Dirac-fermion-assisted interfacial superconductivity in epitaxial topological-insulator/iron-chalcogenide heterostructures

Hemian Yi^#¹, Lun-Hui Hu^#^{1

2}, Yi-Fan Zhao¹, Ling-Jie Zhou¹, Zi-Jie Yan¹, Ruoxi Zhang¹, Wei Yuan¹, Zihao Wang¹, Ke Wang³, Danielle Reifsnyder Hickey^{3

4

5}, Anthony R Richardella¹, John Singleton⁶, Laurel E Winter⁶, Xianxin Wu⁷, Moses H W Chan¹, Nitin Samarth¹, Chao-Xing Liu⁸, Cui-Zu Chang^{9

10}

Affiliations

¹ Department of Physics, The Pennsylvania State University, University Park, PA, 16802, USA.
² Department of Physics and Astronomy, The University of Tennessee, Knoxville, TN, 37996, USA.
³ Materials Research Institute, The Pennsylvania State University, University Park, PA, 16802, USA.
⁴ Department of Chemistry, The Pennsylvania State University, University Park, PA, 16802, USA.
⁵ Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA, 16802, USA.
⁶ National High Magnetic Field Laboratory, Los Alamos, NM, 87544, USA.
⁷ CAS Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing, 100190, China.
⁸ Department of Physics, The Pennsylvania State University, University Park, PA, 16802, USA. cxl56@psu.edu.
⁹ Department of Physics, The Pennsylvania State University, University Park, PA, 16802, USA. cxc955@psu.edu.
¹⁰ Materials Research Institute, The Pennsylvania State University, University Park, PA, 16802, USA. cxc955@psu.edu.

^# Contributed equally.

PMID: 37932274
PMCID: PMC10628154
DOI: 10.1038/s41467-023-42902-2

Dirac-fermion-assisted interfacial superconductivity in epitaxial topological-insulator/iron-chalcogenide heterostructures

Hemian Yi et al. Nat Commun. 2023.

. 2023 Nov 6;14(1):7119.

doi: 10.1038/s41467-023-42902-2.

Authors

Affiliations

¹ Department of Physics, The Pennsylvania State University, University Park, PA, 16802, USA.
² Department of Physics and Astronomy, The University of Tennessee, Knoxville, TN, 37996, USA.
³ Materials Research Institute, The Pennsylvania State University, University Park, PA, 16802, USA.
⁴ Department of Chemistry, The Pennsylvania State University, University Park, PA, 16802, USA.
⁵ Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA, 16802, USA.
⁶ National High Magnetic Field Laboratory, Los Alamos, NM, 87544, USA.
⁷ CAS Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing, 100190, China.
⁸ Department of Physics, The Pennsylvania State University, University Park, PA, 16802, USA. cxl56@psu.edu.
⁹ Department of Physics, The Pennsylvania State University, University Park, PA, 16802, USA. cxc955@psu.edu.
¹⁰ Materials Research Institute, The Pennsylvania State University, University Park, PA, 16802, USA. cxc955@psu.edu.

^# Contributed equally.

PMID: 37932274
PMCID: PMC10628154
DOI: 10.1038/s41467-023-42902-2

Abstract

Over the last decade, the possibility of realizing topological superconductivity (TSC) has generated much excitement. TSC can be created in electronic systems where the topological and superconducting orders coexist, motivating the continued exploration of candidate material platforms to this end. Here, we use molecular beam epitaxy (MBE) to synthesize heterostructures that host emergent interfacial superconductivity when a non-superconducting antiferromagnet (FeTe) is interfaced with a topological insulator (TI) (Bi, Sb)₂Te₃. By performing in-vacuo angle-resolved photoemission spectroscopy (ARPES) and ex-situ electrical transport measurements, we find that the superconducting transition temperature and the upper critical magnetic field are suppressed when the chemical potential approaches the Dirac point. We provide evidence to show that the observed interfacial superconductivity and its chemical potential dependence is the result of the competition between the Ruderman-Kittel-Kasuya-Yosida-type ferromagnetic coupling mediated by Dirac surface states and antiferromagnetic exchange couplings that generate the bicollinear antiferromagnetic order in the FeTe layer.

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Conflict of interest statement

The authors declare no competing interests.

