20,213 research outputs found
Self-similar transmission properties of aperiodic Cantor potentials in gapped graphene
We investigate the transmission properties of quasiperiodic or aperiodic
structures based on graphene arranged according to the Cantor sequence. In
particular, we have found self-similar behaviour in the transmission spectra,
and most importantly, we have calculated the scalability of the spectra. To do
this, we implement and propose scaling rules for each one of the fundamental
parameters: generation number, height of the barriers and length of the system.
With this in mind we have been able to reproduce the reference transmission
spectrum, applying the appropriate scaling rule, by means of the scaled
transmission spectrum. These scaling rules are valid for both normal and
oblique incidence, and as far as we can see the basic ingredients to obtain
self-similar characteristics are: relativistic Dirac electrons, a self-similar
structure and the non-conservation of the pseudo-spin. This constitutes a
reduction of the number of conditions needed to observe self-similarity in
graphene-based structures, see D\'iaz-Guerrero et al. [D. S. D\'iaz-Guerrero,
L. M. Gaggero-Sager, I. Rodr\'iguez-Vargas, and G. G. Naumis,
arXiv:1503.03412v1, 2015]
Interplay between Zeeman interaction and spin-orbit coupling in a two-dimensional semiconductor system
We analyse the interplay between Dresselhaus, Bychkov-Rashba, and Zeeman
interactions in a two-dimensional semiconductor quantum system under the action
of a magnetic field. When a vertical magnetic field is considered, we predict
that the interplay results in an effective cyclotron frequency that depends on
a spin-dependent contribution. For in-plane magnetic fields, we found that the
interplay induces an anisotropic effective gyromagnetic factor that depends on
the orientation of the applied field as well as on the orientation of the
electron momentum.Comment: 5 page
Mixed Integer Linear Programming for Feature Selection in Support Vector Machine
This work focuses on support vector machine (SVM) with feature selection. A
MILP formulation is proposed for the problem. The choice of suitable features
to construct the separating hyperplanes has been modelled in this formulation
by including a budget constraint that sets in advance a limit on the number of
features to be used in the classification process. We propose both an exact and
a heuristic procedure to solve this formulation in an efficient way. Finally,
the validation of the model is done by checking it with some well-known data
sets and comparing it with classical classification methods.Comment: 37 pages, 20 figure
The stratified p-center problem
This work presents an extension of the p-center problem. In this new model,
called Stratified p-Center Problem (SpCP), the demand is concentrated in a set
of sites and the population of these sites is divided into different strata
depending on the kind of service that they require. The aim is to locate p
centers to cover the different types of services demanded minimizing the
weighted average of the largest distances associated with each of the different
strata. In addition, it is considered that more than one stratum can be present
at each site. Different formulations, valid inequalities and preprocessings are
developed and compared for this problem. An application of this model is
presented in order to implement a heuristic approach based on the Sample
Average Approximation method (SAA) for solving the probabilistic p-center
problem in an efficient way.Comment: 32 pages, 1 pictur
The continuum description with pseudo-state wave functions
Benchmark calculations are performed aiming to test the use of two different
pseudo-state bases on the the Multiple Scattering expansion of the total
Transition amplitude (MST) scattering framework. Calculated differential cross
sections for p-6He inelastic scattering at 717 MeV/u show a good agreement
between the observables calculated in the two bases. This result gives extra
confidence on the pseudo-state representation of continuum states to describe
inelastic/breakup scattering.Comment: 4 pages, 2 figures. Published in Physical Review
Scaling in the structure of directory trees in a computer cluster
We describe the topological structure and the underlying organization
principles of the directories created by users of a computer cluster when
storing his/her own files. We analyze degree distributions, average distance
between files, distribution of communities and allometric scaling exponents of
the directory trees. We find that users create trees with a broad, scale-free
degree distribution. The structure of the directories is well captured by a
growth model with a single parameter. The degree distribution of the different
trees has a non-universal exponent associated with different values of the
parameter of the model. However, the distribution of community sizes has a
universal exponent analytically obtained from our model.Comment: refined data analysis and modeling, completely reorganized version, 4
pages, 2 figure
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
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