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A key advantage of utilizing van der Waals materials as defect-hosting platforms for quantum applications is the controllable proximity of the defect to the surface or the substrate for improved light extraction, enhanced coupling with photonic elements, or more sensitive metrology. However, this aspect results in a significant challenge for defect identification and characterization, as the defect's optoelectronic properties depend on the specifics of the atomic environment. Here we explore the mechanisms by which the environment can influence the properties of carbon impurity centres in hexagonal boron nitride (hBN). We compare the optical and electronic properties of such defects between bulk-like and few-layer films, showing alteration of the zero-phonon line energies, modifications to their phonon sidebands, and enhancements of their inhomogeneous broadenings. To disentangle the various mechanisms responsible for these changes, including the atomic structure, electronic wavefunctions, and dielectric screening environment of the defect center, we combine ab-initio calculations based on a density-functional theory with a quantum embedding approach. By studying a variety of carbon-based defects embedded in monolayer and bulk hBN, we demonstrate that the dominant effect of the change in the environment is the screening of the density-density Coulomb interactions within and between the defect orbitals. Our comparative analysis of the experimental and theoretical findings paves the way for improved identification of defects in low-dimensional materials and the development of atomic scale sensors of dielectric environments., 38 pages, 7 figures

Quantifying and controlling the coherent dynamics and couplings of optically active excitations in solids is of paramount importance in fundamental research in condensed matter optics and for their prospective optoelectronic applications in quantum technologies. Here, we perform ultrafast coherent nonlinear spectroscopy of a charge-tunable MoSe$_2$ monolayer. The experiments show that the homogeneous and inhomogeneous line width and the population decay of exciton complexes hosted by this material can be directly tuned by an applied gate bias, which governs the Fermi level and therefore the free carrier density. By performing two-dimensional spectroscopy, we also show that the same bias-tuning approach permits us to control the coherent coupling strength between charged and neutral exciton complexes.

Since their theoretical prediction by Peierls in the 30s, charge density waves (CDW) have been one of the most commonly encountered electronic phases in low dimensional metallic systems. The instability mechanism originally proposed combines Fermi surface nesting and electron-phonon coupling but is, strictly speaking, only valid in one dimension. In higher dimensions, its relevance is questionable as sharp maxima in the static electronic susceptibility \chi(q) are smeared out, and are, in many cases, unable to account for the periodicity of the observed charge modulations. Here, we investigate the quasi twodimensional LaAgSb2, which exhibits two CDW transitions, by a combination of diffuse xray scattering, inelastic x-ray scattering and ab initio calculations. We demonstrate that the CDW formation is driven by phonons softening. The corresponding Kohn anomalies are visualized in 3D through the momentum distribution of the x-ray diffuse scattering intensity. We show that they can be quantitatively accounted for by considering the electronic susceptibility calculated from a Dirac-like band, weighted by anisotropic electron-phonon coupling. This remarkable agreement sheds new light on the importance of Fermi surface nesting in CDW formation., Comment: 20 pages, 9 figures, submitted to Phys. Rev. Rep

Since their theoretical prediction by Peierls in the 1930s, charge density waves (CDWs have been one of the most commonly encountered electronic phases in low-dimensional metallic systems. The instability mechanism originally proposed combines Fermi surface nesting and electron-phonon coupling but is, strictly speaking, only valid in one dimension. In higher dimensions, its relevance is questionable as sharp maxima in the static electronic susceptibility χ(q) are smeared out, and is, in many cases, unable to account for the periodicity of the observed charge modulations. Here, we investigate the quasi-two-dimensional LaAgSb2, which exhibits two CDW transitions, by a combination of diffuse x-ray scattering, inelastic x-ray scattering, and ab initio calculations. We demonstrate that the CDW formation is driven by phonon softening. The corresponding Kohn anomalies are visualized in three dimensions through the momentum distribution of the x-ray diffuse scattering intensity. We show that they can be quantitatively accounted for by considering the electronic susceptibility calculated from a Dirac-like band, weighted by anisotropic electron-phonon coupling. This remarkable agreement sheds new light on the importance of Fermi surface nesting in CDW formation.

