Structural modulation, physical properties, and electronic band structure of the kagome metal UCr$_6$Ge$_6$
The chemical flexibility of the $RM_6X_6$ stoichiometry, where an $f$-block element is intercalated in the CoSn structure type, allows for the tuning of flatbands associated with kagome lattices to the Fermi level and for emergent phenomena due to interactions between the $f$- and $d$-electron lattices. Yet, 5$f$ members of the ``166" compounds are underrepresented compared with 4$f$ members. Here, we report single-crystal growth of UCr$_6$Ge$_6$, which crystallizes in a monoclinically distorted Y$_{0.5}$Co$_3$Ge$_3$-type structure. The real-space character of the modulation, which is unique within the $RM_6X_6$ family, is approximated by a 3$\times$1$\times$2 supercell of the average monoclinic cell. The compound has kagome-lattice flatbands near the Fermi level and a moderately enhanced electronic heat capacity, as evidenced by its low-temperature Sommerfeld coefficient ($γ=86.5$~mJ~mol$^{-1}$~K$^{-2}$) paired with band structure calculations. The small, isotropic magnetization and featureless resistivity of UCr$_6$Ge$_6$ suggest itinerant uranium 5$f$ electrons and Pauli paramagnetism. Angle-resolved photoemission spectroscopy results provide evidence for uranium 5$f$ weight at the Fermi level and for a flatband near the Fermi level associated with the chromium $3d$ kagome lattice. The isotropic magnetic behavior of the uranium 5$f$ electrons starkly contrasts with localized behavior in other uranium 166 compounds, highlighting the high tunability of the magnetic ground state across the material family.