Search

The quantum Hall effect is the seminal example of topological protection, as charge carriers are transmitted through one-dimensional edge channels where backscattering is prohibited. Graphene has made its marks as an exceptional platform to reveal new facets of this remarkable property. However, in conventional Hall bar geometries, topological protection of graphene edge channels is found regrettably less robust than in high mobility semi-conductors. Here, we explore graphene quantum Hall regime at the local scale, using a scanning gate microscope. We reveal the detrimental influence of antidots along the graphene edges, mediating backscattering towards upstream edge channels, hence triggering topological breakdown. Combined with simulations, our experimental results provide further insights into graphene quantum Hall channels vulnerability. In turn, this may ease future developments towards precise manipulation of topologically protected edge channels hosted in various types of two-dimensional crystals., 6 pages, 4 figures

We use scanning gate microscopy to study electron transport through an open, gate-defined resonator in a Ga(Al)As heterostructure. Raster-scanning the voltage-biased metallic tip above the resonator, we observe distinct conductance modulations as a function of the tip position and voltage. Quantum-mechanical simulations reproduce these conductance modulations and reveal their relation to the partial local density of states in the resonator. Our measurements illustrate the current frontier between possibilities and limitations in imaging the local density of states in buried electron systems using scanning gate microscopy., Physical Review Research, 3 (3), ISSN:2643-1564

Observations of electron-hole asymmetry in transport through graphene devices at high magnetic field challenge prevalent models of the graphene quantum Hall effect. Here, we study this asymmetry both in conventional magnetotransport and in scanning gate microscopy maps measured in an encapsulated graphene constriction. We reveal that the presence of upstream modes and local doping in the vicinity of electrical contacts leads to a totally different picture of topological breakdown for electrons and holes, explaining the observed asymmetry., 5 pages, 5 figures

The manuscript describes my research activities on quantum transport in electronic devices using an experimental technique called Scanning Gate Microscopy (SGM). This technique provides spatial information on electron density and electron flow in buried two-dimensional electron gases (2DEG) or to modify these quantities at specific positions in the 2DEG. We investigate III/V semiconductor heterostructures, such as GaAs/AlGaAs or InGaAs/InAlAs, which host 2DEGs with high electron mobility. At low temperature, the elastic mean free path is several microns and the electron transport is fully ballistic in nano-scale devices patterned by electron beam lithography. Since the transport is also phase coherent and the electron wavelength is large, quantum effects are observable and can be studied in various nanostructures forming constrictions, rings, cavities, etc... These effects are usually probed by transport or optical spectroscopy which give mainly access to the energy levels. The spatial distribution of the quantum states, however, cannot be observed directly in these buried nanostructures, as opposed to surface electronic systems where scanning tunneling microscopy can measure the local density of states. The aim of the SGM technique is thus to probe indirectly this spatial distribution by recording the effect of a local electrostatic perturbation on the transport properties of the nanostructure. The sharp tip of an atomic force microscope is used as a local gate which is scanned above the surface. The voltage applied on the tip with respect to the 2DEG induces a local change of the electrostatic potential in the 2DEG, affecting the electron states, and thus the device conductance if these states are involved in the conduction path. With this technique, the dream would be to image directly the wave function in any kind of nanostructure, but the resolution is unfortunately limited by the relatively large distance between the tip and the 2DEG, which is buried several tens of nanometers below the surface. On the other hand, the SGM technique provides very detailed images of conductance changes, with very fine features, because the quantum states are very sensitive to the potential landscape and thus to very small displacements of the tip.In my habilitation thesis, I first describe in details the SGM technique, then review the various experiments that we have done, and finally present a few research projects that are currently developed, or will be studied in the future, in particular on the Kondo effect in quantum dots and on the negative refraction in graphene.