International audience; We perform coherent nonlinear spectroscopy of excitons in single-layers of MoSe2 and WSe2 encapsulated between thin films of hexagonal boron nitride. Employing four-wave mixing microscopy we identify virtually disorder free areas, generating exciton optical response at the homogeneous limit. Focussing on such areas, we measure exciton homogeneous broadening as a function of environmental factors, namely temperature and exciton density. Exploiting FWM imaging, we find that at such locations, nonlinear absorption of the exciton excited states and their coherent couplings can be observed. Using the WSe2 heterostructure, we infer coherence and density dynamics of the exciton 2S state. Owing to its increased radiative lifetime, at low temperatures, the dephasing of the 2S state is longer than of the 1S transition. While scanning various heterostructures across tens of micrometers, we conclude that the disorder, principally induced by strain variations, remain to be present creating spatially varying inhomogeneous broadening.

We perform coherent nonlinear spectroscopy of excitons in single-layers of MoSe$_2$ and WSe$_2$ encapsulated between thin films of hexagonal boron nitride. Employing four-wave mixing microscopy we identify virtually disorder free areas, generating exciton optical response at the homogeneous limit. Focussing on such areas, we measure exciton homogeneous broadening as a function of environmental factors, namely temperature and exciton density. Exploiting FWM imaging, we find that at such locations, nonlinear absorption of the exciton excited states and their coherent couplings can be observed. Using the WSe$_2$ heterostructure, we infer coherence and density dynamics of the exciton 2S state. Owing to its increased radiative lifetime, at low temperatures, the dephasing of the 2S state is longer than of the 1S transition. While scanning various heterostructures across tens of micrometers, we conclude that the disorder, principally induced by strain variations, remain to be present creating spatially varying inhomogeneous broadening.

We investigate dynamics of resonantly excited excitons in single-layers of MoSe$_2$ and WS$_2$ down to 4.5 K. To this end, we measure the delay dependence of the heterodyne four-wave mixing (FWM) amplitude induced by three, short laser pulses. This signal depends not only on the population of optically active excitons, which affects the absorption of the probe, but also on the population of optically inactive states, by interaction-induced energy shift, influencing the refractive index experienced by the probe. As such, it offers insight into density dynamics of excitons which do not directly couple to photons. Reproducing the coherent signal detected in amplitude and phase, the FWM delay dependence is modeled by a coherent superposition of several exponential decay components, with characteristic time constants from 0.1 picosecond up to 1 nanosecond. With increasing excitation intensity and/or temperature, we observe strong interference effects in the FWM field amplitude, resulting in progressively more complex and nonintuitive signal dynamics. We attribute this behaviour to increasingly populated exciton dark states, which change the FWM field phase by the relative effect on absorption and refractive index. We observe that exciton recombination occurs on a significantly longer timescale in WS$_2$ with respect to MoSe$_2$, which is attributed to the dark character of exciton ground state in the former and the bright in the latter.

At cryogenic temperatures, the photoluminescence spectrum of monolayer WSe2features a num-ber of lines related to the recombination of so-called localized excitons. The intensity of these lines strongly decreases with increasing temperature. In order to understand the mechanism behind this phenomenon, we carried out a time-resolved experiment, which revealed a similar trend in the photoluminescence decay time. Our results identify the opening of additional nonradiative relaxation channels as a primary cause of the observed temperature quenching of the localized excitons' photoluminescence.

We review the different results obtained in the last decade in the field of Raman scattering of graphene based systems, with an applied magnetic field. Electronic properties of graphene based systems with an applied magnetic field will first be described. The phonon response in magnetic field, the magneto-phono resonance, will then be introduced and described in different systems, including graphene, multilayer graphene and bulk graphite. Electronic Raman scattering is then be discussed in the context of Landau level spectroscopy, of electron phonon interaction and of electron-electron interaction.

International audience

Resonant Raman scattering is investigated in monolayer WS2 at low temperature with the aid of an unconventional technique, i.e., Raman scattering excitation (RSE) spectroscopy. The RSE spectrum is made up by sweeping the excitation energy, when the detection energy is fixed in resonance with excitonic transitions related to either neutral or charged excitons. We demonstrate that the shape of the RSE spectrum strongly depends on the selected detection energy. The resonance of outgoing light with the neutral exciton leads to an extremely rich RSE spectrum, which displays several Raman scattering features not reported so far, while no clear effect on the associated background photoluminescence is observed. Instead, when the outgoing photons resonate with the negatively charged exciton, a strong enhancement of the related emission occurs. Presented results show that the RSE spectroscopy can be a useful technique to study electron-phonon interactions in thin layers of transition metal dichalcogenides.

