Polaron Formation in the Presence of Rashba Spin-Orbit Coupling: Implications for Spintronics

We study the effects of the Rashba spin-orbit coupling on polaron formation, using a suitable generalization of the momentum average approximation. While previous work on a parabolic band model found that spin-orbit coupling increases the effective mass, we show that the opposite holds for a tight-binding model, unless both the spin-orbit and the electron-phonon couplings are weak. It is thus possible to lower the effective mass of the polaron by increasing the spin-orbit coupling. We also show that when the spin-orbit coupling is large as compared to the phonon energy, the polaron retains only one of the spin-polarized bands in its coherent spectrum. This has major implications for the propagation of spin-polarized currents in such materials, and thus for spintronic applications.

Figure 1

The ground state energy as a function of Rashba SO coupling for three values of the $e$-ph coupling. The inset shows the effective mass on a logarithmic scale as a function of Rashba SO coupling.

Figure 2

Spin-dependent spectral function $\tilde{A}(\mathbf{k},\omega )$, for $\lambda =1$, $\Omega =0.8t$, and $V_s=0.4t$. The right panel shows only the “$+$” band for clarity.

Figure 3

Spin-dependent spectral function $\tilde{A}(\mathbf{k},\omega )$, for $\lambda =1$, $\Omega =0.2t$, and $V_s=0.8t$. The right panel shows only the “$+$” band for clarity.