We demonstrate the emergence of classical features in electronic quantum transport for the scanning gate microscopy response in a cavity defined by a quantum point contact and a micron-sized circular reflector. The branches in electronic flow characteristic of a quantum point contact opening on a two-dimensional electron gas with weak disorder are folded by the reflector, yielding a complex spatial pattern. Considering the deflection of classical trajectories by the scanning gate tip allows to establish simple relationships of the scanning pattern, which are in agreement with recent experimental findings., Comment: 20 pages, 13 figures, revised version, submission to SciPost

We propose a spin-sensitive scanning probe microscopy experiment on double quantum dots in Pauli blockade conditions. Electric spin resonance is induced by an AC voltage applied to the scanning gate which causes lifting of the Pauli blockade of the current. The stationary Hamiltonian eigenstates are used as a basis for description of the spin dynamics with the AC potential of the probe. For the two-electron system we evaluate the transition rates from triplet T+ state to singlet S or triplet T0 states, i.e. to conditions in which the Pauli blockade of the current is lifted. The rates of the spin-flip transitions are consistent with the transition matrix elements and strongly dependent on the tip position. Probing the spin densities and identification of the final transition state are discussed.

We report, for the first time, the direct observation of in-plane (lateral) homojunctions in vertical van der Waals heterostructures and their dominant effect on charge transport properties. Particularly, with an all-van der Waals metal–semiconductor vertical Schottky junction (MoS2/β-In2Se3) as a model system, we use scanning gate microscopy to directly visualize the in-plane homojunction in the MoS2 channel, the formation of which is a result of charge transfer between MoS2 and β-In2Se3, as supported by numerical simulations and electrical transport measurements. Importantly, we find that this gate-tunable in-plane junction controls the vertical Schottky junction characteristics under the forward-bias condition. This is in contrast to the common assumption that the charge transport across the vertical heterojunction interface is the dominant process. Our findings are universally relevant in vertical van der Waals heterostructures where the charge transfer occurs across the interface, providing new insig...

Scanning tunneling microscopy (STM) is an excellent technique to image the surfaces of materials with extreme spatial resolution. However, it is difficult to maintain its imaging quality when applying the technique under the conditions used in many practical processes, such as chemical vapor deposition and catalysis. In this article, we describe two special classes of STM instruments that are capable of maintaining good imaging quality under “difficult” conditions, namely, one for high and variable temperatures and the other for the combination of high temperatures and high gas pressures. In both cases, we discuss the special design features that make these instruments robust with respect to the challenging imaging conditions and provide examples to illustrate how they are applied.

When coherent charge carriers cross micron-scale cavities, their dynamics can be governed by a few resonant states, also called “quantum scars”, determined by the cavity geometry. Quantum scars can be described using theoretical tools but have also been directly imaged in the case of high-quality semiconductor cavities as well as in disordered graphene devices, thanks to scanning gate microscopy (SGM). Here, we discuss spatially resolved SGM images of low-temperature charge transport through a mesoscopic ring fabricated from high-quality monolayer graphene lying on top of hexagonal boron nitride. SGM images are decorated with a pattern of radial scars in the ring area, which is found to evolve smoothly and reappear when varying the charge-carrier energy. The energies separating recurrent patterns are found to be directly related to geometric dimensions of the ring. Moreover, a recurrence is also observed in simulations of the local density of states of a model graphene quantum ring. The observed recurrences are discussed in the light of recent predictions of relativistic quantum scars in mesoscopic graphene cavities.

Scanning probe microscopy (SPM) techniques have been developed and deployed to delineate the internal characteristics of electronic and photonic devices such as doping profiles, electric potential profile, electric field profile, and charge carrier profile, as they play a crucial role in the function and performance of biased and unbiased devices. Two of the most promising techniques, scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM), are now commercially available for two-dimensional (2-D) dopant profiling by the microelectronic and optoelectronic industries, but the techniques delineate only devices at equilibrium. Therefore, to directly observe the internal behavior of operating quantum optoelectronic devices, a new SPM technique, scanning voltage microscopy (SVM), has been developed.

