696 research outputs found
Temperature dependence of D'yakonov-Perel' spin relaxation in zinc blende semiconductor quantum structures
The D'yakonov-Perel' mechanism, intimately related to the spin splitting of
the electronic states, usually dominates the spin relaxation in zinc blende
semiconductor quantum structures. Previously it has been formulated for the two
limiting cases of low and high temperatures. Here we extend the theory to give
an accurate description of the intermediate regime which is often relevant for
room temperature experiments. Employing the self-consistent multiband envelope
function approach, we determine the spin splitting of electron subbands in
n-(001) zinc blende semiconductor quantum structures. Using these results we
calculate spin relaxation rates as a function of temperature and obtain
excellent agreement with experimental data.Comment: 9 pages, 4 figure
Phonon Driven Nonlinear Electrical Behavior in Molecular Devices
Electronic transport in a model molecular device coupled to local phonon
modes is theoretically analyzed. The method allows for obtaining an accurate
approximation of the system's quantum state irrespective of the electron and
phonon energy scales. Nonlinear electrical features emerge from the calculated
current-voltage characteristics. The quantum corrections with respect to the
adiabatic limit characterize the transport scenario, and the polaronic
reduction of the effective device-lead coupling plays a fundamental role in the
unusual electrical features.Comment: 14 pages, 4 figure
Interaction-induced chaos in a two-electron quantum-dot system
A quasi-one-dimensional quantum dot containing two interacting electrons is
analyzed in search of signatures of chaos. The two-electron energy spectrum is
obtained by diagonalization of the Hamiltonian including the exact Coulomb
interaction. We find that the level-spacing fluctuations follow closely a
Wigner-Dyson distribution, which indicates the emergence of quantum signatures
of chaos due to the Coulomb interaction in an otherwise non-chaotic system. In
general, the Poincar\'e maps of a classical analog of this quantum mechanical
problem can exhibit a mixed classical dynamics. However, for the range of
energies involved in the present system, the dynamics is strongly chaotic,
aside from small regular regions. The system we study models a realistic
semiconductor nanostructure, with electronic parameters typical of gallium
arsenide.Comment: 4 pages, 3ps figure
Decoherence due to contacts in ballistic nanostructures
The active region of a ballistic nanostructure is an open quantum-mechanical
system, whose nonunitary evolution (decoherence) towards a nonequilibrium
steady state is determined by carrier injection from the contacts. The purpose
of this paper is to provide a simple theoretical description of the
contact-induced decoherence in ballistic nanostructures, which is established
within the framework of the open systems theory. The active region's evolution
in the presence of contacts is generally non-Markovian. However, if the
contacts' energy relaxation due to electron-electron scattering is sufficiently
fast, then the contacts can be considered memoryless on timescales coarsened
over their energy relaxation time, and the evolution of the current-limiting
active region can be considered Markovian. Therefore, we first derive a general
Markovian map in the presence of a memoryless environment, by coarse-graining
the exact short-time non-Markovian dynamics of an abstract open system over the
environment memory-loss time, and we give the requirements for the validity of
this map. We then introduce a model contact-active region interaction that
describes carrier injection from the contacts for a generic two-terminal
ballistic nanostructure. Starting from this model interaction and using the
Markovian dynamics derived by coarse-graining over the effective memory-loss
time of the contacts, we derive the formulas for the nonequilibrium
steady-state distribution functions of the forward and backward propagating
states in the nanostructure's active region. On the example of a double-barrier
tunneling structure, the present approach yields an I-V curve with all the
prominent resonant features. The relationship to the Landauer-B\"{u}ttiker
formalism is also discussed, as well as the inclusion of scattering.Comment: Published versio
Iron and oxygen isotope and element systematics of magnetite from the Los Colorados IOA deposit, Chile: A paradigm shift for IOA deposits?
