Quantum oscillations of the superconductor LaRu₂P₂ : comparable mass enhancement λ approx 1 in Ru and Fe phosphides

We have studied the angular dependent de Haas-van Alphen oscillations of LaRu$_2$P$_2$ using magnetic torque in pulsed magnetic fields up to 60T. The observed oscillation frequencies are in excellent agreement with the geometry of the calculated Fermi surface. The temperature dependence of the oscillation amplitudes reveals effective masses m*($\alpha$)=0.71 and m*($\beta$)=0.99 m$_e$, which are enhanced over the calculated band mass by $\lambda^{cyc}$ of 0.8. We find a similar enhancement $\lambda^{\gamma} \approx 1$ in comparing the measured electronic specific heat ($\gamma = 11.5$ mJ/mol K$^2$) with the total DOS from band structure calculations. Remarkably, very similar mass enhancements have been reported in other pnictides LaFe$_2$P$_2$, LaFePO ($T_c \approx 4K$), and LaRuPO, independent of whether they are superconducting or not. This is contrary to the common perceptions that the normal state quasi-particle renormalizations reflect the strength of the superconducting paring mechanism and leads to new questions about pairing in isostructural and isoelectronic Ru- and Fe-pnictide superconductors.