PHYSICAL REVIEW B VOLUME 46, NUMBER 14 1 OCTOBER 1992-II Low-energy incommensurate spin excitations in superconducting La, s, Sro»Cuo4 T. R. Thurston, P. M. Gehring, and G. Shirane Department of Physics, Ilrookhauen National Laboratory, Upton, New York 11973 R. J. Birgeneau and M. A. Kastner Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Y. Endoh, M. Matsuda, and K. Yamada Department of Physics, Tohoku University, Sendai 980, Japan H. Kojima and I. Tanaka Inorganic Synthesis Facility, Engineering Faculty, Yamanashi University, Kofu 400, Japan (Received 3 April 1992) Neutron-scattering experiments with markedly improved signal-to-noise ratios have been performed on single-crystal superconducting La& s, Sro»Cu04 (T, =33 K) at energies down to 1. 5 meV. The mea- surernents provide quantitative information on the temperature dependence of g"(q, co). Surprisingly, y"(q~„k, co) is approximately constant below T, for the measured energies 1. 5 meV co ~ 6 meV, that is, for energies much less than the mean-field BCS gap energy of — 10 meV, hence implying other than con- ventional s-wave superconductivity. The superconducting copper oxides La2 „Sr„Cu04 are well known to exhibit unusual short-range magnetic correlations. Early neutron-scattering experiments estab- lished that incommensurate spin correlations' persist for superconducting hole concentrations. Further, the structure factor $(q, co) in T, =33 K superconducting La, 85 Sro» CuOz was found to exhibit a suppression below — 100 K for q near the (n, m)positio. n . in reciprocal space (square lattice notation, unit lattice constant) at co=6 meV, while at to=12 meV, no such anomaly oc- curred. However, a complete characterization of the in- commensurate scattering geometry and a thorough study of the low-energy excitations were not performed in this early work. Recently, Cheong et al. have shown that the positions of the incommensurate peaks are at [n. (1+5), tr(1+5)]. With this information, we have per- formed experiments that probe the low-energy magnetic excitations. The high quality of our data with signal to noise ratios of up to — 5, which represents a marked improvement on other measurements, ' has allowed a detailed charac- terization of the magnetic excitations for energies less than 6 meV. We find that the superconducting transition influences $(q, co) significantly at relatively low energies, co~3 meV. Further, there are substantial spin fluctua- tions in the superconducting state at energies as low as 1. S meV. Indeed, on converting the measured $(q, to) to y"(q, co) we find that y"(q „„, to) is approximately con- stant below T, for energies 1. 5 meV +~~6 meV. Our experiments also show, in agreement with the recent work of Mason et al. , that the width in momentum space of the incommensurate peaks decreases markedly with decreasing energy. The two single crystals used in these experiments were grown using the traveling Aoating-zone method. One of the crystals, sample 1 (labeled KOS-1 in Ref. 2), was ex- amined in previous neutron experiments. Extensive mea- surements characterizing this sample and, in particular, showing its high degree of microscopic homogeneity are reported in Ref. 2. For the current experiments, a second sample, sample 2, of similar quality to sample 1, was mounted and aligned beside sample 1 in order to increase the scattering intensity. The total sample volume was then — 1. 0 cm, and the combined mosaicity was very good, less than -0. 15' half width at half maximum (HWHM). Sample 1 and sample 2 have identical trans- port and magnetic properties. We emphasize here that both crystals undergo a bulk superconducting transition at 33 K with Meissner fractions approaching 100%. The weak-coupling BCS gap in this material is 25 =3. 5k~ T, = 10 meV so that measurements of the spin fluctuations for co ((10 meV are particularly important. Neutron-scattering experiments were carried out on the triple-axis spectrometer H-7 at the Brookhaven High Flux Beam Reactor. For most of the experiments the in- cident neutron energy was fixed at 14. 7 meV and the hor- izontal collimator sequence was 40'-80'-S-80'-80'. This gives an energy resolution of -0.5-meV HWHM. Even though the sample is slightly orthorhornbic below -200 K, in this paper we shall label reciprocal space using tetragonal units (q„,q, q, ). Here q„and q refer to the directions of the square lattice formed by the Cu02 sheets, and q, refers to the layer stacking direction. The magnitude of q„, q, and q, will be given in units where the lattice constants are set at 1. The sample is mounted in two configurations. In the first, scans with q„=q and q, arbitrary can be per- forined. This allows access to the (nm)position. In the. second, the sample is tilted about the (0, 0, 1) axis by an angle P, so that scans with q =q, cos(45+(b), 46 912S 1992 The American Physical Society