Publications

Highlights and most important publications

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Non-universal localization transition in the quantum Hall effect probed through broken-symmetry states of graphene, arXiv:2509.20163

Aifei Zhang, Torsten Röper, Manjari Garg, Kenji Watanabe, Takashi Taniguchi, Carles Altimiras, Patrice Roche, Erwann Bocquillon, Olivier Maillet, François D. Parmentier

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.

Non-ohmic to ohmic crossover in the breakdown of the quantum Hall states in graphene under broadband excitations, arXiv:2509.19978

Torsten Röper, Aifei Zhang, Kenji Watanabe, Takashi Taniguchi, Olivier Maillet, François D. Parmentier, Erwann Bocquillon

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.

Breakdown of the quantum anomalous Hall effect under microwave drives, arXiv:2505.23156

Torsten Röper, Daniel Rosenbach, Achim Rosch, Alexey A. Taskin, Yoichi Ando, Erwann Bocquillon

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)Te 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.

Dynamics of current-induced switching in the quantum anomalous Hall effect, arXiv:2507.19665

Alina Rupp, Daniel Rosenbach, Torsten Röper, Dominik Hoborka, Alexey A. Taskin, Yoichi Ando, Erwann Bocquillon

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.

Propagation, dissipation, and breakdown in quantum anomalous Hall edge states probed by microwave edge plasmons, Phys. Rev. B 110, L161403 (2024)

Torsten Röper, Hugo Thomas, Daniel Rosenbach, Anjana Uday, Gertjan Lippertz, Anne Denis, Pascal Morfin, Alexey A. Taskin, Yoichi Ando and Erwann Bocquillon

The quantum anomalous Hall (QAH) effect, with its single chiral, topologically protected edge state, offers a platform for flying Majorana states as well as nonreciprocal microwave devices. While recent research showed the nonreciprocity of edge plasmons in Cr-doped (BixSb1−x )2Te3, the understanding of their dissipation remains incomplete. Our study explores edge plasmon dissipation in V-doped (BixSb1−x )2Te3 films, analyzing microwave transmission across various conditions. We identify interactions with charge puddles as a primary source, providing insights critical for developing improved QAH-based technologies.

Dynamical Separation of Bulk and Edge Transport in HgTe-Based 2D Topological Insulators, Phys. Rev. Lett. 124, 076802 (2020)

Matthieu C. Dartiailh, S. Hartinger, A. Gourmelon, K. Bendias, H. Bartolomei, H. Kamata, J.-M. Berroir, G. Fève, B. Plaçais, L. Lunczer, R. Schlereth, H. Buhmann, L. W. Molenka, Erwann Bocquillon

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 response 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 suggest that edge states can be addressed selectively on timescales over which bulk carriers are frozen.

Erwann Bocquillon, Russell S. Deacon, Jonas Wiedenmann, Philipp Leubner, Teunis M. Klapwijk, Christoph Brüne, Koji Ishibashi, Hartmut Buhmann and Laurens W. Molenkamp

Gapless Andreev bound states in the quantum spin Hall insulator HgTe, Nat. Nanotech 12, 137–143 (2016)

In recent years, Majorana physics has attracted considerable attention because of 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 2D topological insulator that exhibits the quantum spin Hall (QSH) effect. The a.c. Josephson effect demonstrates that the supercurrent has a 4π periodicity in the superconducting phase difference, as indicated by a doubling of the voltage step for multiple Shapiro steps. In addition, this response like that of a superconducting quantum interference device to a perpendicular magnetic field shows that the 4π-periodic supercurrent originates from states located on the edges of the junction. Both features appear strongest towards the QSH regime, and thus provide evidence for induced topological superconductivity in the QSH edge states.

