Vortex pinning is a crucial factor that determines the critical current of practical superconductors, but its microscopic mechanism has long been elusive. Here using high-resolution scanning tunneling microscopy, we studied the pinning of single vortex by point defect in high-Tc FeSe-based superconductors. We found the defect-vortex interaction drives low-energy vortex bound states away from EF, which effectively lowered the energy of vortex and caused the pinning. It indicates the local pairing near the pinned vortex core is actually enhanced, which is in sharp contrast to the traditional understanding that non-superconducting regions pin the vortex, but is well captured by our microscopic quantum model. Furthermore, we directly obtained the elementary pinning energy from tunneling spectrum and estimated the pinning force via the spatial gradient of pinning energy. The obtained value aligns with the bulk critical current measurement. Our study thus established a general microscopic mechanism of vortex pinning in superconductors.

The STM data was collected by the Nanonis and RHK STM controller software, and the transport data was collected by Quantum Design PPMS DynaCool system.