Direct observation of a one-dimensional static spin modulation in insulating La1.95Sr0.05CuO4

We report the results of an extensive elastic neutron-scattering study of the incommensurate (IC) static spin correlations in La1.95Sr0.05CuO4, which is an insulating spin glass at low temperatures. Recent work by Wakimoto et nl, has revealed the presence of new two-dimensional satellite peaks in La1.95Sr0.05CuO4 at positions rotated by similar to 45 degrees 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)(ortho) and (0,1 +/- 0.064,L)(ortho), that is, the modulation vector is only along the orthorhombic b* axis. This demonstrates unambiguously that La1.95Sr0.05CuO4 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 have 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 similar or equal to x. These results demonstrate that the insulator to superconductor transition in the underdoped regime (0.05 less than or equal to x less than or equal to 0.06) in La2-xSrxCuO4 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.