The discovery of ThCr 2 Si 2 -type A x Fe 2-y Se 2 (A ¼ K, Rb, Cs and Tl) with T c B30 K make much progress in iron-based superconducting field, but their multiple-phase separations are disadvantageous for understanding the origin. On the other hand, for small alkali metals, studies on (Li,Na)FeCu(S,Se) 2 and NaFe 2 À d S 2 show that these compounds possess CaAl 2 Si 2 -type structure, implying that ThCr 2 Si 2 -type structure is unstable for small alkali metal-intercalated FeSe under high temperature. Here we report a new intercalate Na 0.65(1) Fe 1.93(1) Se 2 with T c B37 K, synthesized by low-temperature ammonothermal method. The notable finding is that the Na 0.65(1) Fe 1.93(1) Se 2 shows a ThCr 2 Si 2 -type structure, which is the first instance of small-sized alkali metal intercalates without NH 3 co-intercalation. Besides, the NH 3 -poor Na 0.80(4) (NH 3 ) 0.60 Fe 1.86(1) Se 2 and NH 3 -rich phase with T c s at 45 and 42 K are identified by tuning the concentration of Na-NH 3 solutions. The modulation of interlayer spacing reveals the versatile evolution of structural stability and superconductivity in these intercalates.

We report a finding of a pressure-induced quantum critical transition in K0.8Fe(x)Se2 (x = 1.7 and 1.78) superconductors through in situ high-pressure electrical transport and x-ray diffraction measurements in diamond anvil cells. Transitions from metallic Fermi liquid behavior to non-Fermi liquid behavior and from antiferromagnetism to paramagnetism are found in the pressure range of 9.2-10.3 GPa, in which superconductivity tends to disappear. The change around the quantum critical point from the coexisting antiferromagnetism state and the Fermi liquid behavior to the paramagnetism state and the non-Fermi liquid behavior in the iron-selenide superconductors demonstrates a unique mechanism for their quantum critical transition.

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Widespread adoption of superconducting technologies awaits the discovery of new materials with enhanced properties, especially higher superconducting transition temperatures T c . The unexpected discovery of high T c superconductivity in cuprates suggests that the highest T c s occur when pressure or doping transform the localized and moment-bearing electrons in antiferromagnetic insulators into itinerant carriers in a metal, where magnetism is preserved in the form of strong correlations. The absence of this transition in Fe-based superconductors may limit their T c s, but even larger T c s may be possible in their isostructural Mn analogs, which are antiferromagnetic insulators like the cuprates. It is generally believed that prohibitively large pressures would be required to suppress the effects of the strong Hund's rule coupling in these Mn-based compounds, collapsing the insulating gap and enabling superconductivity. Indeed, no Mn-based compounds are known to be superconductors. The electronic structure calculations and X-ray diffraction measurements presented here challenge these long held beliefs, finding that only modest pressures are required to transform LaMnPO, isostructural to superconducting host LaFeAsO, from an antiferromagnetic insulator to a metallic antiferromagnet, where the Mn moment vanishes in a second pressure-driven transition. Proximity to these charge and moment delocalization transitions in LaMnPO results in a highly correlated metallic state, the familiar breeding ground of superconductivity. correlated electron systems | electronic delocalization transition S uperconductivity with high transition temperatures T c was first found near an electron delocalization transition (EDT) in the cuprates, and subsequently in systems as diverse as quasi-two dimensional organic layer compounds (1), heavy fermions (2, 3), and endohedrally doped fullerides (4). One obstacle to achieving a higher T c in the Fe-based superconductors may be that the parent compounds are metallic (5-7), albeit with quasiparticle mass enhancements (8) that suggest varying degrees of proximity to an EDT (9-11). So far no insulating parent compounds have been identified that can, by analogy to the cuprates, be doped to achieve higher superconducting transition temperatures. It is possible that the recently isolated K 2 Fe 4 Se 5 (12) and La 2 O 2 Fe 2 OðSe; SÞ 2 (13) phases may prove to be the first compounds of this type. In contrast, isostructural Mn-based compounds often have large insulating gaps and ordered moments (14, 15), suggesting their suitability as possible parent compounds. At present there are no known Mn-based superconductors, however, and it is generally believed that the Hund's rule coupling in Mn compounds is prohibitively strong, so that doping will not reduce the overall scale of the correlations to the point at which superconductivity may become possible. The electronic structure calculations and X-ray diffraction measurements presented here show how the interplay of Hund's rule interactions with incr...

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