827 research outputs found
Nanoelectronics
In this chapter we intend to discuss the major trends in the evolution of
microelectronics and its eventual transition to nanoelectronics. As it is well
known, there is a continuous exponential tendency of microelectronics towards
miniaturization summarized in G. Moore's empirical law. There is consensus that
the corresponding decrease in size must end in 10 to 15 years due to physical
as well as economical limits. It is thus necessary to prepare new solutions if
one wants to pursue this trend further. One approach is to start from the
ultimate limit, i.e. the atomic level, and design new materials and components
which will replace the present day MOS (metal-oxide-semi- conductor) based
technology. This is exactly the essence of nanotechnology, i.e. the ability to
work at the molecular level, atom by atom or molecule by molecule, to create
larger structures with fundamentally new molecular orga- nization. This should
lead to novel materials with improved physical, chemi- cal and biological
properties. These properties can be exploited in new devices. Such a goal would
have been thought out of reach 15 years ago but the advent of new tools and new
fabrication methods have boosted the field. We want to give here an overview of
two different subfields of nano- electronics. The first part is centered on
inorganic materials and describes two aspects: i) the physical and economical
limits of the tendency to miniaturiza- tion; ii) some attempts which have
already been made to realize devices with nanometric size. The second part
deals with molecular electronics, where the basic quantities are now molecules,
which might offer new and quite interest- ing possibilities for the future of
nanoelectronicsComment: HAL : hal-00710039, version 2. This version corrects some aspect
concerning the metal-insulator-metal without dot
Discrete Time Quantum Walk Approach to State Transfer
We show that a quantum state transfer, previously studied as a continuous
time process in networks of interacting spins, can be achieved within the model
of discrete time quantum walks with position dependent coin. We argue that due
to additional degrees of freedom, discrete time quantum walks allow to observe
effects which cannot be observed in the corresponding continuous time case.
First, we study a discrete time version of the engineered coupling protocol due
to Christandl et. al. [Phys. Rev. Lett. 92, 187902 (2004)] and then discuss the
general idea of conversion between continuous time quantum walks and discrete
time quantum walks.Comment: 9 pages, 6 figures, comments welcom
Frequency-dependent spontaneous emission rate from CdSe and CdTe nanocrystals: influence of dark states
We studied the rate of spontaneous emission from colloidal CdSe and CdTe
nanocrystals at room temperature. The decay rate, obtained from luminescence
decay curves, increases with the emission frequency in a supra-linear way. This
dependence is explained by the thermal occupation of dark exciton states at
room temperature, giving rise to a strong attenuation of the rate of emission.
The supra-linear dependence is in agreement with the results of tight-binding
calculations.Comment: 11 page
Dramatic impact of pumping mechanism on photon entanglement in microcavity
A theory of entangled photons emission from quantum dot in microcavity under
continuous and pulsed incoherent pumping is presented. It is shown that the
time-resolved two-photon correlations drastically depend on the pumping
mechanism: the continuous pumping quenches the polarization entanglement and
strongly suppresses photon correlation times. Analytical theory of the effect
is presented.Comment: 6 pages, 3 figure
Dirac Cones, Topological Edge States, and Nontrivial Flat Bands in Two-Dimensional Semiconductors with a Honeycomb Nanogeometry
We study theoretically two-dimensional single-crystalline sheets of
semiconductors that form a honeycomb lattice with a period below 10 nm. These
systems could combine the usual semiconductor properties with Dirac bands.
