ABSTRACT
In the previous chapter, we introduced the concept of the longitudinal and transverse relaxation times T1 and T2. One of the major mechanisms that determine T1 and T2 for a single spin – or an ensemble of spins – in a solid is spin-orbit interaction. It is caused by the coupling of a moving electron’s spin with an effective magnetic field due to an electric field in the solid. The electric field could be either microscopic (as in an atom due to the charged nucleus) or macroscopic (due to a global electric field caused by doping in a semiconductor or band structure modulation). Either type will make a moving electron experience an effective magnetic field. The magnetic field will not appear in the laboratory frame, but will appear in the rest frame of the electron due to Lorentz transformation of the electric field. Spin-orbit interaction is important to understand since it not only affects
spin relaxation, but is also at the heart of many spin-based devices that are discussed in Chapter 14. These devices operate by modulating the spin polarizations of charge carriers with an external electric field that controls spinorbit interaction. A simple way of viewing this is that the electric field causes an effective magnetic field via Lorentz transformation, which, in turn, makes spins precess about it. By controlling the electric field (and hence the resulting magnetic field), one can alter the angular frequency of Larmor precession of spins and therefore the angle through which they precess in a given time. This affords control over the spin precession through an external electric field, which is the basis of many spintronic devices. There are essentially two types of spin-orbit interaction that we need to
discuss. One is microscopic or intrinsic (as in an atom) and the other is macroscopic or extrinsic (as in a solid). The latter is usually controllable by external agents and forms the basis of many spintronic devices, but we will start by discussing microscopic spin-orbit interaction first.
