🔬 materials science

Spin-polarized edge modes between different magnet-superconductor-hybrids

By combining two distinct antiferromagnetic magnet-superconductor hybrids, researchers experimentally and theoretically demonstrate the emergence of robust, spin-polarized chiral edge modes at their interface, driven by the connection of topological nodal points and asymmetric lateral decay, thereby establishing a platform for advanced topology-based applications.

Original authors: Felix Zahner, Felix Nickel, Roberto Lo Conte, Tim Drevelow, Roland Wiesendanger, Stefan Heinze, Kirsten von Bergmann

Published 2026-11-02

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Felix Zahner, Felix Nickel, Roberto Lo Conte, Tim Drevelow, Roland Wiesendanger, Stefan Heinze, Kirsten von Bergmann

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 hidden world of quantum materials, scientists are constantly searching for states of matter that are not just stable, but protected by the very laws of physics. Imagine a highway where traffic is forced to move in only one direction, immune to potholes or obstacles; this is the promise of topological states. When these states appear in superconductors—materials that conduct electricity with zero resistance—they become even more intriguing. Researchers are particularly interested in a specific type called topological nodal point superconductors. In these materials, the energy gap that usually separates conducting electrons from non-conducting ones does not close completely everywhere. Instead, it closes at specific points, creating "nodal points." At the edges where these special materials meet ordinary ones, or where two different types of these materials meet, strange new pathways for electrons can appear. These pathways, known as edge modes, are crucial because they could one day carry information without losing energy, a key requirement for the next generation of quantum computers.

A team of researchers in Germany has now taken a significant step toward understanding and controlling these edge modes. By combining two different magnetic layers with a superconducting surface, they discovered a new way to create and manipulate these electronic pathways. The team, working at the University of Hamburg and collaborating with theorists from Kiel and Groningen, focused on a surface made of tantalum, a metal that becomes superconducting at very low temperatures. On this surface, they grew ultra-thin films of manganese, a magnetic metal. They created two distinct regions: one where the manganese formed a single atomic layer, and another where it formed a double layer. Both of these magnetic layers, when placed on the superconducting tantalum, transformed the surface into a topological nodal point superconductor. However, because the layers were of different thicknesses, they possessed slightly different internal electronic structures.

The researchers used a powerful microscope called a spin-polarized scanning tunneling microscope to peer into this setup. This instrument is unique because it can not only see individual atoms but also detect the magnetic orientation, or "spin," of the electrons passing through them. When they scanned the boundary where the single-layer manganese met the double-layer manganese, they found something unexpected. Instead of a quiet transition, there was a bright, active line of electrons right at the junction. These electrons formed a low-energy mode that existed only at the boundary between the two different magnetic films. Crucially, the team found that these electrons were not just flowing; they were highly polarized, meaning their spins were aligned in a specific direction. This was a surprise because the magnetic order of the manganese layers was antiferromagnetic, a state where neighboring atoms point in opposite directions, which usually cancels out any net magnetic effect.

To understand why this happened, the researchers built a detailed computer model of the system. They simulated how electrons move through the single-layer and double-layer regions and how they behave when these two regions meet. The model confirmed that the edge mode connects the specific nodal points of the single-layer system to the nodal points of the double-layer system. The key to the spin polarization lies in how the electrons behave as they move away from the boundary. The model showed that the edge mode decays, or fades away, much more quickly into the single-layer region than into the double-layer region. Because the electrons vanish faster on one side, the magnetic character of that side dominates the signal. This asymmetry creates a net spin polarization, effectively filtering the electrons so that only those with a specific spin orientation remain visible at the boundary.

This finding suggests that the spin polarization of these edge modes is a robust feature that does not depend on the precise details of how the two materials are joined. Whether the magnetic atoms at the boundary are aligned in the same direction or opposite directions, the difference in how the edge mode decays into each material ensures that the electrons remain spin-polarized. The researchers verified this by comparing their experimental images with their simulations, finding a strong match in the intensity and location of the signals. The edge mode appeared as a distinct peak in the electrical conductance right at zero energy, and its strength varied depending on the direction of the magnetic spins at the edge, confirming its magnetic nature.

The implications of this work extend beyond just observing a new phenomenon. It demonstrates that by simply changing the thickness of a magnetic film, or by using different magnetic materials, scientists can create boundaries that host these specialized, spin-polarized currents. This offers a new level of control over topological states. Instead of relying on complex external magnetic fields to tune these properties, researchers might be able to design them directly into the material structure. The ability to create and manipulate spin-polarized edge modes between different topological superconductors opens a path toward more advanced functional designs. It suggests that future devices could be built by stitching together different topological phases, creating a landscape where electronic currents can be guided and filtered with high precision, all without the need for external magnetic control. This work bridges the gap between theoretical predictions and experimental reality, showing that the complex interplay of magnetism and superconductivity can be harnessed to create new, controllable quantum states.

