Nodal-to-nodeless superconducting order parameter in LaFeAs$_{1-x}$P$_x$O synthesized under high pressure

TL;DR

This study reveals that high-pressure synthesis of LaFeAs$_{1-x}$P$_x$O leads to a unique phase diagram with a transition from nodal to nodeless superconducting order parameter, differing from ambient-pressure counterparts.

Contribution

It demonstrates that high-pressure growth induces a radical change in the superconducting gap structure and phase diagram of LaFeAs$_{1-x}$P$_x$O, highlighting pressure's role in tuning electronic properties.

Findings

Superconductivity persists across 0.3 ≤ x ≤ 0.7 in pressure-grown samples.

Superconducting gap changes from nodal to nodeless at x=0.5.

Pressure suppresses magnetic order in these materials.

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 LaFeAs$_{1-x}$P$_x$O 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 macro- 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 $0.3 \leq x \leq 0.7$ 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 $x \geq 0.5$), in clear contrast to other 1111 and 122 iron-based materials grown under ambient-pressure conditions.