In the field of iron-based superconductors, one of the frontier studies is about the pairing mechanism. The recently discovered (Li1−xFex)OHFeSe superconductor with the transition temperature of about 40 K provides a good platform to check the origin of double superconducting gaps and high transition temperature in the monolayer FeSe thin film. Here we report a scanning tunnelling spectroscopy study on the (Li1−xFex)OHFeSe single crystals. The tunnelling spectrum mimics that of the monolayer FeSe thin film and shows double gaps at about 14.3 and 8.6 meV. Further analysis based on the quasiparticle interference allows us to rule out the d-wave gap, and for the first time assign the larger (smaller) gap to the outer (inner) Fermi pockets (after folding) associating with the dxy (dxz/dyz) orbitals, respectively. The gap ratio amounts to 8.7, which demonstrates the strong coupling mechanism in the present superconducting system.

Iron pnictides are the only known family of unconventional high-temperature superconductors besides cuprates. Until recently, it was widely accepted that superconductivity is driven by spin fluctuations and intimately related to the fermiology, specifically, hole and electron pockets separated by the same wavevector that characterizes the dominant spin fluctuations, and supporting order parameters (OP) of opposite signs 1,2 . This picture was questioned after the discovery of intercalated or monolayer form of FeSe-based systems without hole pockets, which seemingly undermines the basis for spin-fluctuation theory and the idea of a signchanging OP [3][4][5][6][7][8][9][10][11] . Using the recently proposed phase-sensitive quasiparticle interference technique, here we show that in LiOH-intercalated FeSe compound the OP does change sign, albeit within the electronic pockets. This result unifies the pairing mechanism of iron-based superconductors with or without the hole Fermi pockets and supports the conclusion that spin fluctuations play the key role in electron pairing.In iron pnictides, it has been widely perceived that superconductivity is driven by spin fluctuations, which supports the sign reversal between order parameters (OP) on the electron and hole pockets 1,2 . The discovery of superconductivity in intercalated or monolayer FeSe at a critical temperature of the order of 40 K rekindled interest in Fe-based superconductivity and sent many theorists back to the drawing board [3][4][5][6][7][8][9][10][11]

By using a hydrothermal method, we have successfully grown crystals of the newly discovered superconductor FeS, which has an isostructure of the iron based superconductor FeSe. The superconductivity appears at about 4.5K, as revealed by both resistive and magnetization measurements. It is found that the upper critical field is relatively low, with however an rather large anisotropy Γ = [(dH ab c2 /dT )/(dH c c2 /dT )]T c ≈ 5.8. A huge magnetoresistivity (290% at 9T and 10K, H c-axis) together with a non-linear behavior of Hall resistivity vs. external field are observed. A two-band model is applied to fit the magnetoresistance and non-linear transverse resistivity, yielding the basic parameters of the electron and hole bands.

Low temperature specific heat has been measured in superconductor β-FeS with Tc = 4.55 K. It is found that the low temperature electronic specific heat Ce/T can be fitted to a linear relation in the low temperature region, but fails to be described by an exponential relation as expected by an s-wave gap. We try fittings to the data with different gap structures and find that a model with one or two nodal gaps can fit the data. Under a magnetic field, the field induced specific heat coefficient ∆γe=[Ce(H)-Ce(0)]/T shows the Volovik relation ∆γe(H) ∝ √ H, suggesting the presence of nodal gap(s) in this material.

Chemical substitution during growth is a well-established method to manipulate electronic states of quantum materials, and leads to rich spectra of phase diagrams in cuprate and iron-based superconductors. Here we report a novel and generic strategy to achieve nonvolatile electron doping in series of (i.e. 11 and 122 structures) Fe-based superconductors by ionic liquid gating induced protonation at room temperature. Accumulation of protons in bulk compounds induces superconductivity in the parent compounds, and enhances the T c largely in some superconducting ones. Furthermore, the existence of proton in the lattice enables the first proton nuclear magnetic resonance (NMR) study to probe directly superconductivity. Using FeS as a model system, our NMR study reveals an emergent high-T c phase with no coherence peak which is hard to measure by NMR with other isotopes. This novel electric-field-induced proton evolution opens up an avenue for manipulation of competing electronic states (e.g. Mott insulators), and may provide an innovative way for a broad perspective of NMR measurements with greatly enhanced detecting resolution.

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We report a high-pressure 75 As NMR study on the heavily hole-doped iron pnictide superconductor KFe2As2 (Tc ≈ 3.8 K). The low-energy spin fluctuations are found to decrease with applied pressure up to 2 GPa, but then increase again, changing in lockstep with the pressure-induced evolution of Tc. Their diverging nature suggests close proximity to a magnetic quantum critical point at a negative pressure of P ≃ −0.6 GPa. Above 2.4 GPa, the 75 As satellite spectra split below 40 K, indicating a breaking of As site symmetry and an incipient charge order. These pressure-controlled phenomena demonstrate the presence of nearly-critical fluctuations in both spin and charge, providing essential input for the origin of superconductivity. Heavily doped FeSCs, whose Fermi surfaces are quite different from optimally doped materials, challenge the existing understanding. KFe 2 As 2 has large hole doping (0.5 hole/Fe), but far from being a regular metal it shows heavy-fermion characteristics below a low coherence temperature of order 60 K [8] and superconductivity at a low but finite T c of 3.8 K [9,10]. The absence of electron pockets around (π, π) [11] suggests that spin fluctuations from interband nesting are unlikely, but low-energy electronic correlations are surprisingly strong. Similarly strong low-energy spin fluctuations [12,13] Recent high-pressure studies of KFe 2 As 2 discovered an anomalous reversal of T c , which has a minimum at 1.8 GPa [18]. Scenarios proposed to explain this include a change of pairing symmetry [18,19] and a k z modulation of the superconducting gap [20]. Although spin fluctuations are essential to FeSC superconductivity, no measurements under pressure have yet been reported.In this Letter, we present a high-pressure study of KFe 2 As 2 by nuclear magnetic resonance (NMR). The 75 As spectra and spin-lattice relaxation rate (1/ 75 T 1 T ) are measured under pressures up to 2.42 GPa, revealing three surprising features. First, 1/ 75 T 1 T is dominated by strong low-energy spin fluctuations, suggesting incipient antiferromagnetic order at a quantum critical point near −0.6 GPa. Second, the spin fluctuations show exactly the same reversal behavior as T c , and indeed identical evolution at all pressures. Third, a line splitting of the 75 As satellite spectra below 40 K for pressures above 2.4 GPa indicates a breaking of four-fold symmetry. This effect is caused by charge order, whose fluctuations we propose are strong around the d 5.5 electron filling of KFe 2 As 2 . This emergent charge order is accompanied by the enhancement of spin fluctuations and hence of T c , demonstrating the importance of nearly-critical charge fluctuations in heavily hole-doped FeSCs.Our KFe 2 As 2 single crystals were synthesized by the self-flux method [21]. We measure very large residual resistivity ratios of 1390, indicating extremely high sample quality. We performed high-pressure NMR measurements using a NiCrAl clamp cell, which reaches a maximum pressure of 2.42 GPa at T = 2 K; to obtain a maximally hydrostatic ...