The solitons at the heart of the study are torons and skyrmion tubes, particle-like configurations of the director field—the local orientation of rod-like molecules—in chiral nematic liquid crystals. In these materials, the director twists with a characteristic pitch set by the molecular chirality, and defects or twisted regions can lock together into stable, localized structures that behave in many ways like particles. Torons, in particular, are triple-twisted, particle-like excitations confined between two bounding glass plates of a liquid-crystal cell. Because their topology is conserved, they cannot simply dissolve into the uniform background; instead, they persist as mobile objects that can be created, moved, and manipulated, for example with laser tweezers.

The experimental team, led by Smalyukh’s group at the University of Colorado Boulder, created individual torons in thin cholesteric cells and then imposed a controlled shear flow by translating one glass plate relative to the other. Using polarization optical microscopy, the researchers tracked the soliton trajectories as the shear rate varied. At low driving speeds, the torons moved essentially along the shear direction, but as the flow intensified, they acquired an increasingly pronounced transverse velocity component, deflecting away from the shear plane much as charged particles deflect in a magnetic field or magnetic skyrmions deflect under current. The team quantified this deflection with a Hall angle, which grew with the Ericksen number, a dimensionless parameter comparing viscous and elastic stresses in the liquid crystal.

Crucially, the experiments revealed that the solitons are not passive tracers of the surrounding fluid. The researchers simultaneously tracked small tracer particles—melamine resin spheres and alumina nanoplates—suspended in the liquid crystal. These tracers, which sample the local flow field, showed no detectable transverse motion across a wide range of shear rates. The torons, by contrast, drifted sideways far more strongly than any underlying transverse mass flow could explain. This discrepancy between the motion of the soliton and the motion of the fluid around it is the central puzzle that the theoretical and numerical parts of the study set out to resolve.

On the theory side, the Lisbon-based team performed a systematic perturbation analysis of the Ericksen–Leslie equations, the continuum description of nematic liquid-crystal hydrodynamics. Treating the shear as a weak perturbation of the static toron configuration, they computed the corrections to the director and velocity fields order by order. Their analysis predicts a weak net transverse mass flow that emerges only at third order in the perturbation parameter, meaning its magnitude scales as the cube of the shear rate. This cubic scaling reflects the nonlinear coupling between director distortion and flow that is characteristic of liquid-crystal rheology, where backflow effects link molecular orientation and fluid motion.

Numerical simulations based on a lattice Boltzmann implementation of the coupled director and flow dynamics confirmed and extended the analytical picture. The simulations reproduced the transverse drift of torons under steady shear, captured the deformation of the soliton as it elongates into finger-like shapes at higher shear rates, and even showed the instability and eventual destruction of torons at very large Ericksen numbers. Importantly, the simulations revealed that in certain parameter regimes the soliton’s transverse motion occurs opposite to the direction of the net transverse mass current predicted by the perturbation theory—a striking result that rules out any simple explanation in which the soliton is merely carried along by a secondary flow.

To rationalize these observations, the authors adapted the Thiele equation, a classical framework developed in the 1970s to describe the steady-state motion of magnetic domains. In its original form, the Thiele equation balances the driving force on a topological soliton against dissipative drag and a gyroscopic term that produces the transverse deflection responsible for the magnetic skyrmion Hall effect. The liquid-crystal version developed in this work replaces the magnetic gyrovector with a term tied to the flow vorticity—the local rotational component of the shear flow surrounding the soliton. This formulation provides a compact physical explanation: it is the vorticity of the backflow generated around the distorted director texture, rather than any net transverse current, that pushes the soliton sideways.

The generality of the effect was tested in several ways. Simulations of skyrmion tubes—solitons that extend through the cell thickness—showed Hall angles comparable to those of torons under identical conditions, and even more complex objects such as skyrmion bags containing multiple antiskyrmions exhibited the same qualitative behavior, with Hall angles of roughly nine to twelve degrees depending on their internal structure. The researchers also examined how boundary conditions at the glass plates influence the effect: tilting the surface anchoring of the director changes both the magnitude and, in some regimes, the sign of the Hall angle. Experiments under a sinusoidal electric field, which stabilizes the solitons against flow-induced distortion, showed torons with Hall angles of about ten degrees, and occasionally small negative angles corresponding to deflection in the opposite transverse direction.

