149 research outputs found
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.
Transport Length Scales in Disordered Graphene-based Materials: Strong Localization Regimes and Dimensionality Effects
We report on a numerical study of quantum transport in disordered two
dimensional graphene and graphene nanoribbons. By using the Kubo and the
Landauer approaches, transport length scales in the diffusive (mean free path,
charge mobilities) and localized regimes (localization lengths) are computed,
assuming a short range disorder (Anderson-type). In agreement with localization
scaling theory, the electronic systems are found to undergo a conventional
Anderson localization in the zero temperature limit. Localization lengths in
weakly disordered ribbons are found to differ by two orders of magnitude
depending on their edge symmetry, but always remain several orders of magnitude
smaller than those computed for 2D graphene for the same disorder strength.
This pinpoints the role of transport dimensionality and edge effects.Comment: 4 pages, Phys. rev. Lett. (in press
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 Communication with Quantum Dot Spins
Single electron spins in quantum dots are attractive for quantum
communication because of their expected long coherence times. We propose a
method to create entanglement between two remote spins based on the coincident
detection of two photons emitted by the dots. Local nodes of several qubits can
be realized using the dipole-dipole interaction between trions in neighboring
dots and spectral addressing, allowing the realization of quantum repeater
protocols. We have performed a detailed feasibility study of our proposal based
on tight-binding calculations of quantum dot properties.Comment: 4 pages, 2 figures, new and improved version, explicit performance
estimate
Magneto-transport Subbands Spectroscopy in InAs Nanowires
We report on magneto-transport measurements in InAs nanowires under large
magnetic field (up to 55T), providing a direct spectroscopy of the 1D
electronic band structure. Large modulations of the magneto-conductance
mediated by an accurate control of the Fermi energy reveal the Landau
fragmentation, carrying the fingerprints of the confined InAs material. Our
numerical simulations of the magnetic band structure consistently support the
experimental results and reveal key parameters of the electronic confinement.Comment: 13 Pages, 5 figure
Transport properties of 2D graphene containing structural defects
We propose an extensive report on the simulation of electronic transport in
2D graphene in presence of structural defects. Amongst the large variety of
such defects in sp carbon-based materials, we focus on the Stone-Wales
defect and on two divacancy-type reconstructed defects. First, based on ab
initio calculations, a tight-binding model is derived to describe the
electronic structure of these defects. Then, semiclassical transport properties
including the elastic mean free paths, mobilities and conductivities are
computed using an order-N real-space Kubo-Greenwood method. A plateau of
minimum conductivity () is progressively
observed as the density of defects increases. This saturation of the decay of
conductivity to is associated with defect-dependent
resonant energies. Finally, localization phenomena are captured beyond the
semiclassical regime. An Anderson transition is predicted with localization
lengths of the order of tens of nanometers for defect densities around 1%.Comment: 17 pages, 17 figures, submitted to Phys. Rev.
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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