Dynamical Dzyaloshinsky-Moriya interaction in KCuF3: Raman evidence for an antiferrodistortive lattice instability
Abstract
In the orbitally ordered, quasi-one dimensional Heisenberg antiferromagnet KCuF3 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 width of the phonon lines above TS follows an activated behavior with an activation energy of about 50 K. Our observations clearly evidence a reduction of the structural symmetry below TS and indicate a strong coupling of lattice and spin fluctuations for T>TS.
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