arXiv subjects

B. Jouault

Publications and source records attributed to B. Jouault.

At least 19 recordsLinked to original sources

arXiv2025

Many-particle hybridization of optical transitions from zero-mode Landau levels in HgTe quantum wells

We present far-infrared magnetospectroscopy measurements of a HgTe quantum well in the inverted band structure regime over the temperature range of 2 to 60 K. The particularly low electron concentration enables us to probe the temperature evolution of all four possible optical transitions originating from zero-mode Landau levels, which are split off from the edges of the electron-like and hole-like bands. By analyzing their resonance energies, we reveal an unambiguous breakdown of the single-particle picture indicating that the explanation of the anticrossing of zero-mode Landau levels in terms of bulk and interface inversion asymmetries is insufficient. Instead, the observed behavior of the optical transitions is well explained by their hybridization driven by electron-electron interaction. We emphasize that our proposed many-particle mechanism is intrinsic to HgTe quantum wells of any crystallographic orientation, including (110) and (111) wells, where bulk and interface inversion asymmetries do not induce the anticrossing of zero-mode Landau levels.

cond-mat.mes-hall↗

arXiv2025

Quasiparticle picture of topological phase transitions induced by interactions

We present a general recipe to describe topological phase transitions in condensed matter systems with interactions. We show that topological invariants in the presence of interactions can be efficiently calculated by means of a non-Hermitian quasiparticle Hamiltonian introduced on the basis of the Green's function. As an example analytically illustrating the application of the quasiparticle concept, we consider a topological phase transition induced by the short-range electrostatic disorder in a two dimensional system described by the Bernevig-Hughes-Zhang model. The latter allows us to explicitly demonstrate the change in the $\mathbb{Z}_2$ topological invariant and explain the quantized values of the longitudinal conductance in a certain range of the Fermi energy and the disorder strength found previously in numerical calculations.

cond-mat.mtrl-sci↗

arXiv2025

Nonlinear Terahertz Electroluminescence from Dirac Landau Polaritons

We report Dirac Landau polaritons observed by terahertz (THz) magnetoreflectivity spectroscopy, demonstrating strong coupling between cyclotron transitions of two-dimensional (2D) Dirac fermions in HgTe quantum wells and optical cavity modes. Under pulsed electrical injection we observe efficient nonlinear electroluminescence, with a strongly out-of-equilibrium polariton distribution dominated by emission from the upper polariton branches. Model analysis of the bias-dependent emission intensity and spectral narrowing indicates a polariton occupancy per mode approaching unity, with a possible contribution from stimulated polariton emission in the spectral region of the upper anticrossing. These results open prospects toward Dirac Landau polariton condensates and low-threshold, tunable THz polariton lasers based on cyclotron emission.

cond-mat.mes-hall↗

arXiv2024

Spin-dependent photovoltage in graphene/MoS2-based field-effect transistors

It has recently been shown that Terahertz sensors can effectively detect the spin resonances of Dirac fermions in graphene. The associated photovoltaic measurement technique allows for the investigation of the intrinsic spin-orbit coupling in graphene as well as its topological properties from microwave to Terahertz frequencies. In this work, using graphene/MoS2-based Field-Effect Transistors, we observed a magnetic resonance photovoltage signal in the Gigahertz range that is independent of the gate bias. The dispersion of the associated spin-flip transitions remains intriguingly unaffected by the MoS2 layer. In parallel, the spin-related signal consistently appears as a drop in photovoltage, regardless of the signal's polarity or origin, whether it arises from plasma wave rectification or thermoelectric effects. This behavior is interpreted as a decrease in the system's spin polarization due to spin-dependent recombination or scattering of photocarriers. Understanding the various photovoltaic signals in highly sensitive Gigahertz/Terahertz sensors paves the way for exploring spin-dependent mechanisms in two-dimensional quantum materials, influenced by proximity effects such as spin-orbit coupling, topology, and magnetism.

cond-mat.mes-hall↗

arXiv2024

Multi-probe analysis to separate edge currents from bulk currents in quantum spin Hall insulators and to analyze their temperature dependence

We present a multi-probe transport analysis that effectively separates bulk and edge currents in large Hall bar devices with standard geometries. Applied to transport measurements on all possible four-probe configurations of six-probe Hall bar devices made of inverted three-layer InAs/GaInSb quantum wells (QWs), our analysis not only reveals the presence of dissipative edge currents in the topological gap, but also allows the temperature dependence of bulk and edge conductivity to be evaluated separately. The temperature dependence of the edge conductivity for Hall bar channels from 10 $\mu$m to 70~$\mu$m in the range of 1.5 K to 45 K is consistent with the theoretical expectation of weakly interacting helical edge electrons with backscattering due to localized magnetic moments of charge impurities. We argue that these charge impurities are naturally associated with intrinsic Ga-antisite defects, which act as double acceptors in InAs/Ga(In)Sb-based QWs.

