A mechanism that has often been considered for spintronic applications is the Rashba effect [1]: Even in the absence of a magnetic field an applied electric field gives rise to a spin splitting of the subbands in a quantum well when the in-plane wave vector k is non-zero. We have recently considered wide modulation-doped InGaSb quantum wells with two weakly interacting electron gases in the interface regions. For a moderate applied field the wave functions become localized to one of the interface regions where the built-in electric field yields a Rashba splitting that is enhanced by an order of magnitude compared to the corresponding case with uniform electric field [2].
We here consider a novel spin flip mechanism in quantum wells with smaller asymmetry such that each wave function has the largest amplitude in one of the interface regions but comparable amplitude at the other interface. The energy separation between the two lowest subband pairs is then comparable to typical Rashba splittings. The anticrossing phenomena occurring in such structures are properly described in our 8x8 k.p approach where the indirect interaction between the electron spin subbands via the hole subbands is included. It can occur that two anticrossing subbands are weakly interacting such that they get very close to each other in energy and the interchange of properties takes place with a very small increase of k.
With a careful design it is possible to have the Fermi energy near such an anticrossing region. With a small change of the applied electric field it moves through the anticrossing region and the expectation value of the spin direction at the Fermi energy in a given spin subband flips. During this process it can also occur that the wave functions of the spin subbands move from one interface region to the other.
The small energy separation between the anticrossing spin subbands seems to imply that very low temperatures are required for observation of this effect. However, we discuss the possibility that the spatial separation between the wave functions can reduce the probability that carriers are thermally excited between such spin subbands.
[1] Y.A. Bychkov and E.I. Rashba, J. Phys. C 17, 6039 (2004).
[2] D.M. Gvozdic and U. Ekenberg, Appl. Phys. Lett. 90, 053105 (2007).
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