81 research outputs found
Electronic response and bandstructure modulation of carbon nanotubes in a transverse electrical field
The electronic properties of carbon nanotubes in a uniform transverse field
are investigated within a single orbital tight-binding model. For doped
nanotubes, the dielectric function is found to depend not only on symmetry of
the tube, but also on radius and Fermi level position. Bandgap opening/closing
is predicted for zigzag tubes, while it is found that armchair tubes always
remain metallic, which is explained by the symmetry in their configuration. The
bandstructures for both types are considerably modified when the field strength
is large enough to mix neighboring subbands.Comment: Accepted for publication in Nanoletters, 8 pages, 3 figure
Metal-Semiconductor Transition in Armchair Carbon Nanotubes by Symmetry Breaking
The electronic band structure of armchair carbon nanotubes may be
considerably modified by potentials with angular dependence. Different angular
modes V_q ~ cos(q*theta) have been studied within a tight-binding scheme. Using
symmetry arguments, we demonstrate a bandgap opening in these metallic
nanotubes when certain selection rules are satisfied for both potential and
nanotube structure. We estimate the bandgap opening as a function of both the
external potential strength and the nanotube radius and suggest an effective
mechanism of metal-semiconductor transition by combination of different forms
of perturbations.Comment: 3 pages, 3 figures, published on AP
A Backscattering Model Incorporating the Effective Carrier Temperature in Nano MOSFET
In this work we propose a channel backscattering model in which increased
carrier temperature at the top of the potential energy barrier in the channel
is taken into account. This model represents an extension of a previous model
by the same authors which highlighted the importance of considering the
partially ballistic transport between the source contact and the top of the
potential energy barrier in the channel. The increase of carrier temperature is
precisely due to energy dissipation between the source contact and the top of
the barrier caused by the high saturation current. To support our discussion,
accurate 2D full band Monte Carlo device simulations with quantum correction
have been performed in double gate nMOSFETs for different geometries (gate
length down to 10 nm), biases and lattice temperatures. Including the effective
carrier temperature is especially important to properly treat the high
inversion regime, where previous backscattering models usually fail
A Universal Framework for Generalized Run Time Assurance with JAX Automatic Differentiation
With the rise of increasingly complex autonomous systems powered by black box
AI models, there is a growing need for Run Time Assurance (RTA) systems that
provide online safety filtering to untrusted primary controller output.
Currently, research in RTA tends to be ad hoc and inflexible, diminishing
collaboration and the pace of innovation. The Safe Autonomy Run Time Assurance
Framework presented in this paper provides a standardized interface for RTA
modules and a set of universal implementations of constraint-based RTA capable
of providing safety assurance given arbitrary dynamical systems and
constraints. Built around JAX, this framework leverages automatic
differentiation to populate advanced optimization based RTA methods minimizing
user effort and error. To validate the feasibility of this framework, a
simulation of a multi-agent spacecraft inspection problem is shown with safety
constraints on position and velocity
Metal-Semiconductor Transition and Fermi Velocity Renormalization in Metallic Carbon Nanotubes
Angular perturbations modify the band structure of armchair (and other
metallic) carbon nanotubes by breaking the tube symmetry and may induce a
metal-semiconductor transition when certain selection rules are satisfied. The
symmetry requirements apply for both the nanotube and the perturbation
potential, as studied within a nonorthogonal -orbital tight-binding
method. Perturbations of two categories are considered: an on-site
electrostatic potential and a lattice deformation which changes the off-site
hopping integrals. Armchair nanotubes are proved to be robust against the
metal-semiconductor transition in second-order perturbation theory due to their
high symmetry, but can develop a nonzero gap by extending the perturbation
series to higher orders or by combining potentials of different types. An
assumption of orthogonality between orbitals is shown to lead to an
accidental electron-hole symmetry and extra selection rules that are weakly
broken in the nonorthogonal theory. These results are further generalized to
metallic nanotubes of arbitrary chirality.Comment: Submitted to Phys. Rev. B, 23 pages, 4 figure
Simulation of a long term memory device with a full bandstructure Monte Carlo approach
Simulations of charging characteristics of a long
term memory device, based on a floating gate structure, are
presented. The analysis requires the inclusion of hot electron effects
and a detailed account of the semiconductor bandstructure,
because device operation is based on the injection of electrons
into the gate oxide high above the silicon conduction band edge.
We have developed a Monte Carlo simulator based on a full
bandstructure approach which accurately accounts for the high
energy tail of the electron distribution function. For practical
simulation of the prototype structure, with 3.0-pm source-drain
separation, the simulator is used as a post-processor on the
potential profile obtained from a PISCES IIB drift-diffusion
solution. The computations are in quantitative agreement with
experimental results for the gate injection current, measured at
fixed drain and gate biases
Carcinogenesis and Metastasis in Liver: Cell Physiological Basis
Hepatocellular carcinoma (HCC) incidence is rising. This paper summarises the current state of knowledge and recent discoveries in the cellular and physiological mechanisms leading to the development of liver cancer, especially HCC, and liver metastases. After reviewing normal hepatic cytoarchitecture and immunological characteristics, the paper addresses the pathophysiological factors that cause liver damage and predispose to neoplasia. Particular attention is given to chronic liver diseases, metabolic syndrome and the impact of altered gut microbiota, disrupted circadian rhythm and psychological stress. Improved knowledge of the multifactorial aetiology of HCC has important implications for the prevention and treatment of this cancer and of liver metastases in general
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