254 research outputs found
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
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
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
Superconducting dome by tuning through a Van Hove singularity in a two-dimensional metal
Chemical substitution is a promising route for the exploration of a rich
variety of doping- and/or disorder-dependent collective phenomena in
low-dimensional quantum materials. Here we show that transition metal
dichalcogenide alloys are ideal platforms to this purpose. In particular, we
demonstrate the emergence of superconductivity in the otherwise metallic
single-layer TaSe by minute electron doping provided by substitutional W
atoms. We investigate the temperature- and magnetic field-dependence of the
superconducting state of TaWSe with electron doping
() using variable temperature (0.34 K - 4.2 K) scanning tunneling
spectroscopy (STS). We unveil the emergence of a superconducting dome spanning
0.003 < < 0.03 with a maximized critical temperature of 0.85 K, a
significant increase from that of bulk TaSe (T = 0.14 K).
Superconductivity emerges from an increase of the density of states (DOS) as
the Fermi surface approaches a van Hove singularity due to doping. Once the
singularity is reached, however, the DOS decreases with , which
gradually weakens the superconducting state, thus shaping the superconducting
dome. Lastly, our doping-dependent measurements allow us to unambiguously track
the development of a Coulomb glass phase triggered by disorder due to W
dopants
Visualization of multifractal superconductivity in a two-dimensional transition metal dichalcogenide in the weak-disorder regime
Eigenstate multifractality is a distinctive feature of non-interacting
disordered metals close to a metal-insulator transition, whose properties are
expected to extend to superconductivity. While multifractality in three
dimensions (3D) only develops near the critical point for specific
strong-disorder strengths, multifractality in 2D systems is expected to be
observable even for weak disorder. Here we provide evidence for multifractal
features in the superconducting state of an intrinsic weakly disordered
single-layer NbSe by means of low-temperature scanning tunneling
microscopy/spectroscopy. The superconducting gap, characterized by its width,
depth and coherence peaks' amplitude, shows a characteristic spatial modulation
coincident with the periodicity of the quasiparticle interference pattern.
Spatial inhomogeneity of the superconducting gap width, proportional to the
local order parameter in the weak-disorder regime, follows a log-normal
statistical distribution as well as a power-law decay of the two-point
correlation function, in agreement with our theoretical model. Furthermore, the
experimental singularity spectrum f() shows anomalous scaling behavior
typical from 2D weakly disordered systems
Experimental observation of thermal fluctuations in single superconducting Pb nanoparticles through tunneling measurements
An important question in the physics of superconducting nanostructures is the
role of thermal fluctuations on superconductivity in the zero-dimensional
limit. Here, we probe the evolution of superconductivity as a function of
temperature and particle size in single, isolated Pb nanoparticles. Accurate
determination of the size and shape of each nanoparticle makes our system a
good model to quantitatively compare the experimental findings with theoretical
predictions. In particular, we study the role of thermal fluctuations (TF) on
the tunneling density of states (DOS) and the superconducting energy gap (D) in
these nanoparticles. For the smallest particles, h < 13nm, we clearly observe a
finite energy gap beyond Tc giving rise to a "critical region". We show
explicitly through quantitative theoretical calculations that these deviations
from mean-field predictions are caused by TF. Moreover, for T << Tc, where TF
are negligible, and typical sizes below 20 nm, we show that D gradually
decreases with reduction in particle size. This result is described by a
theoretical model that includes finite size effects and zero temperature
leading order corrections to the mean field formalism.Comment: Accepted in Physical Review
Electronic and structural characterization of divacancies in irradiated graphene
We provide a thorough study of a carbon divacancy, a fundamental but almost
unexplored point defect in graphene. Low temperature scanning tunneling
microscopy (STM) imaging of irradiated graphene on different substrates enabled
us to identify a common two-fold symmetry point defect. Our first principles
calculations reveal that the structure of this type of defect accommodates two
adjacent missing atoms in a rearranged atomic network formed by two pentagons
and one octagon, with no dangling bonds. Scanning tunneling spectroscopy (STS)
measurements on divacancies generated in nearly ideal graphene show an
electronic spectrum dominated by an empty-states resonance, which is ascribed
to a spin-degenerated nearly flat band of -electron nature. While the
calculated electronic structure rules out the formation of a magnetic moment
around the divacancy, the generation of an electronic resonance near the Fermi
level, reveals divacancies as key point defects for tuning electron transport
properties in graphene systems.Comment: 5 page
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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