Inelastic light scattering measurements of a pressure-induced quantum liquid in K_CuF3

Pressure-dependent, low-temperature inelastic light (Raman) scattering measurements of KCuF₃ show that applied pressure above P ^*∼7kbar suppresses a previously observed structural phase transition temperature to zero temperature in KCuF₃, resulting in the development of a fluctuational (quasielastic) response near T∼0K. This pressure-induced fluctuational response-which we associate with slow fluctuations of the CuF₆ 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 KCuF₃.