New Evidence for the Existence of Exciton Effects at Hyperbolic Critical Points

Abstract

The derivative of the reflectivity spectrum of InSb near the ${E}_{1}$ and ${E}_{1}+{\ensuremath{\Delta}}_{1}$ peaks has been measured at 77\ifmmode^\circ\else\textdegree\fi{}K with a double-beam wavelength-modulation method which emphasizes singularities in the dielectric constant, ${\ensuremath{\epsilon}}_{1}(\ensuremath{\omega})+i{\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$. The experimental line shapes of $\frac{d{\ensuremath{\epsilon}}_{1}}{d\ensuremath{\omega}}$ and $\frac{d{\ensuremath{\epsilon}}_{2}}{d\ensuremath{\omega}}$ cannot be explained as being due to the generally accepted ${M}_{1}$ (hyperbolic) critical points, but are intermediate between the line shapes for ${M}_{1}$ and ${M}_{2}$ critical points. This mixture of line shapes is, in view of our knowledge of the one-electron band structure of InSb, evidence for the contribution of the electron-hole interaction (exciton effects) to the observed optical spectra.