Electron-doped magnetic Weyl semimetal LixCo3Sn2S2 by bulk-gating

This study demonstrates that ionic gating via Li-intercalation in a focused ion beam-fabricated bulk Co3Sn2S2 microdevice enables substantial electron doping and a 200 meV Fermi energy shift while preserving the kagome lattice and magnetic order, thereby extending gate-tuning capabilities to bulk quantum materials.

The Gist

Imagine you have a very special, high-tech crystal that acts like a super-fast highway for electrons. This crystal, called Co₃Sn₂S₂, is a "Weyl semimetal," which is a fancy way of saying it has some magical properties that make it great for future electronics and quantum computers. It's magnetic, and it conducts electricity in a very unique way.

However, there's a catch. Usually, to change how these materials behave (like turning a light switch on or off), scientists have to slice the material into paper-thin flakes. But this crystal is stubborn; it's a "bulk" material, meaning it's hard to slice into thin layers without breaking it. It's like trying to slice a block of granite into a sheet of paper without shattering it.

The Problem:
Scientists want to control the flow of electricity inside this crystal by adding or removing electrons (like adding more cars to the highway). Traditionally, they could only do this on the very surface of a thin slice, or they had to chemically mix other metals into the crystal (like adding salt to soup), which changes the recipe and ruins the original flavor.

The Solution: The "Bulk-Gating" Trick
The researchers in this paper came up with a clever new method called "Bulk-Gating." Here is how they did it, using some creative analogies:

1. The Micro-Scalpel (FIB): Instead of slicing the whole crystal, they used a super-precise laser-like tool called a Focused Ion Beam (FIB) to carve out a tiny, microscopic piece of the crystal (about the width of a human hair). Think of this as using a microscopic scalpel to carve a tiny, perfect Lego brick out of a giant rock.
2. The Electrolyte Bathtub: They placed this tiny brick into a special "bathtub" filled with a liquid electrolyte (a salty liquid that conducts electricity).
3. The Invisible Push (Gating): They applied a voltage (an electrical push) to this liquid. This didn't just push electrons on the surface; it acted like a magnet, pulling Lithium ions (tiny charged atoms) deep inside the crystal structure.
4. The Result: These Lithium ions acted like extra passengers hopping onto the electron highway. They successfully packed over 500 times more electrons into the crystal than before, shifting the energy levels deep inside the material.

The Big Discovery: The "Ghost" Doping
Here is the most surprising part of the story.

Usually, when you add extra passengers (electrons) to a magnetic material, it messes up the magnetic alignment, like adding too many people to a dance floor and making everyone stop dancing. The "Curie Temperature" (the point where the material stops being magnetic) usually drops.

But in this experiment, something magical happened:

• The researchers added a massive amount of electrons.
• The magnetism didn't change at all. The material stayed magnetic at the exact same temperature.

Why? The "Guest Room" Analogy
Think of the crystal structure like a hotel with two types of floors:

• The Dance Floor (Kagome Layer): This is where the magnetic "dancers" (Cobalt atoms) live and spin. They need to stay perfectly organized to keep the magnetism.
• The Hallway (Anion Layer): This is the space between the dance floors.

When the researchers forced the Lithium ions in, they didn't crash the dance floor. Instead, the Lithium ions quietly slipped into the Hallways (the anion layers) and stayed there. They added their "charge" to the system without bumping into the dancers.

Because the Lithium ions stayed in the hallways, the magnetic dancers on the dance floor didn't notice them. The "dance" (magnetism) continued perfectly, even though the "crowd" (electrons) had grown huge.

Why This Matters
This discovery is a game-changer for two reasons:

1. New Materials: It proves we can now control the electricity in "uncuttable" bulk crystals, not just thin films. This opens the door to studying hundreds of new materials that were previously too hard to work with.
2. Understanding Magnetism: It tells us that in this specific crystal, the magnetism is very robust and doesn't rely on the number of electrons. This helps scientists understand the fundamental rules of how magnetism works in these exotic quantum materials.

In a Nutshell:
The team built a tiny, precise device to inject a massive amount of electricity into a stubborn crystal without breaking its magnetic heart. They did this by sneaking the extra charge into the "hallways" of the crystal, leaving the "dance floor" of magnetism completely untouched. This is a major step toward building better, faster, and more powerful quantum computers.

Technical Summary

1. Problem Statement

Gate-controlled carrier modulation is a fundamental technique for tuning material properties (e.g., superconductivity, magnetism, topology). However, current gating methods face significant limitations:

• Electrostatic gating: Limited to a screening length of 1–10 nm, affecting only the surface.
• Intercalation gating: Can penetrate deeper but is typically restricted to ultra-thin films or van der Waals (vdW) materials due to diffusion limitations.
• Bulk limitation: Most quantum materials of interest are "uncleavable" bulk crystals that cannot be exfoliated into thin flakes, making them inaccessible to standard gating techniques.
• Chemical doping limitations: Traditional chemical substitution (e.g., replacing Co with Ni or Sn with Sb) introduces impurities that disrupt the crystal lattice and magnetic interactions, often altering the Curie temperature ($T_C$) and obscuring the intrinsic relationship between carrier density and magnetic properties.

The authors aim to overcome these barriers by developing a method to perform bulk-gating on micro-devices fabricated from single-crystal bulk materials, enabling deep carrier modulation without destroying the host lattice.

