A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature (T c). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving non-magnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe have considerable overlap, how each order affects superconductivity remains perplexing. Here we construct the three-dimensional electronic phase diagram, temperature (T) against pressure (P) and isovalent S-substitution (x), for FeSe1−xSx. By simultaneously tuning chemical and physical pressures, against which the chalcogen height shows a contrasting variation, we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended non-magnetic tetragonal phase emerges, where T c shows a striking enhancement. The completed phase diagram uncovers that high-T c superconductivity lies near both ends of the dome-shaped antiferromagnetic phase, whereas T c remains low near the nematic critical point.

The superconducting transition of FeSe1−xSx with three distinct sulphur concentrations x was studied under hydrostatic pressure up to ∼70 kbar via bulk AC susceptibility. The pressure dependence of the superconducting transition temperature (Tc) features a small dome-shaped variation at low pressures for x = 0.04 and x = 0.12, followed by a more substantial Tc enhancement to a value of around 30 K at moderate pressures. In x = 0.21, a similar overall pressure dependence of Tc is observed, except that the small dome at low pressures is flattened. For all three concentrations, a significant weakening of the diamagnetic shielding is observed beyond the pressure around which the maximum Tc of 30 K is reached near the verge of pressure-induced magnetic phase. This observation points to a strong competition between the magnetic and high-Tc superconducting states at high pressure in this system.

The collection of photoelectrons excited with a synchrotron via a nanotip placed near the surface of a sample is studied. Simulating the electron trajectory, we found that photoelectrons escaping from the surface are too weak to be the only source of electrons contributing to a photocurrent detected with a scanning tunneling microscope tip, as reported recently. The tunneling of low-energy electrons generated with synchrotron irradiation is suggested as an additional channel contributing to the photocurrent at a small separation between tip and sample. An image based on x-ray absorption is expected to attain a resolution comparable to a topographical image.

Ambient-pressure-grown LaO0.5F0.5BiS2 with a superconducting transition temperature Tc ∼ 3 K possesses a highly anisotropic normal state. By a series of electrical resistivity measurements with a magnetic field direction varying between the crystalline c-axis and the ab-plane, we present the first datasets displaying the temperature dependence of the out-of-plane upper critical field H ⊥ c2 (T ), the in-plane upper critical field H / / c2 (T ), as well as the angular dependence of Hc2 at fixed temperatures for ambient-pressure-grown LaO0.5F0.5BiS2 single crystals. The anisotropy of the superconductivity, H / / c2 /H ⊥ c2 , reaches ∼16 on approaching 0 K, but it decreases significantly near Tc. A pronounced upward curvature of H / / c2 (T ) is observed near Tc, which we analyze using a two-gap model. Moreover, H / / c2 (0) is found to exceed the Pauli paramagnetic limit, which can be understood by considering the strong spin-orbit coupling associated with Bi as well as the breaking of the local inversion symmetry at the electronically active BiS2 bilayers. Hence, LaO0.5F0.5BiS2 with a centrosymmetric lattice structure is a unique platform to explore the physics associated with local parity violation in the bulk crystal.

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