Gregorio Millan Institute for Fluid dynamics, Nanoscience & Industrial Mathematics
Universidad Carlos III de Madrid
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Research
Defects and ripples in graphene
We have studied the stability and
evolution of various elastic defects in a
flat graphene sheet and the electronic properties of the most stable
configurations. Our stability studies use elementary periodized
discrete elasticity models of graphene sheets. Two types of
dislocations are found to be stable:
'glide' dislocations consisting of heptagon–pentagon pairs, and
'shuffle' dislocations, an octagon with a dangling bond. Unlike the
most studied case of carbon nanotubes, Stone Wales defects seem to be
dynamically unstable in the planar graphene sheet. This fact has
recently been corroborated by experimental observations. Similar
defects in
which one of the pentagon–heptagon pairs is displaced vertically with
respect to the other one are found to be dynamically stable. Shuffle
dislocations will give rise to local magnetic moments that can provide
an alternative route to magnetism in graphene. In a suspended graphene
sheet, we have included bending effects and the possibility that the
sheet be locally curved upwards or downwards. We find a critical
temperature above which a flat sheet is stable and below which stable
ripples appear. Rippling in the presence of defects is also being
studied.
Nonlinear transport in
nanostructures
Spatially confined Bloch
oscillations in
superlattices
Bloch oscillations
of the current and energy in a n-doped strongly coupled semiconductor
superlattice are damped by scattering but may be sustained by nonlinear
convective and diffusive terms below a certain critical value of the
damping. We have derived equations for the slowly varying amplitude of
the Bloch oscillations coupled to equations for the electron density
and the electric field in a superlattice having a single populated
miniband. Numerical solutions show that stable Bloch oscillations are
confined to the part of the superlattice where the electric field is
large. Applications to Terahertz oscillators and Bloch gain are being
studied. Multiscale methods
Studying
the
impact
of
defects
in
the
macroscopic
properties of a solid
material is a multiscale problem that involves processes taking
place at scales
ranging from the atomic scale to the macroscale. Finding a way to
transfer the
relevant information from the lower scales to the upper scales is
a largely
unsolved problem. A basic problem in this context consists in
developing hybrid
schemes that couple an atomistic description in a localized
region with a
continuum description around it. Discretizing the surrounding continuum
by means
of finite element schemes may give rise to singularities and
reflections by an
abrupt change in the mesh. We are exploring different ways to
couple the atomistic and the continuum region: nonreflecting coupling
conditions based on discrete Green functions, perfectly matched layers,
meshless methods...
Homogeneous and
heterogeneous
vapor condensation in cold walls
We are studying by a combination of singular perturbation and numerical methods the problem of vapor condensation and deposition in a cold wall within simple laminar boundary layer flows. This problem has interest for vapor deposition in combustion chambers, fouling and corrosion in biofuel plants, chemical vapor deposition, outside vapor deposition and aerosol capture by cold plates or rejection by hot ones.
