39 research outputs found
STUDY OF THE MANIPULATION AND MECHANICS OF GRAPHENE
Department of ChemistryGraphene has been the focus of significant research attention for a decade due to its
remarkable mechanical, electrical, and optical properties. It has also been reported that modifying the
morphology of a graphene sheet by introducing curves or wrinkles can tune these properties. In
Chapter I, we present a technique which can remotely control graphene???s morphology using a
magnetic tweezer. Graphene was anchored to a chromium post and a magnetic iron patch was
deposited on the graphene edge. In this way, we modified the morphology of graphene by application
of an external magnetic field. We also demonstrated a microfluidic device which could be switched on
and off by magnetic manipulation of graphene.
Although graphene???s mechanical properties show very high values, the measured response in
the macroscale is much lower than theoretical and experimental microscale values. When the length
scale increases, the probability of encountering defects becomes higher, resulting in lower
performance (if there are defects???and evidently there are). In Chapter II, we have attempted to
determine the nature of graphene???s mechanical performance by identifying regions that contain
defects so as to determine their exact effect on the mechanical performance using the
microelectromechanical system ???Push-to-Pull stage??? inside a scanning electron microscope.ope
Lattice paramenter, lattice disorder and resistivity of carbohydrate doepd MgB2 and their correlation with the transition temperature
The change in the lattice parameters or the lattice disorder is claimed as a
cause of the slight reduction in the transition temperature by carbon doping in
MgB2. In this work, an extensive investigation on the effects of carbohydrate
doping has been carried out. It is found that not only the a-axis but also the
c-axis lattice parameter increases with the sintering temperature. A linear
relation between the unit cell volume and the critical temperature is observed.
Compared with the well known correlation between the lattice strain and the
critical temperature, the X-ray peak broadening itself shows a closer
correlation with the transition temperature. The residual resistivity and the
critical temperature are linearly correlated with each other as well and its
implication is further discussed.Comment: 3 pages. Accepted by Jouranl of nanoscience and Nanotechnology (JNN
Overcoming Catastrophic Forgetting by Neuron-level Plasticity Control
To address the issue of catastrophic forgetting in neural networks, we
propose a novel, simple, and effective solution called neuron-level plasticity
control (NPC). While learning a new task, the proposed method preserves the
knowledge for the previous tasks by controlling the plasticity of the network
at the neuron level. NPC estimates the importance value of each neuron and
consolidates important \textit{neurons} by applying lower learning rates,
rather than restricting individual connection weights to stay close to certain
values. The experimental results on the incremental MNIST (iMNIST) and
incremental CIFAR100 (iCIFAR100) datasets show that neuron-level consolidation
is substantially more effective compared to the connection-level consolidation
approaches.Comment: 8 page
Characteristics of Deforestation in the Democratic People’s Republic of Korea (North Korea) between the 1980s and 2000s
There has been a significant lack of land cover change studies in relation to deforestation in the Democratic People’s Republic of Korea (North Korea). The purpose of this study is to characterise deforestation in North Korea through land cover change trajectory and spatial analysis. We used three 30-m gridded land cover data sets for North Korea representing the conditions of the late 1980s, 1990s, and 2000s, respectively, as well as a digital elevation model. We examined the land cover trajectories during the two decades, i.e. which land cover became which at the pixel level. In addition, we calculated topographic characteristics of deforested pixels. Major findings from the study are summarised as follows: (1) net forest loss in North Korea slowed since the 1990s, whereas land cover changes were active; (2) as a result of deforestation, forest land cover became mostly agricultural and grassland; (3) expansion of agricultural land cover continued during the time; and (4) elevation and slope of deforested areas decreased slightly in the latter decade. The key contribution of the study is that it has demonstrated which land cover became which at the 30-m pixel level, complementing existing studies that examined overall forest stock in North Korea
SelecMix: Debiased Learning by Contradicting-pair Sampling
Neural networks trained with ERM (empirical risk minimization) sometimes
learn unintended decision rules, in particular when their training data is
biased, i.e., when training labels are strongly correlated with undesirable
features. To prevent a network from learning such features, recent methods
augment training data such that examples displaying spurious correlations
(i.e., bias-aligned examples) become a minority, whereas the other,
bias-conflicting examples become prevalent. However, these approaches are
sometimes difficult to train and scale to real-world data because they rely on
generative models or disentangled representations. We propose an alternative
based on mixup, a popular augmentation that creates convex combinations of
training examples. Our method, coined SelecMix, applies mixup to contradicting
pairs of examples, defined as showing either (i) the same label but dissimilar
biased features, or (ii) different labels but similar biased features.
