Combining the advantages of superconducting MgB2 and CaC6 in one material: suggestions from first-principles calculations

TL;DR

This study uses first-principles calculations to explore a layered lithium monoboride phase, Li2B2, which combines key superconducting mechanisms of MgB2 and CaC6, but exhibits weaker electron-phonon coupling due to missing pi states, suggesting doping could enhance its superconductivity.

Contribution

First-principles calculations reveal the electronic and phononic properties of Li2B2, proposing doping strategies to improve its superconducting potential beyond MgB2.

Findings

Li2B2 combines mechanisms of MgB2 and CaC6

Electron-phonon coupling in Li2B2 is weaker than in MgB2 and CaC6

Doping Li2B2 to restore pi electrons could enhance superconductivity

Abstract

We show that a recently predicted layered phase of lithium monoboride, Li2B2, combines the key mechanism for strong electron-phonon coupling in MgB2 (i.e., interaction of covalent B sigma bands with B bond-stretching modes) with the dominant coupling mechanism in CaC6 (i.e., interaction of free-electron-like interlayer states with soft intercalant modes). Yet, surprisingly, the electron-phonon coupling in Li2B2 is calculated to be weaker than in either MgB2 or CaC6. We demonstrate that this is due to the accidental absence of B pi states at the Fermi level in Li2B2. In MgB2, the pi electrons play an indirect but important role in strengthening the coupling of sigma electrons. Doping Li2B2 to restore pi electrons at the Fermi level is expected to lead to a new superconductor that could surpass MgB2 in Tc.