Triplet-singlet relaxation in semiconductor single and double quantum dots

We study the triplet-singlet relaxation in two-electron semiconductor quantum dots. Both single dots and vertically coupled double dots are discussed. In our work, the electron-electron Coulomb interaction, which plays an important role in the electronic structure, is included. The spin mixing is caused by spin-orbit coupling which is the key to the triplet-singlet relaxation. We show that the selection rule widely used in the literature is incorrect unless near the crossing/anticrossing point in single quantum dots. The triplet/singlet relaxation in double quantum dots can be markedly changed by varying barrier height, inter-dot distance, external magnetic field and dot size.Comment: 7 pages, 4 figures, PRB in pres

Asymptotically self-similar propagation of the spherical ionization waves

It is shown that a new type of the self-similar spherical ionization waves may exist in gases. All spatial scales and the propagation velocity of such waves increase exponentially in time. Conditions for existence of these waves are established, their structure is described and approximate analytical relationships between the principal parameters are obtained. It is notable that spherical ionization waves can serve as the simplest, but structurally complete and physically transparent model of streamer in homogeneous electric field.Comment: Corrected typos, the more precise formulas are obtaine

Spin polarization decay in spin-1/2 and spin-3/2 systems

We present a general unifying theory for spin polarization decay due to the interplay of spin precession and momentum scattering that is applicable to both spin-1/2 electrons and spin-3/2 holes. Our theory allows us to identify and characterize a wide range of qualitatively different regimes. For strong momentum scattering or slow spin precession we recover the D'yakonov-Perel result, according to which the spin relaxation time is inversely proportional to the momentum relaxation time. On the other hand, we find that, in the ballistic regime the carrier spin polarization shows a very different qualitative behavior. In systems with isotropic spin splitting the spin polarization can oscillate indefinitely, while in systems with anisotropic spin splitting the spin polarization is reduced by spin dephasing, which is non-exponential and may result in an incomplete decay of the spin polarization. For weak momentum scattering or fast spin precession, the oscillations or non-exponential spin dephasing are modulated by an exponential envelope proportional to the momentum relaxation time. Nevertheless, even in this case in certain systems a fraction of the spin polarization may survive at long times. Finally it is shown that, despite the qualitatively different nature of spin precession in the valence band, spin polarization decay in spin-3/2 hole systems has many similarities to its counterpart in spin-1/2 electron systems.Comment: 4 pages, 1 figure, to appear in Phys. Rev.

Spin orientation of a two-dimensional electron gas by a high-frequency electric field

Coupling of spin states and space motion of conduction electrons due to spin-orbit interaction opens up possibilities for manipulation of the electron spins by electrical means. It is shown here that spin orientation of a two-dimensional electron gas can be achieved by excitation of the carriers with a linearly polarized high-frequency electric field. In (001)-grown quantum well structures excitation with in-plane ac electric field induces orientation of the electron spins along the quantum well normal, with the spin sign and the magnitude depending on the field polarization.Comment: 5 pages, 1 figur

Pauli blockade of the electron spin flip in bulk GaAs

By means of time-resolved optical orientation under strong optical pumping, the k-dependence of the electron spin-flip time (t_sf) in undoped GaAs is experimentally determined. t_sf monotonically decreases by more than one order of magnitude when the electron kinetic energy varies from 2 to 30 meV. At the high excitation densities and low temperatures of the reported experiments the main spin-flip mechanism of the conduction band electrons is the Bir-Aronov-Pikus. By means of Monte-Carlo simulations we evidence that phase-space filling effects result in the blocking of the spin flip, yielding an increase of t_sf with excitation density. These effects obtain values of t_sf up to 30 ns at k=0, the longest reported spin-relaxation time in undoped GaAs in the absence of a magnetic field.Comment: new author added, major changes in section IV (phenomenological model), minor changes throughout the entire manuscrip

Semiclassical theory of weak antilocalization and spin relaxation in ballistic quantum dots

