Quantum geometry induced anomalous chiral transport and hidden symmetry breaking in centrosymmetric 2M-WS2

This study reports the discovery of significant electron magnetochiral anisotropy in centrosymmetric 2M-WS2, revealing a direct link between nonlinear chiral transport, anomalous Nernst response, and the Fermi liquid-to-strange metal transition driven by nontrivial quantum geometry and orbital magnetic moments.

The Gist

Imagine a world made of tiny, flat sheets of material called 2M-WS2. Scientists have long known that these sheets are special because they are "centrosymmetric." In plain English, this means they are perfectly balanced, like a snowflake or a human face: if you flip them over, they look exactly the same. Because of this perfect balance, they usually follow strict rules where electricity flows the same way no matter which direction you push it.

However, this paper reports a surprising discovery: these perfectly balanced sheets are actually breaking their own rules.

Here is the story of what the scientists found, explained through simple analogies:

1. The "One-Way Street" in a Symmetrical City

Usually, if you drive a car down a perfectly symmetrical street, you can go forward or backward with the same ease. But in these 2M-WS2 sheets, the scientists found that electricity behaves like a car on a one-way street.

When they applied a magnetic field (like a giant invisible magnet) and pushed an electric current through the material, the resistance changed depending on the direction of the current. It was easier to push the current one way than the other. This phenomenon is called electronic magnetochiral anisotropy (eMChA).

• The Surprise: This "one-way" behavior usually only happens in materials that are already lopsided (non-centrosymmetric). Finding it in a perfectly symmetrical material like 2M-WS2 is like finding a one-way street in a city built with perfect symmetry. It suggests there is a hidden secret inside the material—a "hidden symmetry breaking" that we couldn't see before.

2. The "Temperature Sweet Spot" (The 25 K Club)

The scientists didn't just find this effect; they found when it happens. They cooled the material down and watched what happened at different temperatures.

They discovered a very specific "sweet spot" around 25 Kelvin (which is about -248°C, or just a few degrees above absolute zero).

• Above 25 K: The electrons behave like a chaotic, strange crowd (what scientists call a "strange metal").
• Below 25 K: The electrons calm down and start behaving like a well-organized marching band (what scientists call a "Fermi liquid").

The Magic Connection:
At exactly this transition point (25 K), three different things happened at the same time:

1. The "one-way street" effect (eMChA) became very strong.
2. A different electrical effect called the Nernst effect (which is like a thermal wind pushing electricity) also spiked to a huge value.
3. The material switched from the "strange" state to the "organized" state.

It's as if the material has a magic switch at 25 K where all these strange behaviors turn on simultaneously, suggesting they are all caused by the same underlying engine.

3. The "Sliding Layers" Theory

So, how does a perfectly symmetrical sheet become lopsided? The scientists used powerful computer simulations (first-principles calculations) to figure it out.

They proposed a mechanism they call "thick-layer sliding."
Imagine a deck of cards. Even if the deck looks perfectly symmetrical from the outside, if you slide the bottom half of the deck slightly to the left, the internal structure changes. The paper suggests that in 2M-WS2, layers of atoms can slide past each other with very little energy cost. This tiny slide doesn't destroy the material, but it creates a hidden twist in the quantum geometry (the shape of the electron's path) that breaks the symmetry just enough to create these weird electrical effects.

4. Why Does This Matter?

The paper suggests that this material is a rare playground for scientists.

• The Mystery of "Strange Metals": There is a big, unsolved puzzle in physics about why "strange metals" (materials that conduct electricity in weird ways) behave the way they do. This material shows a clear link between this "strange" behavior and the hidden symmetry breaking.
• The Quantum Geometry: The study points to "nontrivial quantum geometry" as the culprit. Think of this as the electrons moving on a curved, twisted surface rather than a flat road. This curvature creates the "one-way" traffic and the giant Nernst effect.

Summary

In short, the scientists found that 2M-WS2, a material that looks perfectly symmetrical, actually has a hidden internal twist caused by sliding atomic layers. This twist creates a "one-way street" for electricity and a massive thermal-electric effect, but only when the material is cooled to a specific "magic temperature" of 25 K. This discovery helps scientists understand the mysterious behavior of "strange metals," which is a key piece of the puzzle for understanding high-temperature superconductivity and other complex quantum phenomena.

