Superconducting-Gap Anisotropy of Iron Pnictides Investigated via Combinatorial Microwave Measurements

Tatsunori Okada,Atsutaka Maeda,Hiroshi Eisaki,Masamichi Nakajima,Yoshiya Uwatoko,Akira Iyo,Kazuyuki Matsubayashi,Kentaro Kitagawa,Yoshinori Imai

doi:10.1038/s41598-020-63304-0

Abstract

One of the most significant issues for superconductivity is clarifying the momentum-dependent superconducting gap Δ({boldsymbol{k}}), which is closely related to the pairing mechanism. To elucidate the gap structure, it is essential to investigate Δ({boldsymbol{k}}) in as many different physical quantities as possible and to crosscheck the results obtained in different methods with each other. In this paper, we report a combinatorial investigation of the superfluid density and the flux-flow resistivity of iron-pnictide superconductors; LiFeAs and BaFe2(As1−xPx)2 (x = 0.3, 0.45). We evaluated Δ({boldsymbol{k}}) by fitting these two-independent quantities with a two-band model simultaneously. The obtained Δ({boldsymbol{k}}) are consistent with the results observed in angle-resolved photoemission spectroscopy (ARPES) and scanning-tunneling spectroscopy (STS) studies. We believe our approach is a powerful method for investigating Δ({boldsymbol{k}}) because it does not require a sample with clean surface unlike ARPES and STS experiments, or a rotational magnetic-field system for direct measurements of the angular dependence of thermodynamic quantities.

Highlights

Conventional superconductors and cuprates possess definitive Δ(k) with s- andd-wave symmetry, iron pnictides have multifarious Δ(k) comprised of combinations of gaps with- and without zero points reflecting their multiple-band nature
We investigated the superfluid density and flux-flow resistivity by measuring the microwave surface impedance in the zero-field limit or under finite magnetic field, respectively
We use subscript “h” (“e”) to specify hole-like band component. Unusual phenomena, such as a time-reversal-symmetry-breaking state originating from Josephson-type inter-band interactions among N(>3)-band components[21], may affect on superfluid density and flux-flow resistivity, but considering such exotic contributions is beyond our purpose of this manuscript that to demonstrate a new methodology to evaluate anisotropy of superconducting gaps on multiple-band superconductors from experimentally obtained data

Summary

Introduction

Conventional superconductors and cuprates possess definitive Δ(k) with s- andd-wave symmetry, iron pnictides have multifarious Δ(k) comprised of combinations of gaps with- and without zero points (nodes) reflecting their multiple-band nature. We investigated the superfluid density and flux-flow resistivity by measuring the microwave surface impedance in the zero-field limit or under finite magnetic field, respectively. Unusual phenomena, such as a time-reversal-symmetry-breaking state originating from Josephson-type inter-band interactions among N(>3)-band components[21], may affect on superfluid density and flux-flow resistivity, but considering such exotic contributions is beyond our purpose of this manuscript that to demonstrate a new methodology to evaluate anisotropy of superconducting gaps on multiple-band superconductors from experimentally obtained data.

Results

Conclusion