Optimal coloring and strain-enhanced superconductivity in Li$_n$B$_{n+1}$C$_{n-1}$

TL;DR

This study identifies stable configurations of lithium borocarbides and demonstrates that applying strain can dramatically enhance their superconducting transition temperature from near zero to 37 K.

Contribution

The paper introduces a novel approach combining cluster expansion and strain engineering to achieve high-temperature superconductivity in lithium borocarbides.

Findings

Stable configurations of Li$_2$B$_3$C and Li$_3$B$_4$C$_2$ identified.

Superconducting transition temperature can be increased to 37 K under -5 ext{ extdegree} uniaxial strain.

Electron-phonon coupling is highly sensitive to external strain in these materials.

Abstract

Boron-rich lithium borocarbides are promising candidates for phonon-mediated high-temperature superconductors due to their metallic $\sigma$-bonding electrons. Here, we use the cluster expansion method to identify energetically stable configurations (colorings) of Li$_2$B$_3$C and Li$_3$B$_4$C$_2$, which are characterized by a distinctive pattern of alternating B-B and B-C zigzag chains. Surprisingly, the optimal configuration of Li$_2$B$_3$C exhibits an extremely low superconducting transition temperature of $T_c < 0.03$ K, which is attributed to the suppression of deformation potentials near the Fermi level caused by the specific electron filling of B-B zigzag chains. However, the $\sigma$-bonding electrons at the Fermi level are highly sensitive to external strain or pressure. Specifically, applying a -5\% compressive uniaxial strain can significantly enhance the electron-phonon coupling and the Eliashberg spectral function, boosting up $T_c$ to 37 K. This work not only presents a novel strategy for achieving phonon-mediated high-temperature superconductivity in Li$_n$B$_{n+1}$C$_{n-1}$ compounds but also provides valuable insights into the complex interplay between electronic structure and superconducting interaction.