Frequency dependence of induced spin polarization and spin current in quantum wells

Dynamic response of two-dimensional electron systems with spin-orbit interaction is studied theoretically on the basis of quantum kinetic equation, taking into account elastic scattering of electrons. The spin polarization and spin current induced by the applied electric field are calculated for the whole class of electron systems described by p-linear spin-orbit Hamiltonians. The absence of nonequilibrium intrinsic static spin currents is confirmed for these systems with arbitrary (nonparabolic) electron energy spectrum. Relations between the spin polarization, spin current, and electric current are established. The general results are applied to the quantum wells grown in [001] and [110] crystallographic directions, with both Rashba and Dresselhaus types of spin-orbit coupling. It is shown that the existence of the fixed (momentum-independent) precession axes in [001]-grown wells with equal Rashba and Dresselhaus spin velocities or in symmetric [110]-grown wells leads to vanishing spin polarizability at arbitrary frequency of the applied electric field. This property is explained by the absence of Dyakonov-Perel-Kachorovskii spin relaxation for the spins polarized along these precession axes. As a result, a considerable frequency dispersion of spin polarization at very low frequency in the vicinity of the fixed precession axes is predicted. Possible effects of extrinsic spin-orbit coupling on the obtained results are discussed.Comment: 14 pages, 6 figures; published with minor corrections in Phys. Rev.

Bremsstrahlung Spectrum in alpha Decay

Using our previous approach to electromagnetic emission during tunneling, an explicit, essentially classical, formula describing the bremsstrahlung spectrum in alpha decay is derived. The role of tunneling motion in photon emission is discussed. The shape of the spectrum is a universal function of the ratio Eg/Eo , where Eg is the photon energy and Eo is a characteristic energy depending only on the nuclear charge and the energy of the alpha particle.Comment: 8 pages, 3 figure

Parameter free calculation of hadronic masses from instantons

We propose a non-perturbative calculation scheme which is based on the semi-classical approximation of QCD and can be used to evaluate quantities of interest in hadronic physics. As a first application, we evaluate the mass of the pion and of the nucleon. Such masses are related to a particular combination of Green functions which, in some limit, is dominated by the contribution of \emph{very small-sized} instantons. The size distribution of these pseudo-particles is determined by the 't Hooft tunneling amplitude formula and therefore our calculation is free from any model parameters. We prove that instanton forces generate a light pion and a nucleon with realistic mass ($M_n \sim 970 MeV$). In connection with sum-rules approaches, we discuss the overlap of instantons with pion and nucleon resonances

Phonon-induced decoherence for a quantum dot spin qubit operated by Raman passage

We study single-qubit gates performed via stimulated Raman adiabatic passage (STIRAP) on a spin qubit implemented in a quantum dot system in the presence of phonons. We analyze the interplay of various kinds of errors resulting from the carrier-phonon interaction as well as from quantum jumps related to nonadiabaticity and calculate the fidelity as a function of the pulse parameters. We give quantitative estimates for an InAs/GaAs system and identify the parameter values for which the error is considerably minimized, even to values below $10^{-4}$ per operation.Comment: Final version; considerable extensions; 18 pages, 7 figure

Generation and Measurement of Non Equilibrium Spin Currents in Two Terminal Systems

Generation and measurement of non-equilibrium spin current in two probe configuration is discussed. It is argued and shown that spin current can be generated in two terminal non-magnetic system. Further it is shown that these spin currents can be measured via conductance in two probe configuration when the detector probe is ferromagnetic.Comment: 4 pages, 5 figure

Spin magnetotransport in two-dimensional hole systems

Spin current of two-dimensional holes occupying the ground-state subband in an asymmetric quantum well and interacting with static disorder potential is calculated in the presence of a weak magnetic field H perpendicular to the well plane. Both spin-orbit coupling and Zeeman coupling are taken into account. It is shown that the applied electric field excites both the transverse (spin-Hall) and diagonal spin currents, the latter changes its sign at a finite H and becomes greater than the spin-Hall current as H increases. The effective spin-Hall conductivity introduced to describe the spin response in Hall bars is considerably enhanced by the magnetic field in the case of weak disorder and demonstrates a non-monotonic dependence on H.Comment: 4 pages, 2 figures, published in Phys. Rev.

Long-term Dynamics of the Electron-nuclear Spin System of a Semiconductor Quantum Dot

A quasi-classical theoretical description of polarization and relaxation of nuclear spins in a quantum dot with one resident electron is developed for arbitrary mechanisms of electron spin polarization. The dependence of the electron-nuclear spin dynamics on the correlation time $\tau_c$ of electron spin precession, with frequency $\Omega$, in the nuclear hyperfine field is analyzed. It is demonstrated that the highest nuclear polarization is achieved for a correlation time close to the period of electron spin precession in the nuclear field. For these and larger correlation times, the indirect hyperfine field, which acts on nuclear spins, also reaches a maximum. This maximum is of the order of the dipole-dipole magnetic field that nuclei create on each other. This value is non-zero even if the average electron polarization vanishes. It is shown that the transition from short correlation time to $\Omega\tau_c>1$ does not affect the general structure of the equation for nuclear spin temperature and nuclear polarization in the Knight field, but changes the values of parameters, which now become functions of $\Omega\tau_c$. For correlation times larger than the precession time of nuclei in the electron hyperfine field, it is found that three thermodynamic potentials ($\chi$, $\bm{\xi}$, $\varsigma$) characterize the polarized electron-nuclear spin system. The values of these potentials are calculated assuming a sharp transition from short to long correlation times, and the relaxation mechanisms of these potentials are discussed. The relaxation of the nuclear spin potential is simulated numerically showing that high nuclear polarization decreases relaxation rate.Comment: RevTeX 4, 12 pages, 9 figure

Scale Anomaly Induced Instanton Interaction

The binary interaction of large size instantons in a SU(2) Yang-Mills theory is obtained from the one-loop effective action for the field strength. The instanton interaction is calculated as a function of the instanton separation and in dependence on radius and relative orientation of the instantons. Two equally oriented instantons with radii large compared with the scale defined by the gluon condensate have purely attractive interaction, whereas the interaction of maximal disoriented instantons is repulsive. We argue that the medium range attractive interaction of the instantons generally holds and is solely due to the instability of the perturbative vacuum.Comment: 11 LaTex pages (3 figures available on request), in press by Physics Letters B, UNITUE-THEP-4-199