Vortex penetration along twin boundaries in pristine and proton-irradiated FeSe

Below ${T}_{\mathrm{s}}\ensuremath{\sim}90\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, single-crystal FeSe undergoes a structural phase transition coinciding with the emergence of lattice domain boundaries, termed twin boundaries. Polarized-light microscopy validated the presence of twin boundaries in pristine FeSe and indicated the existence of extensive microsized twining in proton-irradiated FeSe. In twinned FeSe, vortex penetration displays a fractal, mazelike pattern, likely guided by these twin domains. We speculate substantial microtwinning in the proton-irradiation crystals, yet macroscopic twin boundaries persist as primary conduits for vortex motion.