Technical Summary: Spin-Polarized Edge Modes Between Different Magnet-Superconductor Hybrids

Problem Statement
Topological superconductivity (TSC) is a field of significant interest due to the potential for hosting topologically protected states, such as Majorana bound states and edge modes. While magnet-superconductor hybrid (MSH) systems have been established as a promising platform for realizing TSC, previous investigations have primarily focused on boundaries between a topological superconducting phase and a trivial superconducting substrate. A critical gap in understanding remains regarding the interface between two different topological superconducting phases. Specifically, it is unclear whether distinct topological nodal point superconducting (TNPSC) phases, when interfaced, generate edge modes, and if so, what the spin characteristics of these modes are. Furthermore, while edge modes at magnetic-trivial interfaces have been observed, their controllability and spin-polarization properties at the boundary of two different magnetic topological phases require experimental and theoretical clarification.

Methodology
The study employs a combined experimental and theoretical approach using a Mn-monolayer (ML) and Mn-bilayer (BL) system grown on a superconducting Ta(110) surface.

Experimental: Low-temperature spin-polarized scanning tunneling microscopy (SP-STM) and spectroscopy (dI/dU) were conducted at 1.3 K. The samples were prepared in ultra-high vacuum, with Mn deposited on Ta(110) to form pseudomorphic ML and BL regions. SP-STM utilized Cr-bulk tips to resolve magnetic order and spin-polarized electronic states. Zero-bias dI/dU maps and spatially resolved spectroscopy were used to characterize edge modes at the boundaries between ML/BL regions and the Ta substrate, as well as at the ML/BL interface.
Theoretical: A tight-binding model was constructed to simulate the MSH system. The Hamiltonian includes terms for chemical potential ( $\mu$ ), superconducting order parameter ( $\Delta$ ), hopping ( $t$ ), Rashba-type spin-orbit coupling ( $\lambda$ ), and a coupling term ( $J_S$ ) between the superconducting layer and the magnetic moments ( $S$ ) of the Mn layers. The coupling strength $J_S$ was varied to distinguish between the ML and BL systems, based on density functional theory (DFT) calculations of induced magnetic moments. The model utilized stripe geometries to simulate boundaries between different phases (ML/BL and ML/BL vs. Ta). Band structures, spectral functions, and complex band structures were calculated to determine the existence of edge states, their connectivity between nodal points, and their decay characteristics. SP-STM simulations were performed using a spin-polarized generalization of the Tersoff-Hamann model to compare directly with experimental data.

Key Results

Characterization of Individual Phases: Both the Mn ML and Mn BL on Ta(110) exhibit antiferromagnetic order and superconducting gaps induced by proximity to the Ta substrate. Tunnel spectroscopy confirms that both layers are in a TNPSC phase, characterized by nodal points (NPs) in the band structure.
Edge Modes at Trivial Boundaries: Consistent with previous studies, edge modes were observed at the boundaries between the Mn layers and the trivial Ta substrate. The intensity and presence of these modes depended on the crystallographic orientation of the edge (e.g., [001] vs. [111]), confirming the topological nature of the nodal points.
Edge Modes at the ML/BL Interface: The primary experimental finding is the observation of low-energy, spin-polarized edge modes at the boundary between the Mn ML and Mn BL. These modes appear as an increased dI/dU signal at zero bias. Crucially, the signal intensity differs for edges with opposite magnetization directions, demonstrating that the edge mode is spin-polarized.
Theoretical Explanation of Spin Polarization: The tight-binding model confirms that edge states emerge at the interface between the two different TNPSC phases (ML and BL), connecting nodal points of the two distinct systems.
- The model reveals that these edge modes connect NPs from the ML to NPs from the BL.
- The spin polarization arises not from the specific spin configuration at the boundary (whether $\uparrow\uparrow$ or $\uparrow\downarrow$ ), but from an asymmetric lateral decay of the edge mode into the two adjacent bulk systems.
- Calculations of the complex band structure show that the decay constant ( $\kappa$ ) is larger in the ML than in the BL. Consequently, the edge state decays faster into the ML, causing the spin polarization to be dominated by the magnetic termination on the ML side. This results in a net spin polarization regardless of the specific boundary termination.

Significance and Claims
The paper claims to demonstrate the existence of spin-polarized chiral edge modes at the boundary between two different topological nodal point superconductors. The authors assert that this phenomenon is a general effect arising from the mismatch in electronic structures (specifically, different decay lengths) between the two adjacent topological phases.

The significance of this work lies in:

Novel Interface Physics: It establishes that topological edge modes can exist and be spin-polarized at the interface of two distinct topological superconductors, not just between a topological and a trivial phase.
Robustness of Spin Polarization: The spin polarization is shown to be a robust feature determined by the bulk electronic properties (decay constants) of the adjacent materials, rather than being sensitive to the specific atomic-scale details of the boundary termination.
Platform for Applications: The ability to create spin-polarized edge states by interfacing different magnetic layers on a superconductor offers a pathway for advanced functional design. The authors suggest this could enable the tuning of topological edge modes via electrical means (e.g., gating) or strain, providing a new platform for topology-based applications in magnet-superconductor hybrids.

The study concludes that the combination of different antiferromagnetic layers with a superconducting substrate represents a viable strategy for manipulating topological edge modes, opening new directions for the investigation of MSH systems.

Technical Summary: Spin-Polarized Edge Modes Between Different Magnet-Superconductor Hybrids

More like this