J. Wiedenmann, E. Bocquillon, R.S. Deacon, S. Hartinger1, O. Herrmann, T.M. Klapwijk, L. Maier, C. Ames, C. Brüne, C. Gould, A. Oiwa, K. Ishibashi, S. Tarucha, H. Buhmann1 & L.W. Molenkamp

4pi-periodic Josephson supercurrent in HgTe-based topological Josephson junctions, Nat Commun 7, 10303 (2016)

The Josephson effect describes the generic appearance of a supercurrent in a weak link between two superconductors. Its exact physical nature deeply influences the properties of the supercurrent. In recent years, considerable efforts have focused on the coupling of superconductors to the surface states of a three-dimensional topological insulator. In such a material, an unconventional induced p-wave superconductivity should occur, with a doublet of topologically protected gapless Andreev bound states, whose energies vary 4p-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 made of a HgTe weak link. The response is understood as due to a 4p-periodic contribution to the supercurrent, and its amplitude is compatible with the expected contribution of a gapless Andreev doublet. Our work opens the way to more elaborate experiments to investigate the induced superconductivity in a three-dimensional insulator.

E. Bocquillon, V. Freulon, J.-M. Berroir, P. Degiovanni, B. Plaçais, A. Cavanna, Y. Jin, G. Fève

Separation of neutral and charge modes in one-dimensional chiral edge channels, Nat Commun 4, 1839 (2013)

Coulomb interactions have a major role in one-dimensional electronic transport. They modify the nature of the elementary excitations from Landau quasiparticles in higher dimensions to collective excitations in one dimension. Here we report the direct observation of the collective neutral and charge modes of the two chiral co-propagating edge channels of opposite spins of the quantum Hall effect at filling factor 2. Generating a charge density wave at frequency f in the outer channel, we measure the current induced by inter-channel Coulomb interaction in the inner channel after a 3-mm propagation length. Varying the driving frequency from 0.7 to 11 GHz, we observe damped oscillations in the induced current that result from the phase shift between the fast charge and slow neutral eigenmodes. We measure the dispersion relation and dissipation of the neutral mode from which we deduce quantitative information on the interaction range and parameters.

E. Bocquillon, V. Freulon, J.-M Berroir, P. Degiovanni, B. Plaçais, A. Cavanna, Y. Jin, G. Fève

Coherence and Indistinguishability of Single Electrons Emitted by Independent Sources, Science 339, 1054 (2013)

The on-demand emission of coherent and indistinguishable electrons by independent synchronized sources is a challenging task of quantum electronics, in particular regarding its application for quantum information processing. Using two independent on-demand electron sources, we triggered the emission of two single-electron wave packets at different inputs of an electronic beam splitter. Whereas classical particles would be randomly partitioned by the splitter, we observed two-particle interference resulting from quantum exchange. Both electrons, emitted in indistinguishable wave packets with synchronized arrival time on the splitter, exited in different outputs as recorded by the low-frequency current noise. The demonstration of two-electron interference provides the possibility of manipulating coherent and indistinguishable single-electron wave packets in quantum conductors.

 

Theses

Bachelor-, Masters- and PhD theses 

Master Thesis: Resistive ZnO films for suppressing gate screening of edge plasmons (2024)

Dominik Hoborka

The quantum anomalous Hall effect exhibits an insulating bulk and a single chiral edge state hosting non dissipative transport at the edge of the material making it an interesting platform for non-reciprocal microwave devices. Applying a microwave signal to the edge state induces a collective excitation called edge plasmon. The peculiar physics of Coulomb interactions of electrons in one dimension enhances the plasmon velocity. However, careful band engineering of the Fermi energy is required to make transport via the edge state exclusive. Therefore electrostatic gating is beneficial as one can artificially tune the Fermi energy via an electric field. The low resistivity of metallic gate electrodes screens the Coulomb interactions and enhances stray capacitances which makes signal processing difficult. Kumada et al. circumvented these effects using a highly resistive material such as ZnO as a gate electrode on a quantum Hall system. In this work we transfer the idea of Kumada et al. and investigate the effects of highly resistive ZnO gate electrodes on the transport properties of the quantum anomalous Hall insulator V-doped (Bi1–xSbx)2Te3. We develop a growth recipe for highly resistive ZnO thin films using atomic layer deposition and obtain a sheet resistance of 0.6MOhm/cm^-2 by additional doping with Al. Our results show that the screening effects of the plasmon due to the gate are suppressed without enhancing the stray capacitance. However, the tunability of the Fermi energy is hindered due to the formation of Schottky barriers at the interface of ZnO gate electrode and the Pt/Au contact at very low temperatures.

