134 research outputs found
The Josephson heat interferometer
The Josephson effect represents perhaps the prototype of macroscopic phase
coherence and is at the basis of the most widespread interferometer, i.e., the
superconducting quantum interference device (SQUID). Yet, in analogy to
electric interference, Maki and Griffin predicted in 1965 that thermal current
flowing through a temperature-biased Josephson tunnel junction is a stationary
periodic function of the quantum phase difference between the superconductors.
The interplay between quasiparticles and Cooper pairs condensate is at the
origin of such phase-dependent heat current, and is unique to Josephson
junctions. In this scenario, a temperature-biased SQUID would allow heat
currents to interfere thus implementing the thermal version of the electric
Josephson interferometer. The dissipative character of heat flux makes this
coherent phenomenon not less extraordinary than its electric (non-dissipative)
counterpart. Albeit weird, this striking effect has never been demonstrated so
far. Here we report the first experimental realization of a heat
interferometer. We investigate heat exchange between two normal metal
electrodes kept at different temperatures and tunnel-coupled to each other
through a thermal `modulator' in the form of a DC-SQUID. Heat transport in the
system is found to be phase dependent, in agreement with the original
prediction. With our design the Josephson heat interferometer yields
magnetic-flux-dependent temperature oscillations of amplitude up to ~21 mK, and
provides a flux-to-temperature transfer coefficient exceeding ~ 60mK/Phi_0 at
235 mK [Phi_0 2* 10^(-15) Wb is the flux quantum]. Besides offering remarkable
insight into thermal transport in Josephson junctions, our results represent a
significant step toward phase-coherent mastering of heat in solid-state
nanocircuits, and pave the way to the design of novel-concept coherent
caloritronic devices.Comment: 4+ pages, 3 color figure
Thermally driven spin injection from a ferromagnet into a non-magnetic metal
Creating, manipulating and detecting spin polarized carriers are the key
elements of spin based electronics. Most practical devices use a perpendicular
geometry in which the spin currents, describing the transport of spin angular
momentum, are accompanied by charge currents. In recent years, new sources of
pure spin currents, i.e., without charge currents, have been demonstrated and
applied. In this paper, we demonstrate a conceptually new source of pure spin
current driven by the flow of heat across a ferromagnetic/non-magnetic metal
(FM/NM) interface. This spin current is generated because the Seebeck
coefficient, which describes the generation of a voltage as a result of a
temperature gradient, is spin dependent in a ferromagnet. For a detailed study
of this new source of spins, it is measured in a non-local lateral geometry. We
developed a 3D model that describes the heat, charge and spin transport in this
geometry which allows us to quantify this process. We obtain a spin Seebeck
coefficient for Permalloy of -3.8 microvolt/Kelvin demonstrating that thermally
driven spin injection is a feasible alternative for electrical spin injection
in, for example, spin transfer torque experiments
Efficiency of Energy Conversion in Thermoelectric Nanojunctions
Using first-principles approaches, this study investigated the efficiency of
energy conversion in nanojunctions, described by the thermoelectric figure of
merit . We obtained the qualitative and quantitative descriptions for the
dependence of on temperatures and lengths. A characteristic temperature:
was observed. When , . When , tends to a saturation value. The dependence of
on the wire length for the metallic atomic chains is opposite to that for
the insulating molecules: for aluminum atomic (conducting) wires, the
saturation value of increases as the length increases; while for
alkanethiol (insulating) chains, the saturation value of decreases as the
length increases. can also be enhanced by choosing low-elasticity bridging
materials or creating poor thermal contacts in nanojunctions. The results of
this study may be of interest to research attempting to increase the efficiency
of energy conversion in nano thermoelectric devices.Comment: 2 figure
Nonlinear thermoelectric response of quantum dots: renormalized dual fermions out of equilibrium
The thermoelectric transport properties of nanostructured devices continue to
attract attention from theorists and experimentalist alike as the spatial
confinement allows for a controlled approach to transport properties of
correlated matter. Most of the existing work, however, focuses on
thermoelectric transport in the linear regime despite the fact that the
nonlinear conductance of correlated quantum dots has been studied in some
detail throughout the last decade. Here, we review our recent work on the
effect of particle-hole asymmetry on the nonlinear transport properties in the
vicinity of the strong coupling limit of Kondo-correlated quantum dots and
extend the underlying method, a renormalized superperturbation theory on the
Keldysh contour, to the thermal conductance in the nonlinear regime. We
determine the charge, energy, and heat current through the nanostructure and
study the nonlinear transport coefficients, the entropy production, and the
fate of the Wiedemann-Franz law in the non-thermal steady-state. Our approach
is based on a renormalized perturbation theory in terms of dual fermions around
the particle-hole symmetric strong-coupling limit.Comment: chapter contributed to 'New Materials for Thermoelectric
Applications: Theory and Experiment' Springer Series: NATO Science for Peace
and Security Series - B: Physics and Biophysics, Veljko Zlatic (Editor), Alex
Hewson (Editor). ISBN: 978-9400749863 (2012
Effect of Thermoelectric Cooling in Nanoscale Junctions
We propose a thermoelectric cooling device based on an atomic-sized junction.
