Using relativity to improve electronic devices: Ways to enhance the Rashba effect
KTH Applied Physics seminars
Thursday 25 January 2007
to 10:00 at
Ulf Ekenberg (School of Information and Communication Technology, KTH)
There is a rapidly growing interest in spin-related phenomena in solids.
The prospects of combining magnetic properties with the well developed semiconductor technology have lead to a new research field called spintronics.
The spin polarization of electrons has clear analogies to the polarization of light. I will illustrate the function of photonic components and their spintronic counterparts. One of the best known spintronic devices, the Datta-Das spin transistor , was introduced as an analog to the electro-optic modulator. It is based on the so called Rashba effect which is essentially of relativistic origin. Here an electric field produces a spin splitting of subbands in a quantum well which is a convenient way to control a spintronic device. The efforts to implement this transistor in practice have not yet been successful.
Calculations of properties of spintronic devices commonly utilize the Rashba model in which the strength is simply given by a coefficient. We use a more elaborate multiband approach and obtain effects that would not occur in simpler models.
In this talk I will demonstrate some novel ways in which the Rashba splitting can be efficiently controlled. In a wide modulation-doped n-type quantum well one can utilize the strong built-in electric field at the interfaces but turn on the Rashba effect with a much smaller applied field. Anticrossing phenomena can yield a reversal of the spin direction when the in-plane wave vector is increased slightly.
In particular I show that with careful design one can increase the wave-vector splitting relevant for the Datta-Das spin transistor by several orders of magnitude by using holes instead of electrons . We demonstrate how this superefficient Rashba effect can be investigated experimentally. The implications for spintronic devices, in particular the Datta-Das spin transistor, are discussed. The possibility of using holes in spintronics has so far often been discarded because a strong spin-orbit coupling also leads to rapid spin relaxation. We argue that one can get around this problem in hole systems. There is furthermore recent evidence that spin relaxation times can be comparable for electrons and holes