Intersubband-induced spin-orbit interaction in quantum wells

TL;DR

This paper derives and analyzes a new intersubband-induced spin-orbit interaction in symmetric quantum wells, showing its comparable strength to Rashba coupling and its unique behavior under external bias.

Contribution

It provides a detailed derivation of the intersubband spin-orbit Hamiltonian and calculates its strength in realistic quantum well structures, including effects of external bias.

Findings

The intersubband SO coupling is non-zero in symmetric wells and comparable to Rashba.

Rashba couplings for subbands interchange signs abruptly across zero bias.

Dresselhaus coupling remains essentially constant with bias.

Abstract

Recently, we have found an additional spin-orbit (SO) interaction in quantum wells with two subbands [Phys. Rev. Lett. 99, 076603 (2007)]. This new SO term is non-zero even in symmetric geometries, as it arises from the intersubband coupling between confined states of distinct parities, and its strength is comparable to that of the ordinary Rashba. Starting from the $8 \times 8$ Kane model, here we present a detailed derivation of this new SO Hamiltonian and the corresponding SO coupling. In addition, within the self-consistent Hartree approximation, we calculate the strength of this new SO coupling for realistic symmetric modulation-doped wells with two subbands. We consider gated structures with either a constant areal electron density or a constant chemical potential. In the parameter range studied, both models give similar results. By considering the effects of an external applied bias, which breaks the structural inversion symmetry of the wells, we also calculate the strength of the resulting induced Rashba couplings within each subband. Interestingly, we find that for double wells the Rashba couplings for the first and second subbands interchange signs abruptly across the zero bias, while the intersubband SO coupling exhibits a resonant behavior near this symmetric configuration. For completeness we also determine the strength of the Dresselhaus couplings and find them essentially constant as function of the applied bias.