High-temperature superconductivity occurs near antiferromagnetic instabilities and the nematic state. Debate remains on the origin of nematic order in FeSe and its relation with superconductivity. Here, we use transport, neutron scattering and Fermi surface measurements to demonstrate that hydrothermo grown superconducting FeS, an isostructure of FeSe, is a tetragonal paramagnet without nematic order and with a quasiparticle mass significantly reduced from that of FeSe. Only stripe-type spin excitations are observed up to 100 meV. No direct coupling between spin excitations and superconductivity in FeS is found, suggesting that FeS is less correlated and the nematic order in FeSe is due to competing checkerboard and stripe spin fluctuations.npj Quantum Materials (2017) 2:14 ; doi:10.1038/s41535-017-0019-6 INTRODUCTION High-transition temperature superconductivity in copper oxides and iron-based materials occurs near checkerboard and stripe antiferromagnetic (AF) instabilities, respectively. [1][2][3] Although there is also ample evidence for the existence of a nematic order, where a translationally invariant metallic phase spontaneously breaks rotational symmetry, 4-8 and for a nematic quantum critical point near optimal superconductivity in iron-based superconductors, 9, 10 much remains unclear concerning its microscopic origin and its relationship to superconductivity. 2,3 In particular, recent debates focus on whether nematic order in superconducting FeSe below the tetragonal-to-orthorhombic transition temperature T s = 91 K without static AF order 11-13 is due to competing magnetic instabilities or orbital ordering.14-22 Here, we use transport, neutron scattering and Fermi surface measurements to demonstrate that superconducting FeS, an isostructure of FeSe, 23, 24 is a tetragonal paramagnet without nematic order and with a quasiparticle mass significantly reduced from that of FeSe. Our neutron scattering experiments in the energy regime below 100 meV reveal only stripe-type spin fluctuations in FeS that are not directly coupled to superconductivity. These properties suggest that FeS is a weakly correlated analog of FeSe and, moreover, that the nematic order in FeSe is due to the frustrated magnetic interactions underlying the competing checkerboard and stripe spin fluctuations.
16-18A key to understanding the physics of the iron-based superconductors is to determine the role played by magnetism and by electronic nematicity to superconductivity. [1][2][3][5][6][7] In a typical AF ordered iron-pnictide, a tetragonal-to-orthorhombic lattice distortion T s occurs at temperatures above or at the AF ordering temperature T N , 2 and the nematic phase is observed in the paramagnetic orthorhombic phase between T s and T N . [5][6][7] Although iron chalcogenide FeSe single crystals [ Fig. 1a, b] also undergo a nematic transition at T s and become superconducting at T c = 9.3 K, 11 the low-temperature static AF ordered phase is absent. 12,13 This has fueled debates concerning the role of AF order and spin fluctuations...