201 research outputs found
Tight binding description of the electronic response of a molecular device to an applied voltage
We analyze the effect of an external electric field on the electronic
structure of molecules which have been recently studied as molecular wires or
diodes. We use a self-consistent tight binding technique which provides results
in good agreement with ab initio calculations and which may be applied to a
large number of molecules. The voltage dependence of the molecular levels is
mainly linear with slopes intimately related to the electronic structure of the
molecules. We emphasize that the response to the applied voltage is an
important feature which governs the behavior of a molecular device
Assessment of the notions of band offsets, wells and barriers at nanoscale semiconductor heterojunctions
Epitaxially-grown semiconductor heterostructures give the possibility to
tailor the potential landscape for the carriers in a very controlled way. In
planar lattice-matched heterostructures, the potential has indeed a very simple
and easily predictable behavior: it is constant everywhere except at the
interfaces where there is a step (discontinuity) which only depends on the
composition of the semiconductors in contact. In this paper, we show that this
universally accepted picture can be invalid in nanoscale heterostructures
(e.g., quantum dots, rods, nanowires) which can be presently fabricated in a
large variety of forms. Self-consistent tight-binding calculations applied to
systems containing up to 75 000 atoms indeed demonstrate that the potential may
have a more complex behavior in axial hetero-nanostructures: The band edges can
show significant variations far from the interfaces if the nanostructures are
not capped with a homogeneous shell. These results suggest new strategies to
engineer the electronic properties of nanoscale objects, e.g. for sensors and
photovoltaics.Comment: Accepted for publication in Phys. Rev.
Adsorption behavior of conjugated {C}3-oligomers on Si(100) and HOPG surfaces
A pi-conjugated {C}3h-oligomer involving three dithienylethylene branches
bridged at the meta positions of a central benzenic core has been synthesized
and deposited either on the Si(100) surface or on the HOPG surface. On the
silicon surface, scanning tunneling microscopy allows the observation of
isolated molecules. Conversely, by substituting the thiophene rings of the
oligomers with alkyl chains, a spontaneous ordered film is observed on the HOPG
surface. As the interaction of the oligomers is different with both surfaces,
the utility of the Si(100) surface to characterize individual oligomers prior
to their use into a 2D layer is discussed
Kekule versus hidden superconducting order in graphene-like systems: Competition and coexistence
We theoretically study the competition between two possible exotic
superconducting orders that may occur in graphene-like systems, assuming
dominant nearest-neighbor attraction: the gapless hidden superconducting order,
which renormalizes the Fermi velocity, and the Kekule order, which opens a
superconducting gap. We perform an analysis within the mean-field theory for
Dirac electrons, at finite-temperature and finite chemical potential, as well
as at half filling and zero-temperature, first excluding the possibility of the
coexistence of the two orders. In that case, we find the dependence of the
critical (more precisely, crossover) temperature and the critical interaction
on the chemical potential. As a result of this analysis, we find that the
Kekule order is preferred over the hidden order at both finite temperature and
finite chemical potential. However, when the coexistence of the two
superconducting orders is allowed, using the coupled mean-field gap equations,
we find that above a critical value of the attractive interaction a mixed phase
sets in, in which these orders coexist. We show that the critical value of the
interaction for this transition is greater than the critical coupling for the
hidden superconducting state in the absence of the Kekule order, implying that
there is a region in the phase diagram where the Kekule order is favored as a
result of the competition with the hidden superconducting order. The latter,
however, eventually sets in and coexists with the Kekule state. According to
our mean-field calculations, the transition from the Kekule to the mixed phase
is of the second order, but it may become first order when fluctuations are
considered. Finally, we investigate whether these phases could be possible in
honeycomb superlattices of self-assembled semiconducting nanocrystals, which
have been recently experimentally realized with CdSe and PbSe.Comment: 15 pages, 9 figures, published version. Minor changes, new references
adde
Molecular rectifying diodes from self-assembly on silicon
We demonstrate a molecular rectifying junction made from a sequential
self-assembly on silicon. The device structure consists of only one conjugated
