Extensive impurity-scattering study on the pairing symmetry of monolayer FeSe films on ${\mathrm{SrTiO}}_{3}$

Extensive impurity-scattering study on the pairing symmetry of monolayer FeSe films on ${SrTiO}_{3}$

Chong Liu, Jiahao Mao, Hao Ding, Rui Wu, Chenjia Tang, Fangsen Li, Ke He, Wei Li, Can-Li Song, Xu-Cun Ma, Zheng Liu, Lili Wang, and Qi-Kun Xue

Phys. Rev. B 97, 024502 – Published 3 January 2018

Abstract

Determination of the pairing symmetry in monolayer FeSe films on ${SrTiO}_{3}$ is a requisite for understanding the high superconducting transition temperature in this system, which has attracted intense theoretical and experimental studies but remains controversial. Here, by introducing several types of point defects in FeSe monolayer films, we conduct a systematic investigation on the impurity-induced electronic states by spatially resolved scanning tunneling spectroscopy. Ranging from surface adsorption, chemical substitution, to intrinsic structural modification, these defects generate a variety of scattering strengths, which renders insights on the pairing symmetry.

Received 11 August 2017
Revised 15 December 2017

DOI:https://doi.org/10.1103/PhysRevB.97.024502

Physics Subject Headings (PhySH)

Impurities in superconductors Superconducting order parameter Superconductors

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Chong Liu¹, Jiahao Mao^1,2, Hao Ding¹, Rui Wu¹, Chenjia Tang¹, Fangsen Li¹, Ke He^1,3, Wei Li^1,3, Can-Li Song^1,3, Xu-Cun Ma^1,3, Zheng Liu^2,3,*, Lili Wang^1,3,†, and Qi-Kun Xue^1,3,‡

¹State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, People's Republic of China
²Institute for Advanced Study, Tsinghua University, Beijing 100084, People's Republic of China
³Collaborative Innovation Center of Quantum Matter, Beijing 100084, People's Republic of China

^*Corresponding author: zheng-liu@mail.tsinghua.edu.cn
^†liliwang@mail.tsinghua.edu.cn
^‡qkxue@mail.tsinghua.edu.cn

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Vol. 97, Iss. 2 — 1 January 2018

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Figure 1
(a, b, e) Topographic images of monolayer FeSe on ${SrTiO}_{3}$ (001) with native defects, such as Fe vacancies [(a) $V = - 100 mV, I = 200$ pA), ${Se}_{Fe}$ antisite defect [(b), $V = - 80 mV, I = 200$ pA], and Se vacancy [(e), $V = - 100 mV, I = 200$ pA]. (c, d) Tunneling spectra taken along the blue lines in (a) and (b), respectively, with steps of 0.5 $a_{0}$ . The gray dashed lines indicate the double coherence peaks in the energy gaps. (f) Tunneling spectra taken around Se vacancy as marked by the colored dots in (e). The red dashed lines in (d) and (f) label the peak positions of in-gap bound states. For (c), (d), and (f), $V = 40$ mV, $I = 500$ pA.
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Figure 2
(a–c) Topographic images of monolayer ${Fe}_{1 - x} {Co}_{x} Se$ on ${SrTiO}_{3}$ (001) with $x = 0.002, 0.02$ , and 0.1, respectively: (a) $V = 100$ mV, $I = 200$ pA; (b) $V = 100$ mV, $I = 200$ pA; (c) $V = 100$ mV, $I = 100$ pA. (d) Tunneling spectra taken at locations away from Co defects on monolayer ${Fe}_{1 - x} {Co}_{x} Se$ , with a dI/dV curve of pure monolayer FeSe as reference. The arrows indicate the typical double-peak features in the energy gaps ( $V = 40$ mV, $I = 500$ pA for $x = 0$ , 0.002, and 0.02; $V = 40$ mV, $I = 200$ pA for $x = 0.1$ ).
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Figure 3
(a) Topographic image of monolayer ${Fe}_{0.98} {Co}_{0.02} Se$ with single ${Co}_{Fe}$ substitution ( $V = 70$ mV, $I = 500$ pA). (b) Tunneling spectra taken across ${Co}_{Fe}$ substitution ( $V = 500$ mV, $I = 500$ pA, bias modulation amplitud $e = 5$ mV) in a bias range from −500 to 500 mV. The arrow shows the energy shift of the kink. (c) Tunneling spectra taken across ${Co}_{Fe}$ substitution along the blue line in (a) with steps of $a_{0}$ ( $V = 40$ mV, $I = 500$ pA) in a smaller bias range from −40 to 40 mV. The gray lines indicate the double coherence peaks in the energy gaps. (d, e) dI/dV maps for the dashed square area in (a) at 10 and −15 mV, respectively (3 × 3 nm, $V = 40$ mV, $I = 500$ pA). The red dots mark the positions of the dumbbell-like pairs. (f) Topographic image of monolayer FeSe with single ${Cu}_{Fe}$ substitution ( $V = 70$ mV, $I = 500$ pA). (g) Tunneling spectra taken on and far away from ${Cu}_{Fe}$ substitution as marked by the colored dots in (f) ( $V = 40$ mV, $I = 500$ pA).
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Figure 4
(a) Topographic image of surface K atoms adsorbed on monolayer FeSe ( $V = 500$ mV, $I = 50$ pA). (b) Normalized tunneling spectra taken across a K atom along the blue lines in (a) ( $V = 50$ mV, $I = 200$ pA). The normalization is performed by dividing each raw dI/dV curve by its background extracted from a polynomial fitting for $| V | > 20 mV$ . (c) Topographic image of Sr-treated ${SrTiO}_{3}$ (001) substrate ( $V = 300$ mV, $I = 200$ pA). The yellow dots correspond to the (1×1) lattice. (d) Topographic image of monolayer FeSe grown on Sr-treated ${SrTiO}_{3}$ (001) ( $V = 50$ mV, $I = 100$ pA). The arrows in (c) and (d) indicate the in-plane lattice orientation. (e) Tunneling spectra taken on and far away from the Sr-related impurity as marked by the colored dots in (d) ( $V = 50$ mV, $I = 100$ pA). The red dashed line labels the peak position of the in-gap bound state.
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Figure 5
Schematic representations of sign preserving (a) and sign changing (b) and (c) for $Δ (k)$ in monolayer FeSe on ${SrTiO}_{3}$ . Here, the color designates the sign of the gap function (blue: + and red: −). (Adapted from Lee [38].)
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Physical Review B

covering condensed matter and materials physics

Extensive impurity-scattering study on the pairing symmetry of monolayer FeSe films on ${SrTiO}_{3}$

Chong Liu, Jiahao Mao, Hao Ding, Rui Wu, Chenjia Tang, Fangsen Li, Ke He, Wei Li, Can-Li Song, Xu-Cun Ma, Zheng Liu, Lili Wang, and Qi-Kun Xue

Phys. Rev. B 97, 024502 – Published 3 January 2018

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Physical Review B

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Extensive impurity-scattering study on the pairing symmetry of monolayer FeSe films on SrTiO3

Chong Liu, Jiahao Mao, Hao Ding, Rui Wu, Chenjia Tang, Fangsen Li, Ke He, Wei Li, Can-Li Song, Xu-Cun Ma, Zheng Liu, Lili Wang, and Qi-Kun Xue

Phys. Rev. B 97, 024502 – Published 3 January 2018

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Extensive impurity-scattering study on the pairing symmetry of monolayer FeSe films on ${SrTiO}_{3}$