RaphaëlLe Delagrange

The topology of WTe 2 , a transition-metal dichalcogenide with large spin-orbit interactions, is thought to combine type II Weyl semimetal and second-order topological insulator (SOTI) character. The SOTI character should endow WTe 2 multilayer crystals with topologically protected helical states at their hinges, and, indeed, one-dimensional states have been detected thanks to Josephson interferometry. However, the immunity to backscattering conferred to those states by their helical nature has so far not been tested. To probe the topological protection of WTe 2 edge states, we have fabricated superconducting quantum interference devices (SQUIDs) in which the supercurrent through a junction on the crystal edge interferes with the supercurrent through a junction in the bulk of the crystal. We find behaviors ranging from a symmetric SQUID to asymmetric SQUID patterns, including one in which the modulation by magnetic field reveals a sawtoothlike supercurrent versus phase relation for the edge junction, demonstrating that the supercurrent at the edge is carried by ballistic channels over 600 nm, a telltale sign of the SOTI character of multilayer WTe 2 .

Effet Josephson et émission haute fréquence dans un nanotube de carbone dans le régime Kondo: Cette thèse est consacrée au transport quantique à travers une impureté Kondo, formée dans une boîte quantique réalisée dans un nanotube de carbone. L’effet Kondo est ainsi sondé à travers deux situations : en compétition avec l’effet Josephson induit dans le nanotube par des contacts supraconducteurs et à travers son émission haute fréquence. Dans une première série d’expériences, nous avons introduit un nanotube dans un SQUID, afin de mesurer la relation entre son supercourant et la différence de phase supraconductrice à ses bornes. Nous avons mesuré cette relation lorsque les corrélations Kondo et supraconductrices sont du même ordre de grandeur et montré que l’état du système, singulet ou doublet (correspondant respectivement à une jonction 0 ou π) peut alors être contrôlé par la phase supraconductrice. Nous avons également montré que, si un deuxième niveau d’énergie participe au transport des paires de Cooper, la transition 0-π n’est plus une transition du premier ordre comme c’est le cas quand un seul niveau est impliqué. Dans la deuxième partie de la thèse, le nanotube de carbone est couplé, aux fréquences déterminées par un résonateur, à une jonction tunnel supraconductrice servant de détecteur on-chip de bruit haute fréquence. Ceci nous a permis de mesurer le bruit en émission de la boîte quantique dans le régime Kondo avec des couplages aux réservoirs plus ou moins symétriques. Nos mesures posent le problème de l’asymétrie spatiale du bruit mesuré et semblent montrer que, plus le couplage aux réservoirs est symétrique, plus la résonance Kondo est affaiblie dans une situation hors équilibre. Enfin, ce dispositif a été utilisé afin de mesurer l’émission Josephson AC d’un nanotube avec des électrodes supraconductrices, afin de voir ce que devient la compétition entre l’effet Kondo et la supraconductivité à haute fréquence. Ces mesures révèlent une diminution de l’émission Josephson alors que l’on a un maximum de supercourant.

Hole-conjugate states of the fractional quantum Hall effect host counterpropagating edge channels which are thought to exchange charge and energy. These exchanges have been the subject of extensive theoretical and experimental works; in particular, it is yet unclear if the presence of integer quantum Hall edge channels stemming from fully filled Landau levels affects heat equilibration along the edge. In this Letter, we present heat transport measurements in quantum Hall states of graphene demonstrating that the integer channels can strongly equilibrate with the fractional ones, leading to markedly different regimes of quantized heat transport that depend on edge electrostatics. Our results allow for a better comprehension of the complex edge physics in the fractional quantum Hall regime.

We probe the high frequency emission of a carbon nanotube based Josephson junction and compare it to its dc Josephson current. The ac emission is probed by coupling the carbon nanotube to an on-chip detector (a superconductor-insulator-superconductor junction), via a coplanar waveguide resonator. The measurement of the photoassisted current of the detector gives direct access to the signal emitted by the carbon nanotube. We focus on the gate regions that exhibit Kondo features in the normal state and demonstrate that when the dc supercurrent is enhanced by the Kondo effect, the ac Josephson effect is strongly reduced. This result is compared to numerical renormalization group theory and is attributed to a transition between the singlet ground state and the doublet excited state which is enabled only when the junction is driven out-of-equilibrium by a voltage bias.

Subgap states in semiconducting-superconducting nanowire hybrid devices are controversially discussed as potential topologically nontrivial quantum states. One source of ambiguity is the lack of an energetically and spatially well defined tunnel spectrometer. Here, we use quantum dots directly integrated into the nanowire during the growth process to perform tunnel spectroscopy of discrete subgap states in a long nanowire segment. In addition to subgap states with a standard magnetic field dependence, we find topologically trivial subgap states that are independent of the external magnetic field, i.e., that are pinned to a constant energy as a function of field. We explain this effect qualitatively and quantitatively by taking into account the strong spin-orbit interaction in the nanowire, which can lead to a decoupling of Andreev bound states from the field due to a spatial spin texture of the confined eigenstates. This result constitutes an important step forward in the research on superconducting subgap states in nanowires, such as Majorana bound states.

The sensitivity of shot noise to the interplay between Kondo correlations and superconductivity is investigated in a carbon nanotube quantum dot connected to superconducting electrodes. Depending on the gate voltage, the SU(2) and SU(4) Kondo unitary regimes can be clearly identified. We observe enhancement of the shot noise via the Fano factor in the superconducting state. Its divergence at low bias voltage, which is more pronounced in the SU(4) regime than in the SU(2) one, is larger than what is expected from proliferation of multiple Andreev reflections predicted by the existing theories. Our result suggests that the Kondo effect is responsible for this strong enhancement.

The band structure of graphene can be strongly modified if its lattice is aligned with the one of a boron nitride substrate. A moiré superlattice forms, which manifests itself by the appearance of new Dirac points, accompanied by van Hove singularities. In this work, we present supercurrent measurements in a Josephson junction made from such a graphene superlattice in the long and diffusive transport regime, where the critical current depends on the Thouless energy. We can then estimate the specific density of states of the graphene superlattice from the combined measurement of the critical current and the normal state resistance. The result matches with theoretical predictions and highlights the strong increase of the density of states at the van Hove singularities. By measuring the magnetic field dependence of the critical current, we find the presence of edge currents at these singularities. We explain it by the reduction of the Fermi velocity associated with the van Hove singularity, which suppresses the supercurrent in the bulk while the electrons at the edges remain less localized, resulting in an edge supercurrent. We attribute these different behaviors of the edges to defects or chemical doping.