Beyond its fundamental interest, the discovery suggests practical possibilities. Because the transverse drift is controllable—through shear rate, cell geometry, surface anchoring, and applied electric fields—topological solitons in liquid crystals could serve as functional building blocks in microfluidic technologies, acting as self-propelled carriers or as reconfigurable elements whose trajectories are steered by flow rather than by external fields alone. The effect also deepens the analogy between soft matter and magnetic systems, showing that Hall-like dynamics of topological excitations is not tied to electromagnetism or spin, but follows generally from the interplay of topology, dissipation, and vorticity in driven media.

The study, which received support from the Portuguese Foundation for Science and Technology, FAPERJ, CNPq, and the US Department of Energy, establishes a new arena in which the concepts of topological transport can be explored in a soft, transparent, and easily manipulated medium. As the authors show, a simple equation inspired by magnetism, combined with detailed experiments and large-scale simulations, is enough to capture the essence of the phenomenon: when you shear a chiral liquid crystal, its topological solitons refuse to follow the flow—they sidestep it.

Subject of Research: Hall-like transverse drift of topological solitons in sheared chiral liquid crystals

Article Title: A liquid-crystal analogue of the skyrmion Hall effect

Article References: Coelho, R. C. V., Zhao, H., Amaral, G. N. C., Smalyukh, I. I., Telo da Gama, M. M., & Tasinkevych, M. (2026). A liquid-crystal analogue of the skyrmion Hall effect. Nature Physics. https://doi.org/10.1038/s41567-026-03379-0

Image Credits: AI Generated

DOI: 10.1038/s41567-026-03379-0

Keywords: liquid crystals, skyrmions, topological solitons, torons, shear flow, Hall effect, Ericksen-Leslie theory, Thiele equation, microfluidics, soft matter, lattice Boltzmann simulations, cholesteric liquid crystals

The Hall effect, discovered by Edwin Hall in 1879, is one of the foundational measurement tools of condensed matter physics. When a current flows through a conductor in the presence of a perpendicular magnetic field, the Lorentz force pushes charge carriers to one side, producing a voltage across the material at right angles to the current. That transverse voltage encodes the density and sign of the mobile charges and, in its quantized variants, has underpinned decades of research into exotic quantum states. Extending the Hall effect to composite quasiparticles such as trions is therefore more than a technical curiosity: it provides a direct probe of the transport behavior of particles that carry both charge and internal structure.

Trions occupy a fascinating middle ground between ordinary excitons and free electrons. An exciton is electrically neutral, a bound electron–hole pair that can move through a semiconductor much like an atom of light, while a trion—often written X− or X+ depending on whether the extra particle is an electron or a hole—carries net charge equal to that of a single electron. Because trions combine a finite charge with a substantial optical dipole moment, they can be created and detected optically yet still respond to electrical and magnetic forces. This dual character makes them attractive candidates for optoelectronic applications and for fundamental studies of many-body physics, but it also makes their transport properties notoriously difficult to isolate from those of the surrounding charge sea.

The experimental platform used in the new study was an electron–hole double layer, a structure in which two sheets of charge carriers of opposite polarity are separated by an insulating or semiconducting barrier. Electrons reside in one layer and holes in the other, each able to move freely within its own plane, while the Coulomb attraction between the layers binds them into excitons and, in the presence of excess carriers, into trions. Double layers of this kind, typically realized in atomically thin van der Waals heterostructures built from transition metal dichalcogenides or in carefully engineered quantum well systems, allow researchers to tune the balance between free carriers and bound complexes simply by adjusting gate voltages.

When the researchers applied a perpendicular magnetic field to this double layer system, they observed that the trion fluid—collectively, the gas of charged three-particle complexes drifting through the device—developed a transverse voltage characteristic of the Hall effect. Crucially, the measured Hall response could not be explained by the motion of free electrons or free holes alone. Its sign and magnitude tracked the optical signatures of trion formation, indicating that the composite particles themselves were participating in the transport. In effect, the experiment showed that an electrically charged but internally complex quasiparticle can behave, at the level of Hall physics, as a well-defined carrier with its own effective Hall coefficient.