Resonant Raman scattering is investigated in monolayer WS$_2$ at low temperature with the aid of an unconventional spectroscopy technique, $i.e.$, Raman scattering excitation (RSE). The RSE spectrum is made up by sweeping the excitation energy, when the detection energy is fixed in resonance with excitonic transitions related to neutral and/or charged excitons. We demonstrate that the shape of the RSE spectrum strongly depends on a selected detection energy. The out-going resonance with the neutral exciton leads to an extremely rich RSE spectrum displaying several Raman scattering features not reported so far, while no clear effect on the associated background photoluminescence is observed. Instead, a strong enhancement of the emission due to the negatively charged exciton is apparent when the out-going photons resonate with this exciton. Presented results show that the RSE spectroscopy can be a useful technique to study electron-phonon interactions in thin layers of transition metal dichalcogenides., Comment: 9 pages, 5 figures

The crystallographic stacking order in multilayer graphene plays an important role in determining its electronic properties. It has been predicted that a rhombohedral (ABC) stacking displays a conducting surface state with flat electronic dispersion. In such a flat band, the role of electron-electron correlation is enhanced possibly resulting in high Tc superconductivity, charge density wave or magnetic orders. Clean experimental band structure measurements of ABC stacked specimens are missing because the samples are usually too small in size. Here, we directly image the band structure of large multilayer graphene flake containing approximately 14 consecutive ABC layers. Angle-resolved photoemission spectroscopy experiments reveal the flat electronic bands near the K point extends by 0.13 {\AA}-1 at the Fermi level at liquid nitrogen temperature. First-principle calculations identify the electronic ground state as an antiferromagnetic state with a band gap of about 40 meV.

By implementing four-wave mixing (FWM) micro-spectroscopy we measure coherence and population dynamics of the exciton transitions in monolayers of MoSe$_2$. We reveal their dephasing times T$_2$ and radiative lifetime T$_1$ in a sub-picosecond (ps) range, approaching T$_2$=2T$_1$, and thus indicating radiatively limited dephasing at a temperature of 6$\,$K. We elucidate the dephasing mechanisms by varying the temperature and by probing various locations on the flake exhibiting a different local disorder. At a nanosecond range, we observe the residual FWM produced by the incoherent excitons, which initially disperse towards the dark states, but then relax back to the optically active states within the light cone. By introducing polarization-resolved excitation, we infer inter-valley exciton dynamics, showing an initial polarization degree of around 30$\,\%$, constant during the initial sub-picosecond decay, followed by the depolarization on a picosecond timescale. The FWM hyperspectral imaging reveals the doped and undoped areas of the sample, allowing to investigate the neutral exciton, the charged one or both transitions at the same time. In the latter case, we observe the exciton-trion beating in the coherence evolution indicating their coherent coupling.

We study the carrier dynamics in epitaxially grown graphene in the range of photon energies from 10 - 250 meV. The experiments complemented by microscopic modeling reveal that the carrier relaxation is significantly slowed down as the photon energy is tuned to values below the optical phonon frequency, however, owing to the presence of hot carriers, optical phonon emission is still the predominant relaxation process. For photon energies about twice the value of the Fermi energy, a transition from pump-induced transmission to pump-induced absorption occurs due to the interplay of interband and intraband processes.

International audience; Resonant photoluminescence experiments performed on self-assembled InAs/GaAs quantum dots under strong magnetic field up to 28 T give rise to an accurate determination of the interband magneto-optical transitions. As this technique circumvents the homogeneous broadening of the transitions due to the size and composition fluctuations of the dots, the experimental spectra display well-defined peaks. A good agreement is found between the experimental data and calculations using an effective mass model including the coupling between the mixed exciton-LO phonon states. Transitions involving excitonic polarons are clearly identified. Moreover, a light-hole to conduction transition is also evidenced in agreement with previous theoritical predictions.

International audience; We present photoluminescence measurements under strong magnetic field done on a sample with charged (n-doped) quantum dots. We have shown that the broadening of the luminescence line does not give a measure of the QDs size dispersion, and that the coupling of electron/hole pairs with LO phonons is greatly enhanced, because of the presence of the ionised impurities nearby the charged dots.

