Cyclotron-resonant exciton transfer between the nearly free and strongly localized radiative states of a two-dimensional hole gas in a high magnetic field

Avoided crossing of the emission lines of a nearly free positive trion and a cyclotron replica of an exciton bound to an interface acceptor has been observed in the magnetophotoluminescence spectra of $p$-doped GaAs quantum wells. Identification of the localized state depended on the precise mapping of the anticrossing pattern. The underlying coupling is caused by an exciton transfer combined with a resonant cyclotron excitation of an additional hole. The emission spectrum of the resulting magnetically tunable coherent state probes weak localization in the quantum well.

Figure 1

Evolution of the photoluminescence spectrum in the ${\sigma }^{-}$ polarization as a function of magnetic field $B$. In fields around $B_c=13.2\mathrm{T}$, avoided crossings between the pair of exchange-split CR-$AX$ lines and the $X^{+}$ line are clearly observed, demonstrating the coupling via cyclotron-resonant exchange of an electron-hole pair. For better visibility, at each magnetic field, the energy is linearly shifted to the $X^{+}$ resonance (slope 0.7 meV/T, constant 1520.8 meV).

Figure 2

Evolution of the photoluminescence spectra of the narrow symmetric quantum well of width (a) $w=15\mathrm{nm}$ and two wider asymmetric wells of widths (b) $w=18\mathrm{nm}$ and (c) 22 nm in magnetic field $B$, collected in polarization ${\sigma }^{-}$ (higher intensity), at low temperature $T=1.8\mathrm{K}$, and under laser power density $P=20\mathrm{mW}/{\mathrm{cm}}^2$. The identified lines are as follows: exciton ($X$), positive trion family ($X_{\mathrm{s}}^{+},X_{\mathrm{tb}}^{+}$, and $X_{\mathrm{td}}^{+}$), acceptor-bound complexes ($AX$ and $A{X}^{+}$), SU lines, and CRs. The color scheme is the same as in Fig. 1.

Figure 3

Examples of the ${\sigma }^{-}$-polarized high-field photoluminescence spectra of two wells from Fig. 2: (a) $w=15\mathrm{nm}$, symmetric; (b) $w=22\mathrm{nm}$, asymmetric.

Figure 4

Top: Energy-field dependence for the initial and final states of two candidate scenarios for the observed anticrossing pattern of Fig. 1 (left/right: cyclotron replica of a charged/neutral acceptor-bound excitonic complex mixed with a free trion with the two-hole exchange splitting in the final/initial configuration). Magnetic field $B$ is counted from the resonance field $B_c$ and the energy—from the trion position at resonance. Exchange and coupling energies ($\Delta$ and $\delta$) are quoted in meV. Oscillator strengths away from resonance ($I$) are defined relative to the free trion. Bottom: Corresponding field dependences of the emission energy and intensity (indicated by dot radii). Comparison with Fig. 1 allows positive identification of the CR-$AX$ transition.

Figure 5

Schematic of the coupling between the acceptor-bound exciton $AX$ and the nearly free trion $X^{+}$ through the exchange of an electron-hole pair combined with a hole cyclotron transition.