arXiv subjects

Emily Hajigeorgiou

Publications and source records attributed to Emily Hajigeorgiou.

6 recordsLinked to original sources

arXiv2026

Trapping $e/4$ quasiparticles in bilayer graphene

Measuring the charge of the quasiparticles hosted by even-denominator fractional quantum Hall (FQH) states is essential to identify the topology of their ground state. Here, we use a gate-defined antidot in bilayer graphene, with an additional gate to control only the antidot potential, to measure the charge of the quasiparticles trapped around it in even-denominator FQH states. We observe a localized charge of $e/4$ at $ν=-5/2$, $-1/2$, and $3/2$, consistent with the minimal excitation expected for leading candidate even-denominator ground states, and $e/3$ at the hole-conjugate state $ν=2/3$. We further show that increasing the coupling between the antidot-bound states and extended edge states drives a crossover between two regimes, characterized by the minimal-excitation gate-voltage period and approximately twice that period, respectively. We discuss two possible explanations for this crossover: quasiparticle bunching and a crossover between distinct antidot transport regimes. Our results, together with previous observations of the daughter states, show that the even-denominator FQH states in bilayer graphene are compatible with a non-Abelian ground state, and that their quasiparticles can be localized around a quantum Hall antidot, a necessary ingredient for topological quantum computation.

cond-mat.mes-hall↗

arXiv2026

Constriction-induced modulation of charging energy in a quantum Hall cavity

Electronic Fabry-Pérot interferometers (FPIs) operating in the fractional quantum Hall regime are a key platform for probing anyonic braiding statistics, yet interpreting their interference signals is complicated by Coulomb charging effects, which are commonly treated as parasitic, static properties governed by the cavity's geometry and electrostatics. Here, using a gate-defined quantum Hall cavity tuned to the Coulomb-dominated regime, we demonstrate that the charging energy is in fact strongly and non-monotonically modulated by the magnetic field, varying by up to 60% over a range of only 100 mT. The effect appears exclusively when the quantum point contacts (QPCs) forming the cavity are weakly pinched off, i.e., in the strong cavity-to-lead coupling regime. By correlating the charging energy modulation with the QPC magneto-conductance, we attribute this behavior to field-dependent changes in local compressibility and electrostatic screening between the cavity and the leads, driven by the formation of incompressible fractional quantum Hall states within the constrictions. This result establishes QPC constrictions of quantum Hall cavities as active electrostatic elements rather than passive boundaries, revealing a dynamic screening mechanism, with direct consequences for the interpretation of interference measurements and the extraction of anyonic statistics.

cond-mat.mes-hall↗

arXiv2026

Observation of e/4 charge at $ν=1/2$ in GaAs

Even-denominator fractional quantum Hall states (FQHSs) fall outside the standard Laughlin's and Jain's odd-denominator hierarchy. In this work, we study the FQHS $ν=1/2$ in the lowest Landau level. The state is confined within a 70 nm-wide GaAs quantum well, where the electrons exhibit a bilayer-like charge distribution. Inter-layer interactions stabilize the $ν=1/2$ FQHS, which is predicted to host quasiparticles with charge e/4 - with either Abelian or non-Abelian topological order. Here, we report on shot-noise measurements of partitioned quasiparticles at $ν=1/2$, where charge partitioning is generated by a unique etch-defined quantum point contact. Our measurements were performed on two nominally identical devices, at two independent experimental setups. Analysis of shot noise in the weak-backscattering regime in each device reveals quasiparticles with charge e/4. These observations provide a clear benchmark for future studies aimed at probing the topological order of the $ν=1/2$ FQHS and its quasiparticles' exchange statistics.

cond-mat.mes-hall↗

arXiv2025

Quantum Hall Antidot as a Fractional Coulombmeter

The detection of fractionally charged quasiparticles, which arise in the fractional quantum Hall regime, is of fundamental importance for probing their exotic quantum properties. While electronic interferometers have been central to probe their statistical properties, their interpretation is often complicated by bulk-edge interactions. Antidots, potential hills in the quantum Hall regime, are particularly valuable in this context, as they overcome the geometric limitations of conventional designs and act as controlled impurities within a quantum point contact. Furthermore, antidots allow for quasiparticle charge detection through straightforward conductance measurements, replacing the need for more demanding techniques. In this work, we employ a gate-defined bilayer graphene antidot operating in the Coulomb-dominated regime to study quasiparticle tunneling in both integer and fractional quantum Hall states. We show that the gate-voltage period and the oscillation slope directly reveal the charge of the tunneling quasiparticles, providing a practical method to measure fractional charge in graphene. We report direct measurements of fractional charge, finding $q = e/3$ at $ν= 4/3$, 5/3 and 7/3, $q = 2e/3$ at $ν= 2/3$ and $q = 3e/5$ at $ν= 3/5$, while at $ν= 8/3$ we observe signatures of both $e/3$ and $2e/3$ tunneling charge. 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.

cond-mat.mes-hall↗

arXiv2025

Tunable Inter-Edge Interactions in a Bilayer Graphene Quantum Hall Antidot

Electronic interferometers in the quantum Hall regime are one of the best tools to study the statistical properties of localized quasiparticles in the topologically protected bulk. However, since their behavior is probed via chiral edge modes, bulk-to-edge and inter-edge interactions are two important effects that affect the observations. Moreover, almost all kinds of interferometers heavily rely on a pair of high-quality quantum point contacts where the presence of impurities significantly modifies the behavior of such constrictions, which in turn can alter the outcome of the measurements. Antidots, potential hills in the quantum Hall regime, are particularly valuable in this context, as they overcome the geometric limitations of conventional geometries and act as controlled impurities within a quantum point contact. Furthermore, antidots allow for quasiparticle charge detection through simple conductance measurements, replacing the need for complex techniques such as shot noise. Here, we use a gate-defined bilayer graphene antidot, operated in the Coulomb-dominated regime. By varying the antidot potential, we can tune inter-edge interactions, enabling a crossover from a single-dot to a double-dot behavior. In the latter, strong coupling between the two edge states leads to edge-state pairing, resulting in a measured doubling of the tunneling charge. We find that in certain regimes, the inter-edge coupling completely dominates over other energy scales of the system, overshadowing the interference effects these devices are mainly designed to probe. These results highlight the significant role of inter-edge interactions and establish antidots as a versatile platform for exploring quantum Hall interferometry.

cond-mat.mes-hall↗

arXiv2024

Featuring nuanced electronic band structure in gapped multilayer graphene

Moiré systems featuring flat electronic bands exhibit a vast landscape of emergent exotic quantum states, making them one of the resourceful platforms in condensed matter physics in recent times. Tuning these systems via twist angle and the electric field greatly enhances our comprehension of their strongly correlated ground states. Here, we report a technique to investigate the nuanced intricacies of band structures in dual-gated multilayer graphene systems. We utilize the Landau levels of a decoupled monolayer graphene to extract the electric field-dependent bilayer graphene charge neutrality point gap. Then, we extend this method to analyze the evolution of the band gap and the flat bandwidth in twisted mono-bilayer graphene. The band gap maximizes at the same displacement field where the flat bandwidth minimizes, indicating the strongest electron-electron correlation, which is supported by directly observing the emergence of a strongly correlated phase. Moreover, we extract integer and fractional gaps to further demonstrate the strength of this method. Our technique gives a new perspective and paves the way for improving the understanding of electronic band structure in versatile flat band systems.

cond-mat.mes-hall↗