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Register a new userKinetics of spin coherence of electrons in $n$-type InAs quantum wells under intense terahertz laser fields
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Spin kinetics in $n$-type InAs quantum wells under intense terahertz laser fields is investigated by developing fully microscopic kinetic spin Bloch equations via the Floquet-Markov theory and the nonequilibrium Greens function approach, with all the relevant scattering, such as the electron-impurity, electron-phonon, and electron-electron Coulomb scattering explicitly included. We find that a {em finite} steady-state terahertz spin polarization induced by the terahertz laser field, first predicted by Cheng and Wu [Appl. Phys. Lett. {bf 86}, 032107 (2005)] in the absence of dissipation, exists even in the presence of all the scattering. We further discuss the effects of the terahertz laser fields on the spin relaxation and the steady-state spin polarization. It is found that the terahertz laser fields can {em strongly} affect the spin relaxation via hot-electron effect and the terahertz-field-induced effective magnetic field in the presence of spin-orbit coupling. The two effects compete with each other, giving rise to {em non-monotonic} dependence of the spin relaxation time as well as the amplitude of the steady state spin polarization on the terahertz field strength and frequency. The terahertz field dependences of these quantities are investigated for various impurity densities, lattice temperatures, and strengths of the spin-orbit coupling. Finally, the importance of the electron-electron Coulomb scattering on spin kinetics is also addressed.
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In a recent publication, Pfeffer and Zawadzki [cond-mat/0607150; Phys. Rev. B 74, 115309 (2006)] attempted a calculation of electron g factor in III-V heterostructures. The authors emphasize that their outcome is in strong discrepancy with our original result [Ivchenko and Kiselev, Sov. Phys. Semicond. 26, 827 (1992)] and readily conclude that ``the previous theory of the g factor in heterostructures is inadequate. We show here that the entire discrepancy can be tracked down to an additional contribution missing in the incomplete elimination procedure of Pfeffer and Zawadzki. This mistake equally affects their ``exact and approximate results. When the overlooked terms stemming from the nondiagonal Zeeman interaction between light hole and spin-orbit-split valence states are taken into account in the effective electron dispersion, the results of the both approaches applied to the three-level kp model become identical.
We present a theoretical study of the anisotropy of the spin relaxation and decoherence in typical quantum wells with an arbitrary magnetic field. In such systems, the orientation of the magnetic field relative to the main crystallographic directions is crucial, owing to the lack of spin-rotation symmetry. For typical high mobility samples, relaxation anisotropies in the motional narrowing limit owing to the interplay of Rashba and Dresselhaus spin orbit coupling are calculated. We also include the effect of the cubic-in-momentum terms. Although commonly ignored in literature, the latter were experimentally evidenced by the observation of strong anisotropy in spin decoherence measurements by different experimental groups and has long remained unexplained.
A type-II InAs/AlAs$_{0.16}$Sb$_{0.84}$ multiple-quantum well sample is investigated for the photoexcited carrier dynamics as a function of excitation photon energy and lattice temperature. Time-resolved measurements are performed using a near-infrared pump pulse, with photon energies near to and above the band gap, probed with a terahertz probe pulse. The transient terahertz absorption is characterized by a multi-rise, multi-decay function that captures long-lived decay times and a metastable state of for an excess-photon energy of $>100$ meV. For sufficient excess-photon energy, excitation of the metastable state is followed by a transition to the long-lived states. Excitation dependence of the long-lived states map onto a near-direct band gap ($E{_g}$) density of states with an Urbach tail below $E{_g}$. As temperature increases, the long-lived decay times increase $<E{_g}$, due to the increased phonon interaction of the unintentional defect states, and by phonon stabilization of the hot carriers $>E{_g}$. Additionally, Auger (and/or trap-assisted Auger) scattering above the onset of the plateau may also contribute to longer hot-carrier lifetimes. Meanwhile, the initial decay component shows strong dependence on excitation energy and temperature, reflecting the complicated initial transfer of energy between valence-band and defect states, indicating methods to further prolong hot carriers for technological applications.
The carrier spin coherence in a p-doped GaAs/(Al,Ga)As quantum well with a diluted hole gas has been studied by picosecond pump-probe Kerr rotation with an in-plane magnetic field. For resonant optical excitation of the positively charged exciton the spin precession shows two types of oscillations. Fast oscillating electron spin beats decay with the radiative lifetime of the charged exciton of 50 ps. Long lived spin coherence of the holes with dephasing times up to 650 ps. The spin dephasing time as well as the in-plane hole g factor show strong temperature dependence, underlining the importance of hole localization at cryogenic temperatures.
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