Electron–phonon interactions and superconductivity of β -Nb2N thin films

Abstract

Niobium nitride (NbN) has attracted scientific interest due to its diverse physical properties and a variety of structural phases. The structure and superconductivity of the cubic δ-NbN phase are well established, but its hexagonal phases are not explored hitherto. In the present work, we report a simple synthesis route and a detailed study of hexagonal β-Nb2N thin films. Thermal annealing of sputtered grown α-NbN leads to a single phase β-Nb2N at 973 K as confirmed by x-ray diffraction and absorption spectroscopy. The electrical transport measurements revealed a dominance of electron–phonon interactions with a superconducting transition around 4.74 K and an upper critical field [HC2(0)] of 3.99 T. The estimated HC2(0) is well below the calculated Pauli limit, and the Maki parameter value (α < 1) indicates that HC2(0) is dominated by an orbital pair breaking effect. Finally, the obtained value of electron–phonon coupling constant (λ) is in excellent agreement with a weak coupling Bardeen–Cooper–Schrieffer value of conventional superconducting materials.