Europhys. Lett., 70 (2), pp. 237–243 (2005) DOI: 10.1209/epl/i2004-10484-x EUROPHYSICS LETTERS 15 April 2005 Quantum helimagnetism of the frustrated spin- 1 2 chain LiCuVO 4 M. Enderle 1 , C. Mukherjee 2 , B. F˚ ak 3 , R. K. Kremer 4 , J.-M. Broto 5 , H. Rosner 6 , S.-L. Drechsler 7 , J. Richter 8 , J. Malek 7,9 , A. Prokofiev 10 , W. Assmus 10 , S. Pujol 1 , J.-L. Raggazzoni 1 , H. Rakoto 5 , M. Rheinst¨ adter 1 and H. M. Rønnow 11 1 Institut Laue-Langevin - BP 156, 38042 Grenoble C´ edex 9, France 2 Clarendon Laboratory - Parks Road, Oxford, OX1 3PU, UK 3 DRFMC/SPSMS, CEA - 38054 Grenoble, France 4 Max-Planck Institute for Solid State Research - 70569 Stuttgart, Germany 5 Laboratoire National des Champs Magn´ etiques Puls´ es - 31432 Toulouse, France 6 Max-Planck Institute for Chemical Physics of Solids - 01187 Dresden, Germany 7 Institut f¨ ur Festk¨ orpertheorie im IFW Dresden - Dresden, Germany 8 Theoretische Physik, Universit¨ at Magdeburg - Magdeburg, Germany 9 Institute of Physics, ASCR - Prague, Czech Republic 10 Johann Wolfgang von Goethe Universit¨ at - Postfach 111932, 60054 Frankfurt, Germany 11 Paul-Scherrer-Institut - 5232 Villigen PSI, Switzerland received 14 December 2004; accepted in final form 25 February 2005 published online 18 March 2005 PACS. 75.10.Jm – Quantized spin models. PACS. 75.30.-m – Intrinsic properties of magnetically ordered materials. PACS. 71.20.-b – Electron density of states and band structure of crystalline solids. Abstract. – Inelastic neutron scattering, susceptibility, and high-field magnetization identify LiCuVO4 as a nearest-neighbour ferromagnetic, next-nearest-neighbour frustrated, quasi–one- dimensional helimagnet, which is largely influenced by quantum fluctuations. Complementary band structure calculations provide a microscopic model with the correct sign and magnitude of the major exchange integrals. Geometrically frustrated spin- 1 2 systems display a rich variety of exotic ground states, re- flected in gapped or gapless, discrete and/or continuous excitation spectra, and often lack a classical analogue [1]. Even in the perhaps most simple frustration model, the purely one- dimensional (1D) Heisenberg model with competing nearest, J 1 , and next-nearest neighbour interactions, J 2 , the ground state phase diagram is complex and remarkably different from the classical case. This model is identical to the zigzag chain with leg exchange J 2 and rung (zigzag) exchange J 1 . Theoretical work has been focused on the subspace with antifer- romagnetic nearest (nn) and next-nearest neighbour (nnn) interactions, where a dimerized gapped phase [2–4] is surrounded by gapless commensurate [5] and gapped incommensurate c  EDP Sciences