Abrupt change of the superconducting gap structure at the nematic critical point in FeSe _1−x S _x

Abstract

The emergence of the nematic electronic state that breaks rotational symmetry is one of the most fascinating properties of the iron-based superconductors, and has relevance to cuprates as well. FeSe has a unique ground state in which superconductivity coexists with a nematic order without long-range magnetic ordering, providing a significant opportunity to investigate the role of nematicity in the superconducting pairing interaction. Here, to reveal how the superconducting gap evolves with nematicity, we measure the thermal conductivity and specific heat of FeSe S , in which the nematicity is suppressed by isoelectronic sulfur substitution and a nematic critical point (NCP) appears at [Formula: see text] We find that, in the whole nematic regime ([Formula: see text]), the field dependence of two quantities consistently shows two-gap behavior; one gap is small but highly anisotropic with deep minima or line nodes, and the other is larger and more isotropic. In stark contrast, in the tetragonal regime ([Formula: see text]), the larger gap becomes strongly anisotropic, demonstrating an abrupt change in the superconducting gap structure at the NCP. Near the NCP, charge fluctuations of [Formula: see text] and [Formula: see text] orbitals are enhanced equally in the tetragonal side, whereas they develop differently in the orthorhombic side. Our observation therefore directly implies that the orbital-dependent nature of the nematic fluctuations has a strong impact on the superconducting gap structure and hence on the pairing interaction.

Topological ultranodal pair states in iron-based superconductors

“…This is in accordance with the data and conclusions in Ref. 17 for FeSe 1−x S x where, as a function of sulfur doping, all the pockets were posited to become nodal or close to nodal across the nematic quantum critical point. The anisotropic component on each pocket, ∆ ja (k) = ∆ ja (k 2…”

“…We therefore adopt the name ultranodal superconducting state to indicate that the phase space for Bogoliubov quasiparticle excitations is larger than in either the point or line nodal case familiar from studies of unconventional superconductivity Case of FeSe(S). As a concrete example to highlight the physics discussed above, we consider a simplified threepocket model (i = X, Y, Γ) in two dimensions to capture the essential electronic structure of iron-based superconductors, and in particular the FeSe 1−x S x system that has been noted to display anomalous thermodynamics for doping beyond the nematic transition 17 . We set the band dispersions to be quadratic, centered around Γ and X/Y points and the quantities that influence the existence and location of the Bogoliubov Fermi surfaces are the gap functions.…”

Extended Magnetic Dome Induced by Low Pressures in Superconducting FeSe1−xSx

“…In recent years, S substitution has come into focus as an additional tuning parameter for FeSe. The nematically ordered phase of FeSe 1-x S x is suppressed with increasing sulfur content and is no longer present for x > 0.17 [19][20][21][22] where a nematic quantum critical point [23], a topological Lifshitz transition [7], a reduction in electronic correlations [24], and a change in the superconducting pairing state [25] and gap structure [26] are observed. Under high hydrostatic pressures, T c of FeSe 1-x S x exhibits a similarly dramatic increase as observed in FeSe [19,27,28].…”

Anomalous high-magnetic field electronic state of the nematic superconductors FeSe1−xSx

“…As nematicity is suppressed, it creates ideal conditions to explore a potential nematic critical point [17] in the absence of magnetism. The superconducting dome extends outside the nematic state but anisotropic pairing remains robust [18] and a different superconducting state was suggested to be stabilized in the tetragonal phase [19].…”