Rashba

Rashba most commonly refers to the Rashba effect, a phenomenon in condensed matter physics where spin-orbit coupling lifts the spin degeneracy of electron states in systems lacking structural inversion symmetry, such as surfaces, interfaces, and quantum wells. The effect is named after Emmanuel Rashba, who described the mechanism in the 1960s.

Mechanism and mathematical description: In a two-dimensional electron gas, the Rashba interaction can be described by the Hamiltonian HR = αR (σx ky − σy kx), where αR is the Rashba parameter, σi are the Pauli matrices, and k is the in-plane wave vector. This leads to two spin-split branches with energies E±(k) = (ħ^2 k^2)/(2m*) ± αR|k|. The spin orientation winds with momentum, producing spin-momentum locking.

Origin, tunability, and materials: The splitting arises from structural inversion asymmetry, often created by electric fields due to band bending at surfaces or interfaces. The Rashba parameter αR grows with the electric field and can be enhanced by incorporating heavy elements that increase spin-orbit coupling. It has been observed in semiconductor quantum wells (e.g., InAs, GaAs) and at metal surfaces and interfaces; recent work extends to oxide interfaces and topological insulator surfaces.

Applications and context: The Rashba effect is central to spintronics, enabling electrical control of spin with potential devices such as the Datta–Das spin transistor and various spin-orbit torque technologies. It is related to, but distinct from, the Dresselhaus effect, which arises from bulk inversion asymmetry. The concept also provides insights into spin textures in topological quantum materials.