Hydrostatic High-Pressure Studies to 25 GPa on the Model Superconducting Pnictide LaRu$_{2}$P$_{2}$

Prior to the discovery of the Fe-pnictides in 2008, the ruthenium phosphide LaRu$_{2}$P$_{2}$ possessed the highest value of the superconducting transition temperature $T_{c\, }\approx $ 4 K in the entire pnictide family. Recently, there has been renewed interest in this compound in an effort to understand why its $T_{c}$ value is as much as 15x lower than for some Fe-pnictides. Recent soft x-ray angle-resolved photoemission spectroscopy studies [1] have revealed that LaRu$_{2}$P$_{2}$ exhibits Fermi liquid behavior with weak electron-electron correlations. The superconducting properties of LaRu$_{2}$P$_{2}$ are also more isotropic than those of the Fe-pnictides. Unfortunately, the dependence of $T_{c}$ on hydrostatic pressure has not yet been determined, although such studies have an excellent track record for uncovering valuable systematics and pointing the way to higher $T_{c}$ values. We report the first measurement for LaRu$_{2}$P$_{2}$ of the dependence of $T_{c}$ on hydrostatic pressure. In these studies a He-gas system provides pressures to 1 GPa followed by a He-loaded diamond-anvil cell to higher pressures. $T_{c}$ initially increases under pressure, but exhibits a relatively sharp downturn above 2 GPa, indicating a possible structural phase transition. The results of these studies are compared to related work on the Fe-pnictides. [1] Razzoli \textit{et al.,} Phys. Rev. Lett. 108, 257005 (2012).