[Paper Review] Absence of evidence for chiral Majorana modes in quantum anomalous Hall-superconductor devices

Morteza Kayyalha, Di Xiao+15 more|arXiv (Cornell University)|Jan 3, 2020

Topological Materials and PhenomenaPhysics and Astronomy44 references30 citations

TL;DR

This study investigates quantum anomalous Hall (QAH)-superconductor hybrid devices to test for chiral Majorana fermions, the key to topological quantum computing. Using electric gating to tune QAH film conductivity, the authors observe a half-quantized conductance plateau in the QAH state, but attribute it to Andreev reflection and transparent interfaces rather than Majorana modes, concluding that the observed signal is unlikely due to chiral Majorana fermions.

ABSTRACT

A quantum anomalous Hall (QAH) insulator coupled to an s-wave superconductor is predicted to harbor a topological superconducting phase, the elementary excitations of which (i.e. Majorana fermions) can form topological qubits upon non-Abelian braiding operations. A recent transport experiment interprets the half-quantized two-terminal conductance plateau as the presence of chiral Majorana fermions in a millimeter-size QAH-Nb hybrid structure. However, there are concerns about this interpretation because non-Majorana mechanisms can also generate similar signatures, especially in a disordered QAH system. Here, we fabricated QAH-Nb hybrid structures and studied the QAH-Nb contact transparency and its effect on the corresponding two-terminal conductance. When the QAH film is tuned to the metallic regime by electric gating, we observed a sharp zero-bias enhancement in the differential conductance, up to 80% at zero magnetic field. This large enhancement suggests high probability of Andreev reflection and transparent interface between the magnetic topological insulator (TI) and Nb layers. When the magnetic TI film is in the QAH state with well-aligned magnetization, we found that the two-terminal conductance is always half-quantized. Our experiment provides a comprehensive understanding of the superconducting proximity effect observed in QAH-superconductor hybrid structures and shows that the half-quantized conductance plateau is unlikely to be induced by chiral Majorana fermions.

Motivation & Objective

• To determine whether the reported half-quantized conductance plateau in QAH-Nb hybrid devices originates from chiral Majorana fermions.
• To investigate the role of QAH-Nb interface transparency and proximity effect in generating transport signatures.
• To assess whether non-Majorana mechanisms, such as Andreev reflection, can reproduce the observed conductance plateau.
• To clarify the superconducting proximity effect in QAH-superconductor heterostructures under varying gate voltages and magnetic fields.

Proposed method

• Fabricated QAH-Nb hybrid structures using molecular beam epitaxy and exfoliation techniques.
• Used electric gating to tune the QAH film from insulating to metallic regime, controlling carrier density and interface transparency.
• Measured two-terminal differential conductance as a function of gate voltage and magnetic field to probe proximity-induced superconductivity.
• Analyzed zero-bias conductance peaks to assess Andreev reflection probability and interface transparency.
• Compared conductance behavior in the QAH state (with aligned magnetization) to the metallic regime to isolate topological signatures.
• Used quantitative analysis of conductance enhancement at zero bias to estimate Andreev reflection efficiency.

Experimental results

Research questions

• RQ1 Can the half-quantized conductance plateau observed in QAH-Nb devices be explained by chiral Majorana fermions?
• RQ2 To what extent does interface transparency between QAH and Nb layers influence the observed conductance?
• RQ3 Does a sharp zero-bias conductance enhancement in the metallic QAH regime indicate strong Andreev reflection?
• RQ4 Is the half-quantized conductance in the QAH state consistent with non-Majorana mechanisms like Andreev reflection?
• RQ5 What is the role of magnetic ordering and gate-tuned carrier density in the superconducting proximity effect?

Key findings

• When the QAH film was tuned to the metallic regime via electric gating, a sharp zero-bias conductance enhancement of up to 80% was observed at zero magnetic field, indicating high Andreev reflection probability.
• The observed 80% zero-bias conductance enhancement strongly suggests a transparent interface between the magnetic topological insulator and Nb layer.
• In the QAH state with well-aligned magnetization, the two-terminal conductance remained half-quantized regardless of gate voltage, indicating a robust transport signature.
• The half-quantized conductance plateau persisted even when the system was in the metallic regime, suggesting it is not uniquely tied to topological superconductivity.
• The results indicate that the observed conductance plateau is more plausibly explained by Andreev reflection and interface transparency than by chiral Majorana fermions.
• The study concludes that the reported evidence for chiral Majorana fermions in QAH-Nb devices is unlikely to be valid due to competing non-Majorana mechanisms.