Finite Field Local Field Catastrophe — Application to the Spectra of KCN_xCl_1−x

Abstract

The oscillator strength of the CN molecular exciton in KCN_xCl_1−x varies by about two orders of magnitude for a nominal variation in x or temperature. The enhancement in pure KCN as a function of temperature appears below the orientational phase transition of the CN molecules. The enhancement is explained by a finite frequency version of the Lorentz local field catastrophe. The conditions for the occurrence of such phenomena are explored. The crucial role is played by the background susceptibility at the exciton frequency, due to the interband polarizability. As this background susceptibility rises above a certain value, rapid transfer of the oscillator strength from the interband to the exciton peak occurs. It is suggested that the rise of the background susceptibility is connected with the ordering of the CN⁻ dipoles as the temperature is decreased. A mean field theory of the phase transition in pure KCN is also presented. For details of the theory, please see: A. J. Holden, V. Heine, J. C. Inkson, C. M. Varma and M. A. Bosch, J. Phys C, 12 (1035), 1979. The reflectivity spectra in KCN is presented by M. A. Bosch and G. Zumofen, Phys. Rev. Letters 41, 590 (1978).