International audience; It has been known for almost 25 years that high temperature treatment of polar faces of SiC crystals lead to the graphitization of the surface as a consequence of Si preferential sublimation. The group of Walt de Heer in Atlanta has proposed to use this procedure to obtain macroscopic multilayer graphene samples. This has lead to a renewal of interest for he study of the graphitic layers grown on the SiC surfaces. Due to the polar nature of the material, the 6H(4H)-SiC substrate present two non-equivalent surfaces: the (0001) is known as the Si face, and the (000-1) one is the C face. It turns out that the structure and the properties of the multilayer graphene samples obtained on those two surfaces are rather different. In particular the multilayer graphene samples grown on the C-terminated face presents electronic properties that are rather similar to those of monolayer graphene with very high mobility. This is shown by transport and optical experiments under high magnetic field. This surprising result is explained by the rotational stacking sequence, observed experimentally. which leads to an electronic decoupling as demonstrated by ab-initio band structure calculations.

The crystallographic stacking order in multilayer graphene plays an important role in determining its electronic properties. It has been predicted that a rhombohedral (ABC) stacking displays a conducting surface state with flat electronic dispersion. In such a flat band, the role of electron-electron correlation is enhanced, possibly resulting in high ${T}_{\mathrm{c}}$ superconductivity, charge-density wave, or magnetic orders. Clean experimental band-structure measurements of ABC-stacked specimens are missing because the samples are usually too small in size. Here, we directly image the band structure of large multilayer graphene flakes containing approximately 14 consecutive ABC layers. Angle-resolved photoemission spectroscopy experiments reveal the flat electronic bands near the $K$ point extends by $0.13\phantom{\rule{0.16em}{0ex}}{\AA{}}^{\ensuremath{-}1}$ at the Fermi level at liquid nitrogen temperature. First-principles calculations identify the electronic ground state as an antiferromagnetic state with a band gap of about 40 meV.

International audience

Bilayer quantum Hall systems at v_T = 1 support an excitonic ground state. In addition to the usual charged quasiparticles, this system possesses a condensate degree of freedom: exciton transport. Detection of this neutral transport mode is facilitated by the use of the Corbino multiply-connected geometry in which charge transport is suppressed. We here summarize our recent experiments on Corbino devices which directly demonstrate exciton transport across the bulk of the incompressible v_T = 1 quantum Hall state.

Studies of properties of a two-dimensional electron gas in magnetic fields by optical spectroscopy is presented: fractional quantum Hall effect (FQHE) studied by photoluminescence and inelastic light scattering in high fields, and a recently discovered effect of microwave induced resistance oscillations (MIROs) by transport and absorption measurements in low fields. Until now, series of FQHE states, arising due to electron interaction, were not observed in photoluminescence. This work reports on optical detection of a whole family of 1/2 fractional states observed in photoluminescence. The features reflect pattern analogous to the one observed by the magneto--resistivity traces. The strongest features observed even in raw data include fractions 1/3, 2/5, 3/7, 3/5, 2/3 , and 1 on either side of the 1/2 state. The last chapter describes the studies of the new effect of microwave resistance induced oscillations (MIROs). When irradiated with microwaves, the behaviour of 2DEG transport properties deviates from the Shubnikov -- de Haas oscillations. The resistance traces exhibit series of vanishing resistance states. The frequency dependence of transport data shows a gradual changeover from transport traces reflecting MIROs in low microwave frequencies, to traces showing nearly the cyclotron resonance only. The results of the absorption measurements validate description of the MIROs as the result of oscillatory density of states. Also, the existence of two separate absorption processes are suggested to explain the microwave polarisation independence of MIROs, and the crossover from MIROs-like transport data at low frequencies to cyclotron--like traces at higher frequencies., Les propriétés d'un gaz électronique bidimensionnel soumis à champs magnétiques intenses et à bas champs magnétiques sont étudiés par la spectroscopie optique: l'effet Hall quantique fractionnaire (FQHE) par photoluminescence et diffusion inélastique de la lumière, puis un nouvel effet oscillatoire de la résistance induit par micro-ondes (MIROs) par transport et absorption des micro-ondes. Les effets des interactions entre électrons du 2DEG sont à l'origine de FQHE. Jusqu'à maintenant, les expériences d'optiques n'ont pas permis les études des interactions entre électrons sur toute la gamme de fractions. Les fractions 1/3, 2/5, 3/7, 3/5, 2/3 et 1 sont clairement observées dans les spectres non traités et montrent une symétrie autour du facteur de remplissage 1/2. La symétrie des fermions composites ets observée dans les spectres. A bas champ magnétique, sous irradiation micro-onde, les propriétés de transport s'écartent nettement des oscillations bien connues de Shubnikov - de Haas pour évoluer vers une série d'états de résistance zéro. Les résultats des mesures d'absorption des micro-ondes sont présentés pour deux échantillons. L'échantillon de basse mobilité montre seulement une absorption autour de la résonance cyclotron (CR). L' échantillon de haute mobilité montre aussi des signaux d'absorption aux harmoniques de la CR. Les mesures ont permis d'inférer l'existence de deux processus d'absorption différents et séparés. L'absorption non résonante est mieux visible en transport et observée comme MIROs, et l'absorption résonante, mieux observée dans les mesures d'absorption, suit probablement les règles de polarisation de résonance cyclotron.

