Isostructural Spin-Density-Wave and Superconducting Gap Anisotropies in Iron-Arsenide Superconductors

T. T. Han, L. Chen, C. Cai, Y. D. Wang, Z. G. Wang, Z. M. Xin, and Y. Zhang

Phys. Rev. Lett. 124, 247002 – Published 19 June 2020

Abstract

When passing through a phase transition, electronic system saves energy by opening energy gaps at the Fermi level. Delineating the energy gap anisotropy provides insights into the origin of the interactions that drive the phase transition. Here, we report the angle-resolved photoemission spectroscopy (ARPES) study on the detailed gap anisotropies in both the tetragonal magnetic and superconducting phases in ${Sr}_{1 - x} {Na}_{x} {Fe}_{2} {As}_{2}$ . First, we found that the spin-density-wave (SDW) gap is strongly anisotropic in the tetragonal magnetic phase. The gap magnitude correlates with the orbital character of Fermi surface closely. Second, we found that the SDW gap anisotropy is isostructural to the superconducting gap anisotropy regarding to the angular dependence, gap minima locations, and relative gap magnitudes. Our results indicate that the superconducting pairing interaction and magnetic interaction share the same origin. The intraorbital scattering plays an important role in constructing these interactions resulting in the orbital-selective magnetism and superconductivity in iron-based superconductors.

Received 22 January 2020
Accepted 1 June 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.247002

Physics Subject Headings (PhySH)

Spin density waves Superconductivity

Iron-based superconductors

Angle-resolved photoemission spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

T. T. Han ¹, L. Chen ¹, C. Cai ¹, Y. D. Wang¹, Z. G. Wang¹, Z. M. Xin¹, and Y. Zhang^1,2,*

¹International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China
²Collaborative Innovation Center of Quantum Matter, Beijing 100871, China

^*yzhang85@pku.edu.cn

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Vol. 124, Iss. 24 — 19 June 2020

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Images

Figure 1
${Sr}_{1 - x} {Na}_{x} {Fe}_{2} {As}_{2}$ phase diagram and schematic of the magnetic structure and band folding in the magnetic ordered phase. (a) The phase diagram adapted from Ref. [20]. The red vertical lines show the doping levels of our samples. Schematic drawing of the lattice and magnetic structures in the paramagnetic (PM/T), collinear antiferromagnetic (AFM/O), and tetragonal magnetic (AFM/T) phases. The red arrows represent the directions of magnetic moments. (b) The general Fermi surface of iron-based superconductors. The orbital characters of Fermi surface are illustrated using different colors. (c) Schematic of the band folding between $Γ$ and $M$ in the AFM/T phase.
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Figure 2
Characterization of the phase transitions using ARPES and resistivity measurements. (a) Second derivative images of the energy-momentum cuts taken along the $Γ − M$ direction at 70 and 30 K in the sample $# 1$ . (b) Second derivative merged image of the energy distribution curves (EDCs) taken at $k_{1}$ in (a). The AFM/O transition temperatures ( $T_{o}$ ) and AFM/T transition temperature ( $T_{t}$ ) are illustrated using white arrows. (c) Temperature dependence of the in-plane resistivity anisotropy taken on a detwinned sample. (d) Temperature dependence of the peak positions taken from the data in (b). (e) Energy-momentum cuts taken around the $Γ$ point at 70 and 30 K in the sample $# 1$ . (f) The temperature dependence of the EDCs taken at the Fermi crossing ( $k_{F}$ ) in (e). The EDCs were divided by a Fermi-Dirac function to remove the Fermi energy cutoff. (g) The temperature dependence of the energy gap determined using the peak positions in (f). (h),(i) are the same as (e),(f), but taken in the sample $# 2$ . (j) Temperature dependence of the normalized resistivity taken in the sample $# 2$ . (k) The temperature dependence of the superconducting gap determined using the peak positions in (i). All ARPES data were taken using 25 eV photons.
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Figure 3
The SDW gap structure in the AFM/T phase. (a) Fermi surface mappings taken in the AFM/T phase. (b) The symmetrized images of the energy-momentum cuts taken around the $Γ$ point. The cut momenta are illustrated in (a). The data were taken using 25 eV photons with linear vertical polarization. Panels (c),(d) are the same as (b) but taken on the electron pockets around the $M_{X}$ and $M_{Y}$ points, respectively. The data were taken using 21.2 eV photons with mixed polarization. (e) The summarized gap anisotropy on all observable sections of Fermi surface under our experimental setup. (f) The recovered gap anisotropy of all Femi surface sheets. The solid line shows the intraorbital scattering between the $α_{inner}$ and $γ_{inner}$ pockets. The dotted line shows the interorbital scattering between the $α_{outer}$ and $γ_{outer}$ pockets. All data were taken on sample $# 2$ at 15 K.
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Figure 4
The comparison of gap anisotropies taken in the AFM/T and superconducting phases. (a) The SDW gap distributions on different Fermi surface sheets taken in the AFM/T phase on sample $# 1$ at 15 K. The EDCs are selected at the gap minima, which are determined using the Fermi surface mapping, EDCs and MDCs [24]. The EDCs are merged into images to better visualize the gap anisotropy. The measured momenta are shown in the inset panels. (b) The schematic of the SDW gap anisotropies in the AFM/T phase. Panels (c),(d) are the same as (a),(b) but taken in the superconducting phase on sample $# 2$ at 15 K.
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