Technically, the observation is a considerable feat of disentanglement. In an electron–hole double layer, the Hall voltage measured in either layer receives contributions from several populations: the free majority carriers, the minority carriers of opposite sign, the neutral excitons, and the charged trions. The theory developed alongside the experiments accounts for this mixture using a multi-component transport formalism, in which each species contributes its own conductivity tensor, modified by inter-particle scattering and by the mutual attraction between the layers. The Hall response of the trion component depends on its charge, its effective mass—which is dominated by the heavy bound complex rather than the light bare electron—and the statistics of the excess carriers that stabilize it. The agreement between the measured Hall coefficients and these predictions provides quantitative evidence that trions transport as coherent entities rather than as transient associations of separately drifting particles.

The result settles a question that has lingered since trions were first identified in semiconductor optical spectra decades ago. Because a trion is only weakly bound compared with an atom, and because its lifetime in a photoexcited sample is short, many researchers doubted whether trions could sustain the kind of steady-state drift required for a Hall measurement. The new work demonstrates that, in the right regime, a trion fluid behaves hydrodynamically and electrically much like an ordinary charged gas. The effective temperature and density of the trion population can be controlled through optical pumping and electrostatic gating, and under suitable conditions the composite particles establish their own drift velocity in response to an applied electric field, exactly as required for a Hall signal to develop.

Beyond its fundamental significance, the trion Hall effect opens practical avenues. Trions are central to proposals for excitonic and optoelectronic devices in which information is carried by light-matter bound states rather than bare electrons, promising lower-energy operation and new ways to couple optical and electrical signals. A measurable Hall response provides a purely electrical handle on trion populations, complementing optical spectroscopy and enabling faster readout of composite-particle dynamics. It could also serve as a diagnostic in quantum emitter and single-photon technologies, where trion states often mediate the optical response of defect centers and quantum dots. In van der Moiré heterostructures, where flat electronic bands amplify correlation effects, a trion Hall measurement might even reveal interaction-driven phases that are invisible to conventional transport.

The demonstration also connects to broader themes in modern condensed matter research. Hall effects have become a versatile language for characterizing quasiparticles of every description, from Weyl fermions in topological semimetals to strange metals and superconductors. Each new carrier class that exhibits a Hall response adds a fingerprint that can be used to identify and study it. Trions, being charged composites with internal degrees of freedom, sit in an unusual position in this landscape: their Hall coefficient reflects not only their charge-to-mass ratio but also the correlated motion of the constituent electron and hole, potentially encoding information about binding energies, scattering channels and the inter-layer coupling strength. The theoretical framework validated here offers a template for extracting such details in other layered systems where excitonic complexes proliferate.

Looking ahead, the researchers and their colleagues anticipate several extensions. Applying stronger magnetic fields may drive the trion fluid toward quantum Hall regimes, where quantized Hall signatures of composite particles could emerge. Studying how the trion Hall effect evolves as the layer separation shrinks toward the atomic limit could illuminate the crossover from loosely bound three-body states to the strongly correlated exciton fluids seen in recent experiments on monolayer semiconductors. And because the double layer geometry is compatible with contactless detection methods, the approach may generalize to systems where fabricating electrodes is impractical. For now, the observation stands as a clean confirmation of a decade-old theoretical expectation and a reminder that even the most familiar effects in physics can acquire new life when the carriers doing the drifting are as unconventional as charged pairs of light and matter.

Subject of Research: The Hall effect in a trion fluid formed within coupled electron–hole layers of a semiconductor device

Article Title: Trion Hall effect in electron–hole double layers

Article References: Chaturvedi, R., Nguyen, P. X., Knüppel, P., Watanabe, K., Taniguchi, T., Mak, K. F., & Shan, J. (2026). Trion Hall effect in electron–hole double layers. Nature Physics. https://doi.org/10.1038/s41567-026-03444-8

Image Credits: AI Generated

DOI: 10.1038/s41567-026-03444-8

Keywords: trion, Hall effect, exciton, electron–hole double layer, condensed matter physics, quasiparticles, semiconductor, van der Waals heterostructures, quantum Hall physics, optoelectronics, magnetic field, transport phenomena