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

Abstract

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 $\ensuremath{\sim}45\ifmmode^\circ\else\textdegree\fi{}$ 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 $(1,\ifmmode\pm\else\textpm\fi{}{0.064,L)}_{\mathrm{ortho}}$ and $(0,1\ifmmode\pm\else\textpm\fi{}{0.064,L)}_{\mathrm{ortho}},$ 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 $x=0.04$ 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 $\ensuremath{\delta}$ for $x=0.04$ and 0.05, where $\ensuremath{\delta}$ is the distance from $(1/2,1/2)$ in tetragonal reciprocal lattice units, follows the linear relation $\ensuremath{\delta}\ensuremath{\simeq}x.$ These results demonstrate that the insulator to superconductor transition in the underdoped regime $(0.05<~x<~0.06)$ in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ 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.