We develop a semiclassical theory for spin-dependent quantum transport in ballistic quantum dots. The theory is based on the semiclassical Landauer formula, that we generalize to include spin-orbit and Zeeman interaction. Within this approach, the orbital degrees of freedom are treated semiclassically, while the spin dynamics is computed quantum mechanically. Employing this method, we calculate the quantum correction to the conductance in quantum dots with Rashba and Dresselhaus spin-orbit interaction. We find a strong sensitivity of the quantum correction to the underlying classical dynamics of the system. In particular, a suppression of weak antilocalization in integrable systems is observed. These results are attributed to the qualitatively different types of spin relaxation in integrable and chaotic quantum cavities.Comment: 20 page

Electron spin relaxation in bulk III-V semiconductors from a fully microscopic kinetic spin Bloch equation approach

Electron spin relaxation in bulk III-V semiconductors is investigated from a fully microscopic kinetic spin Bloch equation approach where all relevant scatterings, such as, the electron--nonmagnetic-impurity, electron-phonon, electron-electron, electron-hole, and electron-hole exchange (the Bir-Aronov-Pikus mechanism) scatterings are explicitly included. The Elliot-Yafet mechanism is also fully incorporated. This approach offers a way toward thorough understanding of electron spin relaxation both near and far away from the equilibrium in the metallic regime. The dependence of the spin relaxation time on electron density, temperature, initial spin polarization, photo-excitation density, and hole density are studied thoroughly with the underlying physics analyzed. In contrast to the previous investigations in the literature, we find that: (i) In $n$-type materials, the Elliot-Yafet mechanism is {\em less} important than the D'yakonov-Perel' mechanism, even for the narrow band-gap semiconductors such as InSb and InAs. (ii) The density dependence of the spin relaxation time is nonmonotonic and we predict a {\em peak} in the metallic regime in both $n$-type and intrinsic materials. (iii) In intrinsic materials, the Bir-Aronov-Pikus mechanism is found to be negligible compared with the D'yakonov-Perel' mechanism. We also predict a peak in the temperature dependence of spin relaxation time which is due to the nonmonotonic temperature dependence of the electron-electron Coulomb scattering in intrinsic materials with small initial spin polarization. (iv) In $p$-type III-V semiconductors, ...... (the remaining is omitted here due to the limit of space)Comment: 25 pages, 17 figure

Spin dynamics of two-dimensional electrons with Rashba spin-orbit coupling and electron-electron interactions

We study the spin dynamics of two dimensional electron gases (2DEGs) with Rashba spin-orbit coupling by taking account of electron-electron interactions. The diffusion equations for charge and spin densities are derived by making use of the path-integral approach and the quasiclassical Green's function. Analyzing the effect of the interactions, we show that the spin-relaxation time can be enhanced by the electron-electron interaction in the ballistic regime.Comment: accepted for publication in Phys. Rev.

Cyclotron effect on coherent spin precession of two-dimensional electrons

We investigate the spin dynamics of high-mobility two-dimensional electrons in GaAs/AlGaAs quantum wells grown along the $[001]$ and $[110]$ directions by time-resolved Faraday rotation at low temperatures. In measurements on the $(001)$-grown structures without external magnetic fields, we observe coherent oscillations of the electron spin polarization about the effective spin-orbit field. In non-quantizing magnetic fields applied normal to the sample plane, the cyclotron motion of the electrons rotates the effective spin-orbit field. This rotation leads to fast oscillations in the spin polarization about a non-zero value and a strong increase in the spin dephasing time in our experiments. These two effects are absent in the $(110)$-grown structure due to the different symmetry of its effective spin-orbit field. The measurements are in excellent agreement with our theoretical model.Comment: 4 pages, 3 figure

Higher order contributions to Rashba and Dresselhaus effects

We have developed a method to systematically compute the form of Rashba- and Dresselhaus-like contributions to the spin Hamiltonian of heterostructures to an arbitrary order in the wavevector k. This is achieved by using the double group representations to construct general symmetry-allowed Hamiltonians with full spin-orbit effects within the tight-binding formalism. We have computed full-zone spin Hamiltonians for [001]-, [110]- and [111]-grown zinc blende heterostructures (D_{2d},C_{4v},C_{2v},C_{3v} point group symmetries), which are commonly used in spintronics. After an expansion of the Hamiltonian up to third order in k, we are able to obtain additional terms not found previously. The present method also provides the matrix elements for bulk zinc blendes (T_d) in the anion/cation and effective bond orbital model (EBOM) basis sets with full spin-orbit effects.Comment: v1: 11 pages, 3 figures, 8 table