Symmetry Character of theE1Edge in InSb

Interband Landau transitions have been investigated magneto-optically as a function of crystal orientation at the $E_1$ edge of InSb. The energy spacings of the levels are largely independent of crystal orientation for a series of samples with surface normals in a [110] plane. This result is at variance with theories for bulk interband transitions at an anisotropic band edge. A hypothesis involving a bound surface resonance is suggested to explain our measurements.