Phononic and magnetic excitations in the quasi-one-dimensional Heisenberg antiferromagnet KCuF₃

Abstract

The Raman-active phonons and magnetic excitations in the orbitally ordered, quasi-one-dimensional Heisenberg antiferromagnet KCuF₃ are studied as a function of temperature in the range between 3 and 290 K in various scattering configurations. The low-energy Eg and B1g phonon modes show an anomalous softening (~25% and ~13%) between room temperature and the characteristic temperature TS = 50 K. In this temperature range a freezing-in of F– ion dynamic displacements is proposed to occur. In addition, the Eg mode at about 260 cm⁻¹ clearly splits below TS. The widths of the phonon lines above TS follow an activated behavior with an activation energy of about 50 K. Our observations clearly evidence a reduction of the lattice symmetry below TS and indicate a strong coupling of lattice, spin, and orbital fluctuations for T > TS. A strongly polarization dependent quasielastic scattering is observed at temperatures above TN = 39 K due to magnetic-energy fluctuations. For temperatures below TN a rich spectrum of additional modes is observed, with different lineshape, polarization and temperatures dependence. We attribute them to longitudinal and transverse magnetic modes as well as to a continuum of magnetic excitations.