We report bulk superconductivity induced by an isovalent doping of phosphorus in BaFe(2)(As(1-x)P(x))(2). The P-for-As substitution results in shrinkage of the lattice, especially for the FeAs block layers. The resistivity anomaly associated with the spin-density-wave (SDW) transition in the undoped compound is gradually suppressed by the P doping. Superconductivity with a maximum T(c) of 30 K emerges at x = 0.32, coinciding with a magnetic quantum critical point (QCP) which is shown by the disappearance of SDW order and the linear temperature-dependent resistivity in the normal state. The T(c) values were found to decrease with further P doping and no superconductivity was observed down to 2 K for x≥0.77. The appearance of superconductivity in the vicinity of QCP hints at the superconductivity mechanism in iron-based arsenides.
We have studied EuFe2(As0.7P0.3)2 by the measurements of x-ray diffraction, electrical resistivity, thermopower, magnetic susceptibility, magnetoresistance and specific heat. Partial substitution of As with P results in the shrinkage of lattice, which generates chemical pressure to the system. It is found that EuFe2(As0.7P0.3)2 undergoes a superconducting transition at 26 K, followed by ferromagnetic ordering of Eu 2+ moments at 20 K. This finding is the first observation of superconductivity stabilized by internal chemical pressure, and supplies a rare example showing coexistence of superconductivity and ferromagnetism in the ferro-arsenide family.
Two major themes in the physics of condensed matter are quantum critical
phenomena and unconventional superconductivity. These usually occur in the
context of competing interactions in systems of strongly-correlated electrons.
All this interesting physics comes together in the behavior of the recently
discovered iron pnictide compounds that have generated enormous interest
because of their moderately high-temperature superconductivity. The ubiquity of
antiferromagnetic ordering in their phase diagrams naturally raises the
question of the relevance of magnetic quantum criticality, but the answer
remains uncertain both theoretically and experimentally. Here we show that the
undoped iron pnictides feature a novel type of magnetic quantum critical point,
which results from a competition between electronic localization and
itinerancy. Our theory provides a mechanism to understand the
experimentally-observed variation of the ordered moment among the undoped iron
pnictides. We suggest P substitution for As in the undoped iron pnictides as a
means to access this new example of magnetic quantum criticality in an unmasked
fashion. Our findings point to the iron pnictides as a much-needed new setting
for quantum criticality, one that offers a new set of control parameters.Comment: (v3) New abstract, more explanatory material, accepted for PNA
Bad metal properties have motivated a description of the parent iron pnictides as correlated metals on the verge of Mott localization. What has been unclear is whether interactions can push these and related compounds to the Mott-insulating side of the phase diagram. Here we consider the iron oxychalcogenides La2O2Fe2O(Se,S)2, which contain an Fe square lattice with an expanded unit cell. We show theoretically that they contain enhanced correlation effects through band narrowing compared to LaOFeAs, and we provide experimental evidence that they are Mott insulators with moderate charge gaps. We also discuss the magnetic properties in terms of a Heisenberg model with frustrating J1-J2-J2' exchange interactions on a "doubled" checkerboard lattice.
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