arXiv subjects

Erwann Bocquillon

Publications and source records attributed to Erwann Bocquillon.

At least 19 recordsLinked to original sources

arXiv2026

Non-universal localization transition in the quantum Hall effect probed through broken-symmetry states of graphene

The quantum Hall effect hosts quantum phase transitions in which the localization length, that is the size of disorder-induced bulk localized states, is governed by universal scaling from percolation theory. However, this universal character is not systematically observed in experiments, including very recent ones in extremely clean devices. Here we explore this non-universality by systematically measuring the localization length in broken-symmetry quantum Hall states of graphene. Depending on the nature and gap size of these states, we observe differences of up to a tenfold in the minimum localization length, accompanied by clear deviations from universal scaling. Our results, as well as the previously observed non-universality, are fully captured by a simple picture based on the co-existence of localized states from two successive sub-Landau levels.

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arXiv2025

Non-ohmic to ohmic crossover in the breakdown of the quantum Hall states in graphene under broadband excitations

Graphene, through the coexistence of large cyclotron gaps and small spin and valley gaps, offers the possibility to study the breakdown of the quantum Hall effect across a wide range of energy scales. In this work, we investigate the breakdown of the QHE in high-mobility graphene Corbino devices under broadband excitation ranging from DC up to 10 GHz. We find that the conductance is consistently described by variable range hopping (VRH) and extract the hopping energies from both temperature and field-driven measurements. Using VRH thermometry, we are able to distinguish between a cold and hot electron regime, which are dominated by non-ohmic VRH and Joule heating, respectively. Our results demonstrate that breakdown in the quantum Hall regime of graphene is governed by a crossover from non-ohmic, field-driven VRH to ohmic, Joule-heating-dominated transport.

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arXiv2025

Multilayer Cryogenic Powder Filters with Low Parasitic Capacitance

We report the development of a cryogenic powder filter that simultaneously offers high attenuation of radio-frequency (RF) signals in the gigahertz (GHz) range and minimized parasitic capacitance to ground. Conventional powder filters, which consist of a signal line passing through a metal powder-filled housing, attenuate high-frequency signals via the skin effect. However, these designs often suffer from significant parasitic capacitance between the signal line and the grounded chassis, which can compromise the performance of sensitive measurement setups by limiting their frequency bandwidth. In this work, we demonstrate that a multilayer powder filter design effectively achieves both high RF attenuation and reduced parasitic capacitance. This solution suppresses sample heating due to the unintentional intrusion of RF signals through the wiring, without degrading the performance of the measurement setup.

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arXiv2025

Dynamics of current-induced switching in the quantum anomalous Hall effect

Ferromagnetic topological insulators in the quantum anomalous Hall (QAH) regime host chiral, dissipationless edge states whose propagation direction is determined by the internal magnetization. Under suitable conditions, a strong electrical bias can induce magnetization reversal, and thus flip the propagation direction. In this work, we perform time-resolved measurements to investigate the switching dynamics. Our results reveal characteristics consistent with a disordered magnetic landscape and demonstrate that the reversal process is thermally activated, driven by Joule heating during the current pulse. The understanding of the magnetization dynamics in QAH systems opens pathways for local, controlled manipulation of chiral edge states via thermal effects.

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arXiv2025

Breakdown of the quantum anomalous Hall effect under microwave drives

Quantum anomalous Hall (QAH) insulators exhibit chiral dissipationless edge states without an external magnetic field, making them a promising material for quantum metrology and microwave applications. However, the breakdown of the zero-resistance state at low currents hinders progress. We investigate and characterize this breakdown under microwave fields (1-25 GHz) by measuring the increase of longitudinal resistance in RF Hall bars and RF Corbino devices made from V-doped (Bi,Sb)$_2$Te$_3$ films. Our results point to the role of heating of electron-hole puddles under microwave irradiation, thereby fostering hopping transport. Our work offers insights critical for GHz-range QAH applications.

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arXiv2024

Propagation, dissipation and breakdown in quantum anomalous Hall edge states probed by microwave edge plasmons

The quantum anomalous Hall (QAH) effect, with its single chiral, topologically protected edge state, offers a platform for flying Majorana states as well as non-reciprocal microwave devices. While recent research showed the non-reciprocity of edge plasmons in Cr-doped $\mathrm{(Bi_xSb_\text{1-x})_2Te_3}$, the understanding of their dissipation remains incomplete. Our study explores edge plasmon dissipation in V-doped $\mathrm{(Bi_xSb_\text{1-x})_2Te_3}$ films, analyzing microwave transmission across various conditions. We identify interactions with charge puddles as a primary source of dissipation, providing insights critical for developing improved QAH-based technologies.

