Graphene exhibits unique electrical properties on account of its reduced dimensionality and neutrino‐like “massless Dirac fermion” quasiparticle spectrum. When contacted with two superconducting ...electrodes, graphene can support Cooper pair transport, resulting in the well‐known Josephson effect. The current–phase relation in a ballistic graphene Josephson junction is unique, and could provide a signature for the detection of ballistic Dirac fermions. This relation can be measured experimentally either directly via incorporation of graphene in an RF superconducting quantum interference device (SQUID) or indirectly via a dc‐SQUID. We calculate the expected flux modulation of the switching current in the case of the dc‐SQUID and compare the results to a previous experiment. Further experiments investigating the current–phase relation in graphene are promising for the observation of ballistic Dirac fermions.
The scanning tunneling microscope (STM) is a powerful spectroscopic probe of microscopic structures at metal surfaces. Here a series of STM studies are discussed that focus on the interaction of ...two-dimensional (2-d) electrons with surface nanostructures, and on the electronic properties of magnetic adsorbates. Spectroscopic imaging is shown to be a useful method for observing quantum interference patterns of 2-d surface state electrons scattering from natural and atomically fabricated surface structures. Surface state dispersion and scattering phaseshifts are extracted from such images. On Au(111) the interaction of surface state electrons with a reconstruction-induced superlattice is seen to lead to electronic density modulation and a new surface bandstructure. Local spectroscopic measurements at the gold surface are used to extract quantitative details of the superlattice potential. Magnetic scattering at cobalt atoms deposited onto Au(111) is observed to induce electronic fluctuations near the site of these impurities. Spin-dependent interaction between gold conduction electrons and a Co local moment leads to a many-body groundstate known as the Kondo effect. Local spectroscopic measurements of the ‘Kondo resonance’ for individual magnetic impurities are discussed. Atomic manipulation of Kondo impurities is combined with STM spectroscopy to study the interaction between magnetic atoms at a metal surface.
A central question in the field of graphene-related research is how graphene behaves when it is patterned at the nanometre scale with different edge geometries. A fundamental shape relevant to this ...question is the graphene nanoribbon (GNR), a narrow strip of graphene that can have different chirality depending on the angle at which it is cut. Such GNRs have been predicted to exhibit a wide range of behaviour, including tunable energy gaps1, 2 and the presence of one-dimensional (1D) edge states3, 4, 5 with unusual magnetic structure6, 7. Most GNRs measured up to now have been characterized by means of their electrical conductivity, leaving the relationship between electronic structure and local atomic geometry unclear8, 9, 10. Here we present a sub-nanometre-resolved scanning tunnelling microscopy (STM) and spectroscopy (STS) study of GNRs that allows us to examine how GNR electronic structure depends on the chirality of atomically well-defined GNR edges. The GNRs used here were chemically synthesized using carbon nanotube (CNT) unzipping methods that allow flexible variation of GNR width, length, chirality, and substrate11, 12. Our STS measurements reveal the presence of 1D GNR edge states, the behaviour of which matches theoretical expectations for GNRs of similar width and chirality, including width-dependent energy splitting of the GNR edge state. PUBLICATION ABSTRACT
The design of stacks of layered materials in which adjacent layers interact by van der Waals forces has enabled the combination of various two-dimensional crystals with different electrical, optical ...and mechanical properties as well as the emergence of novel physical phenomena and device functionality. Here, we report photoinduced doping in van der Waals heterostructures consisting of graphene and boron nitride layers. It enables flexible and repeatable writing and erasing of charge doping in graphene with visible light. We demonstrate that this photoinduced doping maintains the high carrier mobility of the graphene/boron nitride heterostructure, thus resembling the modulation doping technique used in semiconductor heterojunctions, and can be used to generate spatially varying doping profiles such as p-n junctions. We show that this photoinduced doping arises from microscopically coupled optical and electrical responses of graphene/boron nitride heterostructures, including optical excitation of defect transitions in boron nitride, electrical transport in graphene, and charge transfer between boron nitride and graphene.