Using first-principles approaches, we investigate the working conditions and
the coefficient of performance (COP) of an atomic-scale electronic refrigerator
where the effects of phonon's thermal current and local heating are included.
It is observed that the functioning of the thermoelectric nano-refrigerator is
restricted to a narrow range of driving voltages. Compared with the bulk
thermoelectric system with the overwhelmingly irreversible Joule heating, the
4-Al atomic refrigerator has a higher efficiency than a bulk thermoelectric
refrigerator with the same due to suppressed local heating via the
quasi-ballistic electron transport and small driving voltages. Quantum nature
due to the size minimization offered by atomic-level control of properties
facilitates electron cooling beyond the expectation of the conventional
thermoelectric device theory.Comment: 8 figure
Single and two-particle energy gaps across the disorder-driven superconductor-insulator transition
The competition between superconductivity and localization raises profound
questions in condensed matter physics. In spite of decades of research, the
mechanism of the superconductor-insulator transition (SIT) and the nature of
the insulator are not understood. We use quantum Monte Carlo simulations that
treat, on an equal footing, inhomogeneous amplitude variations and phase
fluctuations, a major advance over previous theories. We gain new microscopic
insights and make testable predictions for local spectroscopic probes. The
energy gap in the density of states survives across the transition, but
coherence peaks exist only in the superconductor. A characteristic pseudogap
persists above the critical disorder and critical temperature, in contrast to
conventional theories. Surprisingly, the insulator has a two-particle gap scale
that vanishes at the SIT, despite a robust single-particle gap.Comment: 7 pages, 5 figures (plus supplement with 4 pages, 5 figures
From thermal rectifiers to thermoelectric devices
We discuss thermal rectification and thermoelectric energy conversion from
the perspective of nonequilibrium statistical mechanics and dynamical systems
theory. After preliminary considerations on the dynamical foundations of the
phenomenological Fourier law in classical and quantum mechanics, we illustrate
ways to control the phononic heat flow and design thermal diodes. Finally, we
consider the coupled transport of heat and charge and discuss several general
mechanisms for optimizing the figure of merit of thermoelectric efficiency.Comment: 42 pages, 22 figures, review paper, to appear in the Springer Lecture
Notes in Physics volume "Thermal transport in low dimensions: from
statistical physics to nanoscale heat transfer" (S. Lepri ed.
Spin-orbit density wave induced hidden topological order in URu2Si2
The conventional order parameters in quantum matters are often characterized
by 'spontaneous' broken symmetries. However, sometimes the broken symmetries
may blend with the invariant symmetries to lead to mysterious emergent phases.
The heavy fermion metal URu2Si2 is one such example, where the order parameter
responsible for a second-order phase transition at Th = 17.5 K has remained a
long-standing mystery. Here we propose via ab-initio calculation and effective
model that a novel spin-orbit density wave in the f-states is responsible for
the hidden-order phase in URu2Si2. The staggered spin-orbit order 'spontaneous'
breaks rotational, and translational symmetries while time-reversal symmetry
remains intact. Thus it is immune to pressure, but can be destroyed by magnetic
field even at T = 0 K, that means at a quantum critical point. We compute
topological index of the order parameter to show that the hidden order is
topologically invariant. Finally, some verifiable predictions are presented.Comment: (v2) Substantially modified from v1, more calculation and comparison
with experiments are include
Effects of interdot hopping and Coulomb blockade on the thermoelectric properties of serially coupled quantum dots
We have theoretically studied the thermoelectric properties of serially
coupled quantum dots (SCQD) embedded in an insulator matrix connected to
metallic electrodes. In the framework of Keldysh Green's function technique,
the Landauer formula of transmission factor is obtained by using the equation
of motion method. Based on such analytical expressions of charge and heat
currents, we calculate the electrical conductance, Seebeck coefficient,
electron thermal conductance and figure of merit (ZT) of SCQD in the linear
response regime. The effects of electron Coulomb interactions on the reduction
and enhancement of ZT are analyzed. We demonstrate that ZT is not a monotonic
increasing function of interdot electron hopping strength (). We also show
that in the absence of phonon thermal conductance, SCQD can reach the Carnot
efficiency as approaches zero.Comment: corrected some argumenet
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