Anomalous Cyclotron Motion in Graphene Superlattice Cavities.

We consider graphene superlattice miniband fermions probed by electronic interferometry in magnetotransport experiments. By decoding the observed Fabry-Pérot interference patterns together with our corresponding quantum transport simulations, we find that the Dirac quasiparticles originating from the superlattice minibands do not undergo conventional cyclotron motion but follow more subtle trajectories. In particular, dynamics at low magnetic fields is characterized by peculiar, straight trajectory segments. Our results provide new insights into superlattice miniband fermions and open up novel possibilities to use periodic potentials in electron optics experiments.

Separation of Specific Single-Enantiomer Single-Wall Carbon Nanotubes in the Large-Diameter Regime.

The enantiomer-level isolation of single-walled carbon nanotubes (SWCNTs) in high concentration and with high purity for nanotubes greater than 1.1 nm in diameter is demonstrated using a two-stage aqueous two-phase extraction (ATPE) technique. In total, five different nanotube species of ∼1.41 nm diameter are isolated, including both metallics and semiconductors. We characterize these populations by absorbance spectroscopy, circular dichroism spectroscopy, resonance Raman spectroscopy, and photoluminescence mapping, revealing and substantiating mod-dependent optical dependencies. Using knowledge of the competitive adsorption of surfactants to the SWCNTs that controls partitioning within the ATPE separation, we describe an advanced acid addition methodology that enables the fine control of the separation of these select nanotubes. Furthermore, we show that endohedral filling is a previously unrecognized but important factor to ensure a homogeneous starting material and further enhance the separation yield, with the best results for alkane-filled SWCNTs, followed by empty SWCNTs, with the intrinsic inhomogeneity of water-filled SWCNTs causing them to be worse for separations. Lastly, we demonstrate the potential use of these nanotubes in field-effect transistors.

Measuring in Situ Length Distributions of Polymer-Wrapped Monochiral Single-Walled Carbon Nanotubes Dispersed in Toluene with Analytical Ultracentrifugation.

The length of a carbon nanotube is an important dimension that has to be adjusted to the requirements of an experiment or application, e.g., through sorting methods. So far, atomic force microscopy (AFM) has been the method of choice for measuring length distributions, despite being an ex situ method with apparent shortcomings. In this work, we explore analytical ultracentrifugation (AUC) as an in situ method for measuring the length distribution of polymer-wrapped (7, 5) single-walled carbon nanotubes dispersed in toluene. This is an AUC study of nanotubes in nonaqueous media, the preferred media for nanotubes used in device fabrication. In AUC, the temporally and spatially dependent change in optical absorption of a sample is measured under centrifugation. The resulting sedimentation curves can be deconvoluted with a standard data processing procedure (SEDFIT), to yield the sedimentation coefficient distribution. However, the conversion of the sedimentation coefficient distribution into a length distribution is nontrivial and requires finding a suitable model for the nanotube friction coefficient. Also, since AUC is based on optical absorption, it yields a volume distribution and not a number distribution as obtained from AFM reference data. By meeting these challenges and finding a surprisingly simple empirical flexible-chain-like model to describe the sedimentation behavior of one specific chiral structure, we suggest AUC as a viable method for measuring in situ nanotube length distributions of nonaqueous dispersions.

Tailoring supercurrent confinement in graphene bilayer weak links.

The Josephson effect is one of the most studied macroscopic quantum phenomena in condensed matter physics and has been an essential part of the quantum technologies development over the last decades. It is already used in many applications such as magnetometry, metrology, quantum computing, detectors or electronic refrigeration. However, developing devices in which the induced superconductivity can be monitored, both spatially and in its magnitude, remains a serious challenge. In this work, we have used local gates to control confinement, amplitude and density profile of the supercurrent induced in one-dimensional nanoscale constrictions, defined in bilayer graphene-hexagonal boron nitride van der Waals heterostructures. The combination of resistance gate maps, out-of-equilibrium transport, magnetic interferometry measurements, analytical and numerical modelling enables us to explore highly tunable superconducting weak links. Our study opens the path way to design more complex superconducting circuits based on this principle, such as electronic interferometers or transition-edge sensors.