🔬 materials science

Observation of stacked vortex structure with number controllable Majorana zero mode in Bi $_{2}$ Te $_{3}$ /FeTe heterojunction

This study demonstrates the creation and magnetic-field-tunable control of Majorana zero modes within stacked vortex structures of stoichiometric Bi $_{2}$ Te $_{3}$ /FeTe heterojunctions, establishing a patterned superconducting platform for fault-tolerant topological quantum computation.

Original authors: Xinru Han, Zhaokun Luo, Quanxin Hu, Xingjian Zhu, Xiaohong Pan, Dan Mu, Xinyu Liang, Zulin Xu, Qiheng Wang, Ryan L. Kong, Mingyuan Li, Zhengliang Zhang, Fazhi Yang, Mojun Pan, Xin Liu, Qiangqiang Gu

Published 2026-10-14

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Xinru Han, Zhaokun Luo, Quanxin Hu, Xingjian Zhu, Xiaohong Pan, Dan Mu, Xinyu Liang, Zulin Xu, Qiheng Wang, Ryan L. Kong, Mingyuan Li, Zhengliang Zhang, Fazhi Yang, Mojun Pan, Xin Liu, Qiangqiang Gu, Baiqing Lv, Gan Wang, Hong Ding

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the quest to build a new kind of computer that can solve problems impossible for today's machines, scientists are looking for a very specific type of particle. This hypothetical particle, known as a Majorana zero mode, acts like a building block for a fault-tolerant quantum computer. Unlike ordinary bits of information that can be easily scrambled by noise or heat, these particles are topologically protected, meaning their information is stored in a way that is inherently robust against errors. The challenge has been finding a material that not only hosts these particles but also allows researchers to create them on demand and control exactly how many exist in a single spot. Without this control, the particles cannot be moved or manipulated to perform calculations, much like having a library of books but no way to open the doors or arrange the shelves.

A team of researchers has now taken a significant step toward solving this puzzle by creating a new type of material interface where these particles can be found and counted. They grew a microscopic sandwich of two different materials: a layer of iron telluride and a single, ultra-thin sheet of bismuth telluride. Neither material is superconducting on its own, but when placed together, they create a superconducting state right at the boundary where they touch. By cooling this structure to near absolute zero and applying a magnetic field, the researchers observed the formation of tiny whirlpools of electricity, known as vortices. Inside these vortices, they found clear evidence of the elusive Majorana zero modes. What makes this discovery particularly powerful is that the researchers could control the number of these modes simply by changing the strength of the magnetic field or the size of the tiny islands of material they were studying.

The researchers used a highly sensitive microscope that can see individual atoms to examine the surface of their new material. They first confirmed that the two layers fit together in a specific, repeating pattern, creating a large-scale grid known as a moiré superlattice. This pattern acts like a subtle landscape that modulates the superconducting properties across the surface. When they applied a magnetic field, distinct whirlpools appeared. Using spectroscopy, a technique that measures how electrons behave at different energy levels, they discovered that these whirlpools have a complex, stacked structure. One part of the vortex behaves like a typical swirling current found in many superconductors, with energy levels that spread out continuously. However, a second, distinct part of the vortex, originating from the iron telluride layer underneath, behaves very differently.

In this lower layer, the researchers observed a sharp peak of electrical conductance right at zero energy, which is a hallmark signature of a Majorana zero mode. Unlike the spreading energy of the upper layer, this zero-energy state remained tightly locked in place, unsplit and stable. Surrounding this central peak were other discrete energy levels that followed a precise, integer-based sequence, further confirming the quantum nature of the state. The team found that these Majorana modes appeared in every vortex they measured. Crucially, they demonstrated that they could tune the system to contain one, two, or three of these Majorana vortices within a single superconducting island. This level of control is achieved by adjusting the magnetic field or by growing islands of specific sizes, allowing the researchers to isolate and manipulate these quantum states with precision.

The significance of this work lies in the purity and controllability of the system. Previous attempts to find these particles often relied on materials with chemical imperfections or alloy mixtures, which made it difficult to distinguish the true signal from background noise. By using a stoichiometric, or chemically perfect, interface between two well-defined crystals, the researchers created a clean environment where the Majorana modes could be observed without ambiguity. They also ruled out the possibility that the top layer alone was responsible for the effect, showing that the unique electronic structure of the iron telluride layer, modified by the presence of the bismuth telluride, was the source of the topological protection. This setup provides a reliable platform where scientists can now study how these particles interact with one another, a necessary step toward eventually braiding them together to perform quantum logic operations.

The ability to engineer these interfaces opens a new path for exploring the fundamental physics of topological quantum matter. The researchers showed that the superconducting properties are confined strictly to the areas where the two materials overlap, meaning the vortices and their associated Majorana modes are naturally isolated within these tiny islands. This confinement prevents the particles from wandering off and allows for the creation of specific configurations, such as pairs or small groups, which are essential for testing theories about how these particles can be used to store and process information. While the path to a working quantum computer remains long, this work provides a clear, controllable, and reproducible method for generating the very particles that could make such a machine possible. The findings suggest that by carefully designing the interface between different materials, scientists can create a stable environment where the exotic rules of quantum mechanics can be harnessed for practical computation.

