Conversione di energia in dispositivi quantistici alla nanoscala

Responsabili di progetto

Fabio Taddei, Liliana Del Carmen Arrachea

Accordo

ARGENTINA - CONICET - Consejo Nacional de Investigaciones Científicas y Técnicas

Bando

CNR/CONICET biennio 2017-2018 2017-2018

Dipartimento

Scienze fisiche e tecnologie della materia

Area tematica

Scienze fisiche e tecnologie della materia

Stato del progetto

Nuovo

Proposta di ricerca

State of the art.
The development of novel quantum technologies is tightly connected to the capability to control and manipulate charge, heat and spin fluxes through thermoelectric effects, both in dc or ac operation [1]. The efficiency of thermoelectric effects and their control are crucial, on the one hand, in nanoelectronics where the miniaturization has to face the problem of evacuating the heat produced by the currents flowing through the circuits. On the other hand, they are crucial in order to realize competitive heat-to-work nanoscale energy converters [1].
The typical device in conventional thermoelectrics consists of a conductor coupled to two reservoirs, which differ in both temperature and chemical potential. A thermal engine converts a temperature difference into electric power. As a consequence of the second law of thermodynamics, the efficiency of this conversion process is limited by the Carnot efficiency. The optimal efficiency that can be reached for a specific device is controlled by the value of its figure of merit ZT.
The thermal engine can also be operated in reverse, realizing a refrigerator, which invests electric power to continuously extract heat from the colder reservoir. The maximal efficiency of this device is bounded by the appropriate Carnot coefficient of performance.
Increasing the efficiency of thermoelectric materials for heat-to-work conversion is one of the main challenges of present-days technology. One of the keys to the success in this field is the ability to modulate, control, and route heat and charge currents, ideally achieving their separate control. This is however by no means obvious as the charge and (the electronic contribution to) the heat are transported by the same carriers.
In order to achieve a separate control of heat and charge currents one should therefore consider more complex devices. The key issue in this context is to assess to which extent this control can be achieved and what are its possible advantages in thermoelectric thermal machines.

Research project.
In this project we will investigate various strategies to enhance thermoelectric effects. Recently, this possibility has been explored by the Italian group investigating multi-terminal hybrid structures, combining metallic and superconducting materials [2,3]. The idea of decoupling heat exchange from particle exchange in multi-terminal devices was also inspected by the Argentinean group [4], where the cooling of a nanomechanical system in the thermoelectric framework was studied.
We will explore the possibility to control coherent nanodevices with time-dependent driving [5,6], thus opening the possibility to make use of quantum pumping schemes [7] whose thermoelectric performance need to be thoroughly investigated.
The physics behind these setups can be understood by analogy to the Archimede's device, a pipe with a rotating screw, which can be used to pump water against gravity. This is a classical analog of an adiabatic quantum pump [7], where ac driving pumps a certain amount of electric charge per cycle. Specifically, this charge can be pumped against an applied dc bias voltage, in which case quantum pump realizes a generator.
Alternatively, these devices can operate involving temperature gradients instead of bias voltages, realizing heat pumps and heat engines.
We will also consider hybrid superconducting systems, especially in multi-terminal setups, which constitute a promising platform stemming from the fact that they naturally allow the separation between charge and heat transport, while preserving (quantum mechanical) phase coherence.
In addition the combination of spin-split superconducting density of states and spin filtering techniques [3] may generate unexpected intense thermoelectric effects and may be crucial in order to develop high sensitivity sensors.
Finally, the investigation of thermoelectric properties in the newly discovered classes of topological materials (such as topological insulators and topological superconductors) is a timely and interesting issue. Indeed, such materials are characterized by topologically protected edge or bound states, insensitive to local perturbations, which are expected to lead to unusual thermoelectric properties. In these systems momentum and spin of the excitations are tightly coupled and the role of entanglement on the thermoelectric properties is also of great interest. Here we will consider thermoelectric properties in these systems and also the impact of electron-electron interactions on thermoelectric performances by considering specific geometries such as quantum dots where Coulomb interactions are relevant.
The project aims to naturally combine the expertise of two groups that have recently studied different but closely related aspects of the problem of the interplay between charge and energy transport in nanoscale systems. The Argentinean group has more experience in quantum transport involving time-dependent driving, while the research of the Italian group has been so far mainly focused on multi-terminals and hybrid devices. Both aspects are the fundamental and complementary ingredients of the present research plan.
The problems to be addressed are very relevant and we believe that the synergy of the cooperation will lead to a qualitative progress in the area. The evolution of this collaboration is very promising regarding the possible extension to a larger network including collaborators from other institutions.

References.
[1] G. Benenti et al., ArXiv:1608.05595 (2016)
[2] F. Mazza et al., NJP 16, 085001 (2014)
[3] F. Giazotto et al., PR Applied 4, 044016 (2015)
[4] L. Arrachea et al., PRB 90, 125450 (2014)
[5] M.F. Ludovico et al., PRB 93, 075136 (2016)
[6] M.F. Ludovico et al., PRB 89, 161306 (2014)
[7] F. Giazotto et al., Nature Phys. 7, 857 (2011)

Obiettivi della ricerca

1. Explore the mechanisms that determine the efficiencies of multi-terminal setups including the effect of ac driving. So far, only mechanisms effectively involving two fluxes were explored (e.g. charge and work for nanomotors). The full situation involving more fluxes and additional possible operational modes still remain an open problem. We will also investigate multi-terminal hybrid devices for heat and charge transport in presence of ac driving.
2. Explore concrete classical mechanisms for implementing the ac driving forces. Here natural candidates are systems obeying a classical dynamics strongly coupled to the electron system. We will explore the coupling to superconducting SQUID-type circuits. In such systems, we will explore different operational modes like motors, generators, heat-pump and heat-engines. Finally, superconductors with spin-split density of states may be also a novel resource to spintronics, opening the possibility to develop new devices where charge, heat an spin transport can be manipulated.
3. Explore the effect of many-body interactions, of magnetic fields, and of the transition to topological phase in hybrid systems. We will study the impact of electronic correlations on thermoelectric efficiencies. The role of topological superconductors will be considered, including the possibility of breