Giant collective Aharonov–Bohm oscillations in a kagome metal

In the layered kagome metal CsV₃Sb₅, researchers have observed ¹ something that, until now, seemed almost impossible: robust quantum interference in the normal, non-superconducting state, persisting over distances of several micrometers. The interference is not the fragile single-particle kind seen in ultra-clean semiconductors at millikelvin temperatures. Instead, it behaves as if the entire stack of kagome layers acts as one giant, phase-coherent interferometer whose conductance oscillates precisely with the magnetic flux threaded between the layers, one quantum of flux per period. This is equivalent to a macroscopic Aharonov–Bohm ring, a tiny loop where particles can be influenced by a magnetic field even in places where the magnetic field is zero. The surprise is that this happens well above the superconducting transition, in a regime where individual electrons scatter after only a few hundred nanometers. The only way to explain the data is that the electrons are moving collectively, carrying a shared quantum phase that survives scattering and spans the whole device.

The intermediate-temperature regime

CsV₃Sb₅ belongs to a family of vanadium-based kagome metals that have kept condensed-matter physicists busy for the past few years. Its structure consists of perfectly stacked two-dimensional kagome lattices, corner-sharing triangles of vanadium atoms sandwiched between cesium and antimony layers. This geometry frustrates conventional electron motion, flattens portions of the band structure, and enhances electron–electron interactions. The phase diagram is already rich: a charge-density-wave order sets in near 94 K, superconductivity appears below about 2.8 K, and various experiments have hinted at an additional electronic rearrangement around 30 K whose nature has remained elusive. The new work homes in on exactly that intermediate-temperature regime and on transport perpendicular to the kagome planes.

The experiment

The experimental devices are microfabricated pillars carved from strain-free single crystals, typically 2–5 μm wide and only a few unit cells thick along the c-axis. Current flows along the stacking direction while a strong magnetic field is applied in the plane of the layers. As the field strength is swept, the out-of-plane resistance shows strikingly periodic oscillations. The period corresponds to one magnetic flux quantum (h/e) penetrating the area between adjacent kagome layers, and it scales inversely with the pillar width in precisely the way expected for Aharonov–Bohm interference around the pillar’s perimeter. In other words, the stack behaves as though electrons traveling upward through different interlayer paths acquire relative phases determined by the enclosed flux, and these phases interfere constructively or destructively at the top contact.

A smoking gun for collective behaviour

Two observations rule out any explanation based on independent electrons preserving their phase memory. First, Hall and longitudinal resistivity measurements show that the in-plane mean free path at these temperatures is only about 560 nm—roughly an order of magnitude shorter than the coherence length implied by the interference. Second, when the direction of the in-plane field is rotated, the oscillation frequency does not vary smoothly. Instead, it jumps discontinuously whenever the field aligns with high-symmetry crystalline directions. These sharp, non-analytic switches are a smoking gun for collective behavior: the underlying electronic state reorients abruptly, dragging the entire interference pattern with it.

Possible microscopic origins

The emerging picture is a phase-coherent but dissipative electronic medium that forms below roughly 30 K. Charge transport between layers remains resistive (there is no zero-resistance supercurrent) yet the relative phases along different paths are locked together over micrometer scales. The coherence appears tied to the same electronic instability that produces the enigmatic 30 K anomaly, and it coexists (and competes) with both the charge-density wave above and the superconducting state below. Possible microscopic origins include fluctuating orbital loop currents circulating around the vanadium triangles, nematic or chiral charge order that breaks rotational symmetry, or an exotic intertwined phase where multiple order parameters fluctuate in concert. Whatever the precise mechanism, the interference provides a bulk thermodynamic probe: any successful theory must now reproduce flux-periodic oscillations with the observed angular discontinuities.

Electrons conspire in CsV₃Sb₅

Kagome lattices have long been celebrated for hosting flat bands, Dirac cones, and strong correlation effects. The discovery of macroscopic many-body interference in the normal state adds a new entry to that list, one that does not require pairing or extreme sample purity. It demonstrates that geometry and interactions alone can combine to make quantum phase coherence a collective, emergent property of the metal itself. Looking ahead, the effect may not be unique to CsV₃Sb₅; similar layered frustrated metals could exhibit analogous behavior, opening a route to phase-sensitive transport phenomena at temperatures far above traditional superconductors.

For now, the experiment delivers a clean, quantitative signature of the long-suspected intermediate phase in this material and forces a rethink of how far quantum interference can extend in interacting, imperfectly clean metals. In CsV₃Sb₅, electrons do not merely scatter, they conspire, turning what should be incoherent hopping into a micrometer-scale quantum symphony whose notes are tuned by magnetic flux.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.