8,728 research outputs found
Semiclassical transport in nearly symmetric quantum dots II: symmetry-breaking due to asymmetric leads
In this work - the second of a pair of articles - we consider transport
through spatially symmetric quantum dots with leads whose widths or positions
do not obey the spatial symmetry. We use the semiclassical theory of transport
to find the symmetry-induced contributions to weak localization corrections and
universal conductance fluctuations for dots with left-right, up-down, inversion
and four-fold symmetries. We show that all these contributions are suppressed
by asymmetric leads, however they remain finite whenever leads intersect with
their images under the symmetry operation. For an up-down symmetric dot, this
means that the contributions can be finite even if one of the leads is
completely asymmetric. We find that the suppression of the contributions to
universal conductance fluctuations is the square of the suppression of
contributions to weak localization. Finally, we develop a random-matrix theory
model which enables us to numerically confirm these results.Comment: (18pages - 9figures) This is the second of a pair of articles (v3
typos corrected - including in equations
Thermodynamic and quantum bounds on nonlinear DC thermoelectric transport
I consider the non-equilibrium DC transport of electrons through a quantum
system with a thermoelectric response. This system may be any nanostructure or
molecule modeled by the nonlinear scattering theory which includes Hartree-like
electrostatic interactions exactly, and certain dynamic interaction effects
(decoherence and relaxation) phenomenologically. This theory is believed to be
a reasonable model when single-electron charging effects are negligible. I
derive three fundamental bounds for such quantum systems coupled to multiple
macroscopic reservoirs, one of which may be superconducting. These bounds
affect nonlinear heating (such as Joule heating), work and entropy production.
Two bounds correspond to the first law and second law of thermodynamics in
classical physics. The third bound is quantum (wavelength dependent), and is as
important as the thermodynamic ones in limiting the capabilities of mesoscopic
heat-engines and refrigerators. The quantum bound also leads to Nernst's
unattainability principle that the quantum system cannot cool a reservoir to
absolute zero in a finite time, although it can get exponentially close.Comment: 8pages (2figs) version2 (PRB version) minor improvements of earlier
versio
Clover Root Curculio Injury and Abundance in Minnesota Alfalfa of Different Stand Age
Root injury and midsummer adult abundance of clover root curculio (CRC), Sitona hispidulus (Fabricius), was surveyed in 24 different aged southeastern Minnesota alfalfa fields. CRC presence was not detected in first year fields 2-3 months following stand establishment. Increased root scarring was observed as stand age increased and most fields 3-5 years old showed heavy CRC root scarring. High CRC adult popUlations and widespread root scarring was found in second year alfalfa fields which suggests that CRC is a mobile insect capable of rapidly colonizing new plantings
Nonlinear thermoelectricity in point-contacts at pinch-off: a catastrophe aids cooling
We consider refrigeration and heat engine circuits based on the nonlinear
thermoelectric response of point-contacts at pinch-off, allowing for
electrostatic interaction effects. We show that a refrigerator can cool to much
lower temperatures than predicted by the thermoelectric figure-of-merit ZT
(which is based on linear-response arguments). The lowest achievable
temperature has a discontinuity, called a fold catastrophe in mathematics, at a
critical driving current I=I_c. For I >I_c one can in principle cool to
absolute zero, when for I<I_c the lowest temperature is about half the ambient
temperature. Heat back-flow due to phonons and photons stop cooling at a
temperature above absolute zero, and above a certain threshold turns the
discontinuity into a sharp cusp. We also give a heuristic condition for when an
arbitrary system's nonlinear response means that its ZT ceases to indicate
(even qualitatively) the lowest temperature to which the system can
refrigerate.Comment: 7 pages 7 figs (changes in this version : Suppl. material expanded &
integrated into text, more references
Quantum coherent three-terminal thermoelectrics: maximum efficiency at given power output
We consider the nonlinear scattering theory for three-terminal thermoelectric
devices, used for power generation or refrigeration. Such systems are quantum
phase-coherent versions of a thermocouple, and the theory applies to systems in
which interactions can be treated at a mean-field level. We consider an
arbitrary three-terminal system in any external magnetic field, including
systems with broken time-reversal symmetry, such as chiral thermoelectrics, as
well as systems in which the magnetic field plays no role. We show that the
upper bound on efficiency at given power output is of quantum origin and is
stricter than Carnot's bound. The bound is exactly the same as previously found
for two-terminal devices, and can be achieved by three-terminal systems with or
without broken time-reversal symmetry, i.e. chiral and non-chiral
thermoelectrics.Comment: 22 pages (final version
Finding the quantum thermoelectric with maximal efficiency and minimal entropy production at given power output
We investigate the nonlinear scattering theory for quantum systems with
strong Seebeck and Peltier effects, and consider their use as heat-engines and
refrigerators with finite power outputs. This article gives detailed
derivations of the results summarized in Phys. Rev. Lett. 112, 130601 (2014).
