Pressure-dependent, low temperature inelastic light (Raman) scattering measurements of KCuF$_3$ show that applied pressure above $P^{*} sim$ 7 kbar suppresses a previously observed structural phase transition temperature to zero temperature in KCuF$_
3$, resulting in the development of a $omegasim$ 0 fluctuational (quasielastic) response near $T sim$ 0 K. This pressure-induced fluctuational response --- which we associate with slow fluctuations of the CuF$_6$ octahedral orientation --- is temperature independent and exhibits a characteristic fluctuation rate that is much larger than the temperature, consistent with quantum fluctuations of the CuF$_6$ octahedra. A model of pseudospin-phonon coupling provides a qualitative description of both the temperature- and pressure-dependent evolution of the Raman spectra of KCuF$_3$.
The propagation of field disturbances is examined in the context of the effective Yang-Mills Lagrangian, which is intended to be applied to QCD systems. It is shown that birefringence phenomena can occur in such systems provided some restrictive cond
itions, as causality, are fulfilled. Possible applications to phenomenology are addressed.
