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 λ 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.