Kane-Fisher weak link physics in the clean scratched-XY model

The nature of the superfluid-insulator transition in 1D has been much debated recently. In particular, to describe the strong disorder regime characterized by weak link proliferation, a scratched-XY model has been proposed [New J. Phys. \textbf{18}, 045018 (2016)], where the transport is dominated by a single anomalously weak link and is governed by Kane-Fisher weak link physics. In this article, we consider the simplest problem to which the scratched-XY model relates: a single weak link in an otherwise \textit{clean} system, with an intensity $J_W$ which decreases algebraically with the size of the system $J_W\sim L^{-\alpha}$. Using a renormalization group approach and a vortex energy argument, we describe the Kane-Fisher physics in this model and show that it leads to a transition from a transparent regime for $K>K_c$ to a perfect cut for $K<K_c$, with an adjustable $K_c=1/(1-\alpha)$ depending on $\alpha$. We check our theoretical predictions with Monte Carlo numerical simulations complemented by finite-size scaling. Our results clarify two important assumptions at the basis of the scratched-XY scenario, the behaviors of the crossover length scale from weak link physics to transparency and of the superfluid stiffness.