38 research outputs found
Electron states of mono- and bilayer graphene on SiC probed by STM
We present a scanning tunneling microscopy (STM) study of a
gently-graphitized 6H-SiC(0001) surface in ultra high vacuum. From an analysis
of atomic scale images, we identify two different kinds of terraces, which we
unambiguously attribute to mono- and bilayer graphene capping a C-rich
interface. At low temperature, both terraces show
quantum interferences generated by static impurities. Such interferences are a
fingerprint of -like states close to the Fermi level. We conclude that the
metallic states of the first graphene layer are almost unperturbed by the
underlying interface, in agreement with recent photoemission experiments (A.
Bostwick et al., Nature Physics 3, 36 (2007))Comment: 4 pages, 3 figures submitte
Quasiparticle Chirality in Epitaxial Graphene Probed at the Nanometer Scale
Graphene exhibits unconventional two-dimensional electronic properties
resulting from the symmetry of its quasiparticles, which leads to the concepts
of pseudospin and electronic chirality. Here we report that scanning tunneling
microscopy can be used to probe these unique symmetry properties at the
nanometer scale. They are reflected in the quantum interference pattern
resulting from elastic scattering off impurities, and they can be directly read
from its fast Fourier transform. Our data, complemented by theoretical
calculations, demonstrate that the pseudospin and the electronic chirality in
epitaxial graphene on SiC(0001) correspond to the ones predicted for ideal
graphene.Comment: 4 pages, 3 figures, minor change
Shaping graphene superconductivity with nanometer precision
Graphene holds great potential for superconductivity due to its pure 2D
nature, the ability to tune its carrier density through electrostatic gating, and
its unique, relativistic-like electronic properties. At present, still far from
controlling and understanding graphene superconductivity, mainly because
the selective introduction of superconducting properties to graphene is
experimentally very challenging. Here, a method is developed that enables
shaping at will graphene superconductivity through a precise control of
graphene-superconductor junctions. The method combines the proximity
effect with scanning tunnelling microscope (STM) manipulation capabilities.
Pb nano-islands are first grown that locally induce superconductivity in
graphene. Using a STM, Pb nano-islands can be selectively displaced, over
different types of graphene surfaces, with nanometre scale precision, in any
direction, over distances of hundreds of nanometres. This opens an exciting
playground where a large number of predefined graphene-superconductor
hybrid structures can be investigated with atomic scale precision. To illustrate
the potential, a series of experiments are performed, rationalized by the
quasi-classical theory of superconductivity, going from the fundamental
understanding of superconductor-graphene-superconductor heterostructures
to the construction of superconductor nanocorrals, further used as “portable”
experimental probes of local magnetic moments in grapheneThe authors acknowledge funding from the Spanish Ministry of Science
and Innovation MCIN/AEI/10.13039/297 501100011033 though grants #
PID2020-115171GB-I00, PID2020-114880GB-I00, PID2019-107338RB-C61
and the “María de Maeztu” Programme for Units of Excellence in R&D
(CEX2018-000805-M, CEX2020-001038-M), the Comunidad de Madrid
NMAT2D-CM program under grant S2018/NMT-4511, the Comunidad de
Madrid, the Spanish State and the European Union by the Recovery, Transformation and Resilience Plan “Materiales Disruptivos Bidimensionales
(2D)” (MAD2D-CM)-UAM3 and the European Union through the Next
Generation EU funds and the Horizon 2020 FET-Open project SPRING
(No. 863098). J. C. C. thanks the German Science Foundation DFG and
SFB 1432 for sponsoring his stay at the University of Konstanz as a Mercator Fello
Electronic structure and the minimum conductance of a graphene layer on SiO2 from density-functional methods.
