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Superconductivity in YRu3B2 and LuRu3B2

Rose Albu Mustaf, Sajilesh K. P., Sanu Mishra, Junze Deng, Yi Jiang, Kaja H. Hiorth, Eeli O. Lamponen, Martin Gutierrez-Amigo, Päivi Törmä, Miguel A. L. Marques, B. Andrei Bernevig, Emilia Morosan

TL;DR

This work reports the experimental realization of superconductivity in two kagome-lattice compounds, YRu$_3$B$_2$ and LuRu$_3$B$_2$, guided by ML-accelerated high-throughput screening and first-principles calculations. Using a combination of DFT/DFPT, Wannier interpolation, and tight-binding mean-field modeling, the authors connect a broadened Ru $d_{x^2-y^2}$-derived quasi-flat band and phonon hardening to reduced electron-phonon coupling, explaining the observed $T_c$ values of $T_c \approx 0.81$ K and $0.95$ K and the near-bulk superconductivity. They show that the conventional, band-dispersion–driven contribution dominates the superfluid weight, with a moderate anisotropy in the London penetration depth, and validate a pipeline that couples rapid ML screening with rigorous ab initio analysis and experimental synthesis. The study demonstrates a practical path to accelerate discovery of superconductors by integrating computational predictions with targeted experiments, even when predicted $T_c$ values are refined downward after detailed phonon treatment.

Abstract

We report the experimental discovery of bulk superconductivity in two kagome lattice compounds, YRu$_3$B$_2$ and LuRu$_3$B$_2$, which were predicted through machine learning-accelerated high-throughput screening combined with first principles calculations. These materials crystallize in the hexagonal CeCo$_3$B$_2$-type structure with planar kagome networks formed by Ru atoms. We observe superconducting critical temperatures of $T_{c} = 0.81$~K for YRu$_3$B$_2$ and $T_{c} = 0.95$~K for LuRu$_3$B$_2$, confirmed through magnetization and specific heat measurements. Both compounds exhibit nearly 100\% superconducting volume fractions, demonstrating bulk superconductivity. Compared with LaRu$_3$Si$_2$, YRu$_3$B$_2$ and LuRu$_3$B$_2$ show a more dispersive Ru local $d_{x^2-y^2}$ quasi-flat band (and thus a reduced DOS at $E_F$) together with an overall hardening of the phonon spectrum, both of which lower the electron-phonon coupling (EPC) constant $λ$. Meanwhile, the dominant real-space EPC between Ru local $d_{x^2-y^2}$ states and the low-frequency Ru in-plane local $x$ branch remains nearly unchanged, indicating that the reduction of $λ$ originates from the $d_{x^2-y^2}$ DOS reduction and the overall phonon hardening. Superfluid weight calculations show that conventional contributions dominate over quantum geometric effects due to the dispersive nature of bands near the Fermi level. This work demonstrates the effectiveness of integrating machine learning screening, first principles theory, and experimental synthesis for accelerating the discovery of new superconducting materials.

Superconductivity in YRu3B2 and LuRu3B2

TL;DR

This work reports the experimental realization of superconductivity in two kagome-lattice compounds, YRuB and LuRuB, guided by ML-accelerated high-throughput screening and first-principles calculations. Using a combination of DFT/DFPT, Wannier interpolation, and tight-binding mean-field modeling, the authors connect a broadened Ru -derived quasi-flat band and phonon hardening to reduced electron-phonon coupling, explaining the observed values of K and K and the near-bulk superconductivity. They show that the conventional, band-dispersion–driven contribution dominates the superfluid weight, with a moderate anisotropy in the London penetration depth, and validate a pipeline that couples rapid ML screening with rigorous ab initio analysis and experimental synthesis. The study demonstrates a practical path to accelerate discovery of superconductors by integrating computational predictions with targeted experiments, even when predicted values are refined downward after detailed phonon treatment.

Abstract

We report the experimental discovery of bulk superconductivity in two kagome lattice compounds, YRuB and LuRuB, which were predicted through machine learning-accelerated high-throughput screening combined with first principles calculations. These materials crystallize in the hexagonal CeCoB-type structure with planar kagome networks formed by Ru atoms. We observe superconducting critical temperatures of ~K for YRuB and ~K for LuRuB, confirmed through magnetization and specific heat measurements. Both compounds exhibit nearly 100\% superconducting volume fractions, demonstrating bulk superconductivity. Compared with LaRuSi, YRuB and LuRuB show a more dispersive Ru local quasi-flat band (and thus a reduced DOS at ) together with an overall hardening of the phonon spectrum, both of which lower the electron-phonon coupling (EPC) constant . Meanwhile, the dominant real-space EPC between Ru local states and the low-frequency Ru in-plane local branch remains nearly unchanged, indicating that the reduction of originates from the DOS reduction and the overall phonon hardening. Superfluid weight calculations show that conventional contributions dominate over quantum geometric effects due to the dispersive nature of bands near the Fermi level. This work demonstrates the effectiveness of integrating machine learning screening, first principles theory, and experimental synthesis for accelerating the discovery of new superconducting materials.

Paper Structure

This paper contains 11 sections, 5 equations, 10 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Experiments
  3. Methods
  4. Results and Discussion
  5. Theory
  6. Methods
  7. Electronic band structure
  8. Phonons and electron-phonon coupling
  9. Mean-field tight-binding model analysis of the superconducting order parameter structure
  10. Superfluid weight and penetration depth calculations
  11. Conclusions

Figures (10)

  • Figure 1: Powder X-ray pattern for LuRu$_3$$B_2$ (red), with calculated pattern (black) and Bragg peak positions (green ticks) for space group P6/mmm. The asterisk denotes residual Ru. Inset: crystallographic unit cell of RRu$_3$B$_2$.
  • Figure 2: Magnetic susceptibility $4 \pi \chi$$_{eff}$ as a function of temperature for YRu$_3$B$_2$ (black) and LuRu$_3$B$_2$ (blue) at H = 5 Oe.
  • Figure 3: Magnetization for (a) YRu$_3$B$_2$ (b) LuRu$_3$B$_2$ as a function of H$_{eff}$ at T = 0.4 K, where H$_{eff}$ = H - N$_d$ M. Inset: linear fit of the low H magnetization showing where M(H) deviates from linearity, giving an estimate for H$_{c1}$.
  • Figure 4: C$_p$(T) data for (a) YRu$_3$B$_2$ and (b) LuRu$_3$B$_2$ in applied fields ranging from 0-800 Oe. Electronic specific heat C$_e$ scaled by temperature T for (c) YRu$_3$B$_2$ and (d) LuRu$_3$B$_2$ showing the entropy conservation construct.
  • Figure 5: H$_{c2}$(T) phase diagram for YRu$_3$B$_2$ (black) and LuRu$_3$B$_2$ (blue), with the G-L fit (\ref{['eq:hc2']}) shown in red.
  • ...and 5 more figures