We study the neutral exciton energy spectrum fine structure and its spin dephasing in transition metal dichalcogenides such as MoS$_2$. The interaction of the mechanical exciton with its macroscopic longitudinal electric field is taken into account.
The splitting between the longitudinal and transverse excitons is calculated by means of the both electrodynamical approach and $mathbf k cdot mathbf p$ perturbation theory. This long-range exciton exchange interaction can induce valley polarization decay. The estimated exciton spin dephasing time is in the picosecond range, in agreement with available experimental data.
The effect of hyperfine interaction on the room-temperature defect-enabled spin filtering effect in GaNAs alloys is investigated both experimentally and theoretically through a master equation approach based on the hyperfine and Zeeman interaction be
tween electron and nuclear spin of the spin filtering defect. We show that the nuclear spin polarization can be tuned through the optically induced spin polarization of conduction band electrons.
Optical and electrical control of the nuclear spin system allows enhancing the sensitivity of NMR applications and spin-based information storage and processing. Dynamic nuclear polarization in semiconductors is commonly achieved in the presence of a
stabilizing external magnetic field. Here we report efficient optical pumping of nuclear spins at zero magnetic field in strain free GaAs quantum dots. The strong interaction of a single, optically injected electron spin with the nuclear spins acts as a stabilizing, effective magnetic field (Knight field) on the nuclei. We optically tune the Knight field amplitude and direction. In combination with a small transverse magnetic field, we are able to control the longitudinal and transverse component of the nuclear spin polarization in the absence of lattice strain i.e. nuclear quadrupole effects, as reproduced by our model calculations.
Hanle effect is ubiquitous in the study of spin-related phenomena and has been used to determine spin lifetime, precession and transport in semiconductors. Here, we report an experimental observation of anomalous Hanle effect in individual self-assem
bled InAs/GaAs quantum dots where we find that a sizeable photo-created electron spin polarization can be maintained in transverse fields as high as 1T until it abruptly collapses. The striking broadening of the Hanle curve by a factor of ~20 and its bistability upon reversal of the magnetic sweep direction points to a novel dynamical nuclear spin polarization mechanism where the effective nuclear magnetic field compensates the transverse applied field. This interpretation is further supported by the measurement of actual electron Zeeman splitting which exhibits an abrupt increase at the Hanle curve collapse. Strong inhomogeneous quadrupolar interactions typical for strained quantum dots are likely to play a key role in polarizing nuclear spins perpendicular to the optically injected spin orientation.
