Niobium-nitride-based nanophononic crystal as a superconductor

Phonon-mediated superconductivity, governed by Cooper pairing, is fundamentally influenced by the phonon band structure of the underlying metallic lattice. We investigate this connection in a niobium-nitride-based nanophononic crystal (NPC) configured as an ultrathin suspended membrane containing a periodic array of nanoscale voids. This architected geometry supports phonon Bragg scattering, which triggers frequency softening and reduced group velocities, leading to an enhancement of the electron–phonon coupling in accordance with Eliashberg theory. To capture these effects with high fidelity, we first perform large-scale lattice-dynamics simulations governed by a moment-tensor potential (MTP) constructed within the framework of machine-learning interatomic potentials (MLIPs)‒trained on density-functional-theory data for an ensemble of niobium nitride nanostructures. We then employ a Brillouin-zone mapping procedure to distill the essential features of the resulting phonon dispersion into a reduced surrogate model representing a uniform niobium-nitride membrane unit cell. This surrogate model enables an estimation of the electron–phonon coupling and the associated superconducting transition temperature directly from ab initio calculations. Anharmonic phonon properties are also analyzed to ensure persistence of wave coherence and the NPC effect. Finally, a systematic parametric study is conducted to elucidate the influence of unit-cell size and geometric features—such as pitch and relative void size—on the resulting phonon spectrum and coupling strength, revealing design pathways for maximizing the critical temperature in phonon-engineered superconductors.