The Hunt for Magnetic Weyl Fermions
A New Particle in the Quantum Playground
The recent discovery of magnetic Weyl fermions represents a triumph in condensed matter physics, revealing a particle once confined to theoretical physics within a special class of magnets1 .
In the quest to understand the universe, physicists often bridge the cosmic and the microscopic. Just as celestial explorers seek new stars, condensed matter physicists hunt for exotic particle-like excitations hidden within the atomic lattice of crystals.
What Are Weyl Fermions? Your Matter FAQ
To appreciate the discovery, let's break down some key concepts.
The Weyl Fermion
Named after the mathematician Hermann Weyl, this is a massless particle that carries a unique property known as chirality, or "handedness." For decades, it was a purely theoretical concept in particle physics, never observed as a fundamental particle in nature5 .
The Magnetic Twist
Most initially discovered Weyl semimetals broke inversion symmetry. The hunt for magnetic Weyl fermions—where the Weyl state is created by the material's own magnetic order—was a major goal1 .
Broken Symmetry Requirement
For Weyl nodes to appear, a material must break either inversion symmetry or time-reversal symmetry2 . Magnetic Weyl fermions emerge when time-reversal symmetry is broken by magnetic order.
Why Fermi Arcs Are a Big Deal
A quintessential signature of a Weyl semimetal is the presence of Fermi arcs2 .
In an ordinary metal, the Fermi surface—a map of all electron states at a specific energy—is composed of closed loops. In a Weyl semimetal, the surface Fermi surface appears as open arcs. Each arc is like a bridge on the surface of the material, connecting the projected locations of two Weyl nodes of opposite chirality in the bulk.
Visualization of Fermi arcs connecting Weyl nodes in momentum space.
Seeing these arcs is like finding the footprint of the hidden Weyl fermions within.
A Breakthrough in a Correlated Metal: The Case of Mn₃Sn
A pivotal moment in this field came in 2017 with the study of a material called Mn₃Sn1 .
Mn₃Sn Properties
- Complex non-collinear antiferromagnetic order
- Strongly interacting "correlated" electrons
- Rich quantum behavior
- Time-reversal symmetry breaking
Research Significance
- First clear evidence of magnetic Weyl fermions
- Demonstrated chiral anomaly in transport
- Opened new avenues for magnetic topology
- Provided a tunable platform for study
Experimental Evidence Timeline
Theoretical Prediction
DFT calculations predicted Weyl nodes in Mn₃Sn's electronic structure1 .
ARPES Confirmation
Angle-resolved photoemission spectroscopy revealed band crossings consistent with Weyl nodes1 .
Transport Signature
Magnetotransport measurements uncovered the chiral anomaly, a key Weyl fermion signature1 .
Correlation Effects
Studies revealed significant electron correlation effects, distinguishing it from simpler Weyl systems1 .
The Experimental Toolkit
The following table outlines the key methods used to unmask the magnetic Weyl fermions in Mn₃Sn1 :
| Experimental Technique | Primary Function | What It Revealed in Mn₃Sn |
|---|---|---|
| Angle-Resolved Photoemission Spectroscopy (ARPES) | Directly maps the electronic band structure and energy-momentum relationship of electrons in a material. | Showed band crossings consistent with Weyl nodes; revealed significant electron correlation effects. |
| Magnetotransport Measurements | Measure the electrical resistance of a material under an applied magnetic field. | Uncovered the "chiral anomaly," a unique signature of Weyl fermions. |
| Density Functional Theory (DFT) | A computational method for modeling the electronic structure of materials. | Provided the theoretical prediction of where Weyl nodes should be located in the Brillouin zone. |
ARPES Data
Transport Response
DFT Prediction
The Smoking Gun: Chiral Anomaly
A tell-tale sign of Weyl fermions is a phenomenon known as the chiral anomaly.
In the world of Weyl physics, this manifests as an unusual electrical behavior: when electric and magnetic fields are applied parallel to each other, the current unexpectedly increases with the magnetic field, leading to a positive magnetoconductance1 . This is the opposite of what happens in most ordinary metals.
Chiral Anomaly Demonstration
In normal metals, resistance increases with magnetic field (negative magnetoconductance).
The observation of this effect in Mn₃Sn provided strong, transport-based evidence that the current was being carried by chiral Weyl fermions1 .
Beyond the First Discovery: A Growing Family
The discovery in Mn₃Sn opened the floodgates. Scientists began exploring other magnetic materials, leading to the identification of new and exciting Weyl systems.
The RAlX Family
The RAlX family (where R is a rare-earth element like Nd or Pr, and X is Si or Ge) has become a particularly fertile playground2 6 .
NdAlSi
A 2023 study provided a stunning visual demonstration of how magnetism can regulate Weyl fermions. Using ARPES, scientists compared the material in its paramagnetic state to its ferrimagnetic state2 8 .
- Observed Fermi arcs and bulk Weyl cones
- Witnessed emergence of new Weyl fermions in magnetic state
- Confirmed theoretical predictions
PrAlSi
A 2025 study revealed a fascinating dichotomy: while the material is a ferromagnetic Weyl semimetal, its famous anomalous Hall effect appears to be disentangled from the Weyl nodes themselves6 .
- Ferromagnetic Weyl semimetal
- Complex relationship between topology and transport
- Shows complexity of magnetic Weyl systems
Material Comparison
| Material | Magnetic Order | Weyl Node Source | Key Finding |
|---|---|---|---|
| Mn₃Sn | Non-collinear antiferromagnet | Time-reversal symmetry breaking | First clear evidence of magnetic Weyl fermions |
| NdAlSi | Ferrimagnet | Both inversion and time-reversal symmetry breaking | Direct observation of magnetic-state-dependent Weyl nodes |
| PrAlSi | Ferromagnet | Both inversion and time-reversal symmetry breaking | Disentangled anomalous Hall effect from Weyl nodes |
The Scientist's Toolkit: Probing the Quantum World
The study of magnetic Weyl semimetals relies on a sophisticated arsenal of tools1 2 5 .
| Tool / Material | Category | Primary Function |
|---|---|---|
| ARPES | Experimental Technique | Directly visualizes electronic band structure, Fermi surfaces, and Fermi arcs. |
| Magnetotransport | Experimental Technique | Measures electrical response to magnetic fields to probe anomalies like the chiral anomaly. |
| DFT Calculations | Theoretical Modeling | Predicts electronic structures, band gaps, and the locations of topological nodes. |
| High-Magnetic Fields | Experimental Condition | Probes quantum limits and reveals Landau level formation. |
| RAlX Family | Material Platform | Provides a tunable system to study the interplay of non-centrosymmetry and magnetism. |
| Mn₃Sn | Material Platform | A prototypical correlated magnet for studying time-reversal symmetry broken Weyl states. |
Conclusion: A New Era of Topological Matter
The experimental discovery of magnetic Weyl fermions marks a milestone in modern physics. It demonstrates our ability to engineer and control exotic quantum states within carefully designed materials.
The implications are profound: the unique properties of these fermions, like their high sensitivity to magnetic fields and the chiral anomaly, make them prime candidates for next-generation technologies in spintronics, low-power electronics, and quantum computation1 .
From a fundamental perspective, these materials provide a unique tabletop laboratory for testing concepts from high-energy physics that would be impossible to study in particle accelerators. The journey of the Weyl fermion, from a mathematical curiosity to a tangible entity in a crystal, underscores a beautiful truth: sometimes, the most profound secrets of the universe are hidden in plain sight, waiting to be discovered in the quantum world of solids.
References
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