Linewidth anomaly of two-dimensional-electron cyclotron resonance in the extreme quantum limit

We report the observation of a striking line narrowing with decreasing T in the cyclotron resonance (CR) of high-mobility two-dimensional electrons in GaAs/${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As in the extreme quantum limit. From 4.2 to 1.2 K, where the dc mobility increases by less than 1%, the CR linewidth can decrease by as much as threefold, accompanied by a downward shift of ∼0.3% in effective mass. Inhomogeneous broadening of Landau levels by long-range potential fluctuations can qualitatively account for the striking features of the data, though not their dependences on the filling factor ν. .AE