It shows how to use the scattering theory to find (i) the quantum
thermoelectric with maximum possible power output, and (ii) the quantum
thermoelectric with maximum efficiency at given power output. The latter
corresponds to a minimal entropy production at that power output. These
quantities are of quantum origin since they depend on system size over
electronic wavelength, and so have no analogue in classical thermodynamics. The
maximal efficiency coincides with Carnot efficiency at zero power output, but
decreases with increasing power output. This gives a fundamental lower bound on
entropy production, which means that reversibility (in the thermodynamic sense)
is impossible for finite power output. The suppression of efficiency by
(nonlinear) phonon and photon effects is addressed in detail; when these
effects are strong, maximum efficiency coincides with maximum power. Finally,
we show in particular limits (typically without magnetic fields) that
relaxation within the quantum system does not allow the system to exceed the
bounds derived for relaxation-free systems, however, a general proof of this
remains elusive.Comment: 20 pages 13 figures. Final version (typos fixed
Suppression of Shot-Noise in Quantum Cavities: Chaos vs. Disorder
We investigate the behavior of the shot-noise power through quantum
mechanical cavities in the semiclassical limit of small electronic wavelength.
In the absence of impurity scattering, the Fano factor , giving the noise to
current ratio, was previously found to disappear as more and more classical,
hence deterministic and noiseless transmission channels open up. We investigate
the behavior of as diffractive impurities are added inside the cavity. We
find that recovers its universal value provided (i) impurities cover the
full cavity so that only a set of zero measure of classical trajectories may
avoid them, and (ii) the impurity scattering rate exceeds the inverse dwell
time through the cavity. If condition (i) is not satisfied, saturates below
its universal value, even in the limit of strong scattering. Our results
corroborate the validity of the two-phase fluid model according to which the
electronic flow splits into two well separated components, a classical
deterministic fluid and a stochastic quantum-mechanical fluid. Only the latter
carries shot-noise.Comment: 6 pages, 3 figures, to appear in the proceedings of the fourth
conference on ``Unsolved Problems of Noise and Fluctuations in Physics,
Biology and High Technology'
Thermoelectric coefficients and the figure of merit for large open quantum dots
We consider the thermoelectric response of chaotic or disordered quantum dots
in the limit of phase-coherent transport, statistically described by random
matrix theory. We calculate the full distribution of the thermoelectric
coefficients (Seebeck and Peltier ), and the thermoelectric figure of
merit , for large open dots at arbitrary temperature and external magnetic
field, when the number of modes in the left and right leads ( and
) are large. Our results show that the thermoelectric coefficients
and are maximal when the temperature is half the Thouless energy, and the
magnetic field is negligible. They remain small, even at their maximum, but
they exhibit a type of universality at all temperatures, in which they do not
depend on the asymmetry between the left and right leads , even though they depend on .Comment: 25 pages [final version - minor typos fixed
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