[Paper]Imaging anisotropic vortex dynamics in FeSe
Our study of imaging anisotropic vortex dynamics in iron-based superconductor FeSe was published in Physical Review B.
When a type-ll superconductor becomes superconducting in an external magnetic field, most fluxes are shielded but some are trapped as quantized fluxes (flux pinning). It is well known that a vortex is trapped in lattice defects and forms a lattice structure in a high field. However, with respect to the dynamics of an isolated vortex, there are many unresolved questions. Here we imaged the anisotropic pinning potential of isolated vortices by measuring ac susceptibility in the area of an isolated vortex with our scanning SQUID microscope, which has a field coil to locally apply an ac magnetic field. Our results show that in a FeSe sample, which has twin boundaries due to the electron nematic order, the vortices are trapped along twin boundaries and have the anisotropic pinning potential associated with them. These results demonstrate the usefulness of our new method to directly measure the pinning potential of the isolated vortices.
The ac susceptibility image shows the observed vortex moves anisotropically. (a)Observed ac susceptibility around an isolated vortex. (b) Simulation of susceptibility with use of a toy model of anisotropic quadratic pinning potential. (c) The difference of observed and simulated values. Reprinted figure from PRB 100, 024514 (2019). Copyright 2019, by the American Physical Society.
Vortices are trapped and move anisotropically along the twin boundaries. (a)Local flux perpendicular to surface. (b)Local ac susceptibility. Reprinted figure from PRB 100, 024514 (2019). Copyright 2019, by the American Physical Society.