Robust surface superconductivity and vortex lattice in the Weyl semimetal $γ$-PtBi$_2$

Abstract: The layered compound $\gamma$-PtBi$2$ is a topological semimetal with Fermi arcs at the surface joining bulk Weyl points. Recent work has found a peculiar surface superconducting state with a critical temperature two orders of magnitude larger than the bulk value. However, no superconducting vortices have been identified, raising questions about the robustness of the phase coherence in this new surface superconducting state. Here we measure the superconducting gap and visualize the vortex lattice through very low temperature Scanning Tunneling Microscopy (STM). We find vortices that are anomalously mobile and show that the extremely weak electrostatic interaction between the vortex charge and the STM tip is sufficient to drag the vortices of the surface superconductor. Our results show that surface superconductivity with $T_c=2.9$ K and $H{c2}\approx 1.8$ T is extraordinarily robust in $\gamma$-PtBi$_2$. Furthermore, quasiparticle scattering is enhanced at the position in the reciprocal space of the Fermi arcs, suggesting a connection between Weyl points and surface superconductivity.