2. Methodology

The study employs a novel combination of Focused Ion Beam (FIB) fabrication and ionic liquid gating to create a micro-device from a bulk single crystal of the magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$.

• Device Fabrication (FIB):
  • A micrometer-scale lamella (1 $\mu$m thick) was carved from a bulk Co$_3$Sn$_2$S$_2$ single crystal using FIB.
  • The lamella was transferred to a SiO$_2$ substrate with Au/Ti electrodes.
  • Tungsten (W) was deposited for electrical connections.
  • Critical Step: The edges of the device were etched to remove unintended W layers that would block ion intercalation, ensuring Li-ions could access the sample side surfaces.
• Ionic Gating Setup:
  • A ring-shaped silicone gel was applied to minimize electrolyte volume and prevent freezing damage.
  • An ionic liquid electrolyte (LiClO$_4$ dissolved in Polyethylene Glycol, PEG) was used.
  • A Pt plate served as the gate electrode.
• Gating Process:
  • A positive gate voltage ($V_G$) up to 4.2 V was applied at 330 K under high vacuum.
  • This drove Li$^+$ ions to intercalate into the bulk crystal, acting as electron donors.
• Characterization:
  • Transport measurements (longitudinal resistivity $\rho_{xx}$, Hall resistivity $\rho_{yx}$) were performed at various temperatures and magnetic fields.
  • Density Functional Theory (DFT) calculations were conducted to model the band structure, carrier density shifts, and Li$^+$ intercalation sites.

3. Key Contributions

1. Demonstration of Bulk-Gating: Successfully demonstrated carrier modulation in a micro-device fabricated from a bulk single crystal, bypassing the need for exfoliation.
2. Massive Electron Doping: Achieved an electron carrier density ($n_e$) exceeding $6.2 \times 10^{21}$ cm$^{-3}$, representing a shift in Fermi energy ($E_F$) of approximately 200 meV.
3. Rigid Band Behavior: Established that the electronic structure shifts according to a rigid band model, with experimental data showing striking agreement with DFT calculations.
4. Decoupling of Magnetism and Carrier Density: Discovered that the Curie temperature ($T_C$) remains nearly constant (~168–170 K) despite a two-order-of-magnitude change in carrier density.

4. Key Results

• Carrier Density Modulation:

  • Applying $V_G = 4.2$ V induced a sign change in the Hall coefficient, confirming a transition from a compensated semimetal to a heavily electron-doped state.
  • The electron density increased from $\sim 7.5 \times 10^{19}$ cm$^{-3}$ (pristine) to $\sim 6.2 \times 10^{21}$ cm$^{-3}$.
  • This corresponds to a chemical composition of Li$_{0.70}$Co$_3$Sn$_2$S$_2$.
  • The process was semi-reversible: reducing $V_G$ to 0 V partially de-intercalated Li, returning $n_e$ to $\sim 1.5 \times 10^{21}$ cm$^{-3}$ (Li$_{0.17}$Co$_3$Sn$_2$S$_2$).

• Anomalous Hall Effect (AHE):

  • The anomalous Hall conductivity ($\sigma_{AH}$) was modulated from 960 $\Omega^{-1}$cm$^{-1}$ (pristine) to 290 $\Omega^{-1}$cm$^{-1}$ (doped).
  • Despite the reduction in $\sigma_{AH}$, the anomalous Hall angle ($\theta_{AH}$) remained high ($\sim 0.11$), indicating the preservation of the Berry curvature associated with Weyl nodes.
  • The experimental $\sigma_{AH}$ values matched DFT predictions based on a rigid band shift, confirming the integrity of the kagome lattice.

• Magnetic Stability ($T_C$):

  • Unlike chemical substitution (Ni or Sb doping) which lowers $T_C$, the Li-intercalated phase showed no significant change in $T_C$ (170 K $\to$ 168 K) despite massive carrier injection.
  • This suggests that Li$^+$ ions preferentially occupy the anion layers (forming Li-S bonds) rather than substituting magnetic Co atoms in the kagome layers.

• Mechanism of Ferromagnetism:

  • The independence of $T_C$ from carrier density implies that the ferromagnetism in Co$_3$Sn$_2$S$_2$ is not carrier-mediated (ruling out simple Stoner or RKKY mechanisms).
  • Instead, the magnetism likely arises from direct exchange between localized magnetic moments or mechanisms typical of narrow-gap magnetic semimetals (e.g., Bloembergen–Rowland or Van Vleck mechanisms).

5. Significance

• New Paradigm for Quantum Materials: This work establishes a pathway to gate-tune "uncleavable" bulk quantum materials, expanding the scope of materials available for electrochemical control beyond 2D vdW crystals.
• Insight into Topological Magnetism: By cleanly doping the system without disrupting the magnetic lattice, the study provides definitive evidence that the ferromagnetism in Co$_3$Sn$_2$S$_2$ is robust against carrier density changes, offering crucial insights into the origin of magnetism in topological kagome magnets.
• Material Design: The finding that Li$^+$ intercalation stabilizes within the anion layer without damaging the magnetic kagome lattice suggests a design principle for future ion-gating studies on other layered magnetic semimetals and superconductors.
• Technological Advancement: The combination of FIB micro-fabrication and ionic gating offers a versatile platform for exploring phase transitions and quantum phenomena in bulk materials that were previously inaccessible to gate-control techniques.