Quantum metal phase in ultrathin niobium films

We study the magnetic field induced transition from superconducting to “dirty” metallic phase in niobium films, of the thickness d varying from 1.1 nm to 20 nm, sandwiched between Si buffer layers. As d is reduced, the structure of the Nb films changes from polycrystalline to amorphous, and the Hall coefficient evolves from positive to negative [1]. In the presence of the magnetic field a crossing point of the resistance isotherms appears, usually associated with the superconductor-insulator transition. In the thinnest films at mK temperatures the resistance saturates at the level smaller than the normal state resistance, signaling the likely presence of quantum metal phase. The power-law scaling of the resistance with the magnetic field is consistent with the Bose-metal model [2]. The evolution of the scaling with d indicates a direct relation of the scaling exponent to the activation energy for vortex pinning, extracted from the Arrhenius law at higher temperatures. We are grateful to Leyi Y. Zhu and Chia-Ling Chien (Johns Hopkins University) for growing the films.
Ref.:[1] I. Zaytseva et al., Phys. Rev. B 90, 060505(R) (2014). [2] D. Das & S. Doniach, Phys. Rev. B 64, 134511 (2001).