Observing Laughlin's pump using quantized edge states in graphene

TL;DR

This paper demonstrates direct observation of Laughlin's charge pump in graphene by fabricating small contacts that induce quantized edge states, revealing spectral flow through conductance oscillations tied to magnetic field and carrier density.

Contribution

The study introduces a graphene-based platform with lithographically defined contacts to directly observe spectral flow in Laughlin's pump, overcoming previous experimental challenges.

Findings

Clear conductance oscillations as a function of magnetic field and density

Oscillation period scales with contact size, confirming quantized charge transfer

Spectral flow associated with Laughlin's pump directly observed

Abstract

Laughlin's thought experiment of quantized charge pumping is central to understanding the integer quantum Hall effect (IQHE) and the topological origin of its conductance quantization. Its direct experimental observation, however, has been hindered by the difficulty of realizing clean electronic edges. We address this by fabricating ultra-small, lithographically defined contacts on graphene. This creates a Corbino-equivalent system, with well-confined inner edge states. Crucially, the small contact size induces strong energy quantization of the edge states. This quantization allows us to directly resolve the spectral flow associated with Laughlin's pump. By tracing the finite-size resonances of the inner edge, we observe clear oscillations in conductance as a function of magnetic field and carrier density. The oscillation period scales with contact size, consistent with quantized charge transfer. Thus, our results provide a direct observation of the spectral flow underlying Laughlin's pump. The simplicity of the graphene platform makes this approach scalable and robust for exploring fundamental topological effects.