Electronic inhomogeneity and band structure in superstructural ${\mathrm{CuO}}_{2}$ planes of infinite-layer ${\mathrm{Sr}}_{0.94}{\mathrm{La}}_{0.06}{\mathrm{CuO}}_{2+y}$ films

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Electronic inhomogeneity and band structure in superstructural ${CuO}_{2}$ planes of infinite-layer ${Sr}_{0.94} {La}_{0.06} {CuO}_{2 + y}$ films

Rui-Feng Wang, Jiaqi Guan, Yan-Ling Xiong, Xue-Feng Zhang, Jia-Qi Fan, Jing Zhu, Can-Li Song, Xu-Cun Ma, and Qi-Kun Xue

Phys. Rev. B 102, 100508(R) – Published 29 September 2020

Abstract

Scanning tunneling microscopy and spectroscopy are utilized to study the atomic-scale structure and electronic properties of infinite-layer ${Sr}_{0.94} {La}_{0.06} Cu O_{2 + y}$ films prepared on $Sr Ru O_{3}$ -buffered $Sr Ti O_{3}$ (001) substrate by ozone-assisted molecular beam epitaxy. Incommensurate structural supermodulation with a period of 24.5 $Å$ is identified on the $Cu O_{2}$ -terminated surface, leading to characteristic stripes running along the $45^{o}$ direction with respect to the Cu-O-Cu bonds. Spatially resolved tunneling spectra reveal substantial inhomogeneity on a nanometer-length scale and emergence of in-gap states at sufficient doping. Despite the Fermi level shifting up to 0.7 eV, the charge-transfer energy gap of the $Cu O_{2}$ planes remains fundamentally unchanged at different doping levels. The occurrence of the $Cu O_{2}$ superstructure is constrained in the surface region and its formation is found to link with oxygen intake that serves as the doping agent of holes in the epitaxial films.

Received 31 July 2020
Revised 1 September 2020
Accepted 18 September 2020

DOI:https://doi.org/10.1103/PhysRevB.102.100508

Physics Subject Headings (PhySH)

Band gap Electronic structure Local density of states

Cuprates High-temperature superconductors Mott insulators

Molecular beam epitaxy Scanning tunneling microscopy X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Rui-Feng Wang¹, Jiaqi Guan¹, Yan-Ling Xiong¹, Xue-Feng Zhang², Jia-Qi Fan ², Jing Zhu², Can-Li Song ^1,3,*, Xu-Cun Ma^1,3,†, and Qi-Kun Xue^1,3,4

¹State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
²Institute of Physics, National Center for Electron Microscopy in Beijing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
³Frontier Science Center for Quantum Information, Beijing 100084, China
⁴Beijing Academy of Quantum Information Sciences, Beijing 100193, China

^*clsong07@mail.tsinghua.edu.cn
^†xucunma@mail.tsinghua.edu.cn

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Vol. 102, Iss. 10 — 1 September 2020

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Figure 1
(a) STM topography of SLCO epitaxial film ( $450 nm \times 450 nm, V = - 3.5 V, I = 20$ pA), decorated by small single-unit-cell islands. (b) XRD pattern around the SLCO(002) diffraction peaks measured using a monochromatic $C K α 1$ radiation with a wavelength of 1.5406 $Å$ . (c) Atomic-resolved STM topographic image of superstructural $Cu O_{2}$ ( $9.2 nm \times 9.2 nm, V = - 0.85 V, I = 30$ pA). The bright spots correspond to the Cu atoms in the top layer. (d) Topographic profiles along the two Cu-O-Cu bond directions ( $a$ and $b$ ), color coded to match the arrowed lines in (c). Black arrows mark the positions of the invisible Cu atoms.
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Figure 2
(a) Spatially resolved tunneling conductance dI/dV spectra (setpoints: $V = - 1.5 V, I = 200$ pA) acquired along the blue arrow in (b). Black and blue triangles mark the onset energies of CTB and UHB, respectively. Color-shaded areas measure spectral weights of CTB (left), IGS (middle), and UHB (right), respectively. Inset illustrates the schematic band structure of doped $Cu O_{2}$ . (b) STM topography of as-prepared SLCO ( $9.7 nm \times 9.7 nm, V = - 1.2 V, I = 10$ pA). (c) Space-dependent variations in spectral weights and $E_{F}$ shift relative to $E_{i}$ . (d) Spatially resolved dI/dV spectra (setpoints: $V = - 1.5 V, I = 200$ pA) acquired along the blue arrow in (e). Black and blue triangles mark the onset energies of CTB and UHB, respectively. (e) STM topography ( $9.7 nm \times 9.7 nm, V = - 0.8 V, I = 10$ pA) of UHV-annealed SLCO films at $510^{o} C$ for 1 h. (f) Space-dependent onset energies of UHB, CTB (top panel), and $Δ_{CT}$ (bottom panel) in (d).
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Figure 3
Onset energy of UHB (red circles) and $Δ_{CT}$ (blue circles) plotted as a function of CTB onset energy. The statistics involve more than 780 dI/dV spectra at varied positions and samples. Gray dashed lines show the best linear fits to the data.
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Figure 4
(a) HAADF-STEM image across the interface between SLCO and SRO along the [001] axis, marked by the dashed line. (b) ABF-STEM image in the same field of view as (a). Inset shows a zoom-in of the SLCO/SRO interface. The magenta arrows mark the oxygen in the SrO layer of SRO, while the blue arrows indicate no apical oxygen in the Sr/La layers of SLCO. (c) Schematic structure of superstructural SLCO films prepared on SRO-buffered STO substrates, with the interfacial stacking of SLCO/SRO and SRO/STO magnified. (d) Possible diagram of the superstructural SLCO near the surface. The magenta arrows mark the incorporated oxygens that serve as the doping agent of holes.
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Physical Review B

covering condensed matter and materials physics

Electronic inhomogeneity and band structure in superstructural ${CuO}_{2}$ planes of infinite-layer ${Sr}_{0.94} {La}_{0.06} {CuO}_{2 + y}$ films

Rui-Feng Wang, Jiaqi Guan, Yan-Ling Xiong, Xue-Feng Zhang, Jia-Qi Fan, Jing Zhu, Can-Li Song, Xu-Cun Ma, and Qi-Kun Xue

Phys. Rev. B 102, 100508(R) – Published 29 September 2020

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Physical Review B

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Electronic inhomogeneity and band structure in superstructural CuO2 planes of infinite-layer Sr0.94La0.06CuO2+y films

Rui-Feng Wang, Jiaqi Guan, Yan-Ling Xiong, Xue-Feng Zhang, Jia-Qi Fan, Jing Zhu, Can-Li Song, Xu-Cun Ma, and Qi-Kun Xue

Phys. Rev. B 102, 100508(R) – Published 29 September 2020

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Electronic inhomogeneity and band structure in superstructural ${CuO}_{2}$ planes of infinite-layer ${Sr}_{0.94} {La}_{0.06} {CuO}_{2 + y}$ films