14 research outputs found
On-Chip Cooling by Heating with Superconducting Tunnel Junctions
Heat management and refrigeration are key concepts for nanoscale devices
operating at cryogenic temperatures. The design of an on-chip mesoscopic
refrigerator that works thanks to the input heat is presented, thus realizing a
solid state implementation of the concept of cooling by heating. The system
consists of a circuit featuring a thermoelectric element based on a
ferromagnetic insulator-superconductor tunnel junction (N-FI-S) and a series of
two normal metal-superconductor tunnel junctions (SINIS). The N-FI-S element
converts the incoming heat in a thermovoltage, which is applied to the SINIS,
thereby yielding cooling. The cooler's performance is investigated as a
function of the input heat current for different bath temperatures. We show
that this system can efficiently employ the performance of SINIS refrigeration,
with a substantial cooling of the normal metal island. Its scalability and
simplicity in the design makes it a promising building block for
low-temperature on-chip energy management applications.Comment: 7 pages, 6 figure
Phase-Tunable Thermal Logic: Computation with Heat
Boolean algebra, the branch of mathematics where variables can assume only
true or false value, is the theoretical basis of classical computation. The
analogy between Boolean operations and electronic switching circuits,
highlighted by Shannon in 1938, paved the way to modern computation based on
electronic devices. The grow of computational power of such devices, after an
exciting exponential -Moore trend, is nowadays blocked by heat dissipation due
to computational tasks, very demanding after the chips miniaturization. Heat is
often a detrimental form of energy which increases the systems entropy
decreasing the efficiency of logic operations. Here, we propose a physical
system able to perform thermal logic operations by reversing the old
heat-disorder epitome into a novel heat-order paradigm. We lay the foundations
of heat computation by encoding logic state variables in temperature and
introducing the thermal counterparts of electronic logic gates. Exploiting
quantum effects in thermally biased Josephson junctions (JJs), we propound a
possible realization of a functionally complete dissipationless logic. Our
architecture ensures high operation stability and robustness with switching
frequencies reaching the GHz
Phase-Tunable Temperature Amplifier
Coherent caloritronics, the thermal counterpart of coherent electronics, has
drawn growing attention since the discovery of heat interference in 2012.
Thermal interferometers, diodes, transistors and nano-valves have been
theoretically proposed and experimentally demonstrated by exploiting the
quantum phase difference between two superconductors coupled through a
Josephson junction. So far, the quantum-phase modulator has been realized in
the form of a superconducting quantum interference device (SQUID) or a
superconducting quantum interference proximity transistor (SQUIPT). Thence, an
external magnetic field is necessary in order to manipulate the heat transport.
Here, we theoretically propose the first on-chip fully thermal caloritronic
device: the phase-tunable temperature amplifier. Taking advantage of a recent
thermoelectric effect discovered in spin-split superconductors coupled to a
spin-polarized system, by a temperature gradient we generate the magnetic flux
controlling the transport through a temperature biased SQUIPT. By employing
commonly used materials and a geometry compatible with state-of-the-art
nano-fabrication techniques, we simulate the behavior of the temperature
amplifier and define a number of figures of merit in full analogy with voltage
amplifiers. Notably, our architecture ensures infinite input thermal impedance,
maximum gain of about 11 and efficiency reaching the 95%. This device concept
could represent a breakthrough in coherent caloritronic devices, and paves the
way for applications in radiation sensing, thermal logics and quantum
information.Comment: 7 pages, 3 figure
High operating temperature in V-based superconducting quantum interference proximity transistors
Here we report the fabrication and characterization of fully superconducting
quantum interference proximity transistors (SQUIPTs) based on the
implementation of vanadium (V) in the superconducting loop. At low temperature,
the devices show high flux-to-voltage (up to 0.52) and
flux-to-current (above 12) transfer functions, with the
best estimated flux sensitivity 2.6
reached under fixed voltage bias, where is the flux quantum. The
interferometers operate up to 2 , with an
improvement of 70 of the maximal operating temperature with respect to
early SQUIPTs design. The main features of the V-based SQUIPT are described
within a simplified theoretical model. Our results open the way to the
realization of SQUIPTs that take advantage of the use of higher-gap
superconductors for ultra-sensitive nanoscale applications that operate at
temperatures well above 1 K.Comment: Published version with Supplementary Informatio
Quasiparticles in superconducting qubits with asymmetric junctions
Designing the spatial profile of the superconducting gap -- gap engineering
-- has long been recognized as an effective way of controlling quasiparticles
in superconducting devices. In aluminum films, their thickness modulates the
gap; therefore, standard fabrication of Al/AlOx/Al Josephson junctions, which
relies on overlapping a thicker film on top of a thinner one, always results in
gap-engineered devices. Here we reconsider quasiparticle effects in
superconducting qubits to explicitly account for the unavoidable asymmetry in
