Abstract
Similar to chemical doping, pressure produces and stabilizes new phases of known materials, whose properties may differ greatly from those of their standard counterparts. Here, by considering a series of LaFeAs1−xPxO iron-pnictides synthesized under high-pressure high-temperature conditions, we investigate the simultaneous effects of pressure and isoelectronic doping in the 1111 family. Results of numerous macroscopic and microscopic technique measurements unambiguously show a radically different phase diagram for the pressure-grown materials, characterized by the lack of magnetic order and the persistence of superconductivity across the whole 0.3 ≤ x ≤ 0.7 doping range. This unexpected scenario is accompanied by a branching in the electronic properties across x = 0.5, involving both the normal and superconducting phases. Most notably, the superconducting order parameter evolves from nodal (for x < 0.5) to nodeless (for x ≥ 0.5), in clear contrast to other 1111 and 122 iron-based materials grown under ambient-pressure conditions.
Highlights
Superconductivity (SC) in LaFePO,[1] a compound first synthesized by Zimmer et al.,[2] sets in at a modest Tc of only 3.2 K
This conclusion was reinforced by later work, where an interpretation based on quantum criticality (QC) was put forward.[16]
LaFePO, the unusual sensitivity to structural modifications, and the occurrence of QC, we investigated a new batch of LaFeAs1−xPxO compounds, grown under high-pressure conditions
Summary
Superconductivity (SC) in LaFePO,[1] a compound first synthesized by Zimmer et al.,[2] sets in at a modest Tc of only 3.2 K. By means of ab initio density-functional methods, the electronic structures and the magnetic properties of LaFePO and LaFeAsO were calculated in considerable detail.[13,14] It turned out that pnictogen atoms play a key role in establishing the Fe–P (or Fe–As) distance, giving rise to an unusual sensitivity of material’s properties to an apparently minor detail.[15] This conclusion was reinforced by later work, where an interpretation based on quantum criticality (QC) was put forward.[16] In a QC scenario, the proximity of iron-based materials to a Mott transition implies that, by increasing the ratio of kinetic energy to Coulomb repulsion, one can pass from an antiferromagnetic to a paramagnetic state
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