530 research outputs found
Electronic Highways in Bilayer Graphene
Bilayer graphene with an interlayer potential difference has an energy gap
and, when the potential difference varies spatially, topologically protected
one-dimensional states localized along the difference's zero-lines. When
disorder is absent, electronic travel directions along zero-line trajectories
are fixed by valley Hall properties. Using the Landauer-B\"uttiker formula and
the non-equilibrium Green's function technique we demonstrate numerically that
collisions between electrons traveling in opposite directions, due to either
disorder or changes in path direction, are strongly suppressed. We find that
extremely long mean free paths of the order of hundreds of microns can be
expected in relatively clean samples. This finding suggests the possibility of
designing low power nanoscale electronic devices in which transport paths are
controlled by gates which alter the inter-layer potential landscape.Comment: 8 pages, 5 figure
Online Algorithms for Geographical Load Balancing
It has recently been proposed that Internet energy costs, both monetary and environmental, can be reduced by exploiting temporal variations and shifting processing to data centers located in regions where energy currently has low cost. Lightly loaded data centers can then turn off surplus servers. This paper studies online algorithms for determining the number of servers to leave on in each data center, and then uses these algorithms to study the environmental potential of geographical load balancing (GLB). A commonly suggested algorithm for this setting is “receding horizon control” (RHC), which computes the provisioning for the current time by optimizing over a window of predicted future loads. We show that RHC performs well in a homogeneous setting, in which all servers can serve all jobs equally well; however, we also prove that differences in propagation delays, servers, and electricity prices can cause RHC perform badly, So, we introduce variants of RHC that are guaranteed to perform as well in the face of such heterogeneity. These algorithms are then used to study the feasibility of powering a continent-wide set of data centers mostly by renewable sources, and to understand what portfolio of renewable energy is most effective
Valley-dependent Brewster angles and Goos-Hanchen effect in strained graphene
We demonstrate theoretically how local strains in graphene can be tailored to
generate a valley polarized current. By suitable engineering of local strain
profiles, we find that electrons in opposite valleys (K or K') show different
Brewster-like angles and Goos-H\"anchen shifts, exhibiting a close analogy with
light propagating behavior. In a strain-induced waveguide, electrons in K and
K' valleys have different group velocities, which can be used to construct a
valley filter in graphene without the need for any external fields.Comment: 5 pages, 4 figure
Valley-Hall Kink and Edge States in Multilayer Graphene
We report on a theoretical study of one-dimensional (1D) states localized at
few-layer graphene system ribbon edges, and at interfaces between few-layer
graphene systems with different valley Hall conductivities. These 1D states are
topologically protected when valley mixing is neglected. We address the
influence on their properties of stacking arrangement, interface structure, and
external electric field perpendicular to the layers. We find that 1D states are
generally absent at multilayer ribbon armchair direction edges, but present
irrespective of crystallographic orientation at any internal valley-Hall
interface of an ABC stacked multilayer.Comment: 5 pages, 3 figure
Current Flow in Random Resistor Networks: The Role of Percolation in Weak and Strong Disorder
We study the current flow paths between two edges in a random resistor
network on a square lattice. Each resistor has resistance ,
where is a uniformly-distributed random variable and controls the
broadness of the distribution. We find (a) the scaled variable , where is the percolation connectedness exponent, fully
determines the distribution of the current path length for all values of
. For , the behavior corresponds to the weak disorder limit and
scales as , while for , the behavior corresponds to
the strong disorder limit with , where
is the optimal path exponent. (b) In the
weak disorder regime, there is a length scale , below which
strong disorder and critical percolation characterize the current path.Comment: 9 pages, 4 figure
Quantum Anomalous Hall Effect in Graphene Proximity Coupled to an Antiferromagnetic Insulator
We propose realizing the quantum anomalous Hall effect by proximity coupling
graphene to an antiferromagnetic insulator that provides both broken
time-reversal symmetry and spin-orbit coupling. We illustrate our idea by
performing ab initio calculations for graphene adsorbed on the (111) surface of
BiFeO3. In this case, we find that the proximity-induced exchange field in
graphene is about 70 meV, and that a topologically nontrivial band gap is
opened by Rashba spin-orbit coupling. The size of the gap depends on the
separation between the graphene and the thin film substrate, which can be tuned
experimentally by applying external pressure.Comment: 5pages, 5 figure
A kriging model for dynamics of mechanical systems with revolute joint clearances
Over the past two decades, extensive work has been conducted on the dynamic effect of
joint clearances in multibody mechanical systems. In contrast, little work has been
devoted to optimizing the performance of these systems. In this study, the analysis of revolute
joint clearance is formulated in terms of a Hertzian-based contact force model. For
illustration, the classical slider-crank mechanism with a revolute clearance joint at the
piston pin is presented and a simulation model is developed using the analysis/design
software MSC.ADAMS. The clearance is modeled as a pin-in-a-hole surface-to-surface dry
contact, with an appropriate contact force model between the joint and bearing surfaces.
Different simulations are performed to demonstrate the influence of the joint clearance
size and the input crank speed on the dynamic behavior of the system with the joint clearance.
In the modeling and simulation of the experimental setup and in the followed parametric
study with a slightly revised system, both the Hertzian normal contact force model
and a Coulomb-type friction force model were utilized. The kinetic coefficient of friction
was chosen as constant throughout the study. An innovative design-of-experiment
(DOE)-based method for optimizing the performance of a mechanical system with the
revolute joint clearance for different ranges of design parameters is then proposed. Based
on the simulation model results from sample points, which are selected by a Latin hypercube
sampling (LHS) method, a polynomial function Kriging meta-model is established
instead of the actual simulation model. The reason for the development and use of the
meta-model is to bypass computationally intensive simulations of a computer model for
different design parameter values in place of a more efficient and cost-effective mathematical
model. Finally, numerical results obtained from two application examples with
different design parameters, including the joint clearance size, crank speed, and contact
stiffness, are presented for the further analysis of the dynamics of the revolute clearance
joint in a mechanical system. This allows for predicting the influence of design parameter
changes, in order to minimize contact forces, accelerations, and power requirements due
to the existence of joint clearance.Fundação para a Ciência e a Tecnologia (FCT
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