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

Gnezdilov, V.; Deisenhofer, J.; Lemmens, P.; Wulferding, D.; Afanasiev, O.; Ghigna, P.; Loidl, A.; Yeremenko, A.

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 <$E E sub g> and <$E B sub 1g> phonon modes show an anomalous softening (25 % and 13 %) between room temperature and the characteristic temperature TS = 50 К. In this temperature range а freezing-in of F<^>- ion dynamic displacements is proposed to occur. In addition, the <$E E sub g> mode at about 260 cm<^>-1 clearly splits below TS. The widths of the phonon lines above TS follow an activated behavior with an activation energy of about 50 K. Our observations clearly evidence a reduction of the lattice symmetry below TS and indicate a strong coupling of lattice, spin, and orbital fluctuations for <$E T~>>~T sub S>. A strongly polarization dependent quasielastic scattering is observed at temperatures above TN = 39 К 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.


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