Stripe order, depinning, and fluctuations in La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ and La$_{1.875}$Ba$_{0.075}$Sr$_{0.050}$CuO$_{4}$

TL;DR

This study uses neutron scattering to investigate stripe correlations, magnetic excitations, and their evolution across structural transitions in specific cuprate superconductors, revealing evidence of dynamical charge stripes and their temperature dependence.

Contribution

It provides detailed neutron scattering data on stripe order and magnetic excitations in La-based cuprates, highlighting the continuous evolution of spin excitations through structural phase transitions.

Findings

Stripe order appears below 50 K in LTT phase.

Magnetic excitations resemble spin waves in the ordered state.

Incommensurability varies with temperature and phase transitions.

Abstract

We present a neutron scattering study of stripe correlations measured on a single crystal of La$_{1.875}$Ba$_{0.125}$CuO$_{4}$. Within the low-temperature-tetragonal (LTT) phase, superlattice peaks indicative of spin and charge stripe order are observed below 50 K. For excitation energies $\hbar\omega\le12$ meV, we have characterized the magnetic excitations that emerge from the incommensurate magnetic superlattice peaks. In the ordered state, these excitations are similar to spin waves. Following these excitations as a function of temperature, we find that there is relatively little change in the {\bf Q}-integrated dynamical spin susceptibility for $\hbar\omega\sim10$ meV as stripe order disappears and then as the structure transforms from LTT to the low-temperature-orthorhombic (LTO) phase. The {\bf Q}-integrated signal at lower energies changes more dramatically through these transitions, as it must in a transformation from an ordered to a disordered state. We argue that the continuous evolution through the transitions provides direct evidence that the incommensurate spin excitations in the disordered state are an indicator of dynamical charge stripes. An interesting feature of the thermal evolution is a variation in the incommensurability of the magnetic scattering. Similar behavior is observed in measurements on a single crystal of La$_{1.875}$Ba$_{0.075}$Sr$_{0.050}$CuO$_{4}$; maps of the scattered intensity in a region centered on the antiferromagnetic wave vector and measured at $\hbar\omega=4$ meV are well reproduced by a model of disordered stripes with a temperature-dependent mixture of stripe spacings. We discuss the relevance of our results to understanding the magnetic excitations in cuprate superconductors.