Anisotropic superconducting gaps in YNi$_2$B$_2$C: A first-principles investigation

TL;DR

This study uses first-principles calculations to analyze the anisotropic superconducting gaps in YNi$_2$B$_2$C, revealing the role of Fermi surface orbital character in gap variation and aligning well with experimental data.

Contribution

It demonstrates that Fermi surface orbital character variation is the key factor behind the anisotropic superconducting gaps in YNi$_2$B$_2$C using density functional theory.

Findings

Calculated phonon frequencies match experimental data.

Superconducting gaps vary with Fermi surface orbital character.

Weak phonon coupling in Ni 3d orbitals suppresses the gap.

Abstract

We calculate superconducting gaps and quasiparticle density of states of YNi$_2$B$_2$C in the framework of the density functional theory for superconductors to investigate the origin of a highly anisotropic superconducting gaps in this material. Calculated phonon frequencies, the quasiparticle density of states, and the transition temperature show good agreement with experimental results. From our calculation of superconducting gaps and orbital character analysis, we establish that the orbital character variation of the Fermi surface is the key factor of the anisotropic gap. Since the electronic states that consist of mainly Ni $3 d$ orbitals couple weakly with phonons, the superconducting gap function is suppressed for the corresponding states, which results in the anisotropy observed in the experiments. These results are hints to increase the transition temperature of materials in the borocarbide family.