Signature of Cooper pairs in the Metallic and Insulating Phases of Homogeneously Disordered Superconducting Ta Films

With the increase of magnetic field or the decrease of sample thickness, homogeneously disordered superconducting Ta films undergo a superconductor-metal-insulator phase transition [1][2]. Each phase displays remarkably different nonlinear current-voltage (I-V) characteristics. The evolution of the nonlinear transport in the insulating phase exhibits a non-monotonic behavior as the magnetic field is increased, which could be evidence of the presence of localized Cooper pairs in the insulating phase [3]. As the metallic phase intervenes the superconducting and insulating states in Ta films, we further suggest that Cooper pairs also exist in the metallic ground state. References: [1] Y. Qin et al., Phys. Rev. B 73, 100505(R) (2006). [2] Y. Li et al., Phys. Rev. B 81, 020505 (R) (2010). [3] Y. S. Li, Supercond. Sci. Technol. 28, 025002 (2015).