Noise Enhanced Stability Phenomenon in Electron Spin Dynamics

Possible utilization of the electron spin as an information carrier in electronic devices is an engaging challenge for future spin-based electronics. In these new devices, the information stored in a system of polarized electron spins, is transferred by applying an external electric field and finally detected. However, each initial non-equilibrium magnetization decays both in time and distance during the transport. Because of increasing miniaturization, to avoid too much intense electric fields, which could lead the system to exhibit a strongly nonlinear physical behavior, applied voltages are very low. Low voltages are subjected to the background noise; hence, it is mandatory to understand the influence of fluctuations on the spin depolarization process in order to guarantee a reliable manipulation, control and detection of information in spin-based devices. The presence of noise is generally considered a disturbance, since strong fluctuations affect the performance of the devices. In the last decade, however, an increasing interest has been directed towards possible constructive aspects of noise in the dynamical response of non-linear systems. Interesting theoretical works on the possibility to improve the ultra-fast magnetization dynamics of magnetic spin systems by including random fields have been recently published. Instead, at the best of our knowledge, the investigation of the role of noise on the electron spin dynamics in semiconductors is still beginning. Preliminary findings show that in semiconductor crystals a fluctuating electric field can strongly modify the spin depolarization length, an essential design parameter in spin-based electronic devices. In this contribution, we investigate the effects of different types of external source of noise on the spin relaxation process in low-doped n-type GaAs crystals. The electron transport is simulated by a Monte Carlo procedure which takes into account all the possible scattering phenomena of hot electrons in the medium and includes the evolution of the spin polarization vector. The effects caused by the addition of external fluctuations are investigated by analyzing the modification of the spin depolarization length. In the presence of a correlated source of noise and for electric field amplitudes greater than the Gunn field, an increase of the spin relaxation length up to 20% is found. This result can be considered a Noise Enhanced Stability (NES) consequence and can be explained in terms of a decrease of the occupation of the L-valleys, where the strength of spin-orbit coupling felt by electrons is at least one order of magnitude greater than that present in the lowest energy valley. If the random component of the driving electric field is modeled with a dichotomous stochastic process (Random telegraph noise) characterized by two discrete levels, the spin depolarization process changes, in a way that critically depends on the jump rate of the stochastic process. We investigate on a possible relationship between the semiconductor characteristic time scales and the noise characteristic time in order to find the most favorable condition for the transmission of information by electron spin.