Order That Doesn't Interfere: Topology in a Thin Film express
A mosaic floor made of many small tiles, each turned in a different direction. It seems that any pattern trying to cross such a floor would get scrambled and vanish. That's how people usually think about disorder in crystals: if atoms aren't arranged in a perfect lattice, delicate quantum effects should fall apart. But in a thin film of iron silicide (ε-FeSi), just 65 nanometers thick, things are different.
In this material, researchers found a robust anomalous Hall effect: a transverse voltage appears without any external magnetic field and doesn't change when cooling from 200 K down to the lowest temperatures. Such constancy is like a compass needle that always points north, even if you shake the compass or turn it upside down. This is a direct sign that the effect doesn't arise from scattering off defects but from the intrinsic geometry of electron clouds — the so-called Berry phase (the twist of an electron's wavefunction when it travels a closed loop in the crystal).
Additional measurements revealed another hallmark of Weyl semimetals — materials where electrons behave like massless particles with a definite "handedness" (chirality). In magnetoresistance and the planar Hall effect, a so-called chiral anomaly appeared: when electric and magnetic fields are parallel, the number of electrons with one "handedness" is no longer conserved, creating an unusual contribution to the current. From the magnitude of the anomalous Hall conductivity, the team even estimated the distance between the two Weyl points in momentum space — about 0.36 in arbitrary units.
The ε-FeSi film turned out to be surprisingly tough against disorder. Its topological character survives at room temperature and doesn't require expensive rare metals. This opens the way to cheap and reliable devices that exploit quantum properties under ordinary conditions. So the mosaic pattern of grains doesn't destroy the delicate dance of electrons: it only adds resilience.
Original abstract
as written by the authors arXiv:2604.17103
Disorder in any form is considered to be highly detrimental to the experimental exploration of novel phenomena in quantum materials with non-trivial band topology. Contrary to established belief, clear topological features are reliably detected in the electron transport of polycrystalline 65-nm-thick \epsilon -FeSi films grown via solid-state reaction of Fe deposited on a Si(100) substrate. The observation of temperature-independent anomalous Hall conductivity \sigma_{xy}^{AHE} \sim const (\sigma_{xx)) (\sigma_{xy}^{AHE} \approx 14 uS/sq.) below 200 K firmly proves the anomalous Hall effect in this compound to be intrinsic and originating from a non-trivial Berry phase. The discovered scaling dominates over the nanoscale (\sim 40 nm) polycrystalline texture and is robust to temperature crossover between bulk and surface modes of electron transport. The non-trivial topological state of \epsilon-FeSi is also confirmed by a chiral anomaly both in anisotropic longitudinal magnetoresistance and planar Hall effect specific for Weyl semimetals. Relating scaled anomalous Hall conductance to a "quantized" Hall response of a Weyl semimetal the distance between two Weyl points has been estimated as (k_{+}^{W}-k_{-}^{W})/(2\pi) \approx 0.36. Our findings confirm the topological origin of electron transport in the polycrystalline \epsilon -FeSi thin films and discover its potential as a new high temperature and noble metal-free Weyl semimetal.
The anomalous Hall effect in this film is so stable that it could serve as a resistance standard: the conductivity remains constant over a wide temperature range, as if frozen.
In Arthur C. Clarke's novel *Childhood's End*, aliens use materials with unusual internal geometry to control gravity. Topological semimetals are a step toward controlling electrons through the geometry of their quantum states.