Scientists detect e/3 charge in graphene’s quantum Hall state

Researchers at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, collaborating with scientists from the National Institute for Materials Science in Japan and the Rudolf Peierls Centre for Theoretical Physics, Parks Road, Oxford, UK, have directly measured a charge of e/3 in graphene using a novel antidot within a graphene sheet that isolates the fractional quantum Hall effect, allowing for straightforward conductance measurements of quasiparticle tunneling.

The work reports fractional charges of q = e/3 at filling factors of 4/3, 5/3, and 7/3, alongside q = 2e/3 at 2/3 and q = 3e/5 at 3/5, while at ν = 8/3 they observe signatures of both e/3 and 2e/3 tunneling charge, offering a practical method to study these exotic particles and opening a pathway to extend antidot-based charge measurements to other van der Waals materials.

Graphene Quantum Hall Antidot Enables Fractional Charge Detection

This novel approach overcomes limitations of conventional designs and allows for quasiparticle charge detection through straightforward conductance measurements, eliminating the need for more complex techniques. The team’s device, described in a recent preprint, utilizes a bilayer graphene sheet with a potential hill created using graphite gates and encapsulated within hexagonal boron nitride. This configuration acts as a controlled impurity within a quantum point contact, enabling the study of fractionally charged quasiparticles that emerge in the fractional quantum Hall regime.

The observed oscillations in conductance, directly linked to the tunneling of these quasiparticles, reveal their charge with clarity. This finding is significant because it directly measures fractional charge, a crucial step toward understanding the exotic quantum properties of these quasiparticles.

The antidot design effectively minimizes these complications, providing a cleaner signal and more reliable interpretation of the results. As the paper explains, this design opens a pathway to extend antidot-based charge measurements to other van der Waals materials, such as rhombohedral multilayer graphene, for probing quasiparticle charge in fractional Chern insulators. The gate-voltage period and the oscillation slope, the researchers show, directly reveal the charge of the tunneling quasiparticles, offering a practical method for measurement.

They observed Coulomb-dominated oscillations at integer filling factors, which provides a basis for comparison with the fractional quantum Hall states. These oscillations, observed as a function of both magnetic field and top gate voltage, confirm the controlled manipulation of quasiparticles within the antidot.

Bilayer Graphene Antidot Design and Fabrication Details

The fabrication of a tunable bilayer graphene antidot, a key component in recent observations of fractional charge, relied on a meticulously crafted van der Waals heterostructure. This approach contrasts with previous attempts using lithographically etched structures, offering a significant advantage in controlling the coupling between extended edge channels and the states localized within the antidot. The device, detailed in a recent preprint, incorporates both top and bottom graphite gates, alongside two side gates, to achieve this precise tunability.

This design overcomes limitations encountered in earlier studies of quantum Hall effects in graphene, which often struggled with complications arising from bulk-edge interactions, particularly when examining hole-like fractional quantum Hall states. A false-color scanning electron microscope image of the fabricated device reveals the antidot area, alongside gold ohmic contacts and palladium connections to the graphite gates; the scale bar in the image is 2 μm.

Fractional Charge Measurements at Filling Factors ν=4/3, 5/3, 7/3

The team’s work, detailed in a recent preprint, centers on a bilayer graphene structure featuring a potential hill within the material’s conductive sheet. This design allows for direct observation of charge tunneling, sidestepping complexities that have previously hampered accurate measurements. These findings confirm theoretical predictions about the behavior of quasiparticles, particles that emerge from the collective behavior of electrons in these extreme quantum states, and provide a crucial validation of the experimental approach.

At a filling factor of ν = 8/3, the team observed signatures of both e/3 and 2e/3 tunneling charge, suggesting a more complex interplay of quasiparticle states at that specific energy level. The ability to isolate these charges within the antidot structure is a significant advancement, as conventional methods often struggle with interference from the material’s edges. The core of this innovation lies in the antidot’s geometry and the way it interacts with the quantum Hall effect.

Antidots act as controlled impurities within a quantum point contact, effectively localizing the quasiparticles and enabling their detection through straightforward conductance measurements. This contrasts with techniques like electronic interferometry, which, while capable of probing quasiparticle statistics, are often complicated by bulk-edge interactions.

Quantum Hall Interferometry Challenges with Bulk-Edge Interactions

The team’s design focuses on isolating quasiparticles within the antidot, allowing for a more straightforward assessment of their properties. The core innovation lies in the ability to precisely control the antidot’s electrostatic environment using graphite gates positioned above, below, and on the sides of the graphene sheet.

The team notes that these interactions hinder accurate interpretation of experimental results; by confining quasiparticles within the antidot, the researchers effectively minimize these disruptive effects, enabling a clearer signal. This approach will be particularly valuable for studying more exotic quantum states, such as those predicted to host non-Abelian quasiparticles, and for verifying their fractional charge.

Gate-Defined Antidot Operation in the Coulomb-Dominated Regime

Bilayer graphene, a material just one atom thick, has yielded direct evidence of fractionally charged quasiparticles using a meticulously crafted antidot structure. The team’s approach centers on a gate-defined antidot, a potential hill within the graphene, allowing for precise control over electron behavior in the quantum Hall regime. This level of control is crucial for studying quasiparticle tunneling in both integer and fractional quantum Hall states, and for probing the charge of those tunneling particles.

The core of the discovery lies in the observation of Coulomb-dominated oscillations in conductance. The team emphasizes that the simplicity of this geometry is a key advantage.

Van der Waals Heterostructures Extend Antidot Platform Versatility

A directly measured fractional charge of e/3 within a graphene quantum Hall system confirms the potential of a novel platform for studying exotic quantum particles. This control is crucial for accurately measuring the charge of tunneling quasiparticles, as evidenced by the direct correlation between gate-voltage period and oscillation slope with the observed charge values. The team’s method relies on observing Coulomb-dominated oscillations in conductance, a phenomenon that directly reveals the charge of the tunneling quasiparticles.

The simplicity and tunability of this design open a pathway to extend antidot-based charge measurements to other van der Waals materials, establishing antidots as a powerful and broadly applicable platform to study the quantum Hall effect. The team’s work suggests that this technique could be instrumental in unraveling the mysteries surrounding non-Abelian anyons and their potential for topological quantum computation.

Antidot’s Potential for Studying Non-Abelian Quantum States

Researchers are now leveraging a novel approach utilizing antidots, potential hills within bilayer graphene, to overcome these limitations and probe these exotic states of matter with greater precision. This work uses findings from earlier studies conducted in gallium arsenide heterostructures, but offers a new level of control and adaptability. Unlike traditional interferometers used to study quasiparticle statistics, this antidot-based method avoids complications caused by bulk-edge interactions, which have previously obscured interpretations of experimental results.

This precise control allows researchers to isolate and study the behavior of individual quasiparticles with greater clarity. The team’s method opens a pathway to extend antidot-based charge measurements to other van der Waals materials, and potentially unlocking new insights into the behavior of non-Abelian quasiparticles, which are predicted to exhibit unique quantum statistical properties.