Spin depolarization of holes and lineshape of the Hanle effect in semiconductor nanostructures

Abstract

We report the statistical effect of random in-plane deformations on the magnetic field depolarization of photoluminescence ͑the Hanle effect͒ in quasi-two-dimensional semiconductor nanostructures such as quantum wells or disklike quantum dots. Provided that the optical orientation signal from the sample is due to the nonequilibrium spin polarization of holes, the lineshape of the Hanle effect becomes non-Lorentzian. This results from a scatter of hole g-factor values, which is immanent to the hole ensembles in such systems. Analysis of the lineshape of the Hanle effect holds promise as a tool for distinguishing between X + and X − trion states in nanostructures.In light of the enhanced interest in studying spin phenomena in low-dimensional semiconductor nanostructures such as quantum wells ͑QWs͒ and quantum dots ͑QDs͒, including details of spin structure, it is convenient to employ optical spin orientation measurements. For bulk semiconductor crystals, the optical orientation signal usually arises due to the nonequilibrium spin polarization of the conduction-band or donor-bound electrons. In the case of holes there exists a strong correlation between spin and momentum, so that the nonequilibrium spin polarization of the holes dissipates after a few momentum changing collisions. 1 Such depolarization typically occurs on a subpicosecond time scale, while typical hole lifetimes are of the order of hundreds of picoseconds. As a rule, therefore, on average the nonequilibrium spin polarization is nearly absent in an ensemble of holes, so that the holes do not contribute to the optical orientation signal in continuous-wave ͑cw͒ experiments.The situation is, however, quite different in twodimensional ͑2D͒ quantum nanostructures. In that case the motion of the holes is restricted by the confinement potential, and the energy spectrum of the valence band is thus fundamentally changed as compared to that of the bulk crystal. Typically, the lowest-energy valence-band state in such 2D situations is the heavy-hole subband, while the light-hole subband is split off due to the difference in effective masses and to biaxial strain in the 2D layer. The Kramers sublevels of the heavy hole Ϯ3 / 2 differ by the value of 3 in the angular-momentum projection. This fact restricts the number of random interactions which can mix these sublevels and initiate the spin flip of the hole. Indeed, while for "warm" holes in a QW spin relaxation remains quite efficient, it slows down noticeably for "cold" holes at the bottom of the band. 2 The possibilities for spin relaxation are further restricted in QDs, where the motion of holes is limited in all three dimensions and the energy spectrum is discrete. One should bear in mind that in QDs ͑and in fact also in QWs͒ the excitonic effects, particularly the electron-hole exchange interaction, become strong, and can mask the pure spin properties of holes of interest in this Brief Report. A convenient way to eliminate the influence of the exchange interaction on spin relaxation of holes ͑and thus on opti...

The Hanle effect and electron spin polarization in InAs/GaAs quantum dots up to room temperature

“…Assuming an approximate average value of 75 ps for T 1 and τ T * 2 , the lower bound of T * 2 can estimated to be 40 ps. Possible mechanisms for the ensemble spin dephasing (T * 2 ) considered in semiconductor QDs are spin dephasing in random fluctuations of the nuclear hyperfine field [26] and a spread in electron and hole g-factors [21,[27][28][29], both of which have been observed to act together at low temperature [30]. To evaluate the relative importance of the two, we conducted measurements of the electron spin polarization degree in a longitudinal magnetic field.…”

“…Such large fluctuation degree was earlier observed for the hole in a negative trion, as the in-plane hole g-value is extremely sensitive to structural distortion that admixes the hh and lh states. As a result, the Hanle curve was described by a non-Lorentzian lineshape that could be regarded as a signature of a hole-related spin depolarization, see [29]. A similar large fluctuation in the g value of electrons is, however, not expected.…”