The Quantum Spin Hall Effect That Persists in Monolayer Ruthenium Trichloride

If you cool a crystal of ruthenium trichloride down to cryogenic temperatures, the electrons usually freeze into a magnetically ordered zigzag pattern. They behave like tiny, locked compass needles, dominated by heavy electron-electron interactions. In three dimensions, this Mott insulating state blocks transport. But peel the material down to a single layer of atoms, and the physics changes.

In 2025, a physics team led by Young-Woo Son measured the quantum spin Hall effect (QSHE) inside a single-layer crystal of ruthenium trichloride ($\alpha\text_3$). Crucially, they did so without applying an external magnetic field.

The finding confirms a long-debated theoretical prediction: topological edge states can survive and remain protected inside a two-dimensional material even when the bulk is completely dominated by strong, correlating electron-electron interactions.

The Trouble with Strong Interactions

In a standard topological insulator, such as mercury telluride, the bulk of the material acts as an insulator while the edges host highly conducting, spin-polarized channels. These edge states are protected by time-reversal symmetry. Electrons of opposite spins travel in opposite directions along the boundaries, completely immune to backscattering from non-magnetic impurities. This is the quantum spin Hall effect, first theorized by Charles Kane and Eugene Mele in 2005.

In most known topological insulators, the electrons behave essentially as independent particles. They navigate the crystal lattice feeling the average electrostatic background, but rarely exchange strong, direct repulsion with one another.

When you introduce strong electron-electron interactions, this neat topological framework usually collapses. In transition metal halides like ruthenium trichloride, the outer $d$-shell electrons of the ruthenium ions repel each other so intensely that they undergo Mott localization. Instead of moving freely, they lock themselves into specific atomic sites.

For years, theorists debated whether the delicate, spin-polarized edge states of the quantum spin Hall effect could ever exist alongside the fierce, local Coulomb repulsions of a Mott insulator. Conventional calculations suggested that these strong interactions would destabilize the topological boundary currents, either by forcing the edges to polarize magnetically or by scattering the edge carriers into the insulating bulk.

Peeling Back the Dimensions

Ruthenium trichloride crystallizes in a honeycomb lattice, mimicking the hexagonal geometry of graphene but with ruthenium atoms coordinated by octahedra of chlorine atoms. Because of its structural layout and heavy atomic weight, it features exceptionally strong spin-orbit coupling.

By exfoliating bulk $\alpha\text_3$ down to a single monolayer, the team led by Young-Woo Son isolated the two-dimensional physics from the three-dimensional stacking forces that typically stabilize the bulk zigzag antiferromagnetic order.

In a single atomic layer, the relativistic spin-orbit coupling and the local crystal field coordinate to open a topological bandgap. Because this gap arises natively from the material's internal relativistic spin-orbit mechanics, the topological protection does not require any external magnetic field to align the electron spins or break time-reversal symmetry.

To measure the edge states, the researchers integrated the monolayer $\alpha\text_3$ into a multi-terminal transport device. Using a specialized mechanical transfer process inside an inert argon glovebox, they placed the delicate monolayer onto a silicon dioxide substrate and patterned sub-micron gold electrodes along its outer boundary.

Guarded Boundaries

When the team ran low-frequency alternating currents through the device at sub-Kelvin temperatures, the bulk of the ruthenium trichloride remained highly resistive, behaving as a strict Mott insulator. Yet, the electrical conductance did not drop to zero.

Instead, the current flowed exclusively along the outer edge of the monolayer. The researchers verified this spatial distribution by measuring the longitudinal conductance across varying channel lengths. While the bulk resistance scaled exponentially with temperature, the boundary transport remained nearly constant, displaying the signature quantized conductance plateaus of the quantum spin Hall effect.

Because the edge states are spin-polarized, electrons with "spin-up" states travel clockwise around the flake, while "spin-down" electrons travel counterclockwise. Because there is no external magnetic field to break time-reversal symmetry, an impurity cannot easily reverse an electron's direction. To scatter backward, the electron would have to flip its spin orientation simultaneously—a process forbidden by the time-reversal symmetry of the crystal's quantum state.

The experimental measurement demonstrates that topological edge currents can peacefully coexist with localized Mott physics. Rather than destroying the topological boundary states, the strong electron-electron interactions in the single-layer $\alpha\text_3$ simply coexist alongside them, leaving the spin-polarized boundary paths intact.

This result changes how physicists search for stable topological phases. Instead of avoiding strongly correlated materials, researchers can now look to transition metal halides and other Mott insulators to find highly robust topological edge states that persist in the absence of bulky, power-hungry external magnets.

Measured.

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