307 research outputs found
Measuring Temperature Gradients over Nanometer Length Scales
When a quantum dot is subjected to a thermal gradient, the temperature of
electrons entering the dot can be determined from the dot's thermocurrent if
the conductance spectrum and background temperature are known. We demonstrate
this technique by measuring the temperature difference across a 15 nm quantum
dot embedded in a nanowire. This technique can be used when the dot's energy
states are separated by many kT and will enable future quantitative
investigations of electron-phonon interaction, nonlinear thermoelectric
effects, and the effciency of thermoelectric energy conversion in quantum dots.Comment: 6 pages, 5 figure
Thermal rectification effect of an interacting quantum dot
We investigate nonlinear thermal transport properties of a single interacting
quantum dot with two energy levels tunnel-coupled to two electrodes using
nonequilibrium Green function method and Hartree-Fock decoupling approximation.
In the case of asymmetric tunnel-couplings to two electrodes, for example, when
the upper level of the quantum dot is open for transport, whereas the lower
level is blocked, our calculations predict a strong asymmetry for the heat
(energy) current, which shows that, the quantum dot system may act as a thermal
rectifier in this specific situation.Comment: 5 pages, 5 figure
Surface Transitions for Confined Associating Mixtures
Thin films of binary mixtures that interact through isotropic forces and
directionally specific "hydrogen bonding" are considered through Monte Carlo
simulations. We show, in good agreement with experiment, that the single phase
of these mixtures can be stabilized or destabilized on confinement. These
results resolve a long standing controversy, since previous theories suggest
that confinement only stabilizes the single phase of fluid mixtures.Comment: LaTeX document, documentstyle[aps,preprint]{revtex}, psfig.sty,
bibtex, 13 pages, 4 figure
Spin-dependent thermoelectric transport through double quantum dots
We study thermoelectric transport through double quantum dots system with
spin-dependent interdot coupling and ferromagnetic electrodes by means of the
non-equilibrium Green function in the linear response regime. It is found that
the thermoelectric coefficients are strongly dependent on the splitting of
interdot coupling, the relative magnetic configurations and the spin
polarization of leads. In particular, the thermoelectric efficiency can achieve
considerable value in parallel configuration when the effective interdot
coupling and tunnel coupling between QDs and the leads for spin-down electrons
are small. Moreover, the thermoelectric efficiency increases with the intradot
Coulomb interactions increasing and can reach very high value at an appropriate
temperature. In the presence of the magnetic field, the spin accumulation in
leads strongly suppresses the thermoelectric efficiency and a pure spin
thermopower can be obtained.Comment: 5 figure
Rectification of electronic heat current by a hybrid thermal diode
We report the realization of an ultra-efficient low-temperature hybrid heat
current rectifier, thermal counterpart of the well-known electric diode. Our
design is based on a tunnel junction between two different elements: a normal
metal and a superconducting island. Electronic heat current asymmetry in the
structure arises from large mismatch between the thermal properties of these
two. We demonstrate experimentally temperature differences exceeding mK
between the forward and reverse thermal bias configurations. Our device offers
a remarkably large heat rectification ratio up to and allows its
prompt implementation in true solid-state thermal nanocircuits and
general-purpose electronic applications requiring energy harvesting or thermal
management and isolation at the nanoscale.Comment: 8 pages, 6 color figure
Delocalized single-photon Dicke states and the Leggett- Garg inequality in solid state systems
We show how to realize a single-photon Dicke state in a large one-dimensional
array of two- level systems, and discuss how to test its quantum properties.
Realization of single-photon Dicke states relies on the cooperative nature of
the interaction between a field reservoir and an array of two-level-emitters.
The resulting dynamics of the delocalized state can display Rabi-like
oscillations when the number of two-level emitters exceeds several hundred. In
this case the large array of emitters is essentially behaving like a
mirror-less cavity. We outline how this might be realized using a
multiple-quantum-well structure and discuss how the quantum nature of these
oscillations could be tested with the Leggett-Garg inequality and its
extensions.Comment: 29 pages, 5 figures, journal pape
Electrical and thermoelectrical transport in Dirac fermions through a quantum dot
We investigate the conductance and thermopower of massless Dirac fermions
through a quantum dot using a pseudogap Anderson model in the non-crossing
approximation. When the Fermi level is at the Dirac point, the conductance has
a cusp where the thermopower changes its sign. When the Fermi level is away
from the Dirac point, the Kondo temperature illustrates a quantum impurity
transition between an asymmetric strong coupling Kondo state and a localized
moment state. The conductance shows a peak near this transition and reaches the
unitary limit at low temperatures. The magnitude of the thermopower exceeds
, and the thermoelectric figure of merit exceeds unity.Comment: 5 pages, 4 figure
Autonomous UAV-based mapping of large-scale urban firefights
This paper describes experimental results from a live-fire data collect designed to demonstrate the ability of IR and acoustic sensing systems to detect and map high-volume gunfire events from tactical UAVs. The data collect supports an exploratory study of the FightSight concept in which an autonomous UAV-based sensor exploitation and decision support capability is being proposed to provide dynamic situational awareness for large-scale battalion-level firefights in cluttered urban environments. FightSight integrates IR imagery, acoustic data, and 3D scene context data with prior time information in a multi-level, multi-step probabilistic-based fusion process to reliably locate and map the array of urban firing events and firepower movements and trends associated with the evolving urban battlefield situation. Described here are sensor results from live-fire experiments involving simultaneous firing of multiple sub/super-sonic weapons (2-AK47, 2-M16, 1 Beretta, 1 Mortar, 1 rocket) with high optical and acoustic clutter at ranges up to 400m. Sensor-shooter-target configurations and clutter were designed to simulate UAV sensing conditions for a high-intensity firefight in an urban environment. Sensor systems evaluated were an IR bullet tracking system by Lawrence Livermore National Laboratory (LLNL) and an acoustic gunshot detection system by Planning Systems, Inc. (PSI). The results demonstrate convincingly the ability for the LLNL and PSI sensor systems to accurately detect, separate, and localize multiple shooters and the associated shot directions during a high-intensity firefight (77 rounds in 5 sec) in a high acoustic and optical clutter environment with no false alarms. Preliminary fusion processing was also examined that demonstrated an ability to distinguish co-located shooters (shooter density), range to <0.5 m accuracy at 400m, and weapon type
Harmonic Sums and Mellin Transforms up to two-loop Order
A systematic study is performed on the finite harmonic sums up to level four.
These sums form the general basis for the Mellin transforms of all individual
functions of the momentum fraction emerging in the quantities of
massless QED and QCD up to two--loop order, as the unpolarized and polarized
splitting functions, coefficient functions, and hard scattering cross sections
for space and time-like momentum transfer. The finite harmonic sums are
calculated explicitly in the linear representation. Algebraic relations
connecting these sums are derived to obtain representations based on a reduced
set of basic functions. The Mellin transforms of all the corresponding Nielsen
functions are calculated.Comment: 44 pages Latex, contract number adde
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