cond-mat.mes-hall↗

arXiv2023

Disorder-induced phase transitions in double HgTe quantum wells

By using the self-consistent Born approximation, we investigate topological phase transitions in double HgTe quantum wells (QWs) induced by the short-range impurities. Following the evolution of the density-of-states and the spectral function, we demonstrate multiple closings and openings of the band-gap with the increase of the disorder strength due to the mutual inversions between the first and second electron-like and hole-like subbands. We show that starting from a band insulator in the clean limit, under the influence of disorder, the double HgTe QW undergoes a transition, first into a semimetal state similar to "bilayer graphene", and then into a high-order topological insulator state with a double band inversion. We find out that all disorder-induced transitions can be fully characterized by introducing a non-Hermitian quasiparticle Hamiltonian encoding the band structure renormalization and quasiparticle decay.

cond-mat.mes-hall↗

arXiv2023

Gate tunable terahertz cyclotron emission from two-dimensional Dirac fermions

Two-dimensional Dirac fermions in HgTe quantum wells close to the topological phase transition can generate significant cyclotron emission that is magnetic field tunable in the Terahertz (THz) frequency range. Due to their relativistic-like dynamics, their cyclotron mass is strongly dependent on their electron concentration in the quantum well, providing a second tunability lever and paving the way for a gate-tunable, permanent-magnet Landau laser. In this work, we demonstrate the proof-of-concept of such a back-gate tunable THz cyclotron emitter at fixed magnetic field. The emission frequency detected at 1.5 Tesla is centered on 2.2 THz and can already be electrically tuned over 250 GHz. With an optimized gate and a realistic permanent magnet of 1.0 Tesla, we estimate that the cyclotron emission could be continuously and rapidly tunable by the gate bias between 1 and 3 THz, that is to say on the less covered part of the THz gap.

cond-mat.mes-hall↗

arXiv2023

Terahertz cyclotron emission from two-dimensional Dirac fermions

Since the emergence of graphene, we have seen several proposals for the realization of Landau lasers tunable over the terahertz frequency range. The hope was that the non-equidistance of the Landau levels from Dirac fermions would suppress the harmful non-radiative Auger recombination. Unfortunately, even with this non-equidistance an unfavorable non-radiative process persists in Landau-quantized graphene, and so far no cyclotron emission from Dirac fermions has been reported. One way to eliminate this last non-radiative process is to sufficiently modify the dispersion of the Landau levels by opening a small gap in the linear band structure. A proven example of such gapped graphene-like materials are HgTe quantum wells close to the topological phase transition. In this work, we experimentally demonstrate Landau emission from Dirac fermions in such HgTe quantum wells, where the emission is tunable by both the magnetic field and the carrier concentration. Consequently, these results represent an advance in the realization of terahertz Landau lasers tunable by magnetic field and gate-voltage.

cond-mat.mes-hall↗

arXiv2022

Temperature Dependent Zero-Field Splittings in Graphene

Graphene is a quantum spin Hall insulator with a 45 $μ$eV wide non-trivial topological gap induced by the intrinsic spin-orbit coupling. Even though this zero-field spin splitting is weak, it makes graphene an attractive candidate for applications in quantum technologies, given the resulting long spin relaxation time. On the other side, the staggered sub-lattice potential, resulting from the coupling of graphene with its boron nitride substrate, compensates intrinsic spin-orbit coupling and decreases the non-trivial topological gap, which may lead to the phase transition into trivial band insulator state. In this work, we present extensive experimental studies of the zero-field splittings in monolayer and bilayer graphene in a temperature range 2K-12K by means of sub-Terahertz photoconductivity-based electron spin resonance technique. Surprisingly, we observe a decrease of the spin splittings with increasing temperature. We discuss the origin of this phenomenon by considering possible physical mechanisms likely to induce a temperature dependence of the spin-orbit coupling. These include the difference in the expansion coefficients between the graphene and the boron nitride substrate or the metal contacts, the electron-phonon interactions, and the presence of a magnetic order at low temperature. Our experimental observation expands knowledge about the non-trivial topological gap in graphene.

