Origin of Subgap States in Normal-Insulator-Superconductor van der Waals Heterostructures.

Superconductivity in van der Waals materials, such as NbSe2 and TaS2, is fundamentally novel due to the effects of dimensionality, crystal symmetries, and strong spin-orbit coupling. In this work, we perform tunnel spectroscopy on NbSe2 by utilizing MoS2 or hexagonal boron nitride (hBN) as a tunnel barrier. We observe subgap excitations and probe their origin by studying various heterostructure designs. We show that the edge of NbSe2 hosts many defect states, which strongly couple to the superconductor and form Andreev bound states. Furthermore, by isolating the NbSe2 edge we show that the subgap states are ubiquitous in MoS2 tunnel barriers but absent in hBN tunnel barriers, suggesting defects in MoS2 as their origin. Their magnetic nature reveals a singlet- or a doublet-type ground state, and based on nearly vanishing g factors or avoided crossings of subgap excitations, we highlight the role of strong spin-orbit coupling.

Compact SQUID Realized in a Double-Layer Graphene Heterostructure.

2D systems that host 1D helical states are advantageous from the perspective of scalable topological quantum computation when coupled to a superconductor. Graphene is particularly promising for its high electronic quality, its versatility in van der Waals heterostructures, and its electron- and hole-like degenerate 0th Landau level. Here we study a compact double-layer graphene SQUID (superconducting quantum interference device), where the superconducting loop is reduced to the superconducting contacts connecting two parallel graphene Josephson junctions. Despite the small size of the SQUID, it is fully tunable by the independent gate control of the chemical potentials in both layers. Furthermore, both Josephson junctions show a skewed current-phase relationship, indicating the presence of superconducting modes with high transparency. In the quantum Hall regime, we measure a well-defined conductance plateau of 2e2/h indicative of counter-propagating edge channels in the two layers.

Optimal architecture for ultralow noise graphene transistors at room temperature.

The fundamental origin of low-frequency noise in graphene field effect transistors (GFETs) has been widely explored but a generic engineering strategy towards low noise GFETs is lacking. Here, we systematically study and eliminate dominant sources of electrical noise to achieve ultralow noise GFETs. We find that in edge contacted, high-quality hexagonal boron nitride (hBN) encapsulated GFETs, the inclusion of a graphite bottom gate and long (⪆1.2 µm) channel-contact distance significantly reduces noise as compared to global Si/SiO2 gated devices. From the scaling of the remaining noise with channel area and its temperature dependence, we attribute this to the traps in hBN. To further screen the charge traps in hBN, we place few layers of MoS2 between graphene and hBN, and demonstrate that the noise is as low as ∼5.2 × 10-9µm2 Hz-1 (corresponding to minimum Hooge parameter ∼5.2 × 10-6) in GFETs at room temperature, which is an order of magnitude lower than the earlier reported values.

Optimising graphene visibility in van der Waals heterostructures.

Graphene constitutes one of the key elements in many functional van der Waals heterostructures. However, it has negligible optical visibility due to its monolayer nature. Here we study the visibility of graphene in various van der Waals heterostructures and include the effects of the source spectrum, oblique incidence and the spectral sensitivity of the detector to obtain a realistic model. A visibility experiment is performed at different wavelengths, resulting in a very good agreement with our calculations. This allows us to reliably predict the conditions for better visibility of graphene in van der Waals heterostructures. The framework and the codes provided in this work can be extended to study the visibility of any 2D material within an arbitrary van der Waals heterostructure.

Marginally Self-Averaging One-Dimensional Localization in Bilayer Graphene.

The combination of a field-tunable band gap, topological edge states, and valleys in the band structure makes insulating bilayer graphene a unique localized system, where the scaling laws of dimensionless conductance g remain largely unexplored. Here we show that the relative fluctuations in lng with the varying chemical potential, in strongly insulating bilayer graphene (BLG), decay nearly logarithmically for a channel length up to L/ξ≈20, where ξ is the localization length. This "marginal" self-averaging, and the corresponding dependence of ⟨lng⟩ on L, suggests that transport in strongly gapped BLG occurs along strictly one-dimensional channels, where ξ≈0.5±0.1 µm was found to be much longer than that expected from the bulk band gap. Our experiment reveals a nontrivial localization mechanism in gapped BLG, governed by transport along robust edge modes.

Current crowding mediated large contact noise in graphene field-effect transistors.

The impact of the intrinsic time-dependent fluctuations in the electrical resistance at the graphene-metal interface or the contact noise, on the performance of graphene field-effect transistors, can be as adverse as the contact resistance itself, but remains largely unexplored. Here we have investigated the contact noise in graphene field-effect transistors of varying device geometry and contact configuration, with carrier mobility ranging from 5,000 to 80,000 cm2 V-1 s-1. Our phenomenological model for contact noise because of current crowding in purely two-dimensional conductors confirms that the contacts dominate the measured resistance noise in all graphene field-effect transistors in the two-probe or invasive four-probe configurations, and surprisingly, also in nearly noninvasive four-probe (Hall bar) configuration in the high-mobility devices. The microscopic origin of contact noise is directly linked to the fluctuating electrostatic environment of the metal-channel interface, which could be generic to two-dimensional material-based electronic devices.

Fermi-edge transmission resonance in graphene driven by a single Coulomb impurity.

The interaction between the Fermi sea of conduction electrons and a nonadiabatic attractive impurity potential can lead to a power-law divergence in the tunneling probability of charge through the impurity. The resulting effect, known as the Fermi edge singularity (FES), constitutes one of the most fundamental many-body phenomena in quantum solid state physics. Here we report the first observation of FES for Dirac fermions in graphene driven by isolated Coulomb impurities in the conduction channel. In high-mobility graphene devices on hexagonal boron nitride substrates, the FES manifests in abrupt changes in conductance with a large magnitude ≈e(2)/h at resonance, indicating total many-body screening of a local Coulomb impurity with fluctuating charge occupancy. Furthermore, we exploit the extreme sensitivity of graphene to individual Coulomb impurities and demonstrate a new defect-spectroscopy tool to investigate strongly correlated phases in graphene in the quantum Hall regime.