The effect of the SiO substrate on a graphene film is investigated using
realistic but computationally convenient energy-optimized models of the
substrate supporting a layer of graphene. The electronic bands are calculated
using density-functional methods for several model substrates. This provides an
estimate of the substrate-charge effects on the behaviour of the bands near
, as well as a variation of the equilibrium distance of the graphene
sheet. A model of a wavy graphene layer is examined as a possible candidate for
understanding the nature of the minimally conducting states in graphene.Comment: 6 pages, 5 figure
Weakly Trapped, Charged, and Free Excitons in Single-Layer MoS2 in the Presence of Defects, Strain, and Charged Impurities
Few- and single-layer MoS2 host substantial densities of defects. They are thought to influence the doping level, the crystal structure, and the binding of electron-hole pairs. We disentangle the concomitant spectroscopic expression of all three effects and identify to what extent they are intrinsic to the material or extrinsic to it, i.e., related to its local environment. We do so by using different sources of MoS2 - a natural one and one prepared at high pressure and high temperature - and different substrates bringing varying amounts of charged impurities and by separating the contributions of internal strain and doping in Raman spectra. Photoluminescence unveils various optically active excitonic complexes. We discover a defect-bound state having a low binding energy of 20 meV that does not appear sensitive to strain and doping, unlike charged excitons. Conversely, the defect does not significantly dope or strain MoS2. Scanning tunneling microscopy and density functional theory simulations point to substitutional atoms, presumably individual nitrogen atoms at the sulfur site. Our work shows the way to a systematic understanding of the effect of external and internal fields on the optical properties of two-dimensional materials
Symmetry Breaking in Few Layer Graphene Films
Recently, it was demonstrated that the quasiparticle dynamics, the
layer-dependent charge and potential, and the c-axis screening coefficient
could be extracted from measurements of the spectral function of few layer
graphene films grown epitaxially on SiC using angle-resolved photoemission
spectroscopy (ARPES). In this article we review these findings, and present
detailed methodology for extracting such parameters from ARPES. We also present
detailed arguments against the possibility of an energy gap at the Dirac
crossing ED.Comment: 23 pages, 13 figures, Conference Proceedings of DPG Meeting Mar 2007
Regensburg Submitted to New Journal of Physic
Neutralization of (NK-cell-derived) B-cell activating factor by Belimumab restores sensitivity of chronic lymphoid leukemia cells to direct and Rituximab-induced NK lysis.
Natural killer (NK) cells are cytotoxic lymphocytes that substantially contribute to the therapeutic benefit of antitumor antibodies like Rituximab, a crucial component in the treatment of B-cell malignancies. In chronic lymphocytic leukemia (CLL), the ability of NK cells to lyse the malignant cells and to mediate antibody-dependent cellular cytotoxicity upon Fc receptor stimulation is compromised, but the underlying mechanisms are largely unclear. We report here that NK-cells activation-dependently produce the tumor necrosis factor family member 'B-cell activating factor' (BAFF) in soluble form with no detectable surface expression, also in response to Fc receptor triggering by therapeutic CD20-antibodies. BAFF in turn enhanced the metabolic activity of primary CLL cells and impaired direct and Rituximab-induced lysis of CLL cells without affecting NK reactivity per se. The neutralizing BAFF antibody Belimumab, which is approved for treatment of systemic lupus erythematosus, prevented the effects of BAFF on the metabolism of CLL cells and restored their susceptibility to direct and Rituximab-induced NK-cell killing in allogeneic and autologous experimental systems. Our findings unravel the involvement of BAFF in the resistance of CLL cells to NK-cell antitumor immunity and Rituximab treatment and point to a benefit of combinatory approaches employing BAFF-neutralizing drugs in B-cell malignancies
One-Dimensional Fermi liquids
I attempt to give a pedagogical overview of the progress which has occurred
during the past decade in the description of one-dimensional correlated
fermions. Fermi liquid theory based on a quasi-particle picture, breaks down in
one dimension because of the Peierls divergence and because of charge-spin
separation. It is replaced by a Luttinger liquid whose elementary excitations
are collective charge and spin modes, based on the exactly solvable Luttinger
model. I review this model and various solutions with emphasis on bosonization
(and its equivalence to conformal field theory), and its physical properties.
The notion of a Luttinger liquid implies that all gapless 1D systems share
these properties at low energies.
Chapters 1 and 2 of the article contain an introduction and a discussion of
the breakdown of Fermi liquid theory. Chapter 3 describes in detail the
solution of the Luttinger model both by bosonization and by Green's functions
methods and summarizes the properties of the model, expressed thorugh
correlation functions. The relation to conformal field theory is discussed.
Chapter 4 of the article introduces the notion of a Luttinger liquid. It
describes in much detail the various mappings applied to realistic models of 1D
correlated fermions, onto the Luttinger model, as well as important corrections
to the Luttinger model properties discussed in Ch.3. Chapter 5 describes
situations where the Luttinger liquid is not a stable fixed point, and where
spin or charge gaps open in at least one channel. Chapter 6 discusses
multi-band and multichain problems, in particular the stability of a Luttinger
liquid with respect to interchain hopping. Ch. 7 gives a brief summary of
experimental efforts to uncover Luttinger liquid correlations in quasi-1D
materials.Comment: uuencoded Latex files and postscript figures, one Readme-file approx
160 pages + 13 figures; to be published by Reports on Progress in Physic