RESONANT NEUTRON SCATTERING FROM YBa2Cu307

We have developed a new scattering geometry for magnetic neutron scattering experiments on YBa2Cu307 in which the phonon background around q (E, -), Rw 40meV is significantly reduced. We use this new approach to study the previously detected &a& magnetic excitation at 4OmeV in the superconducting state in detail. The excitation does not shift substantially in energy up to at least 75K (-0.8T,). Polarized neutron scattering experiments (horizontal minus vertical field) confirm the magnetic origin of the 40meV excitation and put stringent limits on the magnetic scattering intensity in the normal state. Over the past seven years, numerous magnetic neutron scattering experiments on the underdoped Y3a&u30s+x system have been performed and have revealed a wealth of important information on the magnetic ground state and excitations [1,2]. Until recently similar experiments on the fully oxygenated compound YBazCu307 have been more controversial [2,3], because the magnetic cross section is smaller and occurs at higher energies. Since in all neutron scattering experiments on YBa2Cu307 (unpolarized as well as polarized) the background is comparable or larger than the signal, the background subtraction has to be carried out with extreme care. For unpolarized beam experiments, the background from one-phonon and multiphonon scattering is dominant. While multiphonon scattering gives a background which is featureless in energy and momentum, dispersion and dynamical structure factor effects in scattering from single optical phonons generally lead to distinct features of the scattering cross section in energy and momentum which are difficult to distinguish from magnetic excitations. We have therefore begun to develop a detailed understanding of the phonon spectrum with emphasis on the energy range (hw 40meV) in which magnetic excitations had previously been reported [2,3]. Detailed lattice dynamical calculations, together with experimental studies of the temperature and momentum dependence of the scattered intensity, allowed us to separate a phonon involving predominantly c-axis motion of the in-plane oxygen from a magnetic excitation with a similar energy and dynamical structure factor [4]. Our measurements were performed at the High Flux Beam Reactor at Brookhaven at the H7, H8 and H4M triple axis spectrometers. Figure 1 shows the scattering geometry we developed for this purpose. As first reported by Rossat-Mignod et al. [2], the magnetic excitation occurs at -4OmeV and Q = (!(2i+ 11, ;(2j+ l), 1.7(2k+ 1)) with the momen“standard” geometry