Iron Vacancy Tunable Superconductor-Insulator Transition in $\mathrm{FeSe}/{\mathrm{SrTiO}}_{3}$ Monolayer

Iron Vacancy Tunable Superconductor-Insulator Transition in $FeSe / {SrTiO}_{3}$ Monolayer

Cheng-Long Xue, Li-Guo Dou, Yong-Jie Xu, Qian-Qian Yuan, Qi-Yuan Li, Zhen-Yu Jia, Zishuang Li, Ronghua Liu, and Shao-Chun Li

Phys. Rev. Lett. 131, 256002 – Published 20 December 2023

Abstract

The ${Fe}_{4} {Se}_{5}$ with a $\sqrt{5} \times \sqrt{5}$ Fe vacancy order is suggested to be a Mott insulator and the parent state of bulk FeSe superconductor. The iron vacancy ordered state has been considered as a Mott insulator and the parent compound of bulk FeSe-based superconductors. However, for the superconducting $FeSe / {SrTiO}_{3}$ monolayer ( $FeSe / STO$ ) with an interface-enhanced high transition temperature ( $T_{c}$ ), the electronic evolution from its Fe vacancy ordered parent phase to the superconducting state, has not been explored due to the challenge to realize an Fe vacancy order in the $FeSe / STO$ monolayer, even though important to the understanding of superconductivity mechanism. In this study, we developed a new method to generate Fe vacancies within the $FeSe / STO$ monolayer in a tunable fashion, with the assistance of atomic hydrogen. As a consequence, an insulating $\sqrt{5} \times \sqrt{5}$ Fe vacancy ordered monolayer is realized as the parent state. By using scanning tunneling microscopy and scanning tunneling spectroscopy, the spectral evolution from superconductivity to insulator is fully characterized. Surprisingly, a prominent spectral weight transfer occurs, thus implying a strong electron correlation effect. Moreover, the Fe vacancy induced insulating gap exhibits no Mott gap-like features. This work provides new insights in understanding the high- $T_{c}$ superconductivity in $FeSe / STO$ monolayer.

Received 5 April 2023
Revised 15 November 2023
Accepted 21 November 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.256002

Physics Subject Headings (PhySH)

Superconductor-insulator transition

Iron-based superconductors Monolayer films

Molecular beam epitaxy Scanning tunneling microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Cheng-Long Xue¹, Li-Guo Dou ¹, Yong-Jie Xu¹, Qian-Qian Yuan¹, Qi-Yuan Li¹, Zhen-Yu Jia¹, Zishuang Li¹, Ronghua Liu^1,2,3, and Shao-Chun Li ^1,2,3,4,*

¹National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093, China
²Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
³Jiangsu Provincial Key Laboratory for Nanotechnology, Nanjing University, Nanjing 210093, China
⁴Hefei National Laboratory, Hefei 230088, China

^*scli@nju.edu.cn

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Vol. 131, Iss. 25 — 22 December 2023

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Figure 1
Hydrogen exposure induced Fe vacancies in the superconducting $FeSe / STO$ monolayer. (a) Schematic illustration of generating Fe vacancy via hydrogen exposure. (b),(c) Atomically resolved topographic images ( $10 \times 10 {nm}^{2}$ ) of the superconducting $FeSe / STO$ monolayer before and after exposure to a small amount of atomic hydrogen. $U = + 400 mV$ , $I_{t} = 1 nA$ for (b) and $U = + 100 mV$ , $I_{t} = 1 nA$ for (c). The black elliptical in (c) outlines the Fe vacancy. Insets: $d I / d V$ spectra taken on the clean surface (b) and the Fe vacancy (c).
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Figure 2
Topographic evolution of the $FeSe / STO$ monolayer as a function of hydrogen exposure. (a) Topographic images ( $16 \times 16 {nm}^{2}$ ) with randomly distributed Fe vacancies. $U = + 0.2 V$ , $I_{t} = 1 nA$ . (b) Topographic images ( $16 \times 16 {nm}^{2}$ ) of the two distinguishable regions with the different populations of Fe vacancy. $U = + 1 V$ , $I_{t} = 2 nA$ . $N$ region has a low density of Fe vacancies, and $D$ region a high density of Fe vacancies. (c) Topographic images ( $16 \times 16 {nm}^{2}$ ) with ordered Fe vacancies. $U = + 2 V$ , $I_{t} = 3 nA$ . (d) Topographic images ( $16 \times 16 {nm}^{2}$ ) with long-range coherent Fe vacancy order upon the gentle low temperature annealing. $U = + 2 V$ , $I_{t} = 2 nA$ . (e) Atomically resolved STM topographic image ( $4 \times 4 {nm}^{2}$ ) of the $\sqrt{5} \times \sqrt{5}$ order. $U = + 2 V$ and $I_{t} = 1 nA$ . The red square marks the unit cell of the Fe vacancies order. (f) Fast Fourier transform of (e). The red and blue circles mark the $\sqrt{5} \times \sqrt{5}$ order and the lattice Bragg point, respectively.
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Figure 3
Spectral evolution upon hydrogen exposure and phase diagram of the $FeSe / STO$ monolayer. (a)–(c) $d I / d V$ spectra evolution as a function of the Fe vacancy level. The spectra in (a)–(c) are uniformly shifted in the vertical direction for clarity. (d) Electronic phase diagram of the $FeSe / STO$ monolayer as a function of the Fe vacancy level ( $x$ ). Error bars are estimated from the standard deviation of the gap size measured at various locations. The superconductor ( $S$ ), metal ( $M$ ), and insulator ( $I$ ) regions are specified with different colors, respectively.
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Figure 4
Spectral weight evolution of the $FeSe / STO$ monolayer during the superconductor-metal-insulator transition. (a) $d I / d V$ spectra collected as a function of the Fe vacancy level. Each $d I / d V$ spectra is averaged from multiple spectra taken at different locations of the same Fe vacancy level. The spectra are uniformly shifted in the vertical direction for clarity. (b) Series of $d I / d V$ spectra taken from the region of low Fe vacancy level ( $N$ ) to high Fe vacancy level ( $D$ ). (c) Spectral difference obtained by subtracting a spectrum in $N$ region from that in $D$ region. (d),(e) $d I / d V$ intensities of the bump at $\sim - 250 mV$ and the flat bottom at $\sim - 40 mV$ extracted from the positions as marked by the black and blue triangles in (a), respectively.
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Physical Review Letters

Iron Vacancy Tunable Superconductor-Insulator Transition in FeSe/SrTiO3 Monolayer

Cheng-Long Xue, Li-Guo Dou, Yong-Jie Xu, Qian-Qian Yuan, Qi-Yuan Li, Zhen-Yu Jia, Zishuang Li, Ronghua Liu, and Shao-Chun Li

Phys. Rev. Lett. 131, 256002 – Published 20 December 2023

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Iron Vacancy Tunable Superconductor-Insulator Transition in $FeSe / {SrTiO}_{3}$ Monolayer