Nematics are materials made of rod‑like particles that tend to align in the same direction. In active nematics, this alignment is constantly disrupted and renewed because the particles are driven by internal biological or chemical energy. As the orientation field twists and reorganises, it creates topological defects-points where the alignment breaks down. These defects are central to the collective behaviour of active matter, shaping flows, patterns, and self‑organisation.

In this work, the researchers identify an active topological phase transition that separates two distinct regimes of defect organisation. As the system approaches this transition from below, the dynamics slow dramatically: the relaxation of defect density becomes sluggish, fluctuations in the number of defects grow in amplitude and lifetime, and the system becomes increasingly sensitive to small changes in activity. At the critical point, defects begin to interact over long distances, with correlation lengths that grow with system size. This behaviour produces a striking dual‑scaling pattern, defect fluctuations appear uniform at small scales but become anti‑hyperuniform at larger scales, meaning that the number of defects varies far more than expected from a random distribution.

A key finding is that this anti‑hyperuniformity originates from defect clustering. Rather than forming ordered structures or undergoing phase separation, defects tend to appear near existing defects, creating multiscale clusters. This distinguishes the transition from well‑known defect‑unbinding processes such as the Berezinskii-Kosterlitz-Thouless transition in passive nematics or the nematic-isotropic transition in screened active systems. Above the critical activity, the system enters a defect‑laden turbulent state where defects are more uniformly distributed and correlations become short‑ranged and negative.

The researchers confirm these behaviours experimentally using large‑field‑of‑view measurements of endothelial cell monolayers which are the cells that line blood vessels. The same dual‑scaling behaviour, long‑range correlations, and clustering appear in these living tissues, demonstrating that the transition is robust across system sizes, parameter variations, frictional damping, and boundary conditions.

Do you want to learn more about this topic?

Active phase separation: new phenomenology from non-equilibrium physics M E Cates and C Nardini (2025)

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Topological quantum computing is a proposed approach to building quantum computers that aims to solve one of the biggest challenges in quantum technology: error correction.

In conventional quantum systems, qubits are extremely sensitive to their environment and even tiny disturbances can cause errors. Topological quantum computing addresses this by encoding information in the global properties of a system: the topology of certain quantum states.

These systems rely on the use of non-Abelian anyons, exotic quasiparticles that can exist in two-dimensional materials under special conditions.

The main challenge faced by this approach to quantum computing is the creation and control of these quasiparticles.

One possible source of non-Abelian anyons is the fractional quantum Hall state (FQH): an exotic state of matter which can exist at very low temperatures and high magnetic fields.

These states come in two forms: even-denominator and odd-denominator. Here, we’re interested in the even-denominator states – the more interesting but less well understood of the two.

In this latest work, researchers have observed this exotic state in gallium arsenide (GaAs) two-dimensional hole systems.

Typically, FQH states are isotropic, showing no preferred direction. Here, however, the team found states that are strongly anisotropic, suggesting that the system spontaneously breaks rotational symmetry.

This means that it forms a nematic phase – similar to liquid crystals – where molecules align along a direction without forming a rigid structure.

This spontaneous symmetry breaking adds complexity to the state and can influence how quasiparticles behave, interact, and move.

The observation of the existence of spontaneous nematicity in an even-denominator fractional quantum Hall state is the first of its kind.

Although there are many questions left to be answered, the properties of this system could be hugely important for topological quantum computers as well as other novel quantum technologies.

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