99 research outputs found
Group V Mixing Effects in the Structural and Optical Properties of (ZnSi)1/2(P)1/4(As)1/4
We present {\it ab initio} total energy and band structure calculations based
on Density Funtional Theory (DFT) within the Local Density Aproximation (LDA)
on group-V mixing effects in the optoelectronic material
. This compound has been recently proposed by
theoretical design as an optically active material in the 1.5 m (0.8 eV)
fiber optics frequency window and with a monolithic integration with the Si
(001) surface. Our results indicate that alloy formation in the group V planes
would likely occur at typical growth conditions. In addition, desired features
such as in-plane lattice constant and energy gap are virtually unchanged and
the optical oscillator strength for band-to-band transitions is increased by a
factor of 6 due to alloying
Polarons in Carbon Nanotubes
We use ab initio total-energy calculations to predict the existence of
polarons in semiconducting carbon nanotubes (CNTs). We find that the CNTs' band
edge energies vary linearly and the elastic energy increases quadratically with
both radial and with axial distortions, leading to the spontaneous formation of
polarons. Using a continuum model parametrized by the ab initio calculations,
we estimate electron and hole polaron lengths, energies and effective masses
and analyze their complex dependence on CNT geometry. Implications of polaron
effects on recently observed electro- and opto-mechanical behavior of CNTs are
discussed.Comment: Revtex preprint format, 12 pages, 2 eps figures, source in LaTeX.
Accepted for publication in Physical Review Letter
The role of the disorder range and electronic energy in the graphene nanoribbons perfect transmission
Numerical calculations based on the recursive Green's functions method in the
tight-binding approximation are performed to calculate the dimensionless
conductance in disordered graphene nanoribbons with Gaussian scatterers.
The influence of the transition from short- to long-ranged disorder on is
studied as well as its effects on the formation of a perfectly conducting
channel. We also investigate the dependence of electronic energy on the
perfectly conducting channel. We propose and calculate a backscattering
estimative in order to establish the connection between the perfectly
conducting channel (with ) and the amount of intervalley scattering.Comment: 7 pages, 9 figures. To be published on Phys. Rev.
Theory and it ab initio calculation of radiative lifetime of excitons in semiconducting carbon nanotubes
We present theoretical analysis and first-principles calculation of the
radiative lifetime of excitons in semiconducting carbon nanotubes. An intrinsic
lifetime of the order of 10 ps is computed for the lowest optically active
bright excitons. The intrinsic lifetime is however a rapid increasing function
of the exciton momentum. Moreover, the electronic structure of the nanotubes
dictates the existence of dark excitons nearby in energy to each bright
exciton. Both effects strongly influence measured lifetime. Assuming a thermal
occupation of bright and dark exciton bands, we find an effective lifetime of
the order of 10 ns at room temperature, in good accord with recent experiments.Comment: 12 pages, 3 figure
Microscopic model of a phononic refrigerator
We analyze a simple microscopic model to pump heat from a cold to a hot
reservoir in a nanomechanical system. The model consists of a one-dimensional
chain of masses and springs coupled to a back gate through which a
time-dependent perturbation is applied. The action of the gate is to modulate
the coupling of the masses to a substrate via additional springs that introduce
a moving phononic barrier. We solve the problem numerically using
non-equilibrium Green function techniques. For low driving frequencies and for
sharp traveling barriers, we show that this microscopic model realizes a phonon
refrigerator.Comment: 9 pages, 4 figure
Effect of post-growth annealing on the optical properties of InAs/GaAs quantum dots: A tight-binding study
We present an atomistic study of the strain field, the one-particle electronic spectrum and the oscillator strength of the fundamental optical transition in chemically disordered InxGa1−xAs pyramidal quantum dots (QDs). Interdiffusion across the interfaces of an originally “pure” InAs dot buried in a GaAs matrix is simulated through a simple model, leading to atomic configurations where the abrupt heterointerfaces are replaced by a spatially inhomogeneous composition profile x. Structural relaxation and the strain field calculations are performed through the Keating valence force field model, while the electronic and optical properties are determined within the empirical tight-binding approach. We analyze the relative impact of two different aspects of the chemical disorder, namely: (i) the effect of the strain relief inside the QD, and (ii) the purely chemical effect due to the group-III atomic species interdiffusion. We find that these effects may be quantitatively comparable, significantly affecting the electronic and optical properties of the dot. Our results are discussed in comparison with recent luminescence studies of intermixed QDs
Heat pumping in nanomechanical systems
We propose using a phonon pumping mechanism to transfer heat from a cold to a
hot body using a propagating modulation of the medium connecting the two
bodies. This phonon pump can cool nanomechanical systems without the need for
active feedback. We compute the lowest temperature that this refrigerator can
achieve.Comment: 4 pages, 1 figure, published versio
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