20,872 research outputs found
Spectroscopic signatures of different symmetries of the superconducting order parameter in metal-decorated graphene
Motivated by the recent experiments indicating superconductivity in
metal-decorated graphene sheets, we investigate their quasi-particle structure
within the framework of an effective tight-binding Hamiltonian augmented by
appropriate BCS-like pairing terms for p-type order parameter. The normal state
band structure of graphene is modified not only through interaction with
adsorbed metal atoms, but also due to the folding of bands at Brillouin zone
boundaries resulting from a reconstruction.
Several different types of pairing symmetries are analyzed utilizing
Nambu-Gorkov Green's function techniques to show that -symmetric
nearest-neighbor pairing yields the most enhanced superconducting gap. The
character of the order parameter depends on the nature of the atomic orbitals
involved in the pairing process and exhibits interesting angular and radial
asymmetries. Finally, we suggest a method to distinguish between singlet and
triplet type superconductivity in the presence of magnetic substitutional
impurities using scanning tunneling spectroscopy.Comment: Preprint, 15 pages, 4+1 figure
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
Magnetic induction and diffusion mechanisms in a liquid sodium spherical Couette experiment
We present a reconstruction of the mean axisymmetric azimuthal and meridional
flows in the DTS liquid sodium experiment. The experimental device sets a
spherical Couette flow enclosed between two concentric spherical shells where
the inner sphere holds a strong dipolar magnet, which acts as a magnetic
propeller when rotated. Measurements of the mean velocity, mean induced
magnetic field and mean electric potentials have been acquired inside and
outside the fluid for an inner sphere rotation rate of 9 Hz (Rm 28). Using the
induction equation to relate all measured quantities to the mean flow, we
develop a nonlinear least square inversion procedure to reconstruct a fully
coherent solution of the mean velocity field. We also include in our inversion
the response of the fluid layer to the non-axisymmetric time-dependent magnetic
field that results from deviations of the imposed magnetic field from an axial
dipole. The mean azimuthal velocity field we obtain shows super-rotation in an
inner region close to the inner sphere where the Lorentz force dominates, which
contrasts with an outer geostrophic region governed by the Coriolis force, but
where the magnetic torque remains the driver. The meridional circulation is
strongly hindered by the presence of both the Lorentz and the Coriolis forces.
Nevertheless, it contributes to a significant part of the induced magnetic
energy. Our approach sets the scene for evaluating the contribution of velocity
and magnetic fluctuations to the mean magnetic field, a key question for dynamo
mechanisms
Reconstruction of the two-dimensional gravitational potential of galaxy clusters from X-ray and Sunyaev-Zel'dovich measurements
The mass of galaxy clusters is not a direct observable, nonetheless it is
commonly used to probe cosmological models. Based on the combination of all
main cluster observables, that is, the X-ray emission, the thermal
Sunyaev-Zel'dovich (SZ) signal, the velocity dispersion of the cluster
galaxies, and gravitational lensing, the gravitational potential of galaxy
clusters can be jointly reconstructed. We derive the two main ingredients
required for this joint reconstruction: the potentials individually
reconstructed from the observables and their covariance matrices, which act as
a weight in the joint reconstruction. We show here the method to derive these
quantities. The result of the joint reconstruction applied to a real cluster
will be discussed in a forthcoming paper. We apply the Richardson-Lucy
deprojection algorithm to data on a two-dimensional (2D) grid. We first test
the 2D deprojection algorithm on a -profile. Assuming hydrostatic
equilibrium, we further reconstruct the gravitational potential of a simulated
galaxy cluster based on synthetic SZ and X-ray data. We then reconstruct the
projected gravitational potential of the massive and dynamically active cluster
Abell 2142, based on the X-ray observations collected with XMM-Newton and the
SZ observations from the Planck satellite. Finally, we compute the covariance
matrix of the projected reconstructed potential of the cluster Abell 2142 based
on the X-ray measurements collected with XMM-Newton. The gravitational
potentials of the simulated cluster recovered from synthetic X-ray and SZ data
are consistent, even though the potential reconstructed from X-rays shows
larger deviations from the true potential. Regarding Abell 2142, the projected
gravitational cluster potentials recovered from SZ and X-ray data reproduce
well the projected potential inferred from gravitational-lensing observations.
(abridged)Comment: accepted for publication in the journal A&
An elliptic expansion of the potential field source surface model
Context. The potential field source surface model is frequently used as a
basis for further scientific investigations where a comprehensive coronal
magnetic field is of importance. Its parameters, especially the position and
shape of the source surface, are crucial for the interpretation of the state of
the interplanetary medium. Improvements have been suggested that introduce one
or more additional free parameters to the model, for example, the current sheet
source surface (CSSS) model.
Aims. Relaxing the spherical constraint of the source surface and allowing it
to be elliptical gives modelers the option of deforming it to more accurately
match the physical environment of the specific period or location to be
analyzed.
Methods. A numerical solver is presented that solves Laplace's equation on a
three-dimensional grid using finite differences. The solver is capable of
working on structured spherical grids that can be deformed to create elliptical
source surfaces.
Results. The configurations of the coronal magnetic field are presented using
this new solver. Three-dimensional renderings are complemented by
Carrington-like synoptic maps of the magnetic configuration at different
heights in the solar corona. Differences in the magnetic configuration computed
by the spherical and elliptical models are illustrated.Comment: 11 pages, 7 figure
Quantum Mechanics of the Vacuum State in Two-Dimensional QCD with Adjoint Fermions
A study of two-dimensional QCD on a spatial circle with Majorana fermions in
the adjoint representation of the gauge groups SU(2) and SU(3) has been
performed. The main emphasis is put on the symmetry properties related to the
homotopically non-trivial gauge transformations and the discrete axial symmetry
of this model. Within a gauge fixed canonical framework, the delicate interplay
of topology on the one hand and Jacobians and boundary conditions arising in
the course of resolving Gauss's law on the other hand is exhibited. As a
result, a consistent description of the residual gauge symmetry (for
SU(N)) and the ``axial anomaly" emerges. For illustrative purposes, the vacuum
of the model is determined analytically in the limit of a small circle. There,
the Born-Oppenheimer approximation is justified and reduces the vacuum problem
to simple quantum mechanics. The issue of fermion condensates is addressed and
residual discrepancies with other approaches are pointed out.Comment: 44 pages; for hardcopies of figures, contact
[email protected]
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