Uranium $5f$ shell in ${\mathrm{UPd}}_2{\mathrm{Al}}_3$ and ${\mathrm{URu}}_2{\mathrm{Si}}_2$ studied by x-ray magnetic circular dichroism

We report x-ray magnetic circular dichroism (XMCD) measurements performed at the uranium ${\mathrm{M}}_{4,5}$ edges in the paramagnetic phase of the heavy fermion superconductors ${\mathrm{UPd}}_2{\mathrm{Al}}_3$ and ${\mathrm{URu}}_2{\mathrm{Si}}_2.$ The analysis of the spectra with the first sum rule yields the orbital moment of the $5f$ shell for both compounds. The shape of the dichroic spectrum at the ${\mathrm{M}}_5$ edge for the two compounds is qualitatively different: a single lobe is observed for ${\mathrm{URu}}_2{\mathrm{Si}}_2$ and two lobes are detected for ${\mathrm{UPd}}_2{\mathrm{Al}}_3.$ This two lobe structure reflects the strong effect of the interaction of the uranium $5f$ electrons with their environment in the latter compound.