995 research outputs found
The role of atomic vacancies and boundary conditions on ballistic thermal transport in graphene nanoribbons
Quantum thermal transport in armchair and zig-zag graphene nanoribbons are
investigated in the presence of single atomic vacancies and subject to
different boundary conditions. We start with a full comparison of the phonon
polarizations and energy dispersions as given by a fifth-nearest-neighbor
force-constant model (5NNFCM) and by elasticity theory of continuum membranes
(ETCM). For free-edges ribbons we discuss the behavior of an additional
acoustic edge-localized flexural mode, known as fourth acoustic branch (4ZA),
which has a small gap when it is obtained by the 5NNFCM. Then, we show that
ribbons with supported-edges have a sample-size dependent energy gap in the
phonon spectrum which is particularly large for in-plane modes. Irrespective to
the calculation method and the boundary condition, the dependence of the energy
gap for the low-energy optical phonon modes against the ribbon width W is found
to be proportional to 1/W for in-plane, and 1/W for out-of-plane phonon
modes. Using the 5NNFCM, the ballistic thermal conductance and its
contributions from every single phonon mode are then obtained by the non
equilibrium Green's function technique. We found that, while edge and central
localized single atomic vacancies do not affect the low-energy transmission
function of in-plane phonon modes, they reduce considerably the contributions
of the flexural modes. On the other hand, in-plane modes contributions are
strongly dependent on the boundary conditions and at low temperatures can be
highly reduced in supported-edges samples. These findings could open a route to
engineer graphene based devices where it is possible to discriminate the
relative contribution of polarized phonons and to tune the thermal transport on
the nanoscale
Thermomechanical properties of a single hexagonal boron nitride sheet
Using atomistic simulations we investigate the thermodynamical properties of
a single atomic layer of hexagonal boron nitride (h-BN). The thermal induced
ripples, heat capacity, and thermal lattice expansion of large scale h-BN
sheets are determined and compared to those found for graphene (GE) for
temperatures up to 1000 K. By analyzing the mean square height fluctuations and the height-height correlation function we found that the h-BN
sheet is a less stiff material as compared to graphene. The bending rigidity of
h-BN: i) is about 16% smaller than the one of GE at room temperature (300 K),
and ii) increases with temperature as in GE. The difference in stiffness
between h-BN and GE results in unequal responses to external uniaxial and shear
stress and different buckling transitions. In contrast to a GE sheet, the
buckling transition of a h-BN sheet depends strongly on the direction of the
applied compression. The molar heat capacity, thermal expansion coefficient and
the Gruneisen parameter are estimated to be 25.2 J\,mol\,K,
7.2K and 0.89, respectively
Mapas conceptuales como expresión de procesos de interrelación para evaluar la evolución del conocimiento de alumnos universitarios
We here value the necessity of implementing didactics strategies tending to orientate the study of the human organism considered as an integrated and indivisible whole.This work rescues the importance of the use of conceptual maps as an evaluation tool that allows to express the relationships that university students find among the different units of the program of the subject Normal Morphology, by the time of concluding the subject's attendance. In this action-research experience, conceptual maps are also used, incorporating a novel proposal of results' quantification, to evaluate the advances achieved by the students, mainly related to their capacity of interrelating concepts, after being engaged in the intensive study of the discipline, with the purpose of taking the final examination
Propiedades espectroscópicas y electroquÃmicas de complejos de Ni (II) y Cu (II) con algunas bases de Schiff tetradentadas
Se informa sobre la sfntesis y caracterización de complejos de nÃquel (II) y cobre (II) con bases de Schiff tetradentadas derivadas de salicilaldehfdos brornosustituidos y 1,2 diarninoetano y 1,3 diarninopropano. Se realizaron mediciones de voltametr(a c(clica en N,N'-dirnetil forrnarnida y se correlacionaron estos resultados con las estructuras de los complejos en solución
Thermal properties of fluorinated graphene
Large scale atomistic simulations using the reactive force field approach
(ReaxFF) are implemented to investigate the thermomechanical properties of
fluorinated graphene (FG). A new set of parameters for the reactive force field
potential (ReaxFF) optimized to reproduce key quantum mechanical properties of
relevant carbon-fluor cluster systems are presented. Molecular dynamics (MD)
simulations are used to investigate the thermal rippling behavior of FG and its
mechanical properties and compare them with graphene (GE), graphane (GA) and a
sheet of BN. The mean square value of the height fluctuations and the
height-height correlation function for different system sizes and
temperatures show that FG is an un-rippled system in contrast to the thermal
rippling behavior of graphene (GE). The effective Young's modulus of a flake of
fluorinated graphene is obtained to be 273 N/m and 250 N/m for a flake of FG
under uniaxial strain along arm-chair and zig-zag direction, respectively.Comment: To appear in Phys. Rev.
Anderson impurity in the one-dimensional Hubbard model on finite size systems
An Anderson impurity in a Hubbard model on chains with finite length is
studied using the density-matrix renormalization group (DMRG) technique. In the
first place, we analyzed how the reduction of electron density from
half-filling to quarter-filling affects the Kondo resonance in the limit of
Hubbard repulsion U=0. In general, a weak dependence with the electron density
was found for the local density of states (LDOS) at the impurity except when
the impurity, at half-filling, is close to a mixed valence regime. Next, in the
central part of this paper, we studied the effects of finite Hubbard
interaction on the chain at quarter-filling. Our main result is that this
interaction drives the impurity into a more defined Kondo regime although
accompanied in most cases by a reduction of the spectral weight of the impurity
LDOS. Again, for the impurity in the mixed valence regime, we observed an
interesting nonmonotonic behavior. We also concluded that the conductance,
computed for a small finite bias applied to the leads, follows the behavior of
the impurity LDOS, as in the case of non-interacting chains. Finally, we
analyzed how the Hubbard interaction and the finite chain length affect the
spin compensation cloud both at zero and at finite temperature, in this case
using quantum Monte Carlo techniques.Comment: 9 pages, 9 figures, final version to be published in Phys. Rev.
Methods to characterize the real-world use of rollators using inertial sensors – a feasibility study
Rollators are widely used by people with mobility problems, but previous studies have been limited to self-report approaches when evaluating their real-world effectiveness. To support studies based on more robust datasets, a method to estimate mobility parameters, such as gait speed and distance traveled, in the real world is needed. Body-worn sensors offer one approach to the problem, but rollator-mounted sensors have some practical advantages providing direct insight into patterns of walking device used, an under-researched area. We present a novel method to estimate speed and distance traveled from a single rollator-mounted IMU. The method was developed using data collected from ten rollator users performing a series of walking tasks including obstacle negotiation. The IMU data is first pre-processed to account for noise, orientation offset, and rotation-induced accelerations. The method then uses a two-stage approach. First, activity classification is used to separate the rollator data into one of three classes (movement, turning, or other). Subsequently, the speed of movement and distance traveled is estimated, using a separate estimation model for each of the three classes. The results showed high classification accuracy (precision, recall, and F1 statistics all >0.9). Speed estimation showed mean absolute errors below 0.2 m/s. Estimates for distance traveled showed errors which ranged from 5% (straight line walking) to over 70%. The results showed some promise but further work with a larger data set is needed to confirm the performance of our approach
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