Evolution of the superconducting properties inFeSe1−xSx

We present scanning tunneling microscopy and spectroscopy measurements on ${\mathrm{FeSe}}_{1\ensuremath{-}x}{\mathrm{S}}_{x}$ single crystals with $x=0$, $0.04$, and $0.09$. The S substitution into the Se site is equivalent to a positive chemical pressure, since S and Se have the same valence and S has a smaller ionic radius than Se. The subsequent changes in the electronic structure of FeSe induce a decrease of the structural transition temperature and a small increase in the superconducting critical temperature. The evolution of the gaps with increasing S concentration suggests an increase of the hole Fermi surface. Moreover, the vortex core anisotropy, that likely reflects the Fermi surface anisotropy, is strongly suppressed by the S substitution.