Studies of properties of a two-dimensional electron gas in magnetic fields by optical spectroscopy is presented: fractional quantum Hall effect (FQHE) studied by photoluminescence and inelastic light scattering in high fields, and a recently discovered effect of microwave induced resistance oscillations (MIROs) by transport and absorption measurements in low fields. Until now, series of FQHE states, arising due to electron interaction, were not observed in photoluminescence. This work reports on optical detection of a whole family of 1/2 fractional states observed in photoluminescence. The features reflect pattern analogous to the one observed by the magneto--resistivity traces. The strongest features observed even in raw data include fractions 1/3, 2/5, 3/7, 3/5, 2/3 , and 1 on either side of the 1/2 state. The last chapter describes the studies of the new effect of microwave resistance induced oscillations (MIROs). When irradiated with microwaves, the behaviour of 2DEG transport properties deviates from the Shubnikov -- de Haas oscillations. The resistance traces exhibit series of vanishing resistance states. The frequency dependence of transport data shows a gradual changeover from transport traces reflecting MIROs in low microwave frequencies, to traces showing nearly the cyclotron resonance only. The results of the absorption measurements validate description of the MIROs as the result of oscillatory density of states. Also, the existence of two separate absorption processes are suggested to explain the microwave polarisation independence of MIROs, and the crossover from MIROs-like transport data at low frequencies to cyclotron--like traces at higher frequencies.; Les propriétés d'un gaz électronique bidimensionnel soumis à champs magnétiques intenses et à bas champs magnétiques sont étudiés par la spectroscopie optique: l'effet Hall quantique fractionnaire (FQHE) par photoluminescence et diffusion inélastique de la lumière, puis un nouvel effet oscillatoire de la résistance induit par micro-ondes (MIROs) par transport et absorption des micro-ondes. Les effets des interactions entre électrons du 2DEG sont à l'origine de FQHE. Jusqu'à maintenant, les expériences d'optiques n'ont pas permis les études des interactions entre électrons sur toute la gamme de fractions. Les fractions 1/3, 2/5, 3/7, 3/5, 2/3 et 1 sont clairement observées dans les spectres non traités et montrent une symétrie autour du facteur de remplissage 1/2. La symétrie des fermions composites ets observée dans les spectres. A bas champ magnétique, sous irradiation micro-onde, les propriétés de transport s'écartent nettement des oscillations bien connues de Shubnikov - de Haas pour évoluer vers une série d'états de résistance zéro. Les résultats des mesures d'absorption des micro-ondes sont présentés pour deux échantillons. L'échantillon de basse mobilité montre seulement une absorption autour de la résonance cyclotron (CR). L' échantillon de haute mobilité montre aussi des signaux d'absorption aux harmoniques de la CR. Les mesures ont permis d'inférer l'existence de deux processus d'absorption différents et séparés. L'absorption non résonante est mieux visible en transport et observée comme MIROs, et l'absorption résonante, mieux observée dans les mesures d'absorption, suit probablement les règles de polarisation de résonance cyclotron.