🔬 mesoscale physics

Surface-switchable nonreciprocity protected by Fermi arcs in Weyl semimetal TaAs

This study demonstrates the chiral nature of Fermi arcs in the Weyl semimetal TaAs by revealing a surface-switchable, room-temperature nonlinear transport response that serves as a direct probe of these topologically protected surface states.

Original authors: Dong Li, Yuki M. Itahashi, Ying-Ming Xie, Menghu Zhou, Yukako Fujishiro, Kunjie Zheng, Yao Guang, Max T. Birch, Ilya Belopolski, Max Hirschberger, Takahiro Morimoto, Xiao-Xiao Zhang, Zhi-An Ren, Naoto

Published 2026-06-17

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CC BY 4.0

Original authors: Dong Li, Yuki M. Itahashi, Ying-Ming Xie, Menghu Zhou, Yukako Fujishiro, Kunjie Zheng, Yao Guang, Max T. Birch, Ilya Belopolski, Max Hirschberger, Takahiro Morimoto, Xiao-Xiao Zhang, Zhi-An Ren, Naoto Nagaosa, Yoshihiro Iwasa

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

Imagine a crystal of Tantalum Arsenide (TaAs) not as a solid block, but as a bustling city. Inside this city, there are two types of traffic: the massive, chaotic flow of cars on the main highways (the bulk of the material) and a tiny, secret, one-way pedestrian path that exists only on the very edge of the city walls (the surface).

In the world of physics, this material is called a Weyl semimetal. The "secret path" on the edge is known as a Fermi arc. For over a decade, scientists knew these paths existed, but they couldn't prove a specific, weird property they were supposed to have: chirality. In simple terms, chirality means "handedness." Just as your left hand is a mirror image of your right but cannot be superimposed on it, these electron paths were predicted to flow in a specific, one-way direction depending on which side of the crystal you look at.

Here is the problem: The "highways" inside the crystal are so loud and crowded that they drown out the tiny "pedestrian path" on the surface. If you try to listen to the city with a standard microphone (linear transport), you only hear the traffic noise. You can't hear the pedestrians.

The Breakthrough: Listening to the "Echo"

The researchers in this paper found a clever way to isolate the sound of the pedestrians. They used a technique called Focused Ion Beam (FIB) to carve out tiny, microscopic devices from the crystal. Think of this as using a super-precise laser scalpel to cut a tiny slice of the city, exposing both the top wall and the bottom wall simultaneously.

Instead of just pushing electrons through (like a standard current), they pushed them back and forth very fast and measured the echo (nonlinear transport).

The Linear Signal (The Highway Noise): When they measured the basic resistance, the top and bottom surfaces sounded exactly the same. This was because the "highway" traffic inside the crystal was dominating the measurement.
The Nonlinear Signal (The Pedestrian Echo): When they looked at the "echo" (specifically a second-harmonic signal), something magical happened. The signal from the top surface was the exact opposite of the signal from the bottom surface.

The Analogy: Imagine two people walking in a circle. If they are walking normally, they look the same. But if you ask them to walk in a specific "wavy" pattern that depends on which way they are facing, one person might wave their left hand while the other waves their right. The researchers saw this "mirror-image" behavior. This proved that the electrons on the surface were indeed flowing in a specific, chiral direction, just as the theory predicted.

The "Ghost" at the End of the Path

The paper also discovered something even stranger. The signal didn't just behave simply; it had a complex, "wiggly" pattern that changed depending on the angle of the magnetic field.

The researchers realized this was caused by the endpoints of the Fermi arcs. Imagine the Fermi arc as a bridge connecting two islands. The middle of the bridge is smooth, but the very ends where the bridge meets the land are chaotic and "singular." The paper suggests that the electrons get "stuck" or behave wildly at these endpoints, creating a massive, unexpected signal that standard physics equations couldn't fully explain. It's like trying to predict the weather in a tornado; the rules of normal wind don't quite work at the center.

Why This Matters (According to the Paper)

Proof of Concept: This is the first time scientists have unambiguously proven that these surface paths have a "handedness" (chirality) in a Weyl semimetal.
Room Temperature: Usually, these quantum effects are so fragile that they disappear if the material gets even slightly warm. However, because these surface paths are "topologically protected" (like a knot that can't be untied), the effect survived even at room temperature.
A New Tool: The paper establishes that looking for these "echoes" (nonlinear transport) is a powerful new way to study these materials, even when the "noise" of the bulk material is overwhelming.

In summary: The team built a microscopic device to separate the "highway" traffic from the "surface path." By listening to the specific "echo" of the electrons, they proved the surface path flows in a one-way, chiral direction, and they discovered that the ends of this path create a giant, mysterious signal that defies simple explanation. This confirms a decade-old theory and opens a new way to study these exotic materials.

