FeSe exhibits a novel ground state in which superconductivity coexists with a nematic order in the absence of any long-range magnetic order. Here we report an angle-resolved photoemission study on the superconducting gap structure in the nematic state of FeSe 0.93 S 0.07 , without the complication caused by Fermi surface reconstruction induced by magnetic order. We found that the superconducting gap shows a pronounced 2-fold anisotropy around the elliptical hole pocket near the Z point of the Brillouin zone, with gap minima at the endpoints of its major axis, while no detectable gap was observed around the zone center and zone corner. The large anisotropy and nodal gap distribution demonstrate the substantial effects of the nematicity on the superconductivity, and thus put strong constraints on the current theories. PACS numbers: 74.70.Xa, 74.25.Jb, 74.20.Mn The pairing mechanism underlying unconventional superconductivity is often related to the quantum fluctuations of nearby orders. In most Fe-based superconductors, both magnetic and nematic orders appear simultaneously near the superconducting state. Accordingly, both spin-fluctuationmediated and orbital-fluctuation-mediated superconducting pairing mechanisms have been proposed [1][2][3][4][5]. Although intense experimental studies have been conducted [6][7][8][9][10][11][12][13], the exact pairing mechanism of Fe-based superconductors is still under heated debate.FeSe is a unique material with a novel superconducting state. Orbital order develops in the nematic state of FeSe without breaking the translational symmetry as shown by angle resolved photoemission spectroscopy (ARPES) studies [14,15]. The superconductivity coexists with the nematic order without any long range magnetic order [16], thus disentangling the magnetic and orbital orders. Moreover, recent results suggest that FeSe is a quantum paramagnet [4] with coexisting Néel and stripe antiferromagnetic interactions [17,18]. The novel ground state in FeSe provides a fresh perspective for studying the effect of nematic order on the superconducting gap structure in the absence of the Fermi surface reconstruction induced by magnetic order, which helps to reveal the roles of spin and orbital degrees of freedom in unconventional superconductivity. A nodeless superconducting gap structure in FeSe was suggested by previous reports on specific heat [19], Andreev reflection spectroscopy [20], and thermal conductivity measurements [21]. In contrast, scanning tunnelling spectroscopy (STS) studies on FeSe films [22] and transport measurements on bulk FeSe/FeSe 1−x S x crystals with improved quality [23,24] all demonstrate a nodal gap structure. However, due to the low T c and small gap size of FeSe/FeSe 1−x S x single crystals, the gap distribution in momentum-space is still unknown.In this work, we studied the superconducting gap structure of high-quality FeSe 0.93 S 0.07 single crystals (T c = 10 K) with high resolution ARPES [25]. At 6.3 K, both the nematic electronic structure and the superconducting gap ...

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