We report the superconductivity at above 30 K in a FeSe-layer compound K0.8Fe2Se2 (nominal composition) achieved by metal K intercalating in between FeSe layers. It is isostructural to BaFe2As2 and possesses the highest Tc for FeSe-layer materials so far under ambient pressure. Hall effect indicates the carriers are dominated by electron in this superconductor. We confirm that the observed superconductivity at above 30 K is due to this FeSe-based 122 phase. Our results demonstrate that FeSe-layer materials are really remarkable superconductors via structure and carrier modulation.

We have performed an angle-resolved photoemission spectroscopy study of the new iron-based superconductor K0.8Fe1.7Se2 (Tc$\sim${}30\,\,K). Clear band dispersion is observed with the overall bandwidth renormalized by a factor of 2.5 compared to our local density approximation calculations, indicating relatively strong correlation effects. Only an electronlike band crosses the Fermi energy, forming a nearly circular Fermi surface (FS) at M ($\pi${}, 0). The holelike band at $\Gamma${} sinks $\sim${}90\,\,meV below the Fermi energy, with an indirect band gap of 30~meV, to the bottom of the electronlike band. The observed FS topology in this superconductor favors ($\pi${}, $\pi${}) inter-FS scattering between the electronlike FSs at the M points, in sharp contrast to other iron-based superconductors which favor ($\pi${}, 0) inter-FS scattering between holelike and electronlike FSs.

Structural investigations by means of transmission electron microscopy (TEM) on ${\mathrm{K}}_{0.8}$Fe${}_x$Se${}_2$ and KFe${}_x$Se${}_2$, with 1.5 ⩽ $x$ ⩽ 1.8, have revealed a rich variety of microstructure phenomena. Materials with 1.5 ⩽ $x$ ⩽ 1.6 often show a superstructure modulation along the [310] zone-axis direction, and this modulation can be well interpreted by the Fe-vacancy order, which likely yields a superstructure phase of ${\mathrm{K}}_2$Fe${}_4$Se${}_5$. The superconducting ${\mathrm{K}}_{0.8}$Fe${}_x$Se${}_2$ and KFe${}_x$Se${}_2$ (1.7 ⩽ $x$ ⩽ 1.8) materials contain clear phase separation, in particular, along the $c$-axis direction, recognizable as visible parallel lamellae in the crystals; this fact suggests that the superconducting phase could have the Fe-vacancy disordered state.

The recently discovered layered kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, and Cs) with vanadium kagome networks provide a novel platform to explore correlated quantum states intertwined with topological band structures. Here we report the prominent effect of hole doping on both superconductivity and charge density wave (CDW) order, achieved by selective oxidation of exfoliated thin flakes. A superconducting dome is revealed as a function of the effective doping content. The superconducting transition temperature ($T_c$) and upper critical field in thin flakes are significantly enhanced compared with the bulk, which are accompanied by the suppression of CDW. Our detailed analyses establish the pivotal role of van Hove singularities in promoting correlated quantum orders in these kagome metals. Our experiments not only demonstrate the intriguing nature of superconducting and CDW orders, but also provide a novel route to tune the carrier concentration through both selective oxidation and electric gating. This establishes ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ as a tunable 2D platform for the further exploration of topology and correlation among $3d$ electrons in kagome lattices.