Elucidating the nature of the magnetism of a high-temperature superconductor is crucial for establishing its pairing mechanism. The parent compounds of the cuprate and iron-pnictide superconductors exhibit Néel and stripe magnetic order, respectively. However, FeSe, the structurally simplest iron-based superconductor, shows nematic order (T_s=90 K), but not magnetic order in the parent phase, and its magnetic ground state is intensely debated. Here we report inelastic neutron-scattering experiments that reveal both stripe and Néel spin fluctuations over a wide energy range at 110 K. On entering the nematic phase, a substantial amount of spectral weight is transferred from the Néel to the stripe spin fluctuations. Moreover, the total fluctuating magnetic moment of FeSe is ∼60% larger than that in the iron pnictide BaFe₂As₂. Our results suggest that FeSe is a novel S=1 nematic quantum-disordered paramagnet interpolating between the Néel and stripe magnetic instabilities.

FeSe layer-based superconductors exhibit exotic and distinctive properties. The undoped FeSe shows nematicity and superconductivity, while the heavily electron-doped K_xFe_2−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 FeSe_0.93S_0.07 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.

Rhenium(I)-carbonyl-diimine complexes have emerged as promising photocatalysts for carbon dioxide reduction with covalent organic frameworks recognized as perfect sensitizers and scaffold support. Such Re complexes/covalent organic frameworks hybrid catalysts have demonstrated high carbon dioxide reduction activities but with strong excitation energy-dependence. In this paper, we rationalize this behavior by the excitation energy-dependent pathways of internal photo-induced charge transfer studied via transient optical spectroscopies and time-dependent density-functional theory calculation. Under band-edge excitation, the excited electrons are quickly injected from covalent organic frameworks moiety into catalytic Rhenium^I center within picosecond but followed by fast backward geminate recombination. While under excitation with high-energy photon, the injected electrons are located at high-energy levels in Rhenium^I centers with longer lifetime. Besides those injected electrons to Rhenium^I center, there still remain some long-lived electrons in covalent organic frameworks moiety which is transferred back from Rhenium^I. This facilitates the two-electron reaction of carbon dioxide conversion to carbon monoxide.

We investigate the temperature dependence of the lower critical field $H_{c1}\left(T\right)$ of a high-quality FeSe single crystal under static magnetic fields $H$ parallel to the $c$ axis. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods and the corresponding $H_{c1}\left(T\right)$ was deduced by taking into account demagnetization factors. A pronounced change in the $H_{c1}$(T) curvature is observed, which is attributed to anisotopic s-wave or multiband superconductivity. The London penetration depth ${\lambda }_{ab}\left(T\right)$ calculated from the lower critical field does not follow an exponential behavior at low temperatures, as it would be expected for a fully gapped clean $s$-wave superconductor. Using either a two-band model with $s$-wave-like gaps of magnitudes ${\Delta }_1=0.41\pm 0.1$ meV and ${\Delta }_2=3.33\pm 0.25$ meV or a single anisotropic $s$-wave order parameter, the temperature dependence of the lower critical field $H_{c1}\left(T\right)$ can be well described. These observations clearly show that the superconducting energy gap in FeSe is nodeless.

The recent discovery of high-temperature superconductivity in single-layer iron selenide has generated significant experimental interest for optimizing the superconducting properties of iron-based superconductors through the lattice modification. For simulating the similar effect by changing the chemical composition due to S doping, we investigate the superconducting properties of high-quality single crystals of ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ ($x=0$, 0.04, 0.09, and 0.11) using magnetization, resistivity, the London penetration depth, and low temperature specific heat measurements. We show that the introduction of S to FeSe enhances the superconducting transition temperature $T_c$, anisotropy, upper critical field $H_{c2}$, and critical current density $J_c$. The upper critical field $H_{c2}\left(T\right)$ and its anisotropy are strongly temperature dependent, indicating a multiband superconductivity in this system. Through the measurements and analysis of the London penetration depth ${\lambda }_{ab}\left(T\right)$ and specific heat, we show clear evidence for strong coupling two-gap $s$-wave superconductivity. The temperature dependence of ${\lambda }_{ab}\left(T\right)$ calculated from the lower critical field and electronic specific heat can be well described by using a two-band model with $s$-wave-like gaps. We find that a $d$ wave and single-gap BCS theory under the weak-coupling approach cannot describe our experiments. The change of specific heat induced by the magnetic field can be understood only in terms of multiband superconductivity.