The crossover from the superconductivity of the Bardeen-Cooper-Schrieffer (BCS) regime to the Bose-Einstein condensation (BEC) regime holds a key to understanding the nature of pairing and condensation of fermions. It has been mainly studied in ultracold atoms, but in solid systems, fundamentally previously unknown insights may be obtained because multiple energy bands and coexisting electronic orders strongly affect spin and orbital degrees of freedom. Here, we provide evidence for the BCS-BEC crossover in iron-based superconductors FeSe1 − xSx from laser-excited angle-resolved photoemission spectroscopy. The system enters the BEC regime with x = 0.21, where the nematic state that breaks the orbital degeneracy is fully suppressed. The substitution dependence is opposite to the expectation for single-band superconductors, which calls for a new mechanism of BCS-BEC crossover in this system.
Interfacing bulk conducting topological Bi2Se3 films with s-wave superconductors initiates strong superconducting order in the nontrivial surface states. However, bulk insulating topological (Bi1-xSbx)2Te3 films on bulk Nb instead exhibit a giant attenuation of surface superconductivity, even for films only two-layers thick. This massive suppression of proximity pairing is evidenced by ultrahigh-resolution band mappings and by contrasting quantified superconducting gaps with those of heavily n-doped topological Bi2Se3/Nb. The results underscore the limitations of using superconducting proximity effects to realize topological superconductivity in nearly intrinsic systems.
Bogoliubov Fermi surface (BFS) is an unprecedented superconducting gap structure in which the nodal (zero-gap) region forms a two-dimensional surface rather than a line or point. Tetragonal Fe(Se,S) having a large residual density of states is a candidate for hosting BFSs. However, direct evidence has not yet been obtained. Here we report the unique momentum dependence of the superconducting gap in the hole band of FeSe0.78S0.22 by using high-energy-resolution laser-based angle-resolved photoemission spectroscopy. We find highly unusual gap anisotropy with a maximum gap of ~1 meV and widespread zero-gap regions with an accuracy of 200 μeV. Surprisingly, the gap structure breaks the four-fold rotational symmetry of the tetragonal crystal structure. These results reveal the formation of anomalous two-fold symmetric nodal planes, providing evidence for emergent nematic BFSs.
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