Direct Observation of a One Dimensional Static Spin Modulation in Insulating La1.95Sr0.05CuO4

TL;DR

This study uses neutron scattering to reveal that La1.95Sr0.05CuO4 exhibits a one-dimensional static diagonal spin modulation, linking stripe order with the insulator-superconductor transition in underdoped cuprates.

Contribution

It provides direct experimental evidence of one-dimensional diagonal spin modulation in La1.95Sr0.05CuO4 and clarifies its relation to doping and superconductivity.

Findings

La1.95Sr0.05CuO4 has a one-dimensional static diagonal spin modulation.

The incommensurability parameter d follows the linear relation d=x.

The insulator-superconductor transition coincides with a transition from diagonal to collinear stripe order.

Abstract

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. 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 orthorhombic twins at low temperatures with relative populations of 2:1. 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 static diagonal spin modulation at low temperatures, consistent with certain stripe models. 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 d for x=0.04 and 0.05, where d is the distance from (1/2, 1/2) in tetragonal reciprocal lattice units, follows the linear relation d=x. These results demonstrate that the insulator to superconductor transition in the under doped regime (0.05 </= x </= 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.