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arXiv2023

Velocity and confinement of edge plasmons in HgTe-based 2D topological insulators

High-frequency transport in the edge states of the quantum spin Hall (QSH) effect has to date rarely been explored, though it could cast light on the scattering mechanisms taking place therein. We here report on the measurement of the plasmon velocity in topological HgTe quantum wells both in the QSH and quantum Hall (QH) regimes, using harmonic GHz excitations and phase-resolved detection. We observe low plasmon velocities corresponding to large transverse widths, which we ascribe to the prominent influence of charge puddles forming in the vicinity of edge channels. Together with other recent works, it suggests that puddles play an essential role in the edge state physics and probably constitute a main hurdle on the way to clean and robust edge transport.

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arXiv2020

Characterization of helical Luttinger liquids in microwave stepped-impedance edge resonators

Coulomb interaction has important consequences on the physics of quantum spin Hall edge states, weakening the topological protection via two-particle scattering and renormalizing both the velocity and charge of collective plasmon modes compared to that of free electrons. Despite these effects, interactions remain difficult to quantify. We propose here simple and robust edge resonator geometries to characterize Coulomb interaction by means of high-frequency measurements. They rely on a transmission line approach, and take advantage of the impedance mismatch between the edge states and their microwave environment.

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arXiv2019

RF compressibility of topological surface and interface states in metal-hBN-Bi2Se3 capacitors

The topological state that emerges at the surface of a topological insulator (TI) and at the TI-substrate interface are studied in metal-hBN-Bi2Se3 capacitors. By measuring the RF admittance of the capacitors versus gate voltage, we extract the compressibility of the Dirac state located at a gated TI surface. We show that even in the presence of an ungated surface that hosts a trivial electron accumulation layer, the other gated surface always exhibits an ambipolar effect in the quantum capacitance. We succeed in determining the velocity of surface Dirac fermions in two devices, one with a passivated surface and the other with a free surface that hosts trivial states. Our results demonstrate the potential of RF quantum capacitance techniques to probe surface states of systems in the presence of a parasitic density-of-states.

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arXiv2019

Dynamical separation of bulk and edge transport in HgTe-based 2D topological insulators

Topological effects in edge states are clearly visible on short lengths only, thus largely impeding their studies. On larger distances, one may be able to dynamically enhance topological signatures by exploiting the high mobility of edge states with respect to bulk carriers. Our work on microwave spectroscopy highlights the responses of the edges which host very mobile carriers, while bulk carriers are drastically slowed down in the gap. Though the edges are denser than expected, we establish that charge relaxation occurs on short timescales, and suggests that edge states can be addressed selectively on timescales over which bulk carriers are frozen.

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arXiv2018

Microwave studies of the fractional Josephson effect in HgTe-based Josephson junctions

The rise of topological phases of matter is strongly connected to their potential to host Majorana bound states, a powerful ingredient in the search for a robust, topologically protected, quantum information processing. In order to produce such states, a method of choice is to induce superconductivity in topological insulators. The engineering of the interplay between superconductivity and the electronic properties of a topological insulator is a challenging task and it is consequently very important to understand the physics of simple superconducting devices such as Josephson junctions, in which new topological properties are expected to emerge. In this article, we review recent experiments investigating topological superconductivity in topological insulators, using microwave excitation and detection techniques. More precisely, we have fabricated and studied topological Josephson junctions made of HgTe weak links in contact with two Al or Nb contacts. In such devices, we have observed two signatures of the fractional Josephson effect, which is expected to emerge from topologically-protected gapless Andreev bound states. We first recall the theoretical background on topological Josephson junctions, then move to the experimental observations. Then, we assess the topological origin of the observed features and conclude with an outlook towards more advanced microwave spectroscopy experiments, currently under development.

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arXiv2017

Transport spectroscopy of induced superconductivity in the three-dimensional topological insulator HgTe

The proximity-induced superconducting state in the 3-dimensional topological insulator HgTe has been studied using electronic transport of a normal metal-superconducting point contact as a spectroscopic tool (Andreev point contact spectroscopy). By analyzing the conductance as a function of voltage for various temperatures, magnetic fields and gate-voltages, we find evidence, in equilibrium, for an induced order parameter in HgTe of $70\,μ$eV and a niobium order parameter of $1.1\,$meV. To understand the full conductance curve as a function of applied voltage we suggest a non-equilibrium driven transformation of the quantum transport process where the relevant scattering region and equilibrium reservoirs change with voltage. This implies that the spectroscopy probes the superconducting correlations at different positions in the sample, depending on the bias voltage.

