Highly dispersive spin excitations in the chain cuprate Li₂CuO₂

¹  Leibniz-Institut für Festkörper- und Werkstoffforschung (IFW) - Dresden, Germany, EU
²  Institute for Problems of Materials Science - Krzhizhanovskogo 3, 03180 Kiev, Ukraine
³  Institut für Festkörperphysik, Technische Universität Dresden - 01062 Dresden, Germany, EU
⁴  Institute of Physics, ASCR - Prague, Czech Republic, EU
⁵  Max-Planck-Institut für Chemische Physik fester Stoffe - Dresden, Germany, EU
⁶  Institut Laue Langevin - F-38042 Grenoble Cedex 9, France, EU
⁷  Jülich Centre for Neutron Science JCNS - Jülich, Germany, EU

Corresponding authors: w.lorenz@ifw-dresden.de s.l.drechsler@ifw-dresden.de

Received: 29 September 2009
Accepted: 25 October 2009

Abstract

We present an inelastic neutron scattering investigation of Li₂CuO₂ detecting the long-sought quasi-1D magnetic excitations with a large dispersion along the CuO₂-chains studied up to 25 meV. The total dispersion is governed by a surprisingly large ferromagnetic (FM) nearest-neighbor exchange integral J₁ = -228 K. An anomalous quartic dispersion near the zone center and a pronounced minimum near (0,0.11,0.5) r.l.u. (corresponding to a spiral excitation with a pitch angle of about 41°) point to the vicinity of a 3D FM-spiral critical point. The leading exchange couplings are obtained applying standard linear spin-wave theory. The 2nd neighbor inter-chain interaction suppresses a spiral state and drives the FM in-chain ordering below the Néel temperature. The obtained exchange parameters are in agreement with the results for a realistic five-band extended Hubbard Cu O model and L(S)DA+U predictions.