Topological Surface Superconductivity via Josephson Coupling in Bi2Te3/Nb

Abstract

Since discoveries of protected conducting surface states, topological superconducting qubits have enchanted quantum science as prime elements in future fault-tolerant devices, particularly those based on Josephson junctions containing topological insulators. Still, Josephson coupling is often eclipsed by other proximity effects that can dilute topological superconducting pairing at the nontrivial insulator's boundaries. Here, however, using an ultra-low-temperature scanning tunneling microscope, we detect Josephson physics in topological Bi₂Te₃ films on superconducting Nb. At low temperatures, a previously undetected proximity gap varies little with Bi₂Te₃ thickness and the density of states exhibits normal and superconducting components. Such observations are rationalized via Josephson pair tunneling through the (nearly) insulating Bi₂Te₃ bulk, creating a rare, pure topological superconducting sheet. Our findings establish routes toward accessible topological superconducting states in qubits.

Keywords: Josephson junction; Josephson pair tunneling; bulk-carrier doping; proximity effect; topological insulator films; topological superconductor.

1

Preparing and measuring flip-chip Bi₂Te₃/Nb. (A) RHEED patterns of 3–7 QL Bi₂Te₃ on BLG at 300 K, each taken with 8 keV electrons incident along $\stackrel{\circledR }{\mathrm{\Gamma }K}$ . (B) Diagrams showing the flip-chip Bi₂Te₃/Nb structure before cleavage and the measurements conducted on the exposed topological surface after in situ cleavage; the TSS band dispersions (red-and-blue X) at the top surface are depicted schematically. (C,D) Fermi surface maps and photoemission spectra along $\stackrel{\circledR }{\mathrm{\Gamma }K}$ for 3–7 QL Bi₂Te₃/Nb, respectively, all measured with 50 eV photons at 55 K; TSS and CB are labeled in the 5 QL results, and to obtain each map in (C), the ARPES intensity was integrated within ± 10 meV of the Fermi level.

2

Topography of Bi₂Te₃/Nb surfaces at 5 K. (A) STM images for 3 QL Bi₂Te₃/Nb; the top and bottom are respectively a large-scale result (∼100 × 200 nm², V = 200 mV, I = 10 pA) and an atomic-resolved measurement (∼4 × 8 nm², V = −200 mV, I = 50 pA). (B) Topographic maps of 5 QL Bi₂Te₃/Nb (V = 200 mV, I = 20 pA); the top is over a large field of view (∼41.5 × 85 nm²), while the bottom is a small-scale result (∼3.5 × 7 nm²) overlaid with its corresponding fast Fourier transform, which shows Bragg peaks (circled) with hexagonal coordination. (C) Surface topography of 7 QL Bi₂Te₃/Nb; the top is an overview map (∼25 × 50 nm², V = 500 mV, I = 30 pA), and the bottom is an atomic-resolved measurement (∼4.5 × 9 nm², V = – 400 mV, I = 100 pA).

3

Thickness-dependent tunneling spectra at low temperatures. Results are shown for 3–7 QL Bi₂Te₃/Nb (top) and bulk Nb (bottom), each overlaid with a BCS fit (black curve) based on eqs and . The green dashed line in each is the model’s normal component 1 – S.

4

Modeling Josephson coupling in Bi₂Te₃/Nb. (A) Zero-temperature gap Δ (0) versus Bi₂Te₃ thickness (in QL); the light-green dashed curve is an exponential decay assuming the coherence length of Cooper pairs in Sb₂Te₃/Nb (∼4.7 QL), while black dashed lines are guides to the eye to highlight the different gap behavior for Bi₂Te₃/Nb. (B) Superconducting component S as a function of Bi₂Te₃ thickness at T ≈ 0.3 K; the blue curve is an exponential fit. (C) Temperature-dependent tunneling spectra for 3 QL Bi₂Te₃/Nb and their corresponding fits (black curves). The results are consistent with a superconducting transition temperature of 9.3 K. (D) Temperature dependence of S for the 3 QL data in (C). (E) Illustration summarizing Josephson tunneling of Cooper pairs from polycrystalline superconducting Nb to the probed topological surface of flip-chip Bi₂Te₃/Nb. Sb₂Te₃/Nb curve in (A) reproduced from ref . Copyright 2024 American Chemical Society.