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arXiv2017

Dynamical transport measurement of the Luttinger parameter in helical edges states of 2D topological insulators

One-dimensional (1D) electron systems in the presence of Coulomb interaction are described by Luttinger liquid theory. The strength of Coulomb interaction in the Luttinger liquid, as parameterized by the Luttinger parameter K, is in general difficult to measure. This is because K is usually hidden in powerlaw dependencies of observables as a function of temperature or applied bias. We propose a dynamical way to measure K on the basis of an electronic time-of-flight experiment. We argue that the helical Luttinger liquid at the edge of a 2D topological insulator constitutes a preeminently suited realization of a 1D system to test our proposal. This is based on the robustness of helical liquids against elastic backscattering in the presence of time reversal symmetry.

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arXiv2016

Two-particle interferometry in quantum Hall edge channels

Since pioneering works of Hanbury-Brown and Twiss, intensity-intensity correlations have been widely used in astronomical systems, for example to detect binary stars. They reveal statistics effects and two-particle interference, and offer a decoherence-free probe of the coherence properties of light sources. In the quantum Hall edge channels, the concept of quantum optics can transposed to electrons, and an analogous two-particle interferometry can be developed, in order to characterize single-electron states. We review in this article the recent experimental and theoretical progress on this topic.

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arXiv2016

Josephson radiation from gapless Andreev bound states in HgTe-based topological junctions

Frequency analysis of the rf emission of oscillating Josephson supercurrent is a powerful passive way of probing properties of topological Josephson junctions. In particular, measurements of the Josephson emission enables to detect the expected presence of topological gapless Andreev bound states that give rise to emission at half the Josephson frequency $f_J$, rather than conventional emission at $f_J$. Here we report direct measurement of rf emission spectra on Josephson junctions made of HgTe-based gate-tunable topological weak links. The emission spectra exhibit a clear signal at half the Josephson frequency $f_{\rm J}/2$. The linewidths of emission lines indicate a coherence time of $0.3-\SI{4}{ns}$ for the $f_{\rm J}/2$ line, much shorter than for the $f_{\rm J}$ line ($3-\SI{4}{ns}$). These observations strongly point towards the presence of topological gapless Andreev bound states, and pave the way for a future HgTe-based platform for topological quantum computation.

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arXiv2016

Gapless Andreev bound states in the quantum spin Hall insulator HgTe

In recent years, Majorana physics has attracted considerable attention in both theoretical and experimental studies due to exotic new phenomena and its prospects for fault-tolerant topological quantum computation. To this end, one needs to engineer the interplay between superconductivity and electronic properties in a topological insulator, but experimental work remains scarce and ambiguous. Here we report experimental evidence for topological superconductivity induced in a HgTe quantum well, a two-dimensional topological insulator that exhibits the quantum spin Hall effect. The ac Josephson effect demonstrates that the supercurrent has a $4π$-periodicity with the superconducting phase difference as indicated by a doubling of the voltage step for multiple Shapiro steps. In addition, an anomalous SQUID-like response to a perpendicular magnetic field shows that this $4π$-periodic supercurrent originates from states located on the edges of the junction. Both features appear strongest when the sample is gated towards the quantum spin Hall regime, thus providing evidence for induced topological superconductivity in the quantum spin Hall edge states.

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arXiv2015

$4π$-periodic Josephson supercurrent in HgTe-based topological Josephson junctions

The Josephson effect describes the generic appearance of a supercurrent in a weak link between two superconductors. Its exact physical nature however deeply influences the properties of the supercurrent. Detailed studies of Josephson junctions can reveal microscopic properties of the superconducting pairing (spin-triplet correlations, $d$-wave symmetry) or of the electronic transport (quantum dot, ballistic channels). In recent years, considerable efforts have focused on the coupling of superconductors to topological insulators, in which transport is mediated by topologically protected Dirac surface states with helical spin polarization (while the bulk remains insulating). Here, the proximity of a superconductor is predicted to give rise to unconventional induced $p$-wave superconductivity, with a doublet of topologically protected gapless Andreev bound states, whose energies varies $4π$-periodically with the superconducting phase difference across the junction. In this article, we report the observation of an anomalous response to rf irradiation in a Josephson junction with a weak link of the 3D topological insulator HgTe. The response is understood as due to a $4π$-periodic contribution to the supercurrent, and its amplitude is compatible with the expected contribution of a gapless Andreev doublet.

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arXiv2015

Phase-sensitive SQUIDs based on the 3D topological insulator HgTe

Three-dimensional topological insulators represent a new class of materials in which transport is governed by Dirac surface states while the bulk remains insulating. Due to helical spin polarization of the surface states, the coupling of a 3D topological insulator to a nearby superconductor is expected to generate unconventional proximity induced $p$-wave superconductivity. We report here on the development and measurements of SQUIDs on the surface of strained HgTe, a 3D topological insulator, as a potential tool to investigate this effect.

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