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...
We introduce a formalism to compute the neutron magnetic form factor FM(q) within a first-principles density functional theory and dynamical mean field theory. The approach treats spin and orbital interactions on the same footing and reduces to earlier methods in the fully localized or the fully itinerant limit. We test the method on various actinides of current interest NpCoGa5, PuSb and PuCoGa5, and we show that PuCoGa5 is in mixed valent state, which naturally explains the measured magnetic form factor.
On the basis of magnetic, transport, and optical measurements performed on single crystals, we report CaMn2Sb2 to be an antiferromagnetic insulator that exhibits weak ferromagnetic order above the Néel temperature. Magnetic susceptibility measurements reveal the magnitude of the high temperature Curie-Weiss moment to be only half as large as the ground state ordered moment, while electronic structure calculations based on crystallographic measurements suggest a crystal-field induced spin state transition does not occur. The antiferromagnetic state is relatively insensitive to both doping and modest pressures, while the ferromagnetism can be readily tuned by either. Infrared transmission and pressure dependent resistivity measurements suggest proximity to an electronic delocalization transition. We suggest the ferromagnetic state may be the signature of magnetic polarons.
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