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We report the results of an extensive elastic neutron-scattering study of the incommensurate ͑IC͒ static spin correlations in La 1.95 Sr 0.05 CuO 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 1.95 Sr 0.05 CuO 4 at positions rotated by ϳ45°in reciprocal space from those found in superconducting samples. The present neutron-scattering experiments on the same xϭ0.05 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 notation͒ in each domain separately. We find that, in a single twin, only two satellites are observed at (1,Ϯ0.064,L) ortho and (0,1Ϯ0.064,L) ortho , that is, the modulation vector is only along the orthorhombic b* axis. This demonstrates unambiguously that La 1.95 Sr 0.05 CuO 4 has a onedimensional 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 have also reexamined the xϭ0.04 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 xϭ0.04 sample in fact has the same IC structure as the xϭ0.05 sample. The incommensurability parameter ␦ for x ϭ0.04 and 0.05, where ␦ is the distance from (1/2,1/2) in tetragonal reciprocal lattice units, follows the linear relation ␦Ӎx. These results demonstrate that the insulator to superconductor transition in the underdoped regime (0.05рxр0.06) in La 2Ϫx Sr x CuO 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.

We report a detailed study, using neutron scattering, transport and magnetization measurements, of the interplay between superconducting (SC) and spin density wave (SDW) order in La2CuO4+y. Both kinds of order set in below the same critical temperature. However, the SDW order grows with applied magnetic field, whereas SC order is suppressed. Most importantly, the field dependence of the SDW Bragg peak intensity has a cusp at zero field, as predicted by a recent theory of competing SDW and SC order. This leads us to conclude that there is a repulsive coupling between the two order parameters. The question of whether the two kinds of order coexist or microscopically phase separate is discussed.

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