Measurements of charge localization effects in uranium and plutonium intermetallic compounds from $L_{\rm III}$ X-ray absorption spectroscopy

At the heart of the complex behavior of 5$f$-electrons in uranium and plutonium is the variable localization of the 5$f$ orbital. Plutonium, in particular, can behave like a localized lanthanide system or a delocalized transition-metal system, depending on its coordination environment. This behavior gives rise to a variety of novel physical properties in U and Pu compounds: unexpected magnetic susceptibilities, non-Fermi-liquid behavior, superconductivity, etc. To understand the dual nature of U and Pu 5$f$ orbitals, we report measurements of the actinide $L_{\rm III}$-edge X-ray absorption spectra, including high-resolution partial fluorescence yield measurements, for a wide variety of intermetallics, including $\alpha$-U, UCoGa$_5$, UCd$_{11}$, $\alpha-$ and $\delta-$Pu, PuCoGa$_5$, and PuCoIn$_5$. A systematic shift of the $L_{\rm III}$-edge energy is observed, and is correlated to the Sommerfeld coefficient from heat capacity measurements. Based on these results, we argue that this type of experiment is a direct measure of the degree of 5$f$ orbital localization for U and Pu.