Dynamical Dzyaloshinsky-Moriya interaction in KCuF3: Raman evidence for an antiferrodistortive lattice instability

TL;DR

This paper provides Raman spectroscopy evidence of an antiferrodistortive lattice instability in KCuF3, revealing phonon anomalies and symmetry reduction linked to lattice and spin fluctuations around 50 K.

Contribution

It uncovers the dynamical Dzyaloshinsky-Moriya interaction and structural instability in KCuF3 through phonon behavior analysis.

Findings

Anomalous softening of phonon modes between room temperature and 50 K

Splitting of the Eg phonon mode below 50 K

Activated behavior of phonon linewidths indicating symmetry reduction

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 T_S = 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 T_S. The width of the phonon lines above T_S follows an activated behavior with an activation energy of about 50 K. Our observations clearly evidence a reduction of the structural symmetry below T_S and indicate a strong coupling of lattice and spin fluctuations for T>T_S.