Abstract
Monolayers of two-dimensional van der Waals materials exhibit novel electronic phases distinct from their bulk due to the symmetry breaking and reduced screening in the absence of the ...interlayer coupling. In this work, we combine angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy to demonstrate the emergence of a unique insulating 2 × 1 dimer ground state in monolayer 1
T
-IrTe
2
that has a large band gap in contrast to the metallic bilayer-to-bulk forms of this material. First-principles calculations reveal that phonon and charge instabilities as well as local bond formation collectively enhance and stabilize a charge-ordered ground state. Our findings provide important insights into the subtle balance of interactions having similar energy scales that occurs in the absence of strong interlayer coupling, which offers new opportunities to engineer the properties of 2D monolayers.
Semiconducting π-conjugated polymers have attracted significant interest for applications in light-emitting diodes, field-effect transistors, photovoltaics, and nonlinear optoelectronic devices. ...Central to the success of these functional organic materials is the facile tunability of their electrical, optical, and magnetic properties along with easy processability and the outstanding mechanical properties associated with polymeric structures. In this work we characterize the chemical and electronic structure of individual chains of oligo-(E)-1,1′-bi(indenylidene), a polyacetylene derivative that we have obtained through cooperative C1–C5 thermal enediyne cyclizations on Au(111) surfaces followed by a step-growth polymerization of the (E)-1,1′-bi(indenylidene) diradical intermediates. We have determined the combined structural and electronic properties of this class of oligomers by characterizing the atomically precise chemical structure of individual monomer building blocks and oligomer chains (via noncontact atomic force microscopy (nc-AFM)), as well as by imaging their localized and extended molecular orbitals (via scanning tunneling microscopy and spectroscopy (STM/STS)). Our combined structural and electronic measurements reveal that the energy associated with extended π-conjugated states in these oligomers is significantly lower than the energy of the corresponding localized monomer orbitals, consistent with theoretical predictions.
The synthesis of a single‐layer covalent organic framework (COF) with spatially modulated internal potentials provides new opportunities for manipulating the electronic structure of molecularly ...defined materials. Here, the fabrication and electronic characterization of COF‐420: a single‐layer porphyrin‐based square‐lattice COF containing a periodic array of oriented, type II electronic heterojunctions is reported. In contrast to previous donor–acceptor COFs, COF‐420 is constructed from building blocks that yield identical cores upon reticulation, but that are bridged by electrically asymmetric linkers supporting oriented electronic dipoles. Scanning tunneling spectroscopy reveals staggered gap (type II) band alignment between adjacent molecular cores in COF‐420, in agreement with first‐principles calculations. Hirshfeld charge analysis indicates that dipole fields from oriented imine linkages within COF‐420 are the main cause of the staggered electronic structure in this square grid of atomically–precise heterojunctions.
A single‐layer covalent organic framework (COF) that exhibits a staggered internal potential between neighboring bright and dark molecular cores is shown. Donor/acceptor functionality between the structurally identical cores arises from electronic dipoles incorporated into the linkers between them. The resulting two COF sublattices are observed to be offset in energy by 0.25 eV, consistent with first‐principles calculations.
The photon-like behavior of electrons in graphene causes unusual confinement properties that depend strongly on the geometry and strength of the surrounding potential. We report bottom-up synthesis ...of atomically-precise one-dimensional (1D) arrays of point charges on graphene that allow exploration of a new type of supercritical confinement of graphene carriers. The arrays were synthesized by arranging F
TCNQ molecules into a 1D lattice on back-gated graphene, allowing precise tuning of both the molecular charge and the array periodicity. While dilute arrays of ionized F
TCNQ molecules are seen to behave like isolated subcritical charges, dense arrays show emergent supercriticality. In contrast to compact supercritical clusters, these extended arrays display both supercritical and subcritical characteristics and belong to a new physical regime termed "frustrated supercritical collapse". Here carriers in the far-field are attracted by a supercritical charge distribution, but their fall to the center is frustrated by subcritical potentials in the near-field, similar to trapping of light by a dense cluster of stars in general relativity.
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