Strain-Tunable High-$T_c$ Superconductivity in Layered R3m B${}_2$CN

Using DFPT and Wannier-interpolated matrix elements in EPW, we find that isotropic tensile strain of 0–6% narrows the B:2$p$/C:2$p$ bands at the Fermi level while softening the phonon branches most strongly coupled to them, driving the coupling constant $\lambda$ from 0.967 to 2.620. All four strained configurations remain mechanically and dynamically stable, satisfying the Born–Huang criteria with no imaginary phonon modes at any strain level. The fully anisotropic Migdal–Eliashberg equations give a $T_c$ that rises monotonically from 53.5~K to 84.5~K across the strain series, crossing the liquid-nitrogen boiling point at 6% strain. The SISSO-based machine-learning model in EPW agrees with the anisotropic Migdal–Eliashberg $T_c$ values to within 10~K across the strain series.