Identifying such pairs requires comparing examples with respect to unknown
biased features. For this, we utilize an auxiliary contrastive model with the
popular heuristic that biased features are learned preferentially during
training. Experiments on standard benchmarks demonstrate the effectiveness of
the method, in particular when label noise complicates the identification of
bias-conflicting examples.Comment: NeurIPS 202
Theory of proximity effect in superconductor/ferromagnet heterostructures
We present a microscopic theory of proximity effect in the
ferromagnet/superconductor/ferromagnet (F/S/F) nanostructures where S is s-wave
low-T_c superconductor and F's are layers of 3d transition ferromagnetic metal.
Our approach is based on the solution of Gor'kov equations for the normal and
anomalous Green's functions together with a self-consistent evaluation of the
superconducting order parameter. We take into account the elastic
spin-conserving scattering of the electrons assuming s-wave scattering in the S
layer and s-d scattering in the F layers. In accordance with the previous
quasiclassical theories, we found that due to exchange field in the ferromagnet
the anomalous Green's function F(z) exhibits the damping oscillations in the
F-layer as a function of distance z from the S/F interface. In the given model
a half of period of oscillations is determined by the length \xi_m^0 = \pi
v_F/E_ex, where v_F is the Fermi velocity and E_ex is the exchange field, while
damping is governed by the length l_0 = (1/l_{\uparrow} +
1/l_{\downarrow})^{-1} with l_{\uparrow} and l_{\downarrow} being
spin-dependent mean free paths in the ferromagnet. The superconducting
transition temperature T_c(d_F) of the F/S/F trilayer shows the damping
oscillations as a function of the F-layer thickness d_F with period \xi_F =
\pi/\sqrt{m E_ex}, where m is the effective electron mass. We show that strong
spin-conserving scattering either in the superconductor or in the ferromagnet
significantly suppresses these oscillations. The calculated T_c(d_F)
dependences are compared with existing experimental data for Fe/Nb/Fe trilayers
and Nb/Co multilayers.Comment: 13 pages, REVTeX4, 8 PS-figures; improved version, submitted to PR
Manifestation of triplet superconductivity in superconductor-ferromagnet structures
We study proximity effects in a multilayered superconductor/ferromagnet (S/F)
structure with arbitrary relative directions of the magnetization . If
the magnetizations of different layers are collinear the superconducting
condensate function induced in the F layers has only a singlet component and a
triplet one with a zero projection of the total magnetic moment of the Cooper
pairs on the direction. In this case the condensate penetrates the F
layers over a short length determined by the exchange energy . If
the magnetizations are not collinear the triplet component has, in
addition to the zero projection, the projections . The latter component
is even in the momentum, odd in the Matsubara frequency and penetrates the F
layers over a long distance that increases with decreasing temperature and does
not depend on (spin-orbit interaction limits this length). If the thickness
of the F layers is much larger than , the Josephson coupling between
neighboring S layers is provided only by the triplet component, so that a new
type of superconductivity arises in the transverse direction of the structure.
The Josephson critical current is positive (negative) for the case of a
positive (negative) chirality of the vector . We demonstrate that this
type of the triplet condensate can be detected also by measuring the density of
states in F/S/F structures.Comment: 14 pages; 9 figures. Final version, to be published in Phys. Rev.