the gap on the two sides of a Josephson junction. We find that different
regimes can be encountered in which the quasiparticles have either similar
densities in the two junction leads, or are largely confined to the lower-gap
lead. Qualitatively, for similar densities the qubit's excited state population
is lower but its relaxation rate higher than when the quasiparticles are
confined; therefore, there is a potential trade-off between two desirable
properties in a qubit.Comment: Revised version. To be published in PRX Quantu
Negative differential thermal conductance by photonic transport in electronic circuits
The negative differential thermal conductance (NDTC) provides the key
mechanism for realizing thermal transistors. This exotic effect has been the
object of an extensive theoretical investigation, but the implementation is
still limited to a few specific physical systems. Here, we consider a simple
circuit of two electrodes exchanging heat through electromagnetic radiation. We
demonstrate that the existence of an optimal condition for power transmission,
well-known as impedance matching in electronics, provides a natural framework
for engineering NDTC: the heat flux is reduced when the temperature increase is
associated to an abrupt change of the electrode's impedance. As a case study,
we analyze a hybrid structure based on thin-film technology, in which the
increased resistance is due to a superconductor-resistive phase transition. For
typical metallic superconductors operating below K, NDTC reflects in a
temperature drop of the order of a few mK by increasing the power supplied to
the system. Our work draws new routes for implementing a thermal transistor in
nanoscale circuits
Bipolar Thermoelectricity in Bilayer-Graphene/Superconductor Tunnel Junctions
We investigate the thermoelectric properties of a hybrid nanodevice composed
by a 2D carbon based material and a superconductor. This system presents
nonlinear bipolar thermoelectricity as induced by the spontaneous breaking of
the Particle-Hole (PH) symmetry in a tunnel junction between a BiLayer Graphene
(BLG) and a Bardeen-Cooper-Schrieffer (BCS) superconductor. In this scheme, the
nonlinear thermoelectric effect, predicted and observed in SIS' junctions is
not affected by the competitive effect of the Josephson coupling. From a
fundamental perspective, the most intriguing feature of this effect is its
bipolarity, that poses new issues on the nature of thermoelectricity in solid
state systems. The capability to open and control the BLG gap guarantees
improved thermoelectric performances, that reach up to 1 mV/K regarding the
Seebeck coeffcient and a power density of 1 nW/m for temperature
gradients of tens of Kelvins. Furthermore, the externally controlled gating can
also dope the BLG, which is otherwise intrinsically PH symmetric, giving us the
opportunity to investigate the bipolar thermoelectricity even in presence of a
controlled suppression of the PH symmetry. The predicted robustness of this
system could foster further experimental investigations and applications in the
near future, thanks to the available techniques of nano-fabrication
Soliton versus single photon quantum dynamics in arrays of superconducting qubits
Superconducting circuits constitute a promising platform for future
implementation of quantum processors and simulators. Arrays of capacitively
coupled transmon qubits naturally implement the Bose-Hubbard model with
attractive on-site interaction. The spectrum of such many-body systems is
characterised by low-energy localised states defining the lattice analog of
bright solitons. Here, we demonstrate that these bright solitons can be pinned
in the system, and we find that a soliton moves while maintaining its shape.
Its velocity obeys a scaling law in terms of the combined interaction and
number of constituent bosons. In contrast, the source-to-drain transport of
photons through the array occurs through extended states that have higher
energy compared to the bright soliton. For weak coupling between the
source/drain and the array, the populations of the source and drain oscillate
in time, with the chain remaining nearly unpopulated at all times. Such a
phenomenon is found to be parity dependent. Implications of our results for the
actual experimental realisations are discussed
Thermal computation and heat harvesting in hybrid superconducting tunnel junctions
In this thesis the charge and the heat tranport in hybrid superconducting tunnel junctions is discussed. A thermoelectric effect of recent prediction and discovery is exploited to design some proposals for heat harvesting purposes, like a contactless heat engine and a electronic refrigerator. A scheme for a temperature-based computation, based on the transport properties of a thermally-biased superconducting quantum interference proximity transistor (SQUIPT), is also presented. The experimental characterization of the charge transport in a SQUIPT is discussed in the last part of the thesis
Driven microswimmers on a 2D substrate: A stochastic towed sled model
We investigate, both numerically and analytically, the diffusion
properties of a stochastic sled sliding on a substrate, subject to a
constant towing force. The problem is motivated by the growing interest
in controlling transport of artificial microswimmers in 2D geometries at
low Reynolds numbers. We simulated both symmetric and asymmetric towed
sleds. Remarkable properties of their mobilities and diffusion constants
include sidewise drifts and excess diffusion peaks. We interpret our
numerical findings by making use of stochastic approximation techniques.
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