5,535 research outputs found
Time-dependent thermoelectric transport for nanoscale thermal machines
We analyze an electronic nanoscale thermal machine driven by time-dependent
environment: besides bias and gate voltage variations, we consider also the
less prevailing time modulation of the couplings between leads and dot. We
provide energy and heat current expressions in such situations, as well as
expressions for the power exchanged between the dot+leads system and its
outside. Calculations are made in the Keldysh nonequilibrium Green's function
framework. We apply these results to design a cyclic refrigerator,
circumventing the ambiguity of defining energy flows between subsystems in the
case of strong coupling. For fast lead-dot coupling modulation, we observe
transient currents which cannot be ascribed to charge tunneling.Comment: 9 pages, 6 figure
Powerful Coulomb-drag thermoelectric engine
We investigate a thermoelectric nano-engine whose properties are steered by
Coulomb interaction. The device whose design decouples charge and energy
currents is made up of two interacting quantum dots connected to three
different reservoirs. We show that, by tailoring the tunnel couplings, this
setup can be made very attractive for energy-harvesting prospects, due to a
delivered power that can be of the order of the quantum bound [R. S. Whitney,
Phys. Rev. Lett. 112, 130601 (2014); Entropy 18, 208 (2016)], with a
concomitant fair efficiency. To unveil its properties beyond the sequential
quantum master equation, we apply a nonequilibrium noncrossing approximation in
the Keldysh Green's function formalism, and a quantum master equation that
includes cotunneling processes. Both approaches are rather qualitatively
similar in a large operating regime where sequential tunneling alone fails.Comment: Published version. (The discussion about the energy current in QME
has been expanded
Hund and pair-hopping signature in transport properties of degenerate nanoscale devices
We investigate the signature of a complete Coulomb interaction in transport
properties of double-orbital nanoscale devices. We analyze the specific effects
of Hund exchange and pair hopping terms, calculating in particular stability
diagrams. It turns out that a crude model, with partial Coulomb interaction,
may lead to a misinterpretation of experiments. In addition, it is shown that
spectral weight transfers induced by gate and bias voltages strongly influence
charge current. The low temperature regime is also investigated, displaying
inelastic cotunneling associated with the exchange term, as well as Kondo
conductance enhancement.Comment: 5 pages, 4 figure
On strongly coupled quenched QED4, again: chiral symmetry breaking, Goldstone mechanism and the nature of the continuum limit
We explore the possibility of a trivial continuum limit of strongly coupled
quenched QED4 by contrasting our results with a Nambu--Jona Lasinio equation of
state. The data does not compare favorably with such scenario. We study in
detail the interplay of chiral symmetry breaking with the Goldstone mechanism,
and clarify some puzzling features of past results.Comment: Contribution to Lat94, 3 pages, tar-compressed uuencoded ps fil
Conditions for requiring nonlinear thermoelectric transport theory in nanodevices
In this paper, we examine the conditions under which the nonlinear transport
theory is inescapable, when a correlated quantum dot is symmetrically coupled
to two leads submitted to temperature and voltage biases. By detailed numerical
comparisons between nonlinear and linear currents, we show that the claimed
nonlinear behavior in a temperature gradient for the electric current is not so
genuine, and the linear theory made at the operating temperature is unexpectedly robust. This is demonstrated for the single
impurity Anderson model, in different regimes: resonant tunneling, Coulomb
blockade and Kondo regimes
Evidence for a conformal phase in SU(N) gauge theories
We discuss the existence of a conformal phase in SU(N) gauge theories in four
dimensions. In this lattice study we explore the model in the bare parameter
space, varying the lattice coupling and bare mass. Simulations are carried out
with three colors and twelve flavors of dynamical staggered fermions in the
fundamental representation. The analysis of the chiral order parameter and the
mass spectrum of the theory indicates the restoration of chiral symmetry at
zero temperature and the presence of a Coulomb-like phase, depicting a scenario
compatible with the existence of an infrared stable fixed point at nonzero
coupling. Our analysis supports the conclusion that the onset of the conformal
window for QCD-like theories is smaller than Nf=12, before the loss of
asymptotic freedom at sixteen and a half flavors. We discuss open questions and
future directions.Comment: 11 pages, 11 figures; extended analysis, conclusions unchanged.
(version to appear in PRD
Screening Effects in Superfluid Nuclear and Neutron Matter within Brueckner Theory
Effects of medium polarization are studied for pairing in neutron and
nuclear matter. The screening potential is calculated in the RPA limit,
suitably renormalized to cure the low density mechanical instability of nuclear
matter. The selfenergy corrections are consistently included resulting in a
strong depletion of the Fermi surface. All medium effects are calculated based
on the Brueckner theory. The gap is determined from the generalized gap
equation. The selfenergy corrections always lead to a quenching of the gap,
which is enhanced by the screening effect of the pairing potential in neutron
matter, whereas it is almost completely compensated by the antiscreening effect
in nuclear matter.Comment: 8 pages, 6 Postscript figure
Screening of nuclear pairing in nuclear and neutron matter
The screening potential in the and pairing channels in
neutron and nuclear matter in different approximations is discussed. It is
found that the vertex corrections to the potential are much stronger in nuclear
matter than in neutron matter.Comment: 11 pages, 8 figures, revtex4 styl
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