Researchers Find Majorana Modes Unfold across Vortex Fragments

Introduction

Recent work from Shanghai Jiao Tong University, Xiamen University, and the Tsung‑Dao Lee Institute shows that Majorana zero modes (MZMs) in iron‑based superconductors are not confined to a single point. Instead, when hosted by fractional vortices (FVs) the modes unfold over several vortex fragments, forming elongated density profiles. This spatial extension makes the modes observable through two complementary techniques: conventional local tunneling spectroscopy and the non‑local process of crossed Andreev reflection (CAR). The findings suggest a new experimental strategy for confirming MZMs—key ingredients for fault‑tolerant topological quantum computers.

Fractional Vortices and Spatial Unfolding

In a conventional Abrikosov vortex the magnetic flux is quantized in units of the flux quantum, and any bound state, including a Majorana mode, is tightly localized at the vortex core. Fractional vortices, by contrast, carry only a fraction of the flux and can arrange themselves along geometric lines (e.g., Fermat or Torricelli lines). The team observed that MZMs associated with these FVs spread into elliptical contours whose major‑to‑minor axis ratios are less than one. This unfolding is a manifestation of topological protection: the mode’s wavefunction hybridizes across adjacent vortex fragments while remaining immune to local disorder.

Dual Detection Pathways

Local Tunneling Spectroscopy

Scanning tunneling microscopy (STM) measurements reveal a zero‑bias conductance peak that now appears over a broader area rather than at a single point. The extended signal matches theoretical predictions for an unfolded Majorana density, providing a straightforward, spatially resolved confirmation.

Crossed Andreev Reflection

CAR involves injecting an electron into one vortex core and detecting a correlated hole emerging from a neighboring core. Because the unfolded MZM spans multiple cores, the probability of CAR is enhanced, even when the separation exceeds the superconductor’s coherence length. The researchers demonstrated clear CAR signatures that correlate with the locations of the fractional vortices, offering a non‑local verification method that is less susceptible to local noise.

Implications for Topological Quantum Computing

The ability to detect MZMs through both local and non‑local probes reduces reliance on a single, potentially ambiguous measurement. Spatially unfolded modes may also be more robust against material imperfections, a critical factor for scaling up topological qubits. Moreover, the CAR pathway suggests a natural mechanism for coupling distant Majorana sites, which could be harnessed for braiding operations and quantum gate implementations.

Looking Ahead

Future experiments will likely focus on:

1. Material Optimization – refining iron‑based superconductors to enhance spin‑orbit coupling and minimize disorder.
2. Vortex Engineering – deliberately arranging fractional vortices to maximize CAR efficiency and control mode overlap.
3. Device Integration – embedding unfolded MZMs into nanofabricated circuits to test braiding and readout schemes.

By expanding the search space from point‑like to extended signatures, this work opens a practical route toward reliable Majorana detection and, ultimately, the realization of topological quantum processors.

--- This article synthesizes the key concepts from the recent preprint without introducing new data or quotations.