4,544 research outputs found
Improved performance of graphene transistors by strain engineering
By means of numerical simulation, we study in this work the effects of
uniaxial strain on transport properties of strained graphene heterojunctions
and explore the possibility to achieve good performance of graphene transistors
using these hetero-channels. It is shown that a finite conduction-gap can open
in the strain junctions due to the strain-induced deformation of graphene
bandstructure. These hetero-channels are then demonstrated to improve
significantly the operation of graphene field-effect-transistors (FETs). In
particular, ON/OFF current ratio can reach a value of over 10. In graphene
normal FETs, transconductance, though reduced compared to the case of
unstrained devices, is still high while good saturation of current can be
obtained. This results in high voltage gain and high transition frequency of a
few hundreds of GHz for a gate length of 80 nm. In graphene tunneling FETs,
subthreshold swing lower than 30 mV/dec, strong non-linear effects such as gate
controllable negative differential conductance, and current rectification are
observed.Comment: 7 pages, 6 figures, submitte
Conduction gap in graphene strain junctions: direction dependence
It has been shown in a recent study [Nguyen et al., Nanotechnol. \textbf{25},
165201 (2014)] that unstrained/strained graphene junctions are promising
candidates to improve the performance of graphene transistors that is usually
hindered by the gapless nature of graphene. Although the energy bandgap of
strained graphene still remains zero, the shift of Dirac points in the
\textbf{\emph{k}}-space due to strain-induced deformation of graphene lattice
can lead to the appearance of a finite conduction gap of several hundreds meV
in strained junctions with a strain of only a few percent. However, since it
depends essentially on the magnitude of Dirac point shift, this conduction gap
strongly depends on the direction of applied strain and the transport
direction. In this work, a systematic study of conduction gap properties with
respect to these quantities is presented and the results are carefully
analyzed. Our study provides useful information for further investigations to
exploit graphene strained junctions in electronic applications.Comment: 9 pages, 7 figures, submitte
Strain-induced modulation of Dirac cones and van Hove singularities in twisted graphene bilayer
By means of atomistic tight-binding calculations, we investigate the effects
of uniaxial strain on the electronic bandstructure of twisted graphene bilayer.
We find that the bandstructure is dramatically deformed and the degeneracy of
the bands is broken by strain. As a conseqence, the number of Dirac cones can
double and the van Hove singularity points are separated in energy. The
dependence of these effects on the strength of strain, its applied direction
and the twist angle is carefully clarified. As an important result, we
demonstrate that the position of van Hove singularities can be modulated by
strain, suggesting the possibility of observing this phenomenon at low energy
in a large range of twist angle (i.e., larger than ). Unfortunately,
these interesting/important phenomena have not been clarified in the previous
works based on the continuum approximation. While they are in good agreement
with available experiments, our results provide a detailed understanding of the
strain effects on the electronic properties and may motivate other
investigations of electronic transport in this type of graphene lattice.Comment: 8 pages, 7 figures, submitte
Normal forms of vector fields on Poisson manifolds
We study formal and analytic normal forms of radial and Hamiltonian vector
fields on Poisson manifolds near a singular point.Comment: Final versio
Deformation of singular foliations, 1: Local deformation cohomology
In this paper we introduce the notion of deformation cohomology for singular
foliations and related objects (namely integrable differential forms and Nambu
structures), and study it in the local case, i.e., in the neighborhood of a
point
Levi decomposition for smooth Poisson structures
We prove the existence of a local smooth Levi decomposition for smooth
Poisson structures and Lie algebroids near a singular point. In the appendix of
this paper, we show an abstract Nash-Moser normal form theorem, which
generalizes our Levi decomposition result and which may be helpful in the study
of other smooth normal form problems.Comment: 38 pages. The proof of the main theorem is simplified. An appendix
about an abstract Nash-Moser normal form theorem is adde
Enhanced Seebeck effect in graphene devices by strain and doping engineering
In this work, we investigate the possibility of enhancing the thermoelectric
power (Seebeck coefficient) in graphene devices by strain and doping
engineering. While a local strain can result in the misalignment of Dirac cones
of different graphene sections in the k-space, doping engineering leads to
their displacement in energy. By combining these two effects, we demonstrate
that a conduction gap as large as a few hundreds meV can be achieved and hence
the enhanced Seebeck coefficient can reach a value higher than 1.4 mV/K in
graphene doped heterojunctions with a locally strained area. Such
hetero-channels appear to be very promising for enlarging the applications of
graphene devices as in strain and thermal sensors
Larval rearing of the Asian catfish, Pangasius bocourti (Silurlformes, Pangasiidae): Artemia alternative feeding and weaning time
Cold-start Problems in Recommendation Systems via Contextual-bandit Algorithms
In this paper, we study a cold-start problem in recommendation systems where
we have completely new users entered the systems. There is not any interaction
or feedback of the new users with the systems previoustly, thus no ratings are
available. Trivial approaches are to select ramdom items or the most popular
ones to recommend to the new users. However, these methods perform poorly in
many case. In this research, we provide a new look of this cold-start problem
in recommendation systems. In fact, we cast this cold-start problem as a
contextual-bandit problem. No additional information on new users and new items
is needed. We consider all the past ratings of previous users as contextual
information to be integrated into the recommendation framework. To solve this
type of the cold-start problems, we propose a new efficient method which is
based on the LinUCB algorithm for contextual-bandit problems. The experiments
were conducted on three different publicly-available data sets, namely
Movielens, Netflix and Yahoo!Music. The new proposed methods were also compared
with other state-of-the-art techniques. Experiments showed that our new method
significantly improves upon all these methods
Interplay between Aharonov-Bohm interference and parity selective tunneling in zigzag graphene nanoribbon rings
We report a numerical study on Aharonov-Bohm (AB) effect and parity selective
tunneling in pn junctions based on zigzag graphene nanoribbon rings. We find
that when applying a magnetic field to the ring, the AB interference can
reverse the parity symmetry of incoming waves and hence can strongly modulate
the parity selective transmission through the system. Therefore, the
transmission between two states of different parity exhibits the AB
oscillations with a \pi-phase shift, compared to the case of states of same
parity. On this basis, it is shown that interesting effects such as giant (both
positive and negative) magnetoresistance and strong negative differential
conductance can be achieved in this structure. Our study thus presents a new
property of the AB interference, which could be helpful to further understand
the transport properties of graphene mesoscopic-systems.Comment: 6 pages, 5 figures, submitte
- …