Massive Suppression of Proximity Pairing in Topological ${\mathbf{(}{\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}\mathbf{)}}_{2}{\mathrm{Te}}_{3}$ Films on Niobium

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Massive Suppression of Proximity Pairing in Topological ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3}$ Films on Niobium

Joseph A. Hlevyack, Sahand Najafzadeh, Meng-Kai Lin, Takahiro Hashimoto, Tsubaki Nagashima, Akihiro Tsuzuki, Akiko Fukushima, Cédric Bareille, Yang Bai, Peng Chen, Ro-Ya Liu, Yao Li, David Flötotto, José Avila, James N. Eckstein, Shik Shin, Kozo Okazaki, and T.-C. Chiang

Phys. Rev. Lett. 124, 236402 – Published 12 June 2020

Abstract

Interfacing bulk conducting topological ${Bi}_{2} {Se}_{3}$ films with $s$ -wave superconductors initiates strong superconducting order in the nontrivial surface states. However, bulk insulating topological ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3}$ films on bulk Nb instead exhibit a giant attenuation of surface superconductivity, even for films only two layers thick. This massive suppression of proximity pairing is evidenced by ultrahigh-resolution band mappings and by contrasting quantified superconducting gaps with those of heavily $n$ -doped topological ${Bi}_{2} {Se}_{3} / Nb$ . The results underscore the limitations of using superconducting proximity effects to realize topological superconductivity in nearly intrinsic systems.

Received 9 March 2020
Accepted 21 May 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.236402

Physics Subject Headings (PhySH)

Proximity effect Topological insulators Topological superconductors

Topological materials

Angle-resolved photoemission spectroscopy Molecular beam epitaxy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Joseph A. Hlevyack ^1,2,‡, Sahand Najafzadeh^3,‡, Meng-Kai Lin ^1,2, Takahiro Hashimoto³, Tsubaki Nagashima³, Akihiro Tsuzuki³, Akiko Fukushima³, Cédric Bareille³, Yang Bai^1,2, Peng Chen ^1,2,4,5, Ro-Ya Liu^1,2,6, Yao Li ^1,2, David Flötotto⁷, José Avila⁸, James N. Eckstein ^1,2, Shik Shin⁹, Kozo Okazaki^3,*, and T.-C. Chiang ^1,2,†

¹Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
²Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
³Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
⁴Shanghai Center for Complex Physics, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
⁵Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Shenyang National Laboratory for Materials Science, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
⁶Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁷Center for Soft Nanoscience, University of Münster, 48149 Münster, Germany
⁸Synchrotron SOLEIL and Université Paris-Saclay, L’Orme des Merisiers, BP48, 91190 Saint-Aubin, France
⁹Office of University Professor, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan

^*Corresponding author. okazaki@issp.u-tokyo.ac.jp
^†Corresponding author. tcchiang@illinois.edu
^‡These authors contributed equally to this work.

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Vol. 124, Iss. 23 — 12 June 2020

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Figure 1
Flip-chip method and characterizing lightly $n$ -type TIs. (a),(b) Schematic band structures of $n +$ -doped prototypical TIs and lightly $n$ -doped ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3}$ , respectively. Fermi levels are indicated with black dashed lines. (c) RHEED pattern of a 2 QL ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3}$ film ( $x = 0.62$ ) grown on BLG-SiC. (d) ARPES spectra (left) and corresponding second-derivative map (right) of 2 QL ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3}$ taken along $\bar{Γ K}$ with 21.22-eV photons at 18 K. (e) Illustration of flip-chip technique for preparing TIs on superconducting Nb substrates. (f) Photo and diagram of flip-chip sample structure before cleavage. (g) Similar as in (f) but after cleavage. (h) ARPES maps of 2–4 QL ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3} / Nb$ ( $x = 0.62$ ) flip-chip samples taken with 25-eV photons at 80 K. (i) Corresponding second-derivative maps.
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Figure 2
Contrast between surface superconductivity in $n +$ -doped TIs and $n$ -type TIs. (a) ARPES map of 4 QL ${Bi}_{2} {Se}_{3} / Nb$ taken with 6.994-eV photons at $T = 10 K$ . The dashed white lines are guides to the eye that identify the Dirac cone. (b) Detailed ARPES spectra for binding energies near the Fermi level for 4 QL ${Bi}_{2} {Se}_{3} / Nb$ at $T = 10 K$ (top) and $T = 1.5 K$ (bottom). (c) Corresponding symmetrized ARPES spectra, showing superconducting gaps and coherence peaks at all measured momenta at $T = 1.5 K$ . (d) $k$ -space integrated EDCs as a function of temperature (top) and corresponding symmetrized EDCs (bottom), with the $k$ -space integration taken over the Dirac cone (from $- 0.18$ to $0.18 Å^{- 1}$ ). (e)–(h) Corresponding dataset for 4 QL ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3} / Nb$ for $x = 0.62$ , which reveals no superconductivity signatures; the $k$ -space integration in (h) is taken over the Dirac cone (from $- 0 .1$ to $0.1 Å^{- 1}$ ).
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Figure 3
$k$ -space integrated EDCs as a function of temperature and thickness. Results are shown for a Nb reference (top), 4–10 QL ${Bi}_{2} {Se}_{3} / Nb$ (bottom, left), and 2–5 QL ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3} / Nb$ for $x = 0.62$ (bottom, right). The $k$ -space integrations are taken over $- 0.18$ to $0.18 Å^{- 1}$ for ${Bi}_{2} {Se}_{3} / Nb$ and $- 0.1$ to $0.1 Å^{- 1}$ for ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3} / Nb$ . For comparisons between the measurements, the red arrows connect the spectra of 4 and 5 QL ${Bi}_{2} {Se}_{3} / Nb$ to those of ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3} / Nb$ . Clear coherence peaks and leading edge shifts are visible in the Nb reference and ${Bi}_{2} {Se}_{3} / Nb$ for all thicknesses, while none can be identified for all ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3} / Nb$ .
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Figure 4
Symmetrized $k$ -space integrated EDCs as a function of temperature and film thickness. Results are summarized for a Nb reference (top), 4–10 QL ${Bi}_{2} {Se}_{3} / Nb$ (bottom, left), and 2–5 QL ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3} / Nb$ for $x = 0.62$ (bottom, right). The $k$ -space integrations are taken over $- 0.18$ to $0.18 Å^{- 1}$ for ${Bi}_{2} {Se}_{3} / Nb$ and $- 0.1$ to $0.1 Å^{- 1}$ for ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3} / Nb$ . The red arrows are guides in the comparison between the datasets. As the temperature decreases from $T = 10 K$ , a clear superconducting gap develops in the data of the Nb reference and all ${Bi}_{2} {Se}_{3} / Nb$ , while none is evident for all ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3} / Nb$ samples.
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Physical Review Letters

Massive Suppression of Proximity Pairing in Topological (Bi1−xSbx)2Te3 Films on Niobium

Joseph A. Hlevyack, Sahand Najafzadeh, Meng-Kai Lin, Takahiro Hashimoto, Tsubaki Nagashima, Akihiro Tsuzuki, Akiko Fukushima, Cédric Bareille, Yang Bai, Peng Chen, Ro-Ya Liu, Yao Li, David Flötotto, José Avila, James N. Eckstein, Shik Shin, Kozo Okazaki, and T.-C. Chiang

Phys. Rev. Lett. 124, 236402 – Published 12 June 2020

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Massive Suppression of Proximity Pairing in Topological ${({Bi}_{1 - x} {Sb}_{x})}_{2} {Te}_{3}$ Films on Niobium