Transport spectroscopy of NS nanowire junctions with Majorana fermions

Abstract: We investigate transport though normal-superconductor nanowire junctions in the presence of spin-orbit coupling and magnetic field. As the Zeeman field crosses the critical bulk value B_c of the topological transition, a Majorana bound state (MBS) is formed, giving rise to a sharp zero-bias anomaly (ZBA) in the tunneling differential conductance. We identify novel features beyond this picture in wires with inhomogeneous depletion, like the appearance of two MBSs inside a long depleted region for B<B_c. The resulting ZBA is in most cases weakly split and may coexist with Andreev bound states near zero energy. The ZBA may appear without evidence of a topological gap closing. This latter aspect is more evident in the multiband case and stems from a smooth pinch-off barrier. Most of these features are in qualitative agreement with recent experiments [Mourik et al, Science 336, 1003 (2012)]. We also discuss the rich phenomenology of the problem in other regimes which remain experimentally unexplored.

• The paper demonstrates that Majorana bound states produce a distinct zero-bias anomaly in NS nanowire junctions above a critical Zeeman field.
• Inhomogeneous gate-induced potentials foster Majorana signatures even below the theoretical critical field, challenging expected gap-closing behavior.
• The study leverages multiband models to reveal the coexistence of Majorana and Andreev bound states, refining the interpretation of experimental transport data.

Transport Spectroscopy of NS Nanowire Junctions with Majorana Fermions

This paper presents a detailed investigation into the transport properties of normal-superconductor (NS) nanowire junctions that could support the existence of Majorana bound states (MBSs) under certain physical conditions. The authors, Elsa Prada, Pablo San-Jose, and Ramón Aguado, address the nuances of transport spectroscopy in systems with spin-orbit coupling and an applied Zeeman field.

The crux of the work lies in the identification of Majorana signatures and their interplay with other low-energy states such as Andreev bound states (ABSs). The research leverages theoretical models to predict the behavior of hybrid superconductor-semiconductor systems, especially under the influence of inhomogeneous gate-induced potential profiles.

Key Findings

1. Critical Field and ZBA: The onset of MBSs at a Zeeman field exceeding a critical value $B_c$ is characterized by an observable zero-bias anomaly (ZBA) in the differential conductance $dI/dV$. However, the study also identifies scenarios where Majorana states appear below $B_c$ in regions with smooth pinch-off barriers and inhomogeneous depletion profiles.
2. Inhomogeneous Potentials: The study reveals that inhomogeneous gates lead to the formation of MBSs even for $B < B_c$, accompanied by potentially split ZBAs. This diverges from the expected topological gap-closing and reopening process, observable in more uniform systems.
3. Multiband Scenarios: In multiband systems, the absence of a clear gap transition—ordinarily marked by the closing and reopening of the topological superconducting gap—is discussed. Nonetheless, significant phenomenological parallels with experimental observations (e.g., by Mourik et al.) are noted.
4. ABS-Coexistence and Transport Regimes: Certain regimes show the coexistence of MBSs and ABSs, challenging earlier conceptions that link a well-defined TS phase and MBS presence directly with MBS-derived transport features.

Implications and Prospects

This research contributes significantly to understanding emergent phenomena in NS nanowire junctions pertinent to potential topological quantum computing applications. The presence of Majorana modes in inhomogeneous setups opens prospective avenues for designing more practical and tunable quantum information devices. Furthermore, these insights compel a critical re-evaluation of experimental data, urging care in distinguishing genuine Majorana-induced signatures from near-zero energy artifacts caused by trivial states.

The paper also implies that for future experimental realizations, ensuring control over the smoothness of potential barriers and understanding the multiband effects could be vital in accurately identifying Majorana fermions. Moreover, it raises possibilities for probing new, unexplored parameter regimes, which might yield yet unobserved phenomena or confirm current theoretical anticipations.

Conclusion

The paper captures a nuanced view of the transport spectroscopy in NS nanowire junctions, meticulously detailing regimes that could support Majorana fermions amidst a complex background of other low-energy states. It provides a robust theoretical framework that aligns with experimental observations while challenging conventional paradigms regarding gap phenomena in the presence of spin-orbit coupled superconductors. This exploration stands to deepen our grasp of Majorana physics and heralds advancements in developing topologically robust quantum devices.