Both Kelvin Probe Force Microscopy and Scanning Electron Microscopy enable assessment of the effect of electrical bias on the surface potential of the layers of a solar cell. We report on a comprehensive comparison of surface potential measurements on an interdigitated back contact solar cell using these two techniques. Measurements under different values of electrical biases are performed on and between the metallic contacts. They show a good agreement between the surface potential obtained with Kelvin Probe Force Microscopy and the Scanning Electron Microscopy signal. In order to provide an accurate comparison, the scanned areas are adjacent to each other and accurate repositioning is achieved thanks to a nano-indentation between the contacts. We show that measurements under reverse bias are of interest to locate nano-defects and measurements under forward bias are relevant to identify local series resistance issues. We suggest that a setup combining Scanning Electron Microscopy and Kelvin Probe Force Microscopy under different values of the electrical bias should be valuable since the former is a high throughput technique enabling measurements on large scan areas, while the latter is a quantitative, low noise, and unintrusive local technique.

This Minireview focuses exclusively on work with scanning tunneling microscopy to study the self-assembled multilayer films (SAMTs) of organic molecules. The π-conjugated organic molecules form different structures within different monolayers on various substrates. The interplay between molecule-substrate and intermolecular interactions plays a key role in determining the stacking mode of organic multilayer films. Different substrates strongly influence the organic-film growth and electronic properties of the organic molecules. Geometric and electronic structures of SAMTs are important factors that may determine device performance. In addition to the inorganic interface, this Minireview addresses the organic-organic interface. Homo- and hetero-SAMTs of organic molecules are also considered. The subtle interplay between structural and electronic characteristics, on one hand, and functionality and reactivity, on the other hand, are highlighted.

openaire: EC/H2020/785219/EU//GrapheneCore2 We present scanning gate microscopy (SGM) studies of graphene Hall-cross devices where bi-layer graphene (2LG) regions show unexpected signal inversion relative to single-layer graphene (1LG), an observation reproduced via finite element modelling of the current densities. This is attributed to gate-induced charge carrier redistribution between the two layers in 2LG. Hall cross devices were fabricated from epitaxial graphene 6H-SiC(0001) and were covered by 1LG/2LG with the area ratio of 85: 15%, respectively. Local electric-field sensitivity maps of the devices were obtained in two different measurement geometries using electrical SGM with a conductive tip, where it was observed that the voltage of 2LG islands was inverted relative to anticipated reference maps. Finite element modelling of the current densities and voltage response showed good qualitative agreement with the SGM maps when the effect of the gate was reversed for 2LG. The behaviour is attributed to gate-induced charge carrier redistribution between the two layers in 2LG. The model can be used generally as a tool to predict mixed 1LG/2LG response to electric field. Moreover, regions near the corners of the device show the highest sensitivity when the local electric field was applied to the scanning probe microscopy tip. These regions are capable of detecting highly local electric fields down to 110 kV cm(-1).

The branched pattern typically observed through the scanning gate microscopy (SGM) of two dimensional electron gases in the presence of weak, smooth disorder has recently been found to be robust against a very large shift in the Fermi energy of the electron gas. We propose a toy model, where the potential landscape reduces to a single localized feature, that makes it possible to recast the understanding of branch formation through the effect of caustics in an appropriate set of classical trajectories, and it is simple enough to allow for a quantitative analysis of the energy and spatial dependence of the branches. We find the energy stability to be extremely generic, as it rests only upon the assumptions of weak disorder, weak scattering, and the proportionality of the SGM response to the density of classical electron trajectories. Therefore, the robustness against changes of the electron's Fermi energy remains when adopting progressively realistic models of smooth disorder., Comment: 18 pages, 18 figures. Version 2 contains significant revisions with respect to version 1, in accordance with referee suggestions

With the advent of high mobility encapsulated graphene devices, new electronic components ruled by Dirac fermions optics have been envisioned and realized. The main building blocks of electron-optics devices are gate-defined p-n junctions, which guide, transmit and refract graphene charge carriers, just like prisms and lenses in optics. The reflection and transmission are governed by the p-n junction smoothness, a parameter difficult to tune in conventional devices. Here we create p-n junctions in graphene, using the polarized tip of a scanning gate microscope, yielding Fabry-P\'erot interference fringes in the device resistance. We control the p-n junctions smoothness using the tip-to-graphene distance, and show increased interference contrast using smoother potential barriers. Extensive tight-binding simulation reveal that smooth potential barriers induce a pronounced quasi-confinement of Dirac fermions below the tip, yielding enhanced interference contrast. On the opposite, sharp barriers are excellent Dirac fermions transmitters and lead to poorly contrasted interferences. Our work emphasizes the importance of junction smoothness for relativistic electron optics devices engineering.

