Orbital-dependent modulation of the superconducting gap in uniaxially strained ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$

Letter

Orbital-dependent modulation of the superconducting gap in uniaxially strained ${Ba}_{0.6} K_{0.4} {Fe}_{2} {As}_{2}$

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

Phys. Rev. B 104, L060502 – Published 12 August 2021

Abstract

Pairing symmetry which characterizes the superconducting pairing mechanism is normally determined by measuring the superconducting gap structure ( $| Δ_{k} |$ ). Here, we report the measurement of a strain-induced gap modulation ( $\partial | Δ_{k} |$ ) in uniaxially strained ${Ba}_{0.6} K_{0.4} {Fe}_{2} {As}_{2}$ utilizing angle-resolved photoemission spectroscopy and in situ strain tuning. We found that the uniaxial strain drives ${Ba}_{0.6} K_{0.4} {Fe}_{2} {As}_{2}$ into a nematic superconducting state which breaks the fourfold rotational symmetry of the superconducting pairing. The superconducting gap increases on the $d_{y z}$ electron and hole pockets while it decreases on the $d_{x z}$ counterparts. Such orbital selectivity indicates that orbital-selective pairing exists intrinsically in non-nematic iron-based superconductors. The $d_{x z}$ and $d_{y z}$ pairing channels are balanced originally in the pristine superconducting state, but become imbalanced under uniaxial strain. Our results highlight the important role of intraorbital scattering in mediating the superconducting pairing in iron-based superconductors. It also highlights the measurement of $\partial | Δ_{k} |$ as an effective way to characterize the superconducting pairing from a perturbation perspective.

Received 18 July 2020
Revised 3 July 2021
Accepted 27 July 2021

DOI:https://doi.org/10.1103/PhysRevB.104.L060502

Physics Subject Headings (PhySH)

Superconducting gap

Iron-based superconductors

Angle-resolved photoemission spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

L. Chen ¹, T. T. Han ¹, C. Cai¹, Z. G. Wang¹, Y. D. 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. 104, Iss. 6 — 1 August 2021

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Figure 1
(a) Schematic drawing of the observable Fermi surface sheets of ${Ba}_{0.6} K_{0.4} {Fe}_{2} {As}_{2}$ [29, 33]. The inset panel illustrates the working principle of our in situ strain-tuning device [26, 29]. (b) Fermi surface mapping taken at 0 and 500 V. The white arrows highlight the change of photoemission intensity under uniaxial strain. (c) Second derivative images of the energy-momentum cuts taken along the $Γ − M_{X}$ direction at 0 and 500 V. Black dashed lines guide the eye to the band dispersions. (d) is the same as (c) but taken along the $Γ − M_{Y}$ direction. The raw data can be found in the Supplemental Material [29].
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Figure 2
(a) Symmetrized energy-momentum cuts taken across the $γ_{X}$ electron pocket at 0 and 500 V. (b) Strain dependence of the symmetrized energy distribution curves (EDCs) taken at the Fermi crossing ( $k_{F}$ ) of the $γ_{X}$ electron pocket. The $k_{F}$ is determined as the momentum where the gap reaches a minimum. The inset panel shows the strain dependence of the superconducting gap. The gap magnitudes were determined using the gap fitting process [39]. The error bar is estimated by considering the fitting error, the energy resolution, and the error of $k_{F}$ . (c) is a merged image of the symmetrized EDCs shown in (b). (d)–(f) are the same as (a)–(c) but taken on the $γ_{Y}$ electron pocket.
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Figure 3
(a) Symmetrized energy-momentum cuts taken across the $α_{y z}$ hole pocket at 0 and 500 V. (b) Strain dependence of the symmetrized EDCs taken at the $k_{F}$ of the $α_{y z}$ hole pocket. The inset panel shows the strain dependence of the superconducting gap. (c) is a merged image of the symmetrized EDCs shown in (b). (d) Angular dependence of the superconducting gap taken on the $α_{y z}$ hole pocket at 0 V. The symmetrized EDCs taken along the $α_{y z}$ hole pocket are merged into one image to better illustrate the angular distribution of the superconducting gap [29]. (e) is the same as (d), but taken at 500 V.
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Figure 4
(a) Schematic drawing of the actual experimental geometry and corresponding geometry after $90^{\circ}$ rotation. (b) Symmetrized energy-momentum cuts taken at 0 and $- 200$ V. (c) Strain dependence of the symmetrized EDCs taken at the $k_{F}$ of the inner hole pocket. The raw and fitted data are illustrated by the black dots and red lines, respectively. (d) Strain dependence of the superconducting gap on the inner hole pocket.
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Figure 5
Schematic drawing of the gap modulation on the $d_{x z} / d_{y z}$ Fermi surface sheets in uniaxially strained ${Ba}_{0.6} K_{0.4} {Fe}_{2} {As}_{2}$ .
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Physical Review B

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Orbital-dependent modulation of the superconducting gap in uniaxially strained ${Ba}_{0.6} K_{0.4} {Fe}_{2} {As}_{2}$

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

Phys. Rev. B 104, L060502 – Published 12 August 2021

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Physical Review B

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Orbital-dependent modulation of the superconducting gap in uniaxially strained Ba0.6K0.4Fe2As2

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

Phys. Rev. B 104, L060502 – Published 12 August 2021

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Orbital-dependent modulation of the superconducting gap in uniaxially strained ${Ba}_{0.6} K_{0.4} {Fe}_{2} {As}_{2}$