  We investigate the band structure, nematic state and superconducting gap
structure of two selected FeSe single crystals containing different amount of
disorder. Transport and angle-resolved photoemission spectroscopy measurements
show that the small amount of disorder has little effect to the band structure
and the nematic state of FeSe. However, temperature and magnetic field
dependencies of specific heat for the two samples are quite different.
Wave-vector-dependent gap structure are obtained from the three dimensional
field-angle-resolved specific heat measurements. A small gap with two
vertical-line nodes or gap minima along the $k_z$ direction is found only in
the sample with higher quality. Such symmetry-unprotected nodes or gap minima
are found to be smeared out by small amount of disorder, and the gap becomes
isotropic in the sample of lower quality. Our study reveals that the reported
controversy on the gap structure of FeSe is due to the disorder-sensitive
node-like small gap.