cond-mat.mtrl-sci↗

arXiv2022

Electrical control of excitons in GaN/(Al,Ga)N quantum wells

A giant built-in electric field in the growth direction makes excitons in wide GaN/(Al, Ga)N quantum wells spatially indirect even in the absence of any external bias. Significant densities of indirect excitons can accumulate in electrostatic traps imprinted in the quantum well plane by a thin metal layer deposited on top of the heterostructure. By jointly measuring spatially-resolved photoluminescence and photo-induced current, we demonstrate that exciton density in the trap can be controlled via an external electric bias, which is capable of altering the trap depth. Application of a negative bias deepens the trapping potential, but does not lead to any additional accumulation of excitons in the trap. This is due to exciton dissociation instigated by the lateral electric field at the electrode edges. The resulting carrier losses are detected as an increased photo-current and reduced photoluminescence intensity. By contrast, application of a positive bias washes out the electrode-induced trapping potential. Thus, excitons get released from the trap and recover free propagation in the plane that we reveal by spatially-resolved photoluminescence.

cond-mat.quant-gas↗

arXiv2022

Large inverted band-gap in strained three-layer InAs/GaInSb quantum wells

Quantum spin Hall insulators (QSHIs) based on HgTe and three-layer InAs/GaSb quantum wells (QWs) have comparable bulk band-gaps of about 10--18~meV. The former however features a band-gap vanishing with temperature, while the gap in InAs/GaSb QSHIs is rather temperature-independent.Here, we report on the realization of large inverted band-gap in strained three-layer InAs/GaInSb QWs. By temperature-dependent magnetotransport measurements of gated Hall bar devices, we extract a gap as high as 45 meV. Combining local and non-local measurements, we attribute the edge conductivity observed at temperatures up to 40 K to the topological edge channels with equilibration lengths of a few micrometers. Our findings pave the way toward manipulating edge transport at high temperatures in QW heterostructures.

cond-mat.mes-hall↗

arXiv2021

HgTe quantum wells for QHE metrology under soft cryomagnetic conditions: permanent magnets and liquid ${^4He}$ temperatures

HgTe quantum wells with a thickness of ${\sim}$7 nm may have a graphene-like band structure and have been recently proposed to be potential candidates for quantum Hall effect (QHE) resistance standards under the condition of operation in the fields above certain critical field $B_c$, above which the topological phase (with parasitic edge conduction) disappears. We present experimental studies of the magnetoresistance of different of HgTe quantum wells as a function temperature and magnetic field, determining the critical magnetic field $B_c$. We demonstrate that for QWs of specific width $B_c$ becomes low enough to grant observation of remarkably wide QHE plateaus at the filling factor ${v=-1}$ (holes) in relaxed cryomagnetic conditions: while using commercial 0.82 T Neodymium permanent magnets and temperature of a few Kelvin provided by ${^4He}$ liquid system only. Band structure calculations allow us to explain qualitatively observed phenomena due to the interplay between light holes and heavy holes energy sub-bands (side maxima of the valence band). Our work clearly shows that the peculiar band structure properties of HgTe QWs with massless Dirac fermions make them an ideal platform for developing metrological devices with relaxed cryomagnetic conditions.

cond-mat.mes-hall↗

arXiv2021

Orbital selective switching of ferromagnetism in an oxide quasi two-dimensional electron gas

Multi-orbital physics in quasi-two-dimensional electron gases (q2DEGs) triggers unique phenomena not observed in bulk materials, such as unconventional superconductivity and magnetism. Here, we investigate the mechanism of orbital selective switching of the spin-polarization in the oxide q2DEG formed at the (001) interface between the LaAlO$_{3}$, EuTiO$_{3}$ and SrTiO$_{3}$ band insulators. By using density functional theory calculations, transport, magnetic and x-ray spectroscopy measurements, we find that the filling of titanium-bands with 3d$_{xz,yz}$ orbital character in the EuTiO3 layer and at the interface with SrTiO$_{3}$ induces an antiferromagnetic to ferromagnetic switching of the exchange interaction between Eu-4f$^{7}$ magnetic moments. The results explain the observation of the carrier density dependent ferromagnetic correlations and anomalous Hall effect in this q2DEG, and demonstrate how combined theoretical and experimental approaches can lead to a deeper understanding of novel electronic phases and serve as a guide for the materials design for advanced electronic applications.

