[Paper Review] Proximity induced superconductivity in a topological insulator

Philipp Rüßmann, Stefan Blügel|arXiv (Cornell University)|Aug 30, 2022

Topological Materials and Phenomena4 citations

TL;DR

This computational study investigates proximity-induced superconductivity in a Nb/Bi2Te3 heterostructure, revealing that the topological surface state (TSS) at the free surface of the topological insulator experiences a robust superconducting gap due to proximity coupling, while trivial interface states formed by charge-transfer-induced band bending are only weakly proximitized. The key finding is that the TSS remains well-gapped and resilient to Fermi energy fluctuations, but the marginal superconducting gap in trivial states poses a bottleneck for realizing Majorana-based qubits in this platform.

ABSTRACT

Interfacing a topological insulator (TI) with an $s$-wave superconductor (SC) is a promising material platform that offers the possibility to realize a topological superconductor through which Majorana-based topologically protected qubits can be engineered. In our computational study of the prototypical SC/TI interface between Nb and Bi$_2$Te$_3$, we identify the benefits and possible bottlenecks of this potential Majorana material platform. Bringing Nb in contact with the TI film induces charge doping from the SC to the TI, which shifts the Fermi level into the TI conduction band. For thick TI films, this results in band bending leading to the population of trivial TI quantum-well states at the interface. In the superconducting state, we uncover that the topological surface state experiences a sizable superconducting gap-opening at the SC/TI interface, which is furthermore robust against fluctuations of the Fermi energy. We also show that the trivial interface state is only marginally proximitized, potentially obstructing the realization of Majorana-based qubits in this material platform.

Motivation & Objective

• To understand the interplay between interface chemistry, electronic structure, and proximity-induced superconductivity in SC/TI heterostructures.
• To identify the benefits and bottlenecks of using Nb/Bi2Te3 as a platform for topological superconductivity and Majorana zero modes.
• To investigate how charge transfer and band bending at the Nb/Bi2Te3 interface affect the superconducting gap in topological and trivial states.
• To assess the robustness of the induced superconducting gap in the topological surface state against Fermi energy fluctuations.
• To evaluate the feasibility of realizing topologically protected qubits in this material system.

Proposed method

• Density functional theory (DFT) and Bogoliubov-de Gennes (BdG) method are used to model the electronic structure and superconductivity in Nb/Bi2Te3 heterostructures.
• A minimal structural model with six Nb(111) layers and 2–10 quintuple layers of Bi2Te3 is employed to simulate the SC/TI interface.
• The KKR-BdG method is applied with a realistic description of the electronic structure, including a tunable pairing potential to match the experimental Nb gap (0.12 mRy).
• The superconducting gap is scaled to ensure resolution of induced gaps in Bi2Te3, and robustness is tested by varying the coupling constant.
• AiiDA-KKR plugin ensures full data provenance and FAIR-compliant data sharing, with all data and code publicly available.
• Wave function localization and band structure analysis are used to distinguish contributions from TSS, interface states, and bulk states.

Experimental results

Research questions

• RQ1 How does charge transfer from Nb to Bi2Te3 affect the electronic structure and superconducting gap in the topological insulator?
• RQ2 To what extent is the topological surface state (TSS) proximitized compared to trivial interface states formed by band bending?
• RQ3 How robust is the induced superconducting gap in the TSS against variations in the Fermi energy?
• RQ4 What role does wave function hybridization with the superconductor play in determining the strength of proximity-induced pairing?
• RQ5 Why is the trivial interface state only weakly superconducting despite being at the interface?

Key findings

• The topological surface state (TSS) at the free surface of Bi2Te3 experiences a sizable superconducting gap due to proximity coupling with Nb, which is robust against Fermi energy fluctuations.
• Charge transfer from Nb to Bi2Te3 shifts the Fermi level into the conduction band, inducing band bending and populating trivial quantum-well states at the interface in thick films.
• The trivial interface states, formed due to band bending, are only marginally proximitized, acquiring a very small superconducting gap.
• The superconducting gap in the TSS decays exponentially with distance from the Nb/Bi2Te3 interface, consistent with proximity effect expectations.
• The wave function of the TSS shows no overlap with the Nb region, explaining its strong proximitization despite being spatially separated from the superconductor.
• The results are robust to variations in the coupling constant, indicating that the conclusions are not sensitive to this approximation.