Multi-gap and high-Tc superconductivity in metal-atom-free borocarbides: Effects of dimensional confinement and strain engineering

TL;DR

This study combines high-throughput screening and anisotropic Migdal-Eliashberg theory to discover stable borocarbides with high-temperature, multi-gap superconductivity, enhanced by dimensional confinement and strain engineering.

Contribution

It introduces a systematic approach to design high-Tc, multi-gap superconductors in metal-free borocarbides through structural confinement and strain optimization.

Findings

Tc increases from 32 K in bulk to 75 K on surface due to dimensional effects.

External strain can push Tc above 90 K by tuning EPC and phonon frequencies.

Six distinct superconducting gaps are identified, linked to electron-phonon coupling distribution.

Abstract

Pure borocarbides suffer from limited superconducting potential due to intrinsic structural instability, requiring transition/alkali metals as dual-functional stabilizers and dopants. Here, by combining high-throughput screening with anisotropic Migdal-Eliashberg (aME) theory, we identify dynamically stable borocarbides where high-Tc superconductivity predominately originates from E symmetry-selective electron-phonon coupling (EPC). The six distinct superconducting gaps emerge from a staircase distribution or uncoupling of EPC strength across each Fermi surface (FS) sheet, constituting a metal-free system with such high gap multiplicity. Crucially, dimensional reduction from bulk to surface strengthens E-symmetry EPC and enhances Tc from 32 K (3D bulk) to 75 K (2D surface), a result that highlights structural confinement as a key design strategy for observing high Tc. External strain further optimizes the competition between EPC strength and characteristic phonon frequency to achieve Tc > 90 K. This work reveals a systematic correlation between structural dimensionality and gap multiplicity and establishes borocarbide as a tunable platform to engineer both high-Tc and multi-gap superconductivity.