cond-mat.mtrl-sci↗

arXiv2020

Complexity of dipolar exciton Mott transition in GaN/(AlGa)N nanostructures

The Mott transition from a dipolar excitonic liquid to an electron-hole plasma is demonstrated in a wide GaN/(Al,Ga)N quantum well at $T=7$K by means of spatially-resolved magneto-photoluminescence spectroscopy. Increasing optical excitation density we drive the system from the excitonic state, characterized by a diamagnetic behavior and thus a quadratic energy dependence on the magnetic field, to the unbound electron-hole state, characterized by a linear shift of the emission energy with the magnetic field. The complexity of the system requires to take into account both the density-dependence of the exciton binding energy and the exciton-exciton interaction and correlation energy that are of the same order of magnitude. We estimate the carrier density at Mott transition as $n_\mathrm{Mott}\approx 2\times 10^{11}$cm$^{-2}$ and address the role played by excitonic correlations in this process. Our results strongly rely on the spatial resolution of the photoluminescence and the assessment of the carrier transport. We show, that in contrast to GaAs/(Al,Ga)As systems, where transport of dipolar magnetoexcitons is strongly quenched by the magnetic field due to exciton mass enhancement, in GaN/(Al,Ga)N the band parameters are such that the transport is preserved up to $9$T.

cond-mat.mes-hall↗

arXiv2020

Many-particle effects in optical transitions from zero-mode Landau levels in HgTe quantum wells

We report on the far-infrared magnetospectroscopy of HgTe quantum wells with inverted band ordering at different electron concentrations. We particularly focus on optical transitions from zero-mode Landau levels, which split from the edges of electron-like and hole-like bands. We observe a pronounced dependence of the transition energies on the electron concentration varied by persistent photoconductivity effect. This is striking evidence that in addition to the already well-documented crystalline and interface asymmetries, electron-electron interactions also have a significant impact on the usual behavior of the optical transitions from zero mode Landau levels.

cond-mat.mes-hall↗

arXiv2019

Thermal transport driven by charge imbalance in graphene in magnetic field, close to the charge neutrality point at low temperature: Non local resistance

Graphene grown epitaxially on SiC, close to the charge neutrality point (CNP), in an orthogonal magnetic field shows an ambipolar behavior of the transverse resistance accompanied by a puzzling longitudinal magnetoresistance. When injecting a transverse current at one end of the Hall bar, a sizeable non local transverse magnetoresistance is measured at low temperature. While Zeeman spin effect seems not to be able to justify these phenomena, some dissipation involving edge states at the boundaries could explain the order of magnitude of the non local transverse magnetoresistance, but not the asymmetry when the orientation of the orthogonal magnetic field is reversed. As a possible contribution to the explanation of the measured non local magnetoresistance which is odd in the magnetic field, we derive a hydrodynamic approach to transport in this system, which involves particle and hole Dirac carriers, in the form of charge and energy currents. We find that thermal diffusion can take place on a large distance scale, thanks to long recombination times, provided a non insulating bulk of the Hall bar is assumed, as recent models seem to suggest in order to explain the appearance of the longitudinal resistance. In presence of the local source, some leakage of carriers from the edges generates an imbalance of carriers of opposite sign, which are separated in space by the magnetic field and diffuse along the Hall bar generating a non local transverse voltage.

cond-mat.mes-hall↗

arXiv2019

Massless Dirac fermions in III-V semiconductor quantum wells

We report on the clear evidence of massless Dirac fermions in two-dimensional system based on III-V semiconductors. Using a gated Hall bar made on a three-layer InAs/GaSb/InAs quantum well, we restore the Landau levels fan chart by magnetotransport and unequivocally demonstrate a gapless state in our sample. Measurements of cyclotron resonance at different electron concentrations directly indicate a linear band crossing at the $Γ$ point of Brillouin zone. Analysis of experimental data within analytical Dirac-like Hamiltonian allows us not only determing velocity $v_F=1.8\cdot10^5$ m/s of massless Dirac fermions but also demonstrating significant non-linear dispersion at high energies.

cond-mat.mes-hall↗

arXiv2019

Dipolar excitons offer a rich playground for both design of novel optoelectronic devices and fundamental many-body physics

Dipolar excitons offer a rich playground for both design of novel optoelectronic devices and fundamental many-body physics. Wide GaN/(AlGa)N quantum wells host a new and promising realization of dipolar excitons. We demonstrate the in-plane confinement and cooling of these excitons, when trapped in the electrostatic potential created by semitransparent electrodes of various shapes deposited on the sample surface. This result is a prerequisite for the electrical control of the exciton densities and fluxes, as well for studies of the complex phase diagram of these dipolar bosons at low temperature.

physics.app-ph↗