Technical Summary: Observation of Stacked Vortex Structure with Number Controllable Majorana Zero Mode in Bi2Te3/FeTe Heterojunction

Problem and Motivation
The realization of fault-tolerant topological quantum computation relies on the creation and control of Majorana zero modes (MZMs). While various platforms have been proposed, including semiconductor nanowires and iron-based superconductors, significant challenges remain. Nanowire systems often suffer from small topological gaps and low transition temperatures, with conclusive evidence for MZMs still lacking. Conversely, iron-based superconductors like Fe(Te, Se) offer larger superconducting gaps but are plagued by chemical inhomogeneity (alloy disorder) and the polar surface problem, leading to non-uniform MZM distributions. Furthermore, in bulk superconducting substrates, magnetic fields induce dense, uncontrollable Abrikosov vortex lattices, making it difficult to isolate and deterministically manipulate the specific number of vortices required to study Majorana interactions and fusion. There is a critical need for a stoichiometric, structurally well-defined superconducting platform that enables robust topological protection and precise control over the number and spatial distribution of MZMs.

Methodology
The authors employed molecular beam epitaxy (MBE) to grow high-quality heterostructures consisting of a single quintuple layer (1QL) of Bi2Te3 on top of a 20-unit-cell FeTe layer. Neither constituent is superconducting in its pristine form; superconductivity emerges exclusively at the interface. The study utilized low-temperature scanning tunneling microscopy and spectroscopy (STM/S) at 0.4 K to probe the electronic structure. Key experimental conditions included:

Structural Characterization: High-resolution STM to visualize the atomic lattice and the resulting moiré superlattice arising from the mismatch between the hexagonal Te network of Bi2Te3 and the square Te lattice of FeTe.
Spectroscopic Imaging: Mapping the local density of states (LDOS) and differential conductance ( $dI/dV$ ) under perpendicular magnetic fields (up to 1.0 T) to identify vortex cores and bound states.
Parameter Variation: Systematic tuning of the magnetic field and the lateral size of the Bi2Te3 islands to control the number of vortices within the superconducting regions.
Data Analysis: Multi-peak fitting of spectra to extract energy levels, dispersion relations, and spatial decay lengths of vortex bound states. Fourier transform analysis was used to investigate quasiparticle interference (QPI) patterns.

Key Results

Moiré-Modulated Interfacial Superconductivity: The heterostructure exhibits a long-period $7 \times \sqrt{3}$ moiré superlattice. This potential induces a periodic modulation of the superconducting gap size and coherent peak amplitude, confirming that superconductivity is confined to the Bi2Te3-covered regions and is absent in bare FeTe areas.
Stacked-Core Vortex Structure: Under a magnetic field, the system displays a unique "stacked-core" vortex structure. Spectroscopic imaging reveals two distinct, coexisting branches of vortex bound states originating from the different layers:
- Bi2Te3-derived Branch: Exhibits a dispersive, X-shaped pattern extending away from the vortex center, consistent with Caroli–de Gennes–Matricon (CdGM) states in a system with a large Fermi energy.
- FeTe-derived Branch: Remains in the quantum limit, characterized by a spatially unsplit zero-bias conductance peak (ZBCP) with a full width at half maximum of ~0.5 meV. This branch also displays discrete, integer-quantized CdGM states.
Evidence for Majorana Zero Modes: The FeTe-derived branch shows a robust, unsplit ZBCP that persists along different high-symmetry directions from the core center, contrasting with the anisotropic star-shaped profile of the Bi2Te3 branch. The spatial profile of the ZBCP decays with a characteristic length consistent with theoretical predictions for MZMs. The presence of integer-quantized low-energy states alongside the ZBCP provides spectroscopic evidence for MZMs.
Origin of Topology: The MZMs are attributed to the FeTe layer. While single-layer Bi2Te3 has a hybridization gap that annihilates gapless Dirac states, the growth of Bi2Te3 on FeTe creates a Te-rich environment. This is proposed to reduce the influence of excess interstitial Fe (analogous to Te annealing), restoring the non-trivial topological electronic structure of FeTe and driving it into a topological superconducting phase. The top layer Bi2Te3 likely enhances spin-orbit coupling and facilitates charge transfer.
Controllability: By varying the magnetic field and the size of the Bi2Te3 islands, the authors successfully realized configurations containing one, two, and three Majorana vortices. This demonstrates the ability to deterministically control the number of MZMs in isolated superconducting islands.

Significance and Claims
The paper establishes stoichiometric 1QL-Bi2Te3/FeTe as a clean, spatially controllable superconducting interface. Unlike alloyed systems, this platform avoids chemical inhomogeneity, ensuring reproducible vortex Majorana signatures. The confinement of superconductivity to engineered islands allows for the isolation of vortices and the deterministic tuning of their number, addressing a major limitation in previous platforms.

The authors claim that this system provides a controlled platform for exploring Majorana interactions and fusion processes. Specifically, the ability to generate and tune the presence of Majorana edge modes (MEMs) at the edges of superconducting islands by controlling the vortex number offers a pathway toward realizing topological quantum qubits. The observation of stacked vortices with distinct layer-derived bound states also opens new avenues for investigating interactions between different vortex-core states across a superconducting interface. The work represents a step toward moving from isolated spectroscopic signatures to braidable topological qubits in a stoichiometric material system.

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