Anomalous Hanle Effect due to Optically Created Transverse Overhauser Field in Single $\mathrm{InAs}/\mathrm{GaAs}$ Quantum Dots

We report on experimental observations of an anomalous Hanle effect in individual self-assembled $\mathrm{InAs}/\mathrm{GaAs}$ quantum dots. A sizable electron spin polarization photocreated under constant illumination is maintained in transverse magnetic fields as high as $\sim 1\mathrm{T}$, up to a critical field where it abruptly collapses. These striking anomalies of the Hanle curve point to a novel mechanism of dynamic nuclear spin polarization giving rise to an effective magnetic field generated perpendicular to the optically injected electron spin polarization. This transverse Overhauser field, confirmed by the cancellation of electron Zeeman splitting below the critical field, is likely to be a consequence of the strong inhomogeneous quadrupolar interactions typical for strained quantum dots.

Figure 1

(a) Experimental geometry for Hanle effect measurements and schematics of $X^{+}$ spin orientation. (b) Dispersion of the PL intensity around $E_0=1.3522\mathrm{eV}$ for QD1 in a transverse magnetic field under linearly polarized excitation, measured in linear polarization ${\pi }_x$ or ${\pi }_y$. (c) $X^{+}$ PL line in $\sigma \pm$ polarization at ${\mathit{B}}_{\mathrm{ext}}=0$ under ${\sigma }^{+}$ excitation at 1.396 eV.

Figure 2

(a) Hanle depolarization curves of $X^{+}$ trions in 4 different QDs in sample $A$, under a constant ${\sigma }^{+}$ polarized excitation. QD4 was also measured under ${\sigma }^{+}/{\sigma }^{-}$-modulated excitation. Arrows indicate field sweep directions. Curves for QD1 and QD2 are vertically shifted for clarity. (b) Hanle depolarization curves of an $X^{+}$ trion in sample $B$ under different excitation conditions. (c) $X^{+}$ apparent linewidth and nuclear field component $B_{n,z}$ estimated from the ${\sigma }^{+}\mathrm{\text{−}}{\sigma }^{-}$ splitting $|{\Delta }_n|$. The dashed line is the evolution of effective linewidth due to $e$ Zeeman splitting.

Figure 3

(a) Schematics of $X^{+}$ optical transitions in a transverse magnetic field $B_x$ for both special cases $B_{n,x}=-B_x$ and $B_{n,x}=0$, assuming $B_{n,z}=0$. (b) Fit of the QD1 $X^{+}$ PL line measured in linear ${\pi }_y$ polarization by a Lorentzian doublet on both sides of the circular polarization collapse. (c) Diagram of $e$ spin and nuclear field components leading to the experimental observations. (d) Splitting of the QD1 $X^{+}$ line measured in ${\pi }_x$ or ${\pi }_y$ polarization. Dashed lines indicate the $h$ and $e\pm h$ Zeeman splittings. The gray-shaded area represents the resolution limit for the fit in (b).