Technical Summary

Technical Summary: Quantum Geometry Induced Anomalous Chiral Transport and Hidden Symmetry Breaking in Centrosymmetric 2M-WS2

Problem Statement
Chirality and the breaking of left-right symmetry are fundamental properties in nature that significantly influence material behavior. Nonlinear electronic responses, specifically electronic magnetochiral anisotropy (eMChA), serve as sensitive probes for symmetry breaking and nontrivial quantum geometries in solids. Historically, eMChA observations have been restricted to materials lacking inversion symmetry (non-centrosymmetric systems), such as polar semiconductors, chiral conductors, and systems with chiral spin textures. A significant gap exists in understanding whether such effects can emerge in centrosymmetric materials, which would imply the presence of hidden symmetry breaking or exotic quantum orders. Furthermore, the material 2M-WS2, a candidate topological superconductor, exhibits an anomalously large Nernst effect and a transition from a Fermi liquid (FL) to a strange metal (SM) state around 25 K, yet the underlying mechanism connecting these phenomena remains unclear.

Methodology
The study employs a combination of experimental transport measurements, symmetry analysis, and theoretical modeling:

• Sample Preparation: High-quality 2M-WS2 single crystals were synthesized via the topochemical deintercalation of K+ from K0.7WS2. Thin flakes (35–150 nm) were mechanically cleaved and fabricated into Hall bar devices on SiO2/Si substrates using electron beam lithography and evaporation.
• Transport Measurements: The researchers measured the nonlinear electrical response by biasing the devices with a low-frequency (17.777 Hz) alternating current (AC) along the c-axis. Using lock-in detection techniques, they recorded both first-harmonic ($V_{1\omega}$) and second-harmonic ($V_{2\omega}$) voltages under varying magnetic fields ($B$), temperatures ($T$), and current amplitudes ($I$).
• Symmetry Probing: Angular-dependent measurements were performed by rotating the magnetic field in the $xz$, $yz$, and $xy$ planes to determine the orientation of the characteristic polar vector.
• Theoretical Analysis: First-principles calculations based on Density Functional Theory (DFT) were utilized to investigate potential mechanisms for inversion symmetry breaking, including phase transitions and layer sliding. Theoretical models were also applied to link the observed transport phenomena to quantum geometry and orbital magnetic moments at the Fermi surface.

Key Contributions and Results

1. Observation of eMChA in a Centrosymmetric System: The study reports the first observation of a remarkable eMChA in the centrosymmetric candidate topological superconductor 2M-WS2. The measured second-harmonic voltage ($V_{2\omega}$) exhibits the characteristic dependencies of eMChA: it is antisymmetric with respect to the magnetic field ($B$) and the current direction ($I$), and it scales linearly with $B$ and quadratically with $I$. This confirms a hidden symmetry breaking state within the material.
2. Identification of Hidden Symmetry Breaking: Angular dependence measurements reveal that the second-harmonic signal follows the selection rule for polar systems ($R \propto (P \times B) \cdot I$), with the characteristic polar vector $P$ oriented along the $b$-axis. This suggests a spontaneous inversion symmetry breaking in the centrosymmetric 2M-WS2, similar to phenomena observed in polar metals or topological semimetals.
3. Correlation with FL-SM Transition and Nernst Effect: A critical finding is the simultaneous occurrence of three phenomena around the crossover temperature $T_{FL} \approx 25$ K:
   • A peak in the eMChA coefficient ($\gamma$), reaching values up to $0.5 \, \text{T}^{-1}\text{A}^{-1}$, which is significantly higher than in typical polar conductors.
   • An anomalously large Nernst response.
   • The transition from a Fermi liquid to a strange metal state.
     The non-monotonic "hump" behavior of the eMChA at $T_{FL}$ contrasts with the monotonic behavior seen in other topological materials, indicating a unique mechanism linked to the strange metal state.
4. Theoretical Mechanism: Theoretical analysis rules out standard phase transitions (e.g., to $Td$-WS2) or simple lattice distortions due to high energy barriers. Instead, the authors propose a "thick-layer-sliding mechanism" with minimal energy gain. This mechanism subtly perturbs local symmetry while preserving the global 2M framework, generating nontrivial quantum geometry. The calculations suggest that the orbital magnetic moment at the Fermi surface becomes significant during the FL-SM transition, driven by a temperature-induced reconstruction of the Fermi surface (a hidden order), which amplifies both the eMChA and Nernst effects.

Significance
The paper establishes 2M-WS2 as a rare quantum platform for studying the intertwined physics of chiral transport, strange metals, and hidden symmetry breaking. By demonstrating a direct correspondence between nonlinear response, Nernst response, and the FL-SM transition, the work provides new insights into the mechanism of strange metals. The authors suggest that these findings may shed light on unresolved scientific issues regarding strange metals in unconventional high-temperature superconductors, heavy fermion systems, and flat-band materials. The identification of a thick-layer-sliding mechanism as a source of nontrivial quantum geometry offers a new perspective on how subtle structural perturbations can induce profound electronic effects in centrosymmetric materials.