Figures

**Fig. 1. Interfacial superconductivity in MBE-grown (Bi, Sb)₂Te₃/FeTe heterostructures.**
a Schematic of the (Bi, Sb)₂Te₃/FeTe heterostructure. b Cross-sectional ADF-STEM image of the 8QL (Bi, Sb)₂Te₃/50UC FeTe grown on a heat-treated SrTiO₃ (100) substrate. c Temperature dependence of the sheet longitudinal resistance R of 50UC FeTe (black), 8QL Bi₂Te₃/50UC FeTe (red), and 8QL Sb₂Te₃/50UC FeTe (blue) heterostructures. The hump feature in the black curve indicates that the *Néel* temperature T_N of the 50UC FeTe film is ~54 K.

**Fig. 2. Dirac surface states in 8 QL (Bi_1-xSb_x)₂Te₃/50 UC FeTe heterostructures with different Sb concentrations x.**
a–g Top: In vacuo ARPES band map of 8QL (Bi_1-xSb_x)₂Te₃/50UC FeTe/SrTiO₃ heterostructures with x = 0 (a), x = 0.2 (b), x = 0.4 (c), x = 0.6 (d), x = 0.7, x = 0.8 (f), x = 1 (g). Bottom: The corresponding MDCs of the band maps in (a–g). SS, surface states. The dashed lines indicate the positions of the peaks in each momentum distribution curve. The linearly dispersed Dirac surface states are observed in all samples. The Dirac point moves from below to above the chemical potential by increasing x. All ARPES measurements are performed at room temperature.

**Fig. 3. Magnetotransport properties of 8QL (Bi_1-xSb_x)₂Te₃/50UC FeTe heterostructures with interfacial superconductivity.**
a–h Temperature dependence of the sheet longitudinal resistance R measured at μ₀H = 0 T (red) and μ₀H = 9 T (blue) in 8QL (Bi_1-xSb_x)₂Te₃/50UC FeTe/SrTiO₃ heterostructures with x = 0 (a), x = 0.2 (b), x = 0.4 (c), x = 0.7 (d), x = 0.8 (e), x = 0.85 (f), x = 0.95 (g), and x = 1 (h). i Magnetic field μ₀H dependence of R of 8QL (Bi_1−xSb_x)₂Te₃/50UC FeTe heterostructures measured at T = 1.5 K. The magnetic field μ₀H is applied along the out-of-plane direction.

**Fig. 4. Dirac-fermion-assisted interfacial superconductivity in (Bi_1-xSb_x)₂Te₃/FeTe heterostructures.**
a–c The Sb concentration x dependence of the Fermi momentum k_F (a), the onset of superconducting transition temperature T_{c, onset} (blue squares) and the superconducting transition temperature with the zero resistance T_c,0 (red circles) (b), and the upper critical magnetic field μ₀ $H_{c 2, ⊥}$ (c). The error bars in (a) are estimated from the width of the surface state at k_F. The error bars in (b) are estimated to be ~20% of (T_{c, onset} - T_c,0). The error bars in (c) are estimated to be ~40% of [μ₀ $H_{c 2, ⊥}$ (R_normal)- μ₀ $H_{c 2, ⊥}$ (R ~ 0.5R_normal)]. d A phenomenological interpretation of the Dirac-fermion-assisted interfacial superconductivity in (Bi_1-xSb_x)₂Te₃/FeTe heterostructures. The left panel shows the bicollinear antiferromagnetic order of the FeTe layer. $J_{i}$ with $i = 1, 2, 3$ are spin-spin interactions. The middle panel shows the spin-exchange coupling between itinerant Dirac electrons and local spins of FeTe, resulting in the RKKY interaction between two magnetic moments. The right panel shows a possible renormalized spin model in FeTe with a suppressed antiferromagnetic order. e The Dirac fermion-induced RKKY interaction $J_{R K K Y} (k_{F} R_{i j})$ . Here $R_{i j}$ is the next nearest bond of irons, and it reaches its minimal for $k_{F}$ ~ 0 near $x \approx 0.85$ . The error bars in (e) are estimated through the calculation of both $J_{R K K Y} (k_{F} - Δ k_{F})$ and $J_{R K K Y} (k_{F} + Δ k_{F})$ , mirroring the error bars in (a). f The possible $s_{\pm}$ -wave pairing symmetry in real and momentum spaces.

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Dirac-fermion-assisted interfacial superconductivity in epitaxial topological-insulator/iron-chalcogenide heterostructures

Affiliations

Dirac-fermion-assisted interfacial superconductivity in epitaxial topological-insulator/iron-chalcogenide heterostructures

Authors

Affiliations

Abstract

Conflict of interest statement

Figures

References

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