YRu$_3$B$_2$ -- a kagome lattice superconductor

We report the synthesis and physical properties of a polycrystalline, hexagonal boride YRu$_3$B$_2$. Our resistivity and heat capacity measurements indicate that YRu$_3$B$2$ is a weakly coupled superconductor, with critical temperature $T_c$ = 0.63 K and upper critical field $μ_0 H{c2}$ (0)=0.11 T. Density functional theory calculations, together with chemical-bonding analysis, reveal that the electronic states at and near the Fermi energy level are dominated by the Ru kagome sublattice.

💡 Research Summary

This paper presents a comprehensive study on the synthesis, physical properties, and electronic structure of the hexagonal boride YRu3B2, identifying it as a weakly-coupled superconductor derived from a kagome lattice. Polycrystalline samples were successfully synthesized via an arc-melting technique. Structural characterization using powder X-ray diffraction confirmed the CeCo3B2-type structure with space group P6/mmm and lattice parameters a = 5.4826(2) Å and c = 3.02166(1) Å.

Detailed low-temperature measurements of electrical resistivity and heat capacity revealed bulk superconductivity below a critical temperature Tc = 0.63 K (from heat capacity) or 0.7 K (from resistivity midpoint). The material exhibits metallic behavior with a relatively high residual resistivity ratio (RRR) of approximately 10 for a polycrystalline sample. Analysis of the specific heat data yielded a Sommerfeld coefficient γ = 14.9 mJ mol⁻¹ K⁻² and a Debye temperature ΘD = 420 K. Using the McMillan formula, the electron-phonon coupling constant was estimated to be λ_el.-ph. ≈ 0.35-0.43, categorizing YRu3B2 as a weakly-coupled superconductor. Resistivity measurements under magnetic fields allowed for the determination of the upper critical field μ0Hc2(T), which was fitted using the Ginzburg-Landau expression to extract a zero-temperature value of μ0Hc2(0) = 0.11 T.

The core insight of the work comes from density functional theory (DFT) calculations and chemical bonding analysis. The results unequivocally demonstrate that the electronic states at and near the Fermi level are dominated by contributions from the Ru 4d atomic orbitals, which form a kagome sublattice within the crystal structure. While a perfectly flat topological band is not observed, the Fermi level crosses several weakly dispersive bands, and symmetry-protected band crossings are identified near the K point in the Brillouin zone. Crystal Orbital Hamilton Population (COHP) analysis further reveals that occupied states near the Fermi level have strong Ru-Ru antibonding character, which the authors link to a previously observed correlation between antibonding states and superconductivity in other material families.

The study positions YRu3B2 within the broader context of kagome lattice superconductors. It is isostructural to LaRu3Si2, which exhibits a much higher Tc (~7 K) and suggested interplay with charge order. The significant difference in Tc between these related compounds highlights how subtle changes in composition and electronic detail can dramatically alter superconducting properties, even within the same structural framework hosting a kagome network. The paper also reports preliminary doping studies, showing that partial substitution of Y with Sc (up to x ~0.15 in Y1-xScxRu3B2) maintains superconductivity with a similar Tc, whereas the related compound YCo3B2 showed no superconductivity above 0.5 K, underscoring the essential role of Ruthenium in the observed phenomena.