14,873 research outputs found
Nonequilibrium current driven by a step voltage pulse: an exact solution
One of the most important problems in nanoelectronic device theory is to
estimate how fast or how slow a quantum device can turn on/off a current. For
an arbitrary noninteracting phase-coherent device scattering region connected
to the outside world by leads, we have derived an exact solution for the
nonequilibrium, nonlinear, and time-dependent current driven by both up- and
down-step pulsed voltages. Our analysis is based on the Keldysh nonequilibrium
Green's functions formalism where the electronic structure of the leads as well
as the scattering region are treated on an equal footing. A model calculation
for a quantum dot with a Lorentzian linewidth function shows that the
time-dependent current dynamics display interesting finite-bandwidth effects
not captured by the commonly used wideband approximation
Enhancement of parametric pumping due to Andreev reflection
We report properties of parametric electron pumping in the presence of a
superconducting lead. Due to a constructive interference between the direct
reflection and the multiple Andreev reflection, the pumped current is greatly
enhanced. For both quantum point contacts and double barrier structures at
resonance, we obtain exact solutions in the weak pumping regime showing that
, which should be compared with the result of conductance
. Numerical results are also provided for the strong pumping
regime showing interesting Andreev assisted pumping behaviour
Structure and Dielectric Properties of Amorphous High-kappa Oxides: HfO2, ZrO2 and their alloys
High- metal oxides are a class of materials playing an increasingly
important role in modern device physics and technology. Here we report
theoretical investigations of the properties of structural and lattice
dielectric constants of bulk amorphous metal oxides by a combined approach of
classical molecular dynamics (MD) - for structure evolution, and quantum
mechanical first principles density function theory (DFT) - for electronic
structure analysis. Using classical MD based on the Born-Mayer-Buckingham
potential function within a melt and quench scheme, amorphous structures of
high- metal oxides HfZrO with different values of the
concentration , are generated. The coordination numbers and the radial
distribution functions of the structures are in good agreement with the
corresponding experimental data. We then calculate the lattice dielectric
constants of the materials from quantum mechanical first principles, and the
values averaged over an ensemble of samples agree well with the available
experimental data, and are very close to the dielectric constants of their
cubic form.Comment: 5 pages, 4 figure
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