Optical properties of the 1D antiferromagnet KCuF$_3$

We present measurements of the optical properties of KCuF3, a paradigm for orbital ordering [1] and one of the best realizations of a quasi-one dimensional spin chain [2]. We can identify the d-d excitations on the Cu sites and the charge- transfer gap of the system. The observed crystal-field level splitting and the gap value will be compared to recent results obtained by LDA+U calculations [3]. Moreover, we find anomalies in the optical properties already above the magnetic phase transition. These features appear concomitantly with a change in the orbital order parameter as reported by resonant x-ray scattering [4] and indicate a symmetry change already above the Neel temperature [5]. [1] K. I. Kugel and D. I. Khomskii, Sov. Phys. Usp. 25, 231 (1982). [2] B. Lake et al., Nature Materials 4, 329 (2005). [3] S.V. Streltsov et al., Phys.~Rev.~B {\bf 71}, 245114 (2005); I. Leonov, unpublished. [4] L. Paolasini et al., PRL 88, 106403 (2002). [5] N. Binggeli and M. Altarelli, PRB 70, 085117 (2004).