Effect of exchange coupling on coherently controlled spin-dependent transition rates

Journal ArticleThe effect of exchange interactions within spin pairs on spin-dependent transport and recombination rates through localized states in semiconductors during coherent electron-spin resonant excitation is studied theoretically. It is shown that for identical spin systems, significant quantitative differences are to be expected between the results of pulsed electrically/optically detected magnetic resonance (pEDMR/pODMR) experiments, where permutation symmetry is the observable, and the results of pulsed electron-spin resonance (pESR) experiments, with polarization in the x-y plane of the rotating frame as the observable. It is predicted that beat oscillations of the spin nutations and not the nutations themselves dominate the transport or recombination rates when the exchange coupling strength or the field strength of the exciting radiation exceed the difference between the Zeeman energies within the spin pair. Furthermore, while the intensities of the rate oscillations decrease with increasing exchange within the spin pairs, the singlet and triplet signals retain their relative strengths. This means that pEDMR and pODMR experiments allow experimental access to ESR forbidden singlet transitions

Transport and recombination through weakly coupled localized spin pairs in semiconductors during coherent spin excitation

Semi-analytical predictions for the transients of spin-dependent transport and recombination rates through localized states in semiconductors during coherent electron spin excitation are made for the case of weakly spin-coupled charge carrier ensembles. The results show that the on-resonant Rabi frequency of electrically or optically detected spin-oscillation doubles abruptly as the strength of the resonant microwave field gamma B_1 exceeds the Larmor frequency separation within the pair of charge carrier states between which the transport or recombination transition takes place. For the case of a Larmor frequency separation of the order of gamma B_1 and arbitrary excitation frequencies, the charge carrier pairs exhibit four different nutation frequencies. From the calculations, a simple set of equations for the prediction of these frequencies is derived

Irreversible evolution of many-electron systems: From the quantum-Boltzmann equation toward the semi-classical Boltzmann equation

Starting from the quantum-Boltzmann equation derived in a previous paper, we study the irreversible evolution of an electron gas in the one-particle phase space. The connection with phase space is established by expressing one-electron states in terms of the overcomplete and nonorthogonal generating system of coherent states. By using the generalized closure relation for coherent states, as well as the fact that a one-particle operator is completely determined by the ensemble of expectation values for all coherent states, we obtain the master equations in a form that allows us to follow the evolution in phase space. This form of the master equations provides a direct link between the quantum-statistical approach and the semi-classical Boltzmann equation. The latter is obtained after a coarse-graining procedure in the one-particle phase space and by using the fact that the electron-electron interaction, as well as the interactions between the electron gas and the bath subsystems provided by phonons or photons, are local in real space

Irreversible evolution of many-electron systems coupled to a statistical environment

We study the evolution of a many-electron system that is confined in a finite spatial region and coupled to a statistical environment. The latter may be composed of several independent bath subsystems, which are held at some statistical equilibrium. From the master equations describing the evolution of the coarse-grained N-particle density matrix, we obtain the equations, which describe the evolution of the n-particle density operators (D) over bar ((n)) for n < N. These equations are hierarchically coupled through the electron-electron interaction. We show that the hierarchy can be truncated under the assumption that the residual interaction of the electrons in the considered system with the environment introduces a memory loss, which hinders the electronic system to build up more than two-particle correlations. We first consider a weakly excited electronic system, which looses memory but where energy exchange with the statistical environment can be neglected. This is the quantum analog of the classical Boltzmann gas in a box. We derive the master equations, which describe the irreversible evolution of the coarse-grained one-particle density matrix. Based on this result, we show that, in accord with the second law of thermodynamics, the corresponding von Neumann entropy either increases with time or it remains constant. Finally, allowing also for energy exchange with one or more bath subsystems provided, e.g., by phonons or photons, we obtain the corresponding general master equations that describe the evolution of a spatially confined interacting electron gas of metallic density

Influence of disorder on electrically and optically detected electron spin nutation

A numerical study of the influence of disorder in semiconductors on spin-Rabi nutation observed with pulsed electrically or optically detected magnetic-resonance techniques (pEDMR and pODMR, respectively) is presented. It is shown that transient nutation signals of disordered spin ensembles differ from ordered ensembles as inhomogeneously broadened Lande-factor distributions are presented. In contrast to ordered systems, the magnitudes of spin-Rabi nutation and spin-Rabi beat nutation change significantly with a strong dependence of their ratio on the correlation of the Lande factors within the nearest-neighbor spin pairs. An interpretation of these results is given and their application for the investigation of disorder using pEDMR and pODMR is discussed