Amplification of spin-filtering effect by magnetic field in GaAsN alloys

Abstract

We have found that intensity $I$ and circular polarization degree $\rho$ of the edge photoluminescence, excited in GaAsN alloys by circularly polarized light at room temperature, grow substantially in the longitudinal magnetic field $B$ of the order of 1\,kG. This increase depends on the intensity of pumping and, in the region of weak or moderate intensities, may reach a twofold value. In two-charge-state model, which considers spin-dependent recombination of spin-oriented free electrons on deep paramagnetic centers, we included the magnetic-field suppression of spin relaxation of the electrons bound on centers. The model describes qualitatively the rise of $\rho$ and $I$ in a magnetic field under different pump intensities. Experimental dependences $\rho(B)$ and $I(B)$ are shifted with respect to zero of the magnetic field by a value of $\sim$170\,Gauss, while the direction of the shift reverses with change of the sign of circular polarization of pumping. As a possible cause of the discovered shift we consider the Overhauser field, arising due to the hyperfine interaction of an electron bound on a center with nuclei of the crystal lattice in the vicinity of the center.Comment: 8 pages, 6 figures, Submitted to Physical Review

Spin-dependent recombination in GaAs1–x N x alloys at oblique magnetic field

“…The values of J + and J − were measured at room temperature using a high-sensitive polarization analyzer [20]. In the longitudinal field (α = 0, diamonds) the PL polarization and intensity increase as a result of suppression of spin relaxation of the localized electrons, and the increase is described with good accuracy by inverted Lorentzians with half-width at half minimum of B 1/2 ∼ 1 kG [10][11][12][13][14]. The curves ρ(B) and J(B) are asymmetric: their minima are shifted relative to the point B = 0, and the shift changes its sign under reversal of the excitation circular polarization from σ + to σ − .…”

“…Particularly, the electron spin depolarization in the magnetic field perpendicular to the exciting beam (Hanle effect, the Voigt geometry) is described by a superposition of two Lorentzian contours with widths at half maximum differing by two or three orders: the large spin relaxation time of localized electrons (∼1 ns) determines the width of the narrow contour (∼100 G) whereas the short lifetime of free electrons (∼1 ps) sets the width ∼25 kG of the wide contour [8,9]. Also, it has recently been established [10][11][12][13][14] that the magnetic field directed along the exciting beam (the Faraday geometry) can lead to an increase in the efficiency of spin filter and, as a consequence, to a substantial (up to twice) enhancement of the electron polarization and intensity of the edge photoluminescence (PL) at low and moderate pumping rates. This effect is based on the longitudinal-magnetic-field induced suppression of the electron spin depolarization caused by the hyperfine interaction of a localized electron with the nucleus of the paramagnetic center which localizes this electron.…”

Efficient room-temperature nuclear spin hyperpolarization of a defect atom in a semiconductor

“…As schematically shown in Fig. 2b, the application of B z should suppress depolarization of optically generated electron spins S 0 caused by Larmor precession of the electron spins around a randomly fluctuating effective magnetic field (B F ), leading to an increase in electron spin projection oS z 4 along the field and thus spin polarization with increasing field strength 22,23,29 . A nuclear field B N generated by DNP along the direction of the optical spin orientation can either add to or compensate the external field, depending on its relative orientation with B z (that is, parallel or antiparallel).…”

“…Here, oB N 4 denotes an average local nuclear field within the volume of a sample that is subject to optical excitation. As spin polarization and total concentration of conduction-band electrons are inter-connected by the defect-engineered spinfiltering effect [25][26][27][28][29] , see Supplementary Fig. S1, oB N 4 can be conveniently measured by monitoring either circular polarization or total intensity of the band-to-band photoluminescence as a function of B z .…”

“…Our approach exploits the recently discovered defect-engineered spin-filtering effect 25 , which has been demonstrated to be capable of generating strong spin polarization of conduction-band electrons in a semiconductor at RT. Strong conduction-band electron spin polarization of 440% can be obtained at RT in Ga(In)NAs by spin-filtering through Ga i interstitial defects [25][26][27][28][29] . By taking advantage of the spin-filtering effect and going beyond merely generating conduction-band electron spin polarization that was the focal point of the previous studies [25][26][27][28][29] , we shall show in this work that strong and efficient nuclear spin polarization of the core Ga atom of the Ga i defects can be achieved at RT by optical orientation.…”