The depairing critical current density, J_c, determined by the depairing process of Cooper pairs, is crucial for the study of the superconducting mechanism, because it directly provides information on the critical velocity of superfluids, and the magnitude as well as the symmetry of the superconducting (SC) gap. The depairing process occurs when the kinetic energy of the supercurrent exceeds the condensation energy (∝SC gap). However, it is difficult to be achieved, since the vortex flow occurs preceding the depairing at much smaller current density. In addition, it is very difficult for a clean single crystal to achieve this limit since the extremely large current is needed. To solve this problem, micro-fabrication technique is used to reduce the size of the crystal to sub-micrometer scale.

Here, we report the fabrication of a series of in-plane and out-of-plane narrow bridges in Fe(Te,Se) single crystals, with a square-submicrometer cross-section area. The Jc was directly estimated from the transport current-voltage measurements, which reaches a very large value, about one order of magnitude larger than the depinning Jc. Besides, the value of Jc is found to be enhanced in the same bridge by simply reducing its thickness. Those results suggest that we successfully achieve the depairing Jc. The temperature dependence of the depairing Jc was found can be qualitatively described by the Kupriyanov-Lukichev theory. In addition, the anisotropy of the depairing Jc is also obtained. Our study provides a new route to understand the behavior of depairing Jc in iron-based superconductors in a wide temperature range.

Keywords: depairing critical current density, Fe(Te,Se), micro-fabrication, current-voltage measurements