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FeSe layer-based superconductors exhibit exotic and distinctive properties. The undoped FeSe shows nematicity and superconductivity, while the heavily electron-doped KxFe2−ySe2 and single-layer FeSe/SrTiO3 possess high superconducting transition temperatures that pose theoretical challenges. However, a comprehensive study on the doping dependence of an FeSe layer-based superconductor is still lacking due to the lack of a clean means of doping control. Through angle-resolved photoemission spectroscopy studies on K-dosed thick FeSe films and FeSe0.93S0.07 bulk crystals, here we reveal the internal connections between these two types of FeSe-based superconductors, and obtain superconductivity below ∼46 K in an FeSe layer under electron doping without interfacial effects. Moreover, we discover an exotic phase diagram of FeSe with electron doping, including a nematic phase, a superconducting dome, a correlation-driven insulating phase and a metallic phase. Such an anomalous phase diagram unveils the remarkable complexity, and highlights the importance of correlations in FeSe layer-based superconductors.

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 ...

Using angle-resolved photoemission spectroscopy (ARPES), we revealed the surface electronic structure and superconducting gap of (Li 0.8 Fe 0.2 )OHFeSe, an intercalated FeSe-derived superconductor without antiferromagnetic phase or Fe-vacancy order in the FeSe layers, and with a superconducting transition temperature (T c ) ∼ 40 K. We found that (Li 0.8 Fe 0.2 )OH layers dope electrons into FeSe layers. The electronic structure of surface FeSe layers in (Li 0.8 Fe 0.2 )OHFeSe resembles that of Rb x Fe 2−y Se 2 except that it only contains half of the carriers due to the polar surface, suggesting similar quasiparticle dynamics between bulk (Li 0.8 Fe 0.2 )OHFeSe and Rb x Fe 2−y Se 2 . Superconducting gap is clearly observed below T c , with an isotropic distribution around the electron Fermi surface. Compared with A x Fe 2−y Se 2 (A=K, Rb, Cs, Tl/K), the higher T c in (Li 0.8 Fe 0.2 )OHFeSe might be attributed to higher homogeneity of FeSe layers or to some unknown roles played by the (Li 0.8 Fe 0.2 )OH layers.

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At the interface between monolayer FeSe films and SrTiO3 substrates the superconducting transition temperature (Tc) is unexpectedly high, triggering a surge of excitement. The mechanism for the Tc enhancement has been the central question, as it may present a new strategy for seeking out higher Tc materials. To reveal this enigmatic mechanism, by combining advances in high quality interface growth, 16O 18O isotope substitution, and extensive data from angle resolved photoemission spectroscopy, we provide striking evidence that the high Tc in FeSe/SrTiO3 is the cooperative effect of the intrinsic pairing mechanism in the FeSe and interactions between the FeSe electrons and SrTiO3 phonons. Furthermore, our results point to the promising prospect that similar cooperation between different Cooper pairing channels may be a general framework to understand and design high-temperature superconductors.

Extending available body space loading active species and controllably tailoring the d‐band center to Fermi level of catalysts are of paramount importance but extremely challenging for the enhancement of electrocatalytic performance. Herein, a melamine‐bridged self‐construction strategy is proposed to in situ embed Co‐based bimetallic nanoparticles in the body of N‐doped porous carbon spheres (CoM‐e‐PNC), and achieve the controllable tailoring of the d‐band center position by alloying of Co and another transition metal M (M = Ni, Fe, Mn, and Cu). The enrichment and exposure of the active sites in the body interior of porous carbon spheres, and the best balance between the adsorption of OH species and the desorption of O2 induced by optimizing the d‐band center position, collectively enhance the oxygen evolution reaction (OER) performance. Meanwhile, the relationship of d‐band center position and OER activity is found to exhibit the volcano curve rule, where the CoNi‐e‐PNC catalyst shows optimal OER performance with an overpotential of 0.24 V at 10 mA cm−2 in alkaline media, outperforming those of the ever‐reported CoNi‐based catalysts. Besides, CoNi‐e‐PNC catalyst also demonstrates high OER stability with slight current decrease after 100 h.

It is very challenging to effectively exfoliate and functionalize hexagonal boron nitride (h-BN). Here, an efficient exfoliation and functionalization of bulk h-BN was carried out by a ball-milling method using boric acid (BA) as a lubricant and modifier. A series of boric-acid-functionalized boron nitride nanosheets (BNNSs) was successfully produced using this approach. The obtained BNNS thermostable suspension can be easily condensed into a jelly-like dispersion with ultra-high concentration, up to 90 mg ml −1 . The eco-friendly BA was readily and easily recyclable and remarkably reusable during the BNNS exfoliation. Interestingly, by means of a differential-centrifugation technique, the BNNSs could be easily separated and screened with different sizes and thicknesses. These screened BNNS samples also exhibited different levels of functionalization. As a result, filtration membranes made of various well-screened BNNSs exhibited an obviously different rejection rate for pollutant in water. In addition, the different screened BNNS products show a variable ability to dielectric behavior due to their different-level functionalization. We believe that our created boric-acidfunctionalized BNNSs, combined with the smartly screened separation by differential centrifugation, can broaden the future practical applications of BNNS materials.