Quasi-periodic moiré patterns and their effect on electronic properties of twisted bilayer graphene have been intensely studied. At small twist angle θ , due to atomic reconstruction, the moiré superlattice morphs into a network of narrow domain walls separating micron-scale AB and BA stacking regions. We use scanning probe photocurrent imaging to resolve nanoscale variations of the Seebeck coefficient occurring at these domain walls. The observed features become enhanced in a range of mid-infrared frequencies where the hexagonal boron nitride substrate is optically hyperbolic. Our results illustrate the capabilities of the nano-photocurrent technique for probing nanoscale electronic inhomogeneities in two-dimensional materials.

Nanoscale charge control is a key enabling technology in plasmonics, electronic band structure engineering, and the topology of two-dimensional materials. By exploiting the large electron affinity of α-RuCl3, we are able to visualize and quantify massive charge transfer at graphene/α-RuCl3 interfaces through generation of charge-transfer plasmon polaritons (CPPs). We performed nanoimaging experiments on graphene/α-RuCl3 at both ambient and cryogenic temperatures and discovered robust plasmonic features in otherwise ungated and undoped structures. The CPP wavelength evaluated through several distinct imaging modalities offers a high-fidelity measure of the Fermi energy of the graphene layer: EF = 0.6 eV (n = 2.7 × 1013 cm-2). Our first-principles calculations link the plasmonic response to the work function difference between graphene and α-RuCl3 giving rise to CPPs. Our results provide a novel general strategy for generating nanometer-scale plasmonic interfaces without resorting to external contacts or chemical doping.

Hyperbolic Cooper-pair polaritons (HCP) in cuprate superconductors are of fundamental interest due to their potential for providing insights into the nature of unconventional superconductivity. Here, we critically assess an experimental approach using near-field imaging to probe HCP in Bi2Sr2CaCu2O8+x (Bi-2212) in the presence of graphene surface plasmon polaritons (SPP). Our simulations show that inherently weak HCP features in the near-field can be strongly enhanced when coupled to graphene SPP in layered graphene/hexagonal boron nitride (hBN)/Bi-2212 heterostructures. This enhancement arises from our multilayered structures effectively acting as plasmonic cavities capable of altering collective modes of a layered superconductor by modifying its electromagnetic environment. The degree of enhancement can be selectively controlled by tuning the insulating spacer thickness with atomic precision. Finally, we verify the expected renormalization of room-temperature graphene SPP using near-field infrared imaging. Our modeling, augmented with data, attests to the validity of our approach for probing HCP modes in cuprate superconductors.

1Department of Physics, Columbia University, New York, NY, 10027, USA 2Department of Mechanical Engineering, Columbia University, New York, NY, 10027, USA 3Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, 10027, USA 4Department of Physics, Max Planck Institute for Structure and Dynamics of Matter and Center for Free-Electron Laser Science, 22761 Hamburg, Germany 5Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 6Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA 7Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan 8International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 11 Namiki, Tsukuba 305-004, Japan 9Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 10Center for Computational Quantum Physics, Flatiron Institute, New York, New York 10010, USA 11Nano-Bio Spectroscopy Group, Universidad del País Vasco, San Sebastian 20018, Spain 12Department of Physics, University of California San Diego, La Jolla, CA, 92093, USA

Significance The United States performed nuclear testing on Bikini and Enewetak Atolls in the northern Marshall Islands between 1946 and 1958. Fallout from the largest test Bravo, detonated in 1954, spread over a large area, exposing to radiation not only land and ocean but also Marshallese people living in neighboring atolls, including Rongelap and Utirik. Cesium-137, present in the fallout, has a half-life of approximately 30 y and is readily absorbed by food crops, thus representing a health hazard for island inhabitants. In situ measurements of cesium-137 content were made for fruits from 11 islands on four atolls. Contamination remains above limits set by international safety standards in some measured fruits, and several islands display contamination from this human-made radionuclide.

Observations of 12 X-ray binaries that contain black holes within the central parsec of the Galaxy suggest the existence of hundreds more, and even more isolated black holes.