Current-induced spin polarization in anisotropic spin-orbit fields

Current-induced spin polarization is a phenomenon in which carrier spins are oriented when subjected to current flow\footnote{Y. K. Kato, R. C. Myers, A. C. Gossard, and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{93}, 176601 (2004).}. However, the mechanism that produces this spin polarization remains an open question. Existing theory predicts that the spin polarization should be proportional to the spin-orbit splitting yet no clear trend has been observed experimentally. We perform experiments on semiconductor samples designed so that the magnitude and direction of the in-plane current and applied magnetic field can be varied and measure the electrical spin generation efficiency and spin-orbit splitting using optical techniques\footnote{B. M. Norman, C. J. Trowbridge, D. D. Awschalom, and V. Sih, \textit{Phys. Rev. Lett.} \textbf{112}, 056601 (2014).}. Contrary to expectation, the magnitude of the current-induced spin polarization is shown to be larger for momentum directions corresponding to smaller spin-orbit splitting. In addition, angle-dependent measurements demonstrate that the steady-state in-plane spin polarization is not along the direction of the spin-orbit field, which we attribute to anisotropic spin relaxation. Furthermore, we show that this electrically-generated electron spin polarization can produce a nuclear spin hyperpolarization through dynamic nuclear polarization\footnote{C. J. Trowbridge, B. M. Norman, Y. K. Kato, D. D. Awschalom, and V. Sih, \textit{Phys. Rev. B} \textbf{90}, 085122 (2014).}.