Neutron Scattering Studies of the Copper Oxide Superconductor La$_{1.925}$Ba$_{0.075}$CuO$_4$

The interplay between superconductivity and magnetism is a central issue in the study of the high T$_c$ cuprates. The first to be discovered, the Bednorz-M\"{u}ller materials have been much less extensively studied due to the difficulty of growing single crystals. We report recent neutron scattering studies of a large single crystal of La$_{1.925}$Ba$_{0.075}$CuO$_4$. Incommensurate elastic peaks, evidence of static spin stripe order, develop below 40 K and persist into the superconducting state (T$_c \sim 25$ K). Investigations of the low energy incommensurate spin fluctuations show the dynamical spin susceptibility increases as T$_c$ is approached from above, remaining roughly constant below T$_c$. A similar variation in the incommensurability indicates a temperature dependent stripe spacing. We compare our results with studies of La$_{1.875}$Ba$_{0.125}$CuO$_4$ in the normal state [1]. [1] M. Fujita et al, cond-mat/0403396 (2004).