Superconductivity of thin film rock-salt NbO emerging in a free-standing state

Niobium monoxide (NbO) is well-established to possess a pseudo-rock salt structure with 25% ordered vacancies in the Nb and O sublattices for each as its thermodynamically stable configuration [1]. However, our recent research has demonstrated the successful synthesis of rock salt-type NbO thin films on MgO (001) substrates utilizing pulsed laser deposition [2]. These films, when the thickness is constrained to below 100 nm, exhibit tetragonal distortions (c/a >1) due to compressive strain from the substrate, manifesting Mott-insulating behavior. On the other hand, thicker films become more relaxed from the strain state and closer to its original cubic structure (c/a ~1), displaying metallic and superconducting transition with a maximum T_c of 7.4 K.

In this study, we report a novel application of the freestanding film approach [3] to induce superconductivity even in otherwise insulating rock salt-type thin NbO films. The introduction of a water-soluble sacrificial layer, Ba₃Al₂O₆, was found to be useful for getting a NbO film to be in the free-standing state, with the substrate-induced strain effectively mitigated and emergence of superconductivity. The results demonstrate the freestanding film approach as a promising technique for manipulating physical and chemical states in thin film systems, such as the superconducting state in NbO.

[1] Phys. Rev. B 46, 14001 (1992). [2] R. Kimura, K. Kaminaga et al., Chem. Mater. 36, 5028-5036 (2024). [3] ACS Appl. Electron. Mater. 1, 1269−1274 (2019).