Direct observation of a one-dimensional static spin modulation in insulating La 1.95 Sr 0.05 CuO 4

We report the results of an extensive elastic neutron-scattering study of the incommensurate (IC) static spin correlations in La{sub 1.95}Sr{sub 0.05}CuO{sub 4}, which is an insulating spin glass at low temperatures. Recent work by Wakimoto et al. has revealed the presence of new two-dimensional satellite peaks in La{sub 1.95}Sr{sub 0.05}CuO{sub 4} at positions rotated by {approx}45 degree sign in reciprocal space from those found in superconducting samples. The present neutron-scattering experiments on the same x=0.05 crystal employ a narrower instrumental Q resolution and thereby have revealed that the crystal has only two, rather than four, orthorhombic twins at low temperatures with relative populations of 2:1. This has made possible the precise characterization of the IC elastic peaks around (1,0,0) and (0,1,0) (orthorhombic notation) in each domain separately. We find that, in a single twin, only two satellites are observed at (1,{+-}0.064,L){sub ortho} and (0,1{+-}0.064,L){sub ortho}, that is, the modulation vector is only along the orthorhombic b{sup *} axis. This demonstrates unambiguously that La{sub 1.95}Sr{sub 0.05}CuO{sub 4} has a one-dimensional rather than two-dimensional static diagonal spin modulation at low temperatures, consistent with certain stripe models. From the L dependence we conclude that the spin correlations are predominantly two dimensional. We havemore » also reexamined the x=0.04 crystal that previously was reported to show a single commensurate peak. By mounting the sample in the (H,K,0) zone, we have discovered that the x=0.04 sample in fact has the same IC structure as the x=0.05 sample. The incommensurability parameter {delta} for x=0.04 and 0.05, where {delta} is the distance from (1/2,1/2) in tetragonal reciprocal lattice units, follows the linear relation {delta}{approx_equal}x. These results demonstrate that the insulator to superconductor transition in the underdoped regime (0.05{<=}x{<=}0.06) in La{sub 2-x}Sr{sub x}CuO{sub 4} is coincident with a transition from diagonal to collinear static stripes at low temperatures thereby evincing the intimate coupling between the one-dimensional spin density modulation and the superconductivity. (c) 2000 The American Physical Society.« less