Revealing the microscopic mechanism of elementary vortex pinning in superconductors

Vortex pinning is a crucial factor that determines the critical current of
practical superconductors and enables their diverse applications. However,
the underlying mechanism of vortex pinning has long been elusive, lacking
a clear microscopic explanation. Here, using high-resolution scanning
tunneling microscopy, we studied single vortex pinning induced by a point
defect in layered FeSe-based superconductors. We found the defect-vortex
interaction drives low-energy vortex bound states away from EF, creating a
“mini” gap that effectively lowers the system energy and enhances pinning.
By measuring the local density of states, we directly obtained the
elementary pinning energy and estimated the pinning force via the spatial
gradient of pinning energy. The results are consistent with bulk critical
current measurement. Furthermore, we show that a general microscopic
quantum model incorporating defect-vortex interaction can naturally
capture our observation. It suggests that the local pairing near the
pinned vortex core is actually enhanced compared to an unpinned vortex,
which is beyond the traditional understanding that non-superconducting
regions pin vortices. Our study thus unveils a general microscopic
mechanism of vortex pinning in superconductors, and provides insights for
enhancing the critical current of practical superconductors.