Disorder-induced Quantum Phase Transition in Mono-atomic-layer Superconductors

Unlike bulk counterparts, two-dimensional (2D) superconductors are sensitive to disorder. Here, we will present enhancement of the upper critical magnetic field and reduction in critical temperature on Pb mono-atomic-layer superconductors formed on vicinal substrates, where steps work as a disorder against the 2D superconductivity. We revealed how surface steps with an interval shorter than the coherence length ξ affect the superconducting properties by scanning tunneling microscopy (STM) and surface transport measurements. STM exhibited vortices elongated along the steps, that is, Abrikosov-Josephson vortices squeezed normal to the steps due to the reduced ξ. These results demonstrate that vicinal substrates provide a unique platform to manipulate the degree of disorder on 2D superconductors.