Direct observation of a one-dimensional static spin modulation in insulatingLa1.95Sr0.05CuO4

We report the results of an extensive elastic neutron-scattering study of the incommensurate (IC) static spin correlations in ${\mathrm{La}}_{1.95}{\mathrm{Sr}}_{0.05}{\mathrm{CuO}}_4,$ 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 ${\mathrm{La}}_{1.95}{\mathrm{Sr}}_{0.05}{\mathrm{CuO}}_4$ at positions rotated by $\sim 45°$ in reciprocal space from those found in superconducting samples. The present neutron-scattering experiments on the same $x=0.05\mathrm{}$ 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 $\mathrm{}(1,0,0)\mathrm{}$ and $\mathrm{}(0,1,0)\mathrm{}$ (orthorhombic notation) in each domain separately. We find that, in a single twin, only two satellites are observed at $(1,\pm {0.064,L)}_{\mathrm{ortho}}$ and $(0,1\mathrm{}\pm {0.064,L)}_{\mathrm{ortho}},$ that is, the modulation vector is only along the orthorhombic $b^{*}$ axis. This demonstrates unambiguously that ${\mathrm{La}}_{1.95}{\mathrm{Sr}}_{0.05}{\mathrm{CuO}}_4$ 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 have also reexamined the $x=0.04\mathrm{}$ crystal that previously was reported to show a single commensurate peak. By mounting the sample in the $\mathrm{}(H,K,0\mathrm{})\mathrm{}$ zone, we have discovered that the $x=0.04\mathrm{}$ sample in fact has the same IC structure as the $x=0.05\mathrm{}$ sample. The incommensurability parameter $\delta$ for $x=0.04\mathrm{}$ and 0.05, where $\delta$ is the distance from $\mathrm{}(1/2,1/2)\mathrm{}$ in tetragonal reciprocal lattice units, follows the linear relation $\delta \simeq x.\mathrm{}$ These results demonstrate that the insulator to superconductor transition in the underdoped regime $(0.05\mathrm{}<~x<~0.06)$ in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ 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.