Spin Correlations among the Charge Carriers in an Ordered Stripe Phase

We have observed a diffuse component to the low-energy magnetic excitation spectrum of stripe-ordered ${\mathrm{L}\mathrm{a}}_{5/3}{\mathrm{S}\mathrm{r}}_{1/3}{\mathrm{N}\mathrm{i}\mathrm{O}}_4$ probed by neutron inelastic scattering. The diffuse scattering forms a square pattern with sides parallel and perpendicular to the stripe directions. The signal is dispersive, with a maximum energy of $\sim 10\mathrm{m}\mathrm{e}\mathrm{V}$. Probed at 2 meV, the scattering decreases in strength with increasing temperature, and is barely visible at 100 K. We argue that the signal originates from dynamic, quasi-one-dimensional, antiferromagnetic correlations among the stripe electrons.

Figure 1

Diffuse neutron inelastic scattering from ${\mathrm{L}\mathrm{a}}_{5/3}{\mathrm{S}\mathrm{r}}_{1/3}{\mathrm{N}\mathrm{i}\mathrm{O}}_4$. (a) Contour plot of the intensity in the $(h,k,0)$ plane in reciprocal space at an energy transfer of 2.5 meV. The plot is derived from a $25\times 35$ rectangular grid of data points. (b) Diagram of the $(h,k,0)$ plane in reciprocal space. Circles are stripe superlattice zone centers. Diagonal lines indicate where diffuse scattering would be observed if the stripes behaved as 1D AF chains. Filled circles and full lines are for $[1,-1,0]$ stripe domains; open circles and broken lines are for [1,1,0] stripe domains. The dotted lines marked A and B are scan directions used in the experiment. (c) [1,1,0] stripe domain showing the square lattice of Ni sites with local 1D AF correlations on the stripes, as proposed here.

Figure 2

Constant-energy scans along lines equivalent to those marked A and B in Fig. 1b. The polarized-neutron data in (b) show the SF and NSF channels recorded with the neutron polarization parallel to the scattering vector.

Figure 3

(a) Constant-energy scans along the line marked B in Fig. 1b. Successive scans have been displaced vertically by 2000 counts for clarity. (b) Dispersion of the peak positions from a series of constant-energy scans including those shown in (a). The line is the lower bound to the excitation spectrum of a spin-$\frac{1}{2}$ Heisenberg AF chain calculated from Eq. (1) with $J=3.2\mathrm{m}\mathrm{e}\mathrm{V}$. The vertical dotted lines indicate one period of the lower bound curve.

Figure 4

Temperature dependence of the diffuse scattering measured in a scan along line B in Fig. 1b. The energy was fixed at 2 meV. Successive scans have been displaced vertically by 400 counts for clarity.