Lattice distortions in KCuF3: a paradigm shift?

Abstract

KCuF 3 is the paradigmatic compound for the co-operative Jahn-Teller effect. But do we really know its structure?Co-operative Jahn-Teller distortions are ubiquitous. But do we really know where they come from? Usually they go along with other distortions and ordering phenomena, which make it difficult to identify the actual driving mechanism. KCuF 3 is believed to be a beautiful exception, the cleanest realization of a co-operative Jahn-Teller system. Its CuF 6 octahedra are slightly compressed along z, and have a long (l) and a short (s) CuF bond in the xy plane. The electronic configuration of Cu is 3d9 , a single hole in the e g states; the distortions split the otherwise degenerate e g orbitals, and the hole goes into the |s 2 -z 2 state. The spatial alternation of l and s bonds in all direction produces the orbital pattern shown in Fig. 1. Despite its simple structure, the origin of the co-operative Jahn-Teller distortion in KCuF 3 remained a puzzle. Is it driven by electron-phonon coupling or by many-body superexchange [1]? Early-on static mean-field LDA+U calculations showed that the stability of the Jahn-Teller distortion (total energy gain) is strongly enhanced by Coulomb repulsion, a result recently confirmed by dynamical mean-field (DMFT) calculations [2]. Does this mean that many-body super-exchange is driving the co-operative distortion? The answer remained elusive for half a century. Only recently, by using DMFT and a new approach to separate super-exchange and electron-phonon coupling effects, the puzzle was finally solved. It was shown [3,4] that many-body super-exchange alone gives a critical temperature of about 350 K, very large, but far too low to explain experimental facts: the co-operative Jahn-Teller distortion persists up to T OO ∼ 800 K, and probably at even higher temperatures. Hence the static Jahn-Teller distortions must be driven by electronphonon coupling. *

Electronic Structure and Magnetic Coupling of Pure and Mg-Doped KCuF3

“…This experimental result is in nice agreement with the LDA + DMFT calculations [2] and supports the conclusion 2 Advances in Condensed Matter Physics that the electron phonon coupling is the most important interaction in stabilizing the peculiar structure of KCuF 3 . The subtle interplay between spin, orbital, and vibrational degrees of freedom of this material is still the subject of experimental and theoretical investigations [7][8][9][10][11][12][13][14][15][16][17][18], adopting a variety of techniques ranging from inelastic Raman and X-rays scattering and absorption, [11,13], neutron scattering [10,14], DFT based electronic structure computations [8], and lattice dynamics [7,15] and, more recently, calculations within the variational Green's function formalism [17,18].…”