Understanding the re-entrant superconducting phase diagram of an iron-pnictide Ca$_4$Al$_2$O$_6$Fe$_2$(As$_{1-x}$P$_x$)$_2$

TL;DR

This study combines first-principles calculations and experiments to explore how momentum space frustration influences the coexistence and competition of superconductivity and antiferromagnetism in an iron-pnictide compound.

Contribution

It introduces a theoretical scenario linking momentum space frustration to suppressed superconductivity, supported by pressure experiments on Ca4Al2O6Fe2(As1-xPx)2.

Findings

Superconductivity is suppressed in the intermediate x regime due to momentum space frustration.

Hydrostatic pressure experiments show antiferromagnetism is smeared out without inducing superconductivity.

Theoretical calculations align with experimental results, supporting the proposed frustration scenario.

Abstract

Recently, a very rich phase diagram has been obtained for an iron-based superconductor Ca4Al2O6Fe2(As1-xPx)2. It has been revealed that nodeless (x=0) and nodal (x = 1) superconductivity are separated by an antiferromagnetic phase. Here we study the origin of this peculiar phase diagram using a five orbital model constructed from first principles band calculation, and applying the fluctuation exchange approximation assuming spin fluctuation mediated pairing. Based on the calculation results, we propose a scenario where the frustration in momentum space degrades superconductivity in the intermediate x regime, while antiferromangetism takes place due to a very good nesting. In order to see whether the present theoretical scenario is consistent with the actual nature of the competition between superconductivity and antiferromagnetism, we also perform hydrostatic pressure experiment for Ca4Al2O6Fe2(As1-xPx)2. In the intermediate x regime where antiferromagnetism occurs at ambient pressure, applying hydrostatic pressure smears out the antiferromagnetic transition, but superconductivity does not take place. This supports our scenario that superconductivity is suppressed by the momentum space frustration in the intermediate x regime, apart from the presence of the antiferromangnetism.