Publications

While extensively studied in normal metals, semimetals, and semiconductors, the superconducting (SC) proximity effect remains elusive in the emerging field of flat-band systems. In this study, we probe proximity-induced superconductivity in Josephson junctions (JJs) formed between superconducting NbTiN electrodes and twisted bilayer graphene (TBG) weak links. Here, the TBG acts as a highly tunable topological flat-band system, which, due to its twist-angle-dependent bandwidth, allows us to probe the SC proximity effect at the crossover from the dispersive to the flat-band limit. Contrary to our original expectations, we find that the induced superconductivity remains strong even in the flat-band limit and gives rise to broad, dome-shaped SC regions, in the filling-dependent phase diagram. In addition, we find that, unlike in conventional JJs, the critical current 𝐼𝑐 strongly deviates from a scaling with the normal state conductance 𝐺𝑁. We attribute these findings to the onset of strong electron interactions, which can give rise to an excess critical current. By also studying the dependence of 𝐼𝑐 on the filling and twist angle across multiple samples, we further uncover the importance of quantum geometric terms as well as multiband pairing mechanisms in describing the induced superconductivity in the TBG flat bands as their bandwidth decreases. To the best of our knowledge, our results present the first detailed study of the SC proximity effect in the flat-band limit and shed new light on the mechanisms that drive the formation of SC domes in flat-band systems.

Post-transfer in-depth morphological characterization of graphene grown by chemical vapor deposition (CVD) is of great importance to evaluate the quality and to understand the origin of defects in the transferred sheets. Herein, a semi-dry transfer technique is used to peel off millimeter-sized CVD graphene flakes from polycrystalline copper foils and transfer them onto SiO2/Si substrates. We take advantage of the unique feature of this semi-dry process: it preserves the copper substrate, enabling location-specific morphological comparisons between graphene and copper at various stages of the transfer. Thanks to a combination of morphological characterization techniques, this leads to trace and elucidate the origin of various post-transfer graphene defects (cracks, wrinkles, holes, tears). Specifically, thermally induced wrinkles are shown to evolve into nanoscale cracks, while copper surface steps lead to folds. Furthermore, we find that the macroscale topography of the copper foil also plays a critical role in defect formation. This work provides guidelines on how to correctly interpret the post-transfer morphology of graphene films on relevant substrates and how to properly assess their quality. This contributes to the optimization of both the graphene CVD growth and transfer processes for future applications.

We use a combination of molecular dynamics and quantum transport simulations to investigate the upper limit of spin transport in suspended graphene. We find that thermally-induced atomic-scale corrugations are the dominant factor, limiting spin lifetimes to 10 ns by inducing a strongly-varying local spin–orbit coupling. These extremely short-range corrugations appear even when the height profile appears to be smooth, suggesting they may be present in any graphene device. We discuss our results in the context of experiments, and briefly consider approaches to suppress these short-range corrugations and further enhance spin lifetimes in graphene-based spin devices.

Flat bands in moiré systems are exciting new playgrounds for the generation and study of exotic many-body physics phenomena in low-dimensional materials. Such physics is attributed to the vanishing kinetic energy and strong spatial localization of the flat-band states. Here, we use numerical simulations to examine the electronic transport properties of such flat bands in magic-angle twisted bilayer graphene in the presence of disorder. We find that while a conventional downscaling of the mean free path with increasing disorder strength occurs at higher energies, in the flat bands the mean free path can actually increase with increasing disorder strength. This phenomenon is also captured by the disorder-dependent quantum metric, which is directly linked to the ground state localization. This disorder-induced delocalization suggests that weak disorder may have a strong impact on the exotic physics of magic-angle bilayer graphene and other related moiré systems.