Topology is a powerful recent concept asserting that quantum states could be globally protected against local perturbations. Dissipationless topologically protected states are thus of major fundamental interest as well as of practical importance in metrology and quantum information technology. Although topological protection can be robust theoretically, in realistic devices it is often fragile against various dissipative mechanisms, which are difficult to probe directly because of their microscopic origins. By utilizing scanning nanothermometry, we visualize and investigate microscopic mechanisms undermining the apparent topological protection in the quantum Hall state in graphene. Our simultaneous nanoscale thermal and scanning gate microscopy shows that the dissipation is governed by crosstalk between counterpropagating pairs of downstream and upstream channels that appear at graphene boundaries because of edge reconstruction. Instead of local Joule heating, however, the dissipation mechanism comprises two distinct and spatially separated processes. The work generating process that we image directly and which involves elastic tunneling of charge carriers between the quantum channels, determines the transport properties but does not generate local heat. The independently visualized heat and entropy generation process, in contrast, occurs nonlocally upon inelastic resonant scattering off single atomic defects at graphene edges, while not affecting the transport. Our findings offer a crucial insight into the mechanisms concealing the true topological protection and suggest venues for engineering more robust quantum states for device applications., Comment: Main text: 12 pages, 3 figures. Supplementary information: 19 pages, 9 figures

Hydrodynamic flow in two-dimensional electron systems has so far been probed only by dc transport and scanning gate microscopy measurements. In this work we discuss theoretically signatures of the hydrodynamic regime in near-field optical microscopy. We analyze the dispersion of acoustic plasmon modes in two-dimensional electron liquids using a non-local conductivity that takes into account the effects of (momentum-conserving) electron-electron collisions, (momentum-relaxing) electron-phonon and electron-impurity collisions, and many-body interactions beyond the celebrated Random Phase Approximation. We derive the dispersion and, most importantly, the damping of acoustic plasmon modes and their coupling to a near-field probe, identifying key experimental signatures of the crossover between collisionless and hydrodynamic regimes., Comment: 14 pages, 4 figures

A counter-intuitive behavior analogous to the Braess paradox is encountered in a two-terminal mesoscopic network patterned in a two-dimensional electron system (2DES). Decreasing locally the electron density of one channel of the network paradoxically leads to an increased network electrical conductance. Our low temperature scanning gate microscopy experiments reveal different occurrences of such puzzling conductance variations, thanks to tip-induced localized modifications of electron flow throughout the network’s channels in the ballistic and coherent regime of transport. The robustness of the puzzling behavior is inspected by varying the global 2DES density, magnetic field and the tip-surface distance. Depending on the overall 2DES density, we show that either Coulomb Blockade resonances due to disorder-induced localized states or Fabry-Perot interferences tuned by the tip-induced electrostatic perturbation are at the origin of transport inefficiencies in the network, which are lifted when gradually closing one channel of the network with the tip.

We present a perturbative approach to the conductance change caused by a weakly invasive scattering potential in a two-dimensional electron gas. The resulting expressions are used to investigate the relationship between the conductance change measured in scanning gate microscopy as a function of the position of a scattering tip and local electronic quantities like the current density. We use a semiclassical approach to treat the case of a strong hard-wall scatterer in a half-plane facing a reflectionless channel. The resulting conductance change is consistent with the numerically calculated quantum conductance.

A novel nanosensor is demonstrated based on domain-wall (DW) nucleation in perpendicular-anisotropy CoFeB/Pt components, with an electrical read-out based on the anomalous Hall effect. Notch-shaped constrictions within perpendicular CoFeB/Pt wires are used to control the local anisotropy. The successful detection of a single NdFeB microbead is demonstrated, based on the variations in DW nucleation in a notched nanowire in the presence of the bead. Maps of the sensor response obtained using scanning gate microscopy measurements are presented. We describe a number of potential advantages of the nucleation-sensing technique as compared with existing DW-based magnetic bead sensors using in-plane magnetized materials.