Inelastic light scattering measurements of a pressure-induced quantum liquid in KCuF${}_3$

Pressure-dependent, low temperature inelastic light (Raman) scattering measurements of KCuF${}_3$ show that applied pressure above $P^{\ast }\sim$ 7 kbar suppresses a previously observed structural phase transition temperature to zero temperature in KCuF${}_3$, resulting in the development of a $\omega \sim$ 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${}_3$.