The ability to tune the energy gap in bilayer graphene makes it the perfect playground for the study of the effects of internal electric fields, such as the crystalline field, which are developed when other layered materials are deposited on top of it. Here, we introduce a novel device architecture allowing simultaneous control over the applied displacement field and the crystalline alignment between two materials. Our experimental and numerical results confirm that the crystal field and electrostatic doping due to the interface reflect the 120° symmetry of the bilayer graphene/BN heterostructure and are highly affected by the commensurate state. These results provide unique insight into the role of twist angle in the development of internal crystal fields and intrinsic electrostatic doping in heterostructures. Our results highlight the importance of layer alignment, beyond the existence of a moiré superlattice, to understand the intrinsic properties of a heterostructure.

The collision of two electrons at a beam splitter provides a method for studying their coherence and indistinguishability. Its realization requires the on-demand generation and synchronization of single electrons. In this work, we demonstrate the coherent collision of single electrons, generated by voltage pulses, in a graphene Mach-Zehnder interferometer. By measuring shot noise resulting from the collisions, we unveil fundamental characteristics of colliding electrons, highlighting the complementarity between the indistinguishable and distinguishable parts of their wave functions. The former is manifested through fermionic Hong-Ou-Mandel destructive interference, whereas the latter is discerned through double-winding Aharonov-Bohm interference in the noise. The interference visibilities of around 60% enable comprehensive quantum state tomography. Our findings may place coherent operations involving flying qubits within reach in graphene.

The moiré superconductor magic-angle twisted bilayer graphene (MATBG) shows exceptional properties, with an electron (hole) ensemble of only ~1011 carriers per square centimeter, which is five orders of magnitude lower than traditional superconductors (SCs). This results in an ultralow electronic heat capacity and a large kinetic inductance of this truly two-dimensional SC, providing record-breaking parameters for quantum sensing applications, specifically thermal sensing and single-photon detection. To fully exploit these unique superconducting properties for quantum sensing, here, we demonstrate a proof-of-principle experiment to detect single near-infrared photons by voltage biasing an MATBG device near its superconducting phase transition.We observe complete destruction of the SC state upon absorption of a single infrared photon even in a 16–square micrometer device, showcasing exceptional sensitivity. Our work offers insights into the MATBG-photon interaction and demonstrates pathways to use moiré superconductors as an exciting platform for revolutionary quantum devices and sensors.

Copper foil impurities are hampering scalable production of high-quality graphene by chemical vapor deposition (CVD). Here, we conduct a thorough study on the origin of these unavoidable contaminations at the surface of copper after the CVD process. We identify two distinct origins for the impurities. The first type is intrinsic impurities, originating from the manufacturing process of the copper foils, already present at the surface before any high-temperature treatment, or buried into the bulk of copper foils. The buried impurities diffuse towards the copper surface during high-temperature treatment and precipitate. The second source is external: silica contamination arising from the quartz tube that also precipitate on copper. The problem of the extrinsic silica contamination is readily solved upon using an adequate confinement the copper foil samples. The intrinsic impurities are much more difficult to remove since they appear spread in the whole foil. Nevertheless, electropolishing proves particularly efficient in drastically reducing the issue.

Hexagonal boron-nitride (h-BN) provides an ideal substrate for supporting graphene devices to achieve fascinating transport properties, such as Klein tunneling, electron optics and other novel quantum transport phenomena. However, depositing graphene on h-BN creates moiré superlattices, whose electronic properties can be significantly manipulated by controlling the lattice alignment between layers. In this work, the effects of these moiré structures on the transport properties of graphene are investigated using atomistic simulations. At large misalignment angles (leading to small moiré cells), the transport properties (most remarkably, Klein tunneling) of pristine graphene devices are conserved. On the other hand, in the nearly aligned cases, the moiré interaction induces stronger effects, significantly affecting electron transport in graphene. In particular, Klein tunneling is significantly degraded. In contrast, strong Fabry-Pérot interference (accordingly, strong quantum confinement) effects and non-linear I-V characteristics are observed. P-N interface smoothness engineering is also considered, suggesting as a potential way to improve these transport features in graphene/h-BN devices.