Universality in Systems with Power-Law Memory and Fractional Dynamics
There are a few different ways to extend regular nonlinear dynamical systems
by introducing power-law memory or considering fractional
differential/difference equations instead of integer ones. This extension
allows the introduction of families of nonlinear dynamical systems converging
to regular systems in the case of an integer power-law memory or an integer
order of derivatives/differences. The examples considered in this review
include the logistic family of maps (converging in the case of the first order
difference to the regular logistic map), the universal family of maps, and the
standard family of maps (the latter two converging, in the case of the second
difference, to the regular universal and standard maps). Correspondingly, the
phenomenon of transition to chaos through a period doubling cascade of
bifurcations in regular nonlinear systems, known as "universality", can be
extended to fractional maps, which are maps with power-/asymptotically
power-law memory. The new features of universality, including cascades of
bifurcations on single trajectories, which appear in fractional (with memory)
nonlinear dynamical systems are the main subject of this review.Comment: 23 pages 7 Figures, to appear Oct 28 201
Thermoelectric properties of the bismuth telluride nanowires in the constant-relaxation-time approximation
Electronic structure of bismuth telluride nanowires with the growth
directions [110] and [015] is studied in the framework of anisotropic effective
mass method using the parabolic band approximation. The components of the
electron and hole effective mass tensor for six valleys are calculated for both
growth directions. For a square nanowire, in the temperature range from 77 K to
500 K, the dependence of the Seebeck coefficient, the electron thermal and
electrical conductivity as well as the figure of merit ZT on the nanowire
thickness and on the excess hole concentration are investigated in the
constant-relaxation-time approximation. The carrier confinement is shown to
play essential role for square nanowires with thickness less than 30 nm. The
confinement decreases both the carrier concentration and the thermal
conductivity but increases the maximum value of Seebeck coefficient in contrast
to the excess holes (impurities). The confinement effect is stronger for the
direction [015] than for the direction [110] due to the carrier mass difference
for these directions. The carrier confinement increases maximum value of ZT and
shifts it towards high temperatures. For the p-type bismuth telluride nanowires
with growth direction [110], the maximum value of the figure of merit is equal
to 1.3, 1.6, and 2.8, correspondingly, at temperatures 310 K, 390 K, 480 K and
the nanowire thicknesses 30 nm, 15 nm, and 7 nm. At the room temperature, the
figure of merit equals 1.2, 1.3, and 1.7, respectively.Comment: 13 pages, 7 figures, 2 tables, typos added, added references for
sections 2-
Combinatorial approach to identify electronically cloaked hollow nanoparticles
The possibility of designing core-shell nanoparticles that are “invisible” to the conduction electrons has been demonstrated recently. A total scattering cross section smaller than 0.01% of the physical cross section was demonstrated by artificially adjusting the parameters of the barrier and the well in a core-shell geometry. In this paper, we aim to extend the developed concept and find realistic material combinations that satisfy the cloaking criteria. We report designs of hollow nanoparticles that could be used to realize the cloaking concept in III–V semiconductor host matrices. Such particles could be used in advanced materials design to enhance and tune the electrical and the thermoelectric properties of a given host matrix. This paper may also contribute to defect engineering by coating defect sites with a proper cloaking layer.United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-FG02-09ER46577
Microbial U isotope fractionation depends on U(VI) reduction rate
U isotope fractionation may serve as an accurate proxy for U(VI) reduction in both modern and ancient environments, if the systematic controls on the magnitude of fractionation (ε) are known. We model the effect of U(VI) reduction kinetics on U isotopic fractionation during U(VI) reduction by a novel Shewanella isolate, Shewanella sp. (NR), in batch incubations. The measured ε values range from 0.96 ± 0.16 to 0.36 ± 0.07‰ and are strongly dependent on the U(VI) reduction rate. The ε decreases with increasing reduction rate constants normalized by cell density and initial U(VI). Reactive transport simulations suggest that the rate dependence of ε is due to a two-step process, where diffusive transport of U(VI) from the bulk solution across a boundary layer is followed by enzymatic reduction. Our results imply that the spatial decoupling of bulk U(VI) solution and enzymatic reduction should be taken into account for interpreting U isotope data from the environment
Silicon-based molecular electronics
Molecular electronics on silicon has distinct advantages over its metallic
counterpart. We describe a theoretical formalism for transport through
semiconductor-molecule heterostructures, combining a semi-empirical treatment
of the bulk silicon bandstructure with a first-principles description of the
molecular chemistry and its bonding with silicon. Using this method, we
demonstrate that the presence of a semiconducting band-edge can lead to a novel
molecular resonant tunneling diode (RTD) that shows negative differential
resistance (NDR) when the molecular levels are driven by an STM potential into
the semiconducting band-gap. The peaks appear for positive bias on a p-doped
and negative for an n-doped substrate. Charging in these devices is compromised
by the RTD action, allowing possible identification of several molecular
highest occupied (HOMO) and lowest unoccupied (LUMO) levels. Recent experiments
by Hersam et al. [1] support our theoretical predictions.Comment: Author list is reverse alphabetical. All authors contributed equally.
Email: rakshit/liangg/ ghosha/[email protected]
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