Auger recombination of excitons in semimagnetic quantum dot structure in a magnetic field

Magnetoluminescence of $\mathrm{Cd}\mathrm{Mn}\mathrm{Se}\text{−}\mathrm{Zn}\mathrm{Se}$ disk-shaped quantum dots in a magnetic field up to $11\mathrm{T}$ is measured in the Faraday and Voigt geometries at liquid He temperatures and various levels of laser excitation. Within the range of $B=0–11\mathrm{T}$ the intensity of the quantum dot photoluminescence increases strongly (up to two orders of magnitude) in the Faraday geometry and only slightly ($\sim 1.5$ times) in the Voigt geometry. To explain the observed dependence on magnetic field direction the selection rules for the Auger recombination of excitons with excitation of Mn-ion are revised. The magnetic field dependence of the quantum yield of quantum dot exciton emission and its anisotropy are shown to appear due to the dependence of Auger recombination of uncharged excitons with excitation of Mn-ion on the spin of both exciton and Mn-ion. The anisotropy is provided by a quick spin relaxation of photoexcited excitons into the ground state which spin structure depends on the direction of the magnetic field.

Figure 1

A sketch of the sample under study. Directions of the magnetic field in “Faraday” and “Voigt” geometries are shown by arrows.

Figure 2

PL spectra of the $\mathrm{Cd}\mathrm{Mn}\mathrm{Se}\text{−}\mathrm{Zn}\mathrm{Se}$ QDs sample at the laser excitation power $4\mathrm{mW}$ in the (a) Faraday and (b) Voigt geometries. PL signal of Mn ions internal transitions recorded under $P\simeq 0.8\mathrm{mW}\mathrm{ps}$ laser excitation is shown in the insert.

Figure 3

The Zeeman shift of the PL signal vs magnetic field in the Faraday geometry at the laser excitation power $P=4\mathrm{mW}$. The solid line is the fit by Eq. (11) with parameters $B_{\mathrm{ex}}=2.6\mathrm{T}$, $T_{\mathrm{eff}}\simeq 7\mathrm{K}$, $E_{\mathit{mp}}\simeq 17\mathrm{meV}$, $E_0\simeq 2.39\mathrm{eV}$.

Figure 4

PL intensity $I\left(B\right)∕I\left(0\right)$ vs magnetic field in Faraday and Voigt geometries at various laser excitation powers. The solid lines show the results of the fitting with the use of Eq. (7). The best fit was found at $T\simeq 2.4\mathrm{K}$ for the $P=0.1\mathrm{mW}$ and at $T\simeq 6.5\mathrm{K}$ for the $P=4\mathrm{mW}$ curves. Parameters $N_A{\tau }_0{W}_{1,-3∕2}=7.2\times {10}^7$ and $N_A{\tau }_0{W}_{1,-1∕2}=1\times {10}^7$ are the same for both curves.

Figure 5

Polarization degree of the PL signal vs magnetic field in the Faraday and Voigt geometries. The excitation power $P\simeq 2\mathrm{mW}$.

Figure 6

Schematic picture of the Auger transitions between initial and final states of the combined system with the lowest in the magnetic field $\mathbf{B}\Vert \mathbf{0}\mathbf{z}$ (a) “bright” and (b) “dark” exciton states. $V_{\mathrm{in}}$ is the interaction operator [see. Eq. (2)]. Allowed Auger transitions are shown by arrows.