Anharmonic lattice dynamics and superconductivity in strained bulk and surface niobium

Using first-principles calculations, we investigate how homogeneous strain and crystallographic surface orientation modify the vibrational and superconducting properties of niobium. For bulk Nb, tensile strain strongly softens the phonon spectrum and enhances the electron--phonon coupling, increasing the superconducting transition temperature from 9.5 K at equilibrium to 14.5 K at $\sim\!6\%$ lattice expansion. For the low-index Nb(001), Nb(110), and Nb(111) surfaces, harmonic phonon calculations exhibit imaginary modes, showing that anharmonic lattice effects are essential. To treat these effects efficiently, we train Nb-specific machine-learning interatomic potentials on bulk and slab first-principles configurations and use them to accelerate stochastic self-consistent harmonic approximation calculations, thereby obtaining anharmonically renormalized phonon modes that are combined with density-functional perturbation theory electron--phonon matrix elements to construct the Eliashberg spectral function. Among the clean free-standing slabs considered here, Nb(001) exhibits the strongest electron--phonon coupling and the highest calculated transition temperature of 10.0 K, while Nb(110) and Nb(111) show progressively reduced pairing strength. Finally, by analyzing the Eliashberg spectral function and the functional derivative $\delta T_\text{c}/\delta\alpha^2F(\omega)$, we identify the phonon energy ranges most effective for superconducting pairing. Our results show that strain, surface termination, and anharmonic phonon renormalization provide complementary and interrelated microscopic routes for tuning superconductivity in Nb.