82 research outputs found
Role of pseudospin in quasiparticle interferences in epitaxial graphene probed by high-resolution scanning tunneling microscopy
Pseudospin, an additional degree of freedom related to the honeycomb
structure of graphene, is responsible of many of the outstanding electronic
properties found in this material. This article provides a clear understanding
of how such pseudospin impacts the quasiparticle interferences of monolayer
(ML) and bilayer (BL) graphene measured by low temperature scanning tunneling
microscopy and spectroscopy. We have used this technique to map, with very high
energy and space resolution, the spatial modulations of the local density of
states of ML and BL graphene epitaxialy grown on SiC(0001), in presence of
native disorder. We perform a Fourier transform analysis of such modulations
including wavevectors up to unit-vectors of the reciprocal lattice. Our data
demonstrate that the quasiparticle interferences associated to some particular
scattering processes are suppressed in ML graphene, but not in BL graphene.
Most importantly, interferences with 2qF wavevector associated to intravalley
backscattering are not measured in ML graphene, even on the images with highest
resolution. In order to clarify the role of the pseudospin on the quasiparticle
interferences, we use a simple model which nicely captures the main features
observed on our data. The model unambiguously shows that graphene's pseudospin
is responsible for such suppression of quasiparticle interferences features in
ML graphene, in particular for those with 2qF wavevector. It also confirms
scanning tunneling microscopy as a unique technique to probe the pseudospin in
graphene samples in real space with nanometer precision. Finally, we show that
such observations are robust with energy and obtain with great accuracy the
dispersion of the \pi-bands for both ML and BL graphene in the vicinity of the
Fermi level, extracting their main tight binding parameters
The missing atom as a source of carbon magnetism
Atomic vacancies have a strong impact in the mechanical, electronic and
magnetic properties of graphene-like materials. By artificially generating
isolated vacancies on a graphite surface and measuring their local density of
states on the atomic scale, we have shown how single vacancies modify the
electronic properties of this graphene-like system. Our scanning tunneling
microscopy experiments, complemented by tight binding calculations, reveal the
presence of a sharp electronic resonance at the Fermi energy around each single
graphite vacancy, which can be associated with the formation of local magnetic
moments and implies a dramatic reduction of the charge carriers' mobility.
While vacancies in single layer graphene naturally lead to magnetic couplings
of arbitrary sign, our results show the possibility of inducing a macroscopic
ferrimagnetic state in multilayered graphene samples just by randomly removing
single C atoms.Comment: Accepted for publication in Physical Review Letter
Point defects on graphene on metals
Understanding the coupling of graphene with its local environment is critical
to be able to integrate it in tomorrow's electronic devices. Here we show how
the presence of a metallic substrate affects the properties of an atomically
tailored graphene layer. We have deliberately introduced single carbon
vacancies on a graphene monolayer grown on a Pt(111) surface and investigated
its impact in the electronic, structural and magnetic properties of the
graphene layer. Our low temperature scanning tunneling microscopy studies,
complemented by density functional theory, show the existence of a broad
electronic resonance above the Fermi energy associated with the vacancies.
Vacancy sites become reactive leading to an increase of the coupling between
the graphene layer and the metal substrate at these points; this gives rise to
a rapid decay of the localized state and the quenching of the magnetic moment
associated with carbon vacancies in free-standing graphene layers
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
Hydrogen physisorption channel on graphene: A highway for atomic H diffusion
We study the adsorption of atomic hydrogen on graphene by combining scanning tunneling microscopy experiments and first principle calculations. Our results reveal the existence of a physisorption channel over the graphene layer, dominated by van der Waals forces and thus homogeneous over the whole atomic lattice, where atomic hydrogen can move freely. Such physisorption channel is essential to understand the final configuration of hydrogen atoms chemisorbed on graphene. We find that ∼95% of chemisorbed H atoms form non-magnetic dimers even for very dilute concentrationsThis work was supported by Spain’s Ministerio de Economía y Competitividad under grants; PCIN-2015-030; FIS2015-64886-C5-5-P, MDM-2014-0377 and FIS2016-80434-P, by AEI and FEDER under projects MAT2016-80907-P and MAT2016-77852-C2-2-R (AEI/FEDER, UE), by the Fundación Ramón Areces, by the European Union through the FLAG ERA program and structural funds and by the Comunidad de Madrid MAD2D-CM program under grant S2013/MIT-3007
Unraveling the intrinsic and robust nature of van hove singularities in twisted bilayer graphene by scanning tunneling microscopy and theoretical analysis
Extensive scanning tunneling microscopy and spectroscopy experiments complemented by first-principles and parametrized tight binding calculations provide a clear answer to the existence, origin, and robustness of vanHove singularities (vHs) in twisted graphene layers. Our results are conclusive: vHs due to interlayer coupling are ubiquitously present in a broad range (from 1º to 10º) of rotation angles in our graphene on 6H-SiC(000-1) samples. From the variation of the energy separation of the vHs with the rotation angle we are able to recover the Fermi velocity of a graphene monolayer as well as the strength of the interlayer interaction. The robustness of the vHs is assessed both by experiments, which show that they survive in the presence of a third graphene layer, and by calculations, which test the role of the periodic modulation and absolute value of the interlayer distance. Finally, we clarify the role of the layer topographic corrugation and of electronic effects in the apparent moiré contrast measured on the STM imagesThis work was supported by Spain’s MICINN under Grants No. MAT2010-14902, No. CSD2010-00024, and No. CSD2007-00050, and by Comunidad de Madrid under Grant No. S2009/MAT-1467. M. M. U., I. B., P. M, J.-Y.V., L. M., and J. M. G.-R. also acknowledge the PHC Picasso program for financial support (Project No. 22885NH). I. B. was supported by a Ramón y Cajal project of the Spanish MEC. L. M., P. M., and J.-Y.V. acknowledge support from Fondation Nanosciences (Dispograph project
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