Importance of both spin and orbital fluctuations in BaFe2(As1-xPx)2 : Evidence from superconducting gap anisotropy

TL;DR

This study uses ARPES to reveal that both spin and orbital fluctuations significantly influence the superconducting gap anisotropy in BaFe2(As1-xPx)2, suggesting their combined role in high-Tc superconductivity.

Contribution

It provides direct experimental evidence that both spin and orbital fluctuations are crucial in shaping the superconducting gap in iron pnictides.

Findings

Superconducting gap shows large anisotropy on electron Fermi surface.

Hole Fermi surfaces exhibit nearly isotropic gaps.

Results support a modified s+- gap with nodal loops influenced by both fluctuations.

Abstract

In the iron pnictide superconductors, two distinct unconventional mechanisms of superconductivity have been put forth: One is mediated by spin fluctuations leading to the s+- state with sign change of superconducting gap between the hole and electron bands, and the other is orbital fluctuations which favor the s++ state without sign reversal. Here we report direct observation of peculiar momentum-dependent anisotropy in the superconducting gap from angle-resolved photoemission spectroscopy (ARPES) in BaFe2(As1-xPx)2 (Tc=30 K). The large anisotropy found only in the electron Fermi surface (FS) and the nearly isotropic gap on the entire hole FSs are together consistent with modified s+- gap with nodal loops, which can be theoretically reproduced by considering both spin and orbital fluctuations whose competition generates the gap modulation. This indicates that these two fluctuations are nearly equally important to the high-Tc superconductivity in this system.