Nature of the intensification of a cyclotron resonance in potassium in a   normal magnetic field
  The cyclotron-resonance peak which has been observed [G. A. Baraff et al., Phys. Rev. Lett. 22, 590 (1969) ] in potassium in a magnetic field directed perpendicular to the surface may be due to an effect of zero-curvature points of the Fermi surface on the cyclotron orbit of effective electrons. 
Natalya A. Zimbovskaya and Vsevolod I. Okulov