The attempt to understand cuprate superconductors is complicated by the presence of multiple strong interactions. While many believe that antiferromagnetism is important for the superconductivity, there has been revived interest in the role of electron-lattice coupling. 1,2,3,4The recently studied conventional superconductor MgB 2 has a very strong electron-lattice coupling, involving a particular vibrational mode (phonon), that was predicted by standard theory and confirmed quantitatively by experiment. 5Here we present inelastic scattering measurements that show a similarly strong anomaly in the Cu-O bondstretching phonon in the cuprate superconductors La 2-x Sr x CuO 4 (with x=0.07, 0.15). This is in contrast to conventional theory, which does not predict such

No abstract

Both the static and dynamic properties of the antiferromagnetic state in YBa2Cu306. &5 have been reinvestigated by neutron scattering. The crystal studied exhibited a Neel temperature of 410 + 3 K, with a continuous transition below 15 K to a magnetic structure with a doubled unit cell along the c axis. The Cu magnetic form factor has been extracted from magnetic Bragg peak intensities measured at 15 K, and it is shown to have the large anisotropy expected for a Cu 3d 2 y2 state. The form-factor anisotropy can explain much of the Q dependence of the inelastic magnetic cross section that has been observed in superconducting YBaqCu306+ . A search for the optical spinwave modes at excitation energies up to 60 meV was unsuccessful. Analysis of acoustic spin-wave measurements yields an intralayer superexchange energy J~~o f 120 + 20 meV (without correction for quantum renormalization) with an anisotropy o. "=(7+ 1) x 10, and an effective next-nearestlayer exchange J&2 of 0.04 6 0.01 meV. A lower limit for the intra-bilayer exchange, Jzz, of 8 meV is established. Use of theoretical and experimental results for Q-integrated spin-wave intensities in the antiferromagnet, together with a crystal volume normalization based on phonon measurements, allows us to put previous measurements of spin Quctuations in superconducting YBaqCu306 6 on an absolute scale. The results for the superconductor are shown to be consistent with the relaxation rates determined by nuclear magnetic resonance.

No abstract

No abstract