critical slowdown in dynamics occurs, we find that an external magnetic field can restore the fast dynamics. We discuss the implication of the fast dynamics in the application of heat assisted magnetic recording.Comment: 11 Pages, 7 Figure

Exciton spin decay modified by strong electron-hole exchange interaction

We study exciton spin decay in the regime of strong electron-hole exchange interaction. In this regime the electron spin precession is restricted within a sector formed by the external magnetic field and the effective exchange fields triggered by random spin flips of the hole. Using Hanle effect measurements, we demonstrate that this mechanism dominates our experiments in CdTe/(Cd,Mg)Te quantum wells. The calculations provide a consistent description of the experimental results, which is supported by independent measurements of the parameters entering the model.Comment: 5 pages, 3 figure

Temperature dependence of D'yakonov-Perel' spin relaxation in zinc blende semiconductor quantum structures

The D'yakonov-Perel' mechanism, intimately related to the spin splitting of the electronic states, usually dominates the spin relaxation in zinc blende semiconductor quantum structures. Previously it has been formulated for the two limiting cases of low and high temperatures. Here we extend the theory to give an accurate description of the intermediate regime which is often relevant for room temperature experiments. Employing the self-consistent multiband envelope function approach, we determine the spin splitting of electron subbands in n-(001) zinc blende semiconductor quantum structures. Using these results we calculate spin relaxation rates as a function of temperature and obtain excellent agreement with experimental data.Comment: 9 pages, 4 figure

Phonitons as a sound-based analogue of cavity quantum electrodynamics

A quantum mechanical superposition of a long-lived, localized phonon and a matter excitation is described. We identify a realization in strained silicon: a low-lying donor transition (P or Li) driven solely by acoustic phonons at wavelengths where high-Q phonon cavities can be built. This phonon-matter resonance is shown to enter the strongly coupled regime where the "vacuum" Rabi frequency exceeds the spontaneous phonon emission into non-cavity modes, phonon leakage from the cavity, and phonon anharmonicity and scattering. We introduce a micropillar distributed Bragg reflector Si/Ge cavity, where Q=10^5-10^6 and mode volumes V<=25*lambda^3 are reachable. These results indicate that single or many-body devices based on these systems are experimentally realizable.Comment: Published PRL version. Note that the previous arXiv version has more commentary, figures, etc. Also see http://research.tahan.com

Spin relaxation at the singlet-triplet crossing in a quantum dot

We study spin relaxation in a two-electron quantum dot in the vicinity of the singlet-triplet crossing. The spin relaxation occurs due to a combined effect of the spin-orbit, Zeeman, and electron-phonon interactions. The singlet-triplet relaxation rates exhibit strong variations as a function of the singlet-triplet splitting. We show that the Coulomb interaction between the electrons has two competing effects on the singlet-triplet spin relaxation. One effect is to enhance the relative strength of spin-orbit coupling in the quantum dot, resulting in larger spin-orbit splittings and thus in a stronger coupling of spin to charge. The other effect is to make the charge density profiles of the singlet and triplet look similar to each other, thus diminishing the ability of charge environments to discriminate between singlet and triplet states. We thus find essentially different channels of singlet-triplet relaxation for the case of strong and weak Coulomb interaction. Finally, for the linear in momentum Dresselhaus and Rashba spin-orbit interactions, we calculate the singlet-triplet relaxation rates to leading order in the spin-orbit interaction, and find that they are proportional to the second power of the Zeeman energy, in agreement with recent experiments on triplet-to-singlet relaxation in quantum dots.Comment: 29 pages, 14 figures, 1 tabl

Effect of strain on stripe phases in the Quantum Hall regime

Spontaneous breaking of rotational symmetry and preferential orientation of stripe phases in the quantum Hall regime has attracted considerable experimental and theoretical effort over the last decade. We demonstrate experimentally and theoretically that the direction of high and low resistance of the two-dimensional (2D) hole gas in the quantum Hall regime can be controlled by an external strain. Depending on the sign of the in-plane shear strain, the Hartree-Fock energy of holes or electrons is minimized when the charge density wave (CDW) is oriented along [110] or [1-10] directions. We suggest that shear strains due to internal electric fields in the growth direction are responsible for the observed orientation of CDW in pristine electron and hole samples.Comment: 10 pages, 3 figure