Spin relaxation in CdTe quantum dots

We have measured photoluminescence (PL) spectra and time-resolved PL in CdTe quantum dots under the longitudinal magnetic field up to 10 T. Circular polarization of PL increases with increasing magnetic field, while its linear polarization remains zero under linearly polarized excitation. This behavior cannot be explained by the anisotropic exchange interaction of excitons. Time-resolved PL measurements clarified that this behavior is caused by the suppression of spin relaxation induced by the longitudinal magnetic field. We believe that this behavior is related to the hyperfine interaction of electron spin with magnetic momenta of lattice nuclei.

Figure 1

Circularly polarized PL spectra (a) in co-circular (solid lines) and cross-circular (dotted lines) geometries, at various magnetic fields. (b) Circularly polarized PL spectra in four geometries at 4 T. Linearly polarized excitation gives the same intensity of right- and left-circularly polarized PL ($\mathrm{lin}∕{\sigma }^{+}$ and $\mathrm{lin}∕{\sigma }^{-}$, respectively). (c) Linearly polarized PL spectra in co- and cross-linear geometries along [110] direction.

Figure 2

Magnetic field dependence of the circular polarization of QDs emission peak and phonon resonance ${\mathrm{LO}}_1$ in steady-state PL spectra.

Figure 3

Time dependence of the circularly polarized PL for co-circular (solid lines) and cross-circular (dotted lines) geometries at various magnetic fields (0–8 T) at 10 K. Smooth curves are fittings by a combination of a rise function and two exponential decay functions, whose decay times are related to ${\tau }_r$ and ${\tau }_s$.

Figure 4

Magnetic field dependence of the spin relaxation rate. The experimental values from time-resolved PL (circles) and the estimated values from steady-state PL spectra (squares) are shown together for comparison. The solid line is the theoretical fitting according to Eq. (2).