EDSR was first observed experimentally with free carriers in indium antimonide (InSb), a semiconductor with strong spin–orbit coupling. Observations made under different experimental conditions allowed demonstrate and investigate various mechanisms of EDSR. In a dirty material, Bell observed a motionally narrowed EDSR line at ω ω S frequency against a background of a wide cyclotron resonance band. MacCombe et al. working with high quality InSb observed isotropic EDSR driven by the ( r s o ⋅ ⋅ E ~ ~ ) mechanism at the combinational frequency ω ω C + ω ω S where ω ω C is the cyclotron frequency. Strongly anisotropic EDSR band due to inversion-asymmetry k 3 Dresselhaus spin–orbit coupling was observed in InSb at the spin-flip frequency ω ω S by Dobrowolska et al. spin–orbit coupling in n -Ge that manifests itself through strongly anisotropic electron g -factor results in EDSR through breaking translational symmetry by inhomogeneous electric fields which mixes wave functions of different valleys. Infrared EDSR observed in semimagnetic semiconductor Cd 1 − − x Mn x Se was ascribed to spin–orbit coupling through inhomogeneous exchange field. EDSR with free and trapped charge carriers was observed and studied at a large variety of three-dimensional (3D) systems including dislocations in Si, an element with notoriously weak spin–orbit coupling. All above experiments were performed in the bulk of three-dimensional (3D) systems.