Structural modulation, physical properties, and electronic band structure of the kagome metal UCr$_6$Ge$_6$

TL;DR

This work develops a detailed synthesis and multi-faceted characterization of the actinide-containing kagome material $UCr_6Ge_6$, revealing a distinct monoclinic modulation that can be modeled by a $3×1×2$ supercell within an average $C2/m$ cell. Combined experimental measurements (magnetic susceptibility, heat capacity, and resistivity) with first-principles calculations show kagome flatbands near the Fermi level and a moderately enhanced Sommerfeld coefficient $\gamma$, consistent with an increased DOS at $E_F$ from both Cr and U states. The magnetism is isotropic and consistent with itinerant uranium $5f$ electrons exhibiting Pauli paramagnetism, though a crystal-field singlet ground state remains possible; transport shows anisotropy with conduction preferentially along the direction normal to the kagome layers. Overall, the results demonstrate the potential to tune electronic band structure and magnetic ground states in 5$f$ 166 systems, expanding kagome physics into actinide chemistry and offering a platform for exploring correlated flatband phenomena.

Abstract

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. 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.

Figures (7)

• Figure 1: (a) Average unit cell of UCr$_6$Ge$_6$ with half-occupancy uranium and germanium sites (b) View down the $c$ axis showing the kagome layer of chromium atoms (c) $h0l$ precession image for single crystal XRD on UCr$_6$Ge$_6$ (d) Zoomed in view of the $h0l$ precession image showing the average cell reflections circled in pink and the modulation reflections at $l=\pm0.5$ circled in white
• Figure 2: A 3$\times$1$\times$2 supercell that approximates the real-space modulation of UCr$_6$Ge$_6$ is viewed down the average cell's (a) $c$ axis and (b) $b$ axis. Dashed and solid white lines indicate the bounds of the 3$\times$1$\times$2 supercell and the average cell, respectively. (c) The building blocks of the supercell are cages with half- (gray), 88%- (blue), and 12%- (orange) occupied U centers and Ge vertices. In the actual crystal, to maintain the correct stoichiometry and reasonable bond distances, the Ge vertices and the U center cannot be occupied in neighboring cages along $c$. Atom colors follow Fig. \ref{['fig:cell']}.
• Figure 3: EDS maps of the surface of a polished UCr$_6$Ge$_6$ crystal (black=no intensity). A Sn streak due to residual flux is apparent but localized. The bulk of the material is the expected 1:6:6 stoichiometry.
• Figure 4: (a) The zero-field-cooled and field-cooled magnetic susceptibilities of UCr$_6$Ge$_6$ with field parallel to ($H{\parallel}ab$) and perpendicular to ($H{\parallel}c$) the kagome plane contain a ferro(ferri)magnetic transition at 30 K from an impurity. (b) The 1.8 K magnetization of UCr$_6$Ge$_6$ is linear, featureless, and isotropic up to 6.5 T.
• Figure 5: The heat capacity ($C_\mathrm{p}/T$) of UCr$_6$Ge$_6$ is shown with (inset) a Sommerfeld coefficient ($\gamma$) fit to the low-temperature heat capacity, giving 86.5 mJ mol$^{-1}$ K$^{-2}$.