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 present here measurements of the magnetization M, ac susceptibility χ ′ , electrical resistivity ρ, and specific heat C in single crystals of metallic YFe2Al10. The magnetic susceptibility follows a Curie-Weiss temperature dependence for 75 K≤T≤750 K, with a fluctuating Fe moment of 0.45 µB/Fe, and the ac susceptibility χ ′ diverges at lower temperatures χ ′ ∼T −1.28±0.04 when the ac field is in the basal plane. The field B and temperature T dependencies of the magnetization M are well described by the scaling expression MT −β =F(B/T β+γ ) for 1.8 K≤T≤30 K and for fields larger than 0.1 T. These results indicate that strong quasi-two dimensional critical fluctuations are present that can be suppressed by magnetic fields. The magnetic and electronic parts of the specific heat CM show a similar divergence for 0.4 K≤T≤12 K, where CM /T∼T −0.47±0.03 . The divergences in χ ′ and CM /T indicate that YFe2Al10 is located near a quantum critical point, and no magnetic order is observed above 0.09 K. We argue that our results are inconsistent with quantum impurity or disorder models, suggesting instead that YFe2Al10 is on the verge of bulk magnetic ordering, and that the critical fluctuations that are associated with this quantum critical point lead to the divergencies in CM /T and χ ′ .

Infrared transmission and electrical resistivity measurements reveal that single crystals of LaMnPO1−xFx (x ≤ 0.28) are insulating. The optical gap obtained from transmission measurements is nearly unaffected by doping, decreasing only slightly from 1.3 eV in undoped LaMnPO to 1.1 eV for x = 0.04. The activation gaps obtained from electrical resistivity measurements are smaller by at least an order of magnitude, signalling the presence of states within the optical gap. At low temperatures, the resistivity is described well by variable range hopping conduction between these localized gap states. Analysis of the hopping conduction suggests that the gap states become slightly more delocalized with fluorine content, although metallic conduction is not observed even for fluorine concentrations as large as x = 0.28.Correlation gap insulators are distinct from simple band insulators in that the former require an electronelectron interaction to open a band gap at the Fermi level. This interaction typically stems from strong Coulomb repulsion (U) between the charge carriers, which may or may not be accompanied by further restrictions on orbital occupancy that are derived from the often subtle interplay between crystal field splitting and Hund's rules. These correlations can be weakened by carrier doping or by the application of high pressures [1], in either case resulting in the transformation of the correlation gap insulator into a correlated metal at a Mott-like electronic delocalization transition (EDT). It is believed that the correlation gap itself remains essentially unchanged by this process and that the EDT involves the delocalization of intrinsic states located within the gap [2]. Initially these states are highly localized by strong Coulomb interactions. Their density increases as the EDT is approached, however, and a band of correlated states forms as the correlations become weaker. In this way, the ingap states found in a correlation gap insulator are the intrinsic precursors of the correlated metal that forms once the EDT occurs, although the gap itself remains almost unchanged. Qualitatively speaking, this scenario is consistent with the wealth of experimental information available on the cuprates, where charge doping drives the magnetically ordered and insulating parent compounds metallic and ultimately superconducting [3].The more recent discovery of high superconducting critical temperatures (T c ) in the iron pnictide family, most notably T c = 55 K in F-doped SmFeAsO with the ZrCuSiAs structure [4], often draws comparison to the cuprate superconductors [5,6]. Nearly all compounds of the iron pnictide family so far reported are * jsimonson@bnl.gov metals, however, falling on the itinerant side of a possible EDT. Two insulating exceptions have been reported, but neither provides the perfect analogue to La 2 CuO 4 : (K,Tl)Fe x Se 2 [7] is subject to phase segregation and chemical inhomogeneity [8], while a doping study of compounds within the R 2 O 2 Fe 2 O(Se, S) 2 (R = La, Ce, Pr, Nd, Sm) system [9,10] h...

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We present measurements of the specific heat, magnetization, magnetocaloric effect and magnetic neutron diffraction carried out on single crystals of antiferromagnetic Yb3Pt4, where highly localized Yb moments order at TN = 2.4 K in zero field. The antiferromagnetic order was suppressed to TN → 0 by applying a field of 1.85 T in the ab plane. Magnetocaloric effect measurements show that the antiferromagnetic phase transition is always continuous for TN > 0, although a pronounced step in the magnetization is observed at the critical field in both neutron diffraction and magnetization measurements. These steps sharpen with decreasing temperature, but the related divergences in the magnetic susceptibility are cut off at the lowest temperatures, where the phase line itself becomes vertical in the field-temperature plane. As TN → 0, the antiferromagnetic transition is increasingly influenced by a quantum critical endpoint, where TN ultimately vanishes in a first order phase transition.