Highly dispersive spin excitations in the chain cuprate Li

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 $3d$ O $2p$ model and L(S)DA+U predictions