Depairing critical current density and the vortex-free state in FeSe nanobridges

The depairing limit and the vortex-free state in a superconductor is crucial for both the study of supercurrent related physics and the application eliminating noise linked to vortex motion. In this work, we report the evidence of depairing limit and the vortex-free state achieved by geometric constraint in FeSe superconductors. A series of narrow bridges with varying widths at the same location of a single crystal were prepared by the ``pickup'' method using successive focused ion beam millings. By simply reducing the width of bridge, the magnitude of critical current density $({J}_{\mathrm{c}})$ is enhanced more than one order, evidencing the achievement of depairing limit. Moreover, in the bridge with a width smaller than the penetration depth $(\ensuremath{\lambda})$, ${J}_{\mathrm{c}}$ is found to be robust against magnetic field up to 1 kOe. The field-robust ${J}_{\mathrm{c}}$ is a strong piece of evidence for vortex-free state, which is created by the enhancement of lower critical fields due to geometric constraint.