Technical Summary: Surface-switchable nonreciprocity protected by Fermi arcs in Weyl semimetal TaAs

Problem Statement
Weyl semimetals host topologically protected surface states known as Fermi arcs, which connect bulk Weyl nodes in momentum space. While the chirality of bulk Weyl bands has been confirmed through phenomena like the chiral anomaly and Weyl orbits, the chiral (unidirectional) nature of the Fermi arcs themselves has remained experimentally unresolved. This is primarily due to the overwhelming contribution of metallic bulk states to charge transport, which masks the subtle signals from surface states. Furthermore, conventional nonlinear transport responses (such as electronic magnetochiral anisotropy) are inherently weak in bulk samples, making the detection of surface-specific chirality difficult.

Methodology
To address these challenges, the authors utilized Tantalum Arsenide (TaAs), a prototypical Weyl semimetal with broken inversion symmetry. They employed focused ion beam (FIB) techniques to fabricate micro-scale devices (approx. 100 × 50 × 4 µm³) that allow simultaneous transport measurements on opposing topological surfaces (top and bottom) via the device sidewalls. This fabrication method avoids the use of FIB-deposited asymmetric platinum electrodes, which can introduce extrinsic nonreciprocal signals.

The study involved:

Linear Transport: Shubnikov–de Haas (SdH) quantum oscillation measurements to verify the presence of bulk Weyl fermions and characterize the bulk carrier properties.
Nonlinear Transport: AC transport measurements using lock-in amplifiers to detect the second-harmonic resistance ( $R_{2\omega}$ ), which serves as a probe for nonreciprocal transport. Measurements were conducted across a range of temperatures (2 K to 300 K), magnetic fields (up to 9 T), and current densities.
Comparative Analysis: Measurements were performed on topological surfaces (top/bottom) versus topologically trivial surfaces (left/right) to isolate surface contributions.
Thickness Dependence: A series of devices with varying thicknesses (from 10 µm down to 0.45 µm) were fabricated to distinguish between bulk-dominated and surface-dominated transport channels.

Key Results

Linear vs. Nonlinear Response: In the linear regime (first-harmonic resistance, $R_{1\omega}$ ), transport is dominated by bulk states, showing nearly identical behavior on top and bottom surfaces. In contrast, the nonlinear response ( $R_{2\omega}$ ) reveals distinct surface-specific contributions.
Surface-Switchable Sign Reversal: The most critical observation is that the $R_{2\omega}$ signal exhibits opposite signs on the top and bottom surfaces. This sign reversal is consistent with the theoretical prediction that Fermi arcs on opposing surfaces possess reversed Fermi velocities. This behavior rules out bulk polarization origins and confirms the surface nature of the nonlinear signal.
Robustness: The nonlinear signal persists up to room temperature and remains robust even in devices with significant FIB-induced surface damage (amorphous layers), supporting the topological protection of the Fermi arcs.
Higher-Order Nonreciprocity: The angular dependence of $R_{2\omega}$ reveals anomalous higher-order components. While low-field data fits a simple $\sin(\theta)$ dependence, high-field data (above ~1 T) clearly resolves a $\sin(3\theta)$ component, and in some devices, even higher-order terms ( $\sin(5\theta)$ , $\sin(7\theta)$ ). These higher-order terms are reproducible across devices and correlate with the breaking of mirror symmetry by the magnetic field.
Theoretical Discrepancy: A theoretical model based on conventional Boltzmann transport (considering Zeeman shifts and Fermi arc elongation) qualitatively captures the existence of higher-order terms but fails to reproduce the experimental magnitudes quantitatively. The model requires unphysically large g-factors (>1000) to match the low-field threshold for the third-order term. The authors attribute this discrepancy to the singular nature of Fermi arc endpoints, where quantum geometric quantities diverge, a factor not included in the semiclassical treatment.

Significance and Claims
The paper claims to provide the first unambiguous experimental demonstration of the chiral nature of Fermi arcs in Weyl semimetals through nonlinear transport. By establishing that nonlinear transport can isolate surface contributions even when bulk conduction dominates linear transport, the work introduces a new, direct probe for topological surface states.

The authors highlight that the observed surface-switchable, room-temperature nonlinear response is topologically protected, offering a new functionality for Weyl semimetals. They suggest that these findings open opportunities for exploring nonlinear transport phenomena and device concepts across a broad class of topological semimetals, given the abundance of natural materials predicted to host such states. The work positions Weyl semimetals as promising platforms for surface-switchable topological rectification and room-temperature nonlinear electronics, distinct from topological insulators which often require carrier doping to generate resistive responses.

The Breakthrough: Listening to the "Echo"

The "Ghost" at the End of the Path

Why This Matters (According to the Paper)

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