High-energy magnetic excitations from dynamic stripes in La1.875 Ba0.125 Cu O4

We use inelastic neutron scattering to study the temperature dependence of magnetic excitations (for energies up to 100 meV) in the cuprate La1.875 Ba0.125 Cu O4. This compound exhibits stripe order below a temperature of ∼50 K; previous measurements have shown that the magnetic excitations of the stripe-ordered phase have an hour-glass-like dispersion, with a saddle point at ∼50 meV. Here, we compare measurements in the disordered phase taken at 65 and 300 K. At energies on the scale of kB T, there is substantial momentum broadening of the signal, and the low-energy incommensurate features can no longer be resolved at 300 K. In contrast, there is remarkably little change in the scattered signal for energies greater than kB T. In fact, the momentum-integrated dynamic susceptibility is almost constant with temperature. The continuity of the magnetic excitations at E>50 meV indicates that the character of the local antiferromagnetic spin correlations is independent of stripe ordering. Based on the smooth evolution of the lower-energy excitations into the spin waves of the stripe-ordered phase, we infer the existence of dynamic stripe correlations in the disordered phase. We reconsider the nature of the magnetic dispersion, and we discuss the correspondences between the thermal evolution of magnetic stripe correlations with other electronic properties of the cuprates.