We present scanning tunneling microscopy and spectroscopy measurements on FeSe1−xSx single crystals with x = 0, x=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.PACS numbers: 74.70. Xa, 74.25.Jb, 74.55.+v, INTRODUCTIONFeSe has the simplest crystallographic structure among the family of iron-based superconductors and investigations into this system may yield a better understanding of the origin of superconductivity in iron pnictides/chalcogenides. FeSe undergoes a structural phase transition from tetragonal to orthorhombic at T s ≈ 90K, but differently from other Fe-based superconductors this is not accompanied by a transition to a longrange magnetic order phase [1]. Below T s a strong electronic anisotropy develops that is detected through an anisotropic Fermi surface, an anisotropic resistivity and is sensitive to external parameters such as in-plane strain [2]. The dramatic changes in the electronic properties cannot be explained in terms of the small changes in the lattice parameters of the order of 0.1%, therefore, this phase is driven by electronic degree of freedom, namely the electronic nematic order [3]. This C4 symmetry breaking, which breaks the rotational symmetry without changing the translational symmetry of the lattice may play an important role in understanding the superconducting state that sets in below T c = 8.5K [4]. In Fe-based superconductors, the nematic phase is usually regarded as the orthorombic phase sandwiched between the structural transition and the onset of the long-range magnetic order. However, in the case of FeSe no longrange magnetic order has been observed, which makes FeSe an ideal system for the study of the nematic order even in the superconducting state. Indeed, the strong Abrikosov vortex core anisotropy observed in FeSe [5] has been regarded as a manifestation of the nematic order in this material [6,7].The origin of the nematicity in Fe-based superconductors is still a subject of intense debate. It has been proposed that nematicity could be driven either by orbital ordering of the Fe d electrons or by spin fluctuations. Furthermore, orbital order with interorbital electron-electron interactions would favor a sign preserving s-wave superconducting order parameter [8,9], while spin fluctuations with strong intraorbital interaction would favor a signchanging s ± -wave or d-wave pairing [3,[10][11][12][13][14][15][16][17]. The experimental findings in Fe-based superconductors so far have supported the latter scenario [18,19]. Howev...

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