HIGHER ORDER EQUATIONS AND CONSTITUENT FIELDS

We consider a simple wave equation of fourth degree in the D'Alembertian operator. It contains the main ingredients of a general Lorentz-invariant higher order equation, namely, a normal bradyonic sector, a tachyonic state and a pair of complex conjugate modes. The propagators are respectively the Feynman causal function and three Wheeler-Green functions (half-advanced and half-retarded). The latter are Lorentz-invariant and consistent with the elimination of tachyons and complex modes from free asymptotic states. We also verify the absence of absorptive parts from convolutions involving Wheeler propagators.

Raman Scattering in GaSe

Raman spectra have been obtained from both the ε and γ polytypes of GaSe. The frequencies of the phonons at the zone center have been obtained for both compounds at 80 K and at 295 K. All modes have been assigned on the basis of their observed polarization dependence to an irreducible representation of the appropriate point group. A comparison of the spectra of the two polytypes has provided further confirmation of previous identifications of the various modes in ε-GaSe. Excellent agreement has been obtained between the observed and predicted dependence of the polar mode frequencies with phonon propagation direction. No evidence of conjugate modes has been observed in any of the GaSe spectra.