Electric-dipole spin resonance and spin orbit coupling effects in odd-integer quantum Hall edge channels

We have observed remarkable multiple-line electron spin-resonance spectra in sensitive terahertz laser photoresponse measurements of the two-dimensional electron gas of an asymmetric InAs quantum well in the integer quantum Hall effect regime. Near filling factor 7 with the magnetic field oriented at large angles θ from the normal to the sample surface, rather than the expected single-electron spin-resonance line, we observed several sharp pairs of features at fields across the corresponding quantum Hall plateau. At a unique angle θc, the dominant central pair merges into a single line close to the estimated magnetic field of electron spin resonance. For θ>θc this line splits into two sharper features whose separation in magnetic field increases with increasing θ. Surprisingly, for θ<θc the central feature disappears. The explanation of this behavior, as well as the observation of additional pairs of sharp features with larger magnetic-field separations, is based on strong spin orbit coupling effects and the concomitant effective magnetic fields associated with pairs of oppositely directed, persistent quantum Hall edge currents combined with the behavior of edge channels near the center of the odd plateaus. Modeling the splitting of the spin resonances due to these effective spin orbit magnetic fields is in reasonable agreement with observations. These results show that it is possible to probe the widths of quantum Hall edge channels through the spectral specificity of the electron spin resonance and possibly manipulate spins with THz photons having wavelengths several orders of magnitude larger than the edge-channel widths.