(p) group and an alkyl spacer chain. We obtain rectification ratios up to 37
and threshold voltages for rectification between -0.3V and -0.9V. We show that
rectification occurs from resonance through the highest occupied molecular
orbital of the p-group in good agreement with our calculations and internal
photoemission spectroscopy. This approach allows us to fabricate molecular
rectifying diodes compatible with silicon nanotechnologies for future hybrid
circuitries
Modelling of spin decoherence in a Si hole qubit perturbed by a single charge fluctuator
Spin qubits in semiconductor quantum dots are one of the promizing devices to
realize a quantum processor. A better knowledge of the noise sources affecting
the coherence of such a qubit is therefore of prime importance. In this work,
we study the effect of telegraphic noise induced by the fluctuation of a single
electric charge. We simulate as realistically as possible a hole spin qubit in
a quantum dot defined electrostatically by a set of gates along a silicon
nanowire channel. Calculations combining Poisson and time-dependent
Schr\"odinger equations allow to simulate the relaxation and the dephasing of
the hole spin as a function of time for a classical random telegraph signal. We
show that dephasing time is well given by a two-level model in a wide
range of frequency. Remarkably, in the most realistic configuration of a low
frequency fluctuator, the system has a non-Gaussian behavior in which the phase
coherence is lost as soon as the fluctuator has changed state. The Gaussian
description becomes valid only beyond a threshold frequency , when
the two-level system reacts to the statistical distribution of the fluctuator
states. We show that the dephasing time at this threshold
frequency can be considerably increased by playing on the orientation of the
magnetic field and the gate potentials, by running the qubit along "sweet"
lines. However, remains bounded due to dephasing induced
by the non-diagonal terms of the stochastic perturbation Hamiltonian. Our
simulations reveal that the spin relaxation cannot be described cleanly in the
two-level model because the coupling to higher energy hole levels impacts very
strongly the spin decoherence. This result suggests that multi-level
simulations including the coupling to phonons should be necessary to describe
the relaxation phenomenon in this type of qubit
Quantum confinement effects in Pb Nanocrystals grown on InAs
In the recent work of Ref.\cite{Vlaic2017-bs}, it has been shown that Pb
nanocrystals grown on the electron accumulation layer at the (110) surface of
InAs are in the regime of Coulomb blockade. This enabled the first scanning
tunneling spectroscopy study of the superconducting parity effect across the
Anderson limit. The nature of the tunnel barrier between the nanocrystals and
the substrate has been attributed to a quantum constriction of the electronic
wave-function at the interface due to the large Fermi wavelength of the
electron accumulation layer in InAs. In this manuscript, we detail and review
the arguments leading to this conclusion. Furthermore, we show that, thanks to
this highly clean tunnel barrier, this system is remarkably suited for the
study of discrete electronic levels induced by quantum confinement effects in
the Pb nanocrystals. We identified three distinct regimes of quantum
confinement. For the largest nanocrystals, quantum confinement effects appear
through the formation of quantum well states regularly organized in energy and
in space. For the smallest nanocrystals, only atomic-like electronic levels
separated by a large energy scale are observed. Finally, in the intermediate
size regime, discrete electronic levels associated to electronic wave-functions
with a random spatial structure are observed, as expected from Random Matrix
Theory.Comment: Main 12 pages, Supp: 6 page
Phonon-limited carrier mobility and resistivity from carbon nanotubes to graphene
Under which conditions do the electrical transport properties of
one-dimensional (1D) carbon nanotubes (CNTs) and 2D graphene become equivalent?
We have performed atomistic calculations of the phonon-limited electrical
mobility in graphene and in a wide range of CNTs of different types to address
this issue. The theoretical study is based on a tight-binding method and a
force-constant model from which all possible electron-phonon couplings are
computed. The electrical resistivity of graphene is found in very good
agreement with experiments performed at high carrier density. A common
methodology is applied to study the transition from 1D to 2D by considering
CNTs with diameter up to 16 nm. It is found that the mobility in CNTs of
increasing diameter converges to the same value, the mobility in graphene. This
convergence is much faster at high temperature and high carrier density. For
small-diameter CNTs, the mobility strongly depends on chirality, diameter, and
existence of a bandgap.Comment: 12 page
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