Master Thesis: Towards Josephson Radiation Measurements (2024)

Ricardo Gioia Alvarez

This work will deal with Al/Al2O3/Al and Nb/Al2O3/Nb (SIS), as well as Nb/BiSbTeSe2/Nb junctions (S/TI/S) as test subjects. In order to gain more information on these junctions, this thesis aims towards a reliable and understandable platform to study their characteristics and to probe their performance. Additionally, the second goal is to successfully measure the radiation emitted by these junctions and to observe higher harmonics or subharmonics in SIS or S/TI/S junctions, the latter of which would hint at the presence of Majorana bound states.

Bachelor Thesis: Broad frequency study of Al-doped zinc oxide thin film capacitor devices (2025)

Timo Lautenschlager

Resistive gating is a useful tool for suppressing screening effects of charge modes triggered at high frequencies. Additionally, it can help to reduce parasitic stray coupling between on-chip high-frequency contacts while preserving the ability to tune the charge density of the device. For this purpose, aluminum-doped zinc oxide (AZO) seems a good candidate as one can control its electrical properties, especially the sheet resistance. In this thesis, AZO is studied by fabricating metaloxide-semiconductor (MOS) capacitors with different dielectrics (Al2O3, HfO2, and SiO2). Moreover the devices are electrically characterized to evaluate the suitability of the dielectrics for integration with AZO gates. The fabrication process enabled reliable deposition of dielectric and AZO films in a single step. Measurements revealed the feasibility of all 3 oxides as dielectrics in AZO gates. AZO films showed resistances between 15 kΩ and 100 kΩ, with values affected by the dielectric choice. Low-temperature experiments demonstrated a resistance increase by about two orders of magnitude and a temperature dependence of capacitance. These findings suggest the need for optimizing additional parameters to further lower the resistances obtained at low temperatures.

Bachelor Thesis: High-Frequency Filtering Systems for Noise Measurements (2024)

Paul Ruhrberg

The thesis is separated into three main parts. The first chapter focuses on using electrical components to construct a circuit known as a low-pass filter. Therefore, basic electronics knowledge is reiterated, followed by the theory of low-pass filters, and concluded with the design and construction of such a circuit. The second chapter introduces the copper powder filter. A short introduction to the theory of metal powder filters is followed by the design and optimization process. The third chapter combines both filter stages and focuses on the application of such a filter at low temperatures. A conclusion and outlook is given at the end of this thesis.

Bachelor Thesis: Low-Temperature Atomic Layer Deposition of Hafnium Oxide Gate Dielectrics (2024)

Benjamin Roth

The quantum anomalous Hall effect (QAHE), first experimentally observed in 2013, is a fascinating quantum phenomenon that continues to be a focus of active research. The QAHE is characterized by a single chiral edge state. To suppress bulk transport, the Fermi level is tuned in the bulk’s band gap by electrostatic gating. However, the deposition of gate material often requires elevated temperatures, at which our samples degrade. Hence, we develop and optimize the low-temperature (< 100 ◦C) atomic layer deposition of hafnium oxide (HfO2) gate dielectrics. We found relative permittivities ϵr of 6 to 10, which were significantly lower than in other studies. Current-voltage measurements showed strong asymmetry, with electrical breakdowns occurring at Ebd ∼ 1.5MVcm−1 and up to Ebd ∼ 6MVcm−1 depending on the voltage polarity. Finally, we deposited HfO2 at 80 ◦C on a quantum anomalous Hall insulator (QAHI), where a successful gating effect was shown at T ≈ 14 mK. These results highlight the potential for atomic layer deposition of hafnium oxide for temperature-sensitive gating applications.