Using atomistic tight-binding calculations, we show that both the atomic
lattice and the overall geometry influence the band structure, revealing
materials with unusual electronic properties. In rocksalt Pb chalcogenides, the
expected Dirac-type features are clouded by a complex band structure. However,
in the case of zinc-blende Cd-chalcogenide semiconductors, the honeycomb
nanogeometry leads to rich band structures, including, in the conduction band,
Dirac cones at two distinct energies and nontrivial flat bands and, in the
valence band, topological edge states. These edge states are present in several
electronic gaps opened in the valence band by the spin-orbit coupling and the
quantum confinement in the honeycomb geometry. The lowest Dirac conduction band
has S-orbital character and is equivalent to the pi-pi* band of graphene but
with renormalized couplings. The conduction bands higher in energy have no
counterpart in graphene; they combine a Dirac cone and flat bands because of
their P-orbital character. We show that the width of the Dirac bands varies
between tens and hundreds of meV. These systems emerge as remarkable platforms
for studying complex electronic phases starting from conventional
semiconductors. Recent advancements in colloidal chemistry indicate that these
materials can be synthesized from semiconductor nanocrystals.Comment: 12 pages, 12 figure
Ab initio calculation of the binding energy of impurities in semiconductors: Application to Si nanowires
We discuss the binding energy E_b of impurities in semiconductors within
density functional theory (DFT) and the GW approximation, focusing on donors in
nanowires as an example. We show that DFT succeeds in the calculation of E_b
from the Kohn-Sham (KS) hamiltonian of the ionized impurity, but fails in the
calculation of E_b from the KS hamiltonian of the neutral impurity, as it
misses most of the interaction of the bound electron with the surface
polarization charges of the donor. We trace this deficiency back to the lack of
screened exchange in the present functionals
Role of local fields in the optical properties of silicon nanocrystals using the tight binding approach
The role of local fields in the optical response of silicon nanocrystals is
analyzed using a tight binding approach. Our calculations show that, at
variance with bulk silicon, local field effects dramatically modify the silicon
nanocrystal optical response. An explanation is given in terms of surface
electronic polarization and confirmed by the fair agreement between the tight
binding results and that of a classical dielectric model. From such a
comparison, it emerges that the classical model works not only for large but
also for very small nanocrystals. Moreover, the dependence on size of the
optical response is discussed, in particular treating the limit of large size
nanocrystals.Comment: 4 pages, 4 figure
Topological states in multi-orbital HgTe honeycomb lattices
Research on graphene has revealed remarkable phenomena arising in the
honeycomb lattice. However, the quantum spin Hall effect predicted at the K
point could not be observed in graphene and other honeycomb structures of light
elements due to an insufficiently strong spin-orbit coupling. Here we show
theoretically that 2D honeycomb lattices of HgTe can combine the effects of the
honeycomb geometry and strong spin-orbit coupling. The conduction bands,
experimentally accessible via doping, can be described by a tight-binding
lattice model as in graphene, but including multi-orbital degrees of freedom
and spin-orbit coupling. This results in very large topological gaps (up to 35
meV) and a flattened band detached from the others. Owing to this flat band and
the sizable Coulomb interaction, honeycomb structures of HgTe constitute a
promising platform for the observation of a fractional Chern insulator or a
fractional quantum spin Hall phase.Comment: includes supplementary materia
Interband, intraband and excited-state direct photon absorption of silicon and germanium nanocrystals embedded in a wide band-gap lattice
Embedded Si and Ge nanocrystals (NCs) in wide band-gap matrices are studied
theoretically using an atomistic pseudopotential approach. From small clusters
to large NCs containing on the order of several thousand atoms are considered.
Effective band-gap values as a function of NC diameter reproduce very well the
available experimental and theoretical data. It is observed that the highest
occupied molecular orbital for both Si and Ge NCs and the lowest unoccupied
molecular orbital for Si NCs display oscillations with respect to size among
the different irreducible representations of the point group to which
these spherical NCs belong. Based on this electronic structure, first the
interband absorption is thoroughly studied which shows the importance of
surface polarization effects that significantly reduce the absorption when
included. This reduction is found to increase with decreasing NC size or with
increasing permittivity mismatch between the NC core and the host matrix.
Reasonable agreement is observed with the experimental absorption spectra where
available. The deformation of spherical NCs into prolate or oblate ellipsoids
are seen to introduce no pronounced effects for the absorption spectra. Next,
intraconduction and intravalence band absorption coefficients are obtained in
the wavelength range from far-infrared to visible region. These results can be
valuable for the infrared photodetection prospects of these NC arrays. Finally,
excited-state absorption at three different optical pump wavelengths, 532 nm,
355 nm and 266 nm are studied for 3- and 4 nm-diameter NCs. This reveals strong
absorption windows in the case of holes and a broad spectrum in the case of
electrons which can especially be relevant for the discussions on achieving
gain in these structures.Comment: Published version, 13 pages, 15 figures, local field effects include
Effect of quantum confinement on the dielectric function of PbSe
Monolayers of lead selenide nanocrystals of a few nanometers in height have been made by electrodeposition on a Au(111) substrate. These layers show a thickness-dependent dielectric function, which was determined using spectroscopic ellipsometry. The experimental results are compared with electronic structure calculations of the imaginary part of the dielectric function of PbSe nanocrystals. We demonstrate that the size-dependent variation of the dielectric function is affected by quantum confinement at well-identifiable points in the Brillouin zone, different from the position of the band-gap transition
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