Quantized Charge Pumping Directly Observed in Topological Edge States.

The integer quantum Hall effect (IQHE), a phenomenon where electrical conductance becomes precisely quantized in two-dimensional electron systems subjected to strong magnetic fields, continues to yield fundamental insights into the nature of topological states of matter. A long-standing theoretical prediction, Laughlin’s pump, describes the spectral flow of edge states responsible for this quantization, yet direct experimental verification has proved elusive due to the challenges in creating sufficiently clean and well-defined edges. Now, researchers led by Bjarke S. Jessen from the Technical University of Denmark, alongside colleagues from Columbia University, ETH Zurich, the National Institute for Materials Science in Japan, and the University of Konstanz, report a direct observation of this spectral flow. Their work, detailed in a recent publication entitled ‘Observing Laughlin’s pump using quantized edge states in graphene’, utilises lithographically defined contacts on graphene to create a Corbino-equivalent system, enabling the resolution of quantized edge states and the observation of conductance oscillations consistent with quantized charge transfer. The team’s approach offers a scalable platform for investigating fundamental topological effects.

Researchers report the direct observation of spectral flow, a key signature of topological charge transport, within a Corbino disk geometry. This observation validates a central prediction stemming from Laughlin’s thought experiment, a cornerstone in understanding the integer quantum Hall effect, a phenomenon where electron transport occurs in quantized steps despite the presence of disorder.

The experiment utilises a two-dimensional electron gas (2DEG), a system confining electrons to move in a plane, fabricated with ultra-small contacts to create a Corbino-equivalent configuration. This geometry effectively isolates inner edge states, which are conducting pathways existing at the boundaries of the 2DEG, allowing for clearer observation of the spectral flow. The Corbino disk, a circular sample with radial contacts, facilitates this isolation by confining electron movement.

Numerical simulations, performed using the Kwant software package, model the two-terminal conductance – a measure of how easily current flows – as a function of both applied magnetic field and carrier density, the number of charge carriers per unit volume. These simulations provide a theoretical benchmark against which to interpret the experimental data.

Precise fabrication and measurement techniques yield experimental data subsequently analysed using several methods. Fourier analysis identifies peaks corresponding to different edge state modes, relating these to their velocity. Conductance mapping, plotting conductance at fixed energies, visualises patterns within the data. Researchers trace coordinates within these maps to determine the group velocity of the edge states, the speed at which information is carried. Finally, model fitting compares the experimental data to the theoretical predictions, validating the findings and refining parameter estimations.

The results demonstrate clear oscillations in conductance as a function of both magnetic field and carrier density. Critically, the oscillation period scales with the size of the contacts, a behaviour consistent with quantized charge transfer. These observations directly validate the theoretical predictions of spectral flow, confirming the predicted behaviour of charge carriers in this topological system.

Further analysis reveals that disorder, imperfections within the material, broadens the spectral flow and diminishes the distinction between individual edge state modes. This finding highlights the interplay between topological protection and material imperfections, a crucial consideration for future device applications.

This research provides a direct experimental confirmation of a fundamental concept in topological physics and establishes a simple, scalable platform for exploring and potentially utilising topological phenomena in future electronic devices. The relative simplicity of the experimental setup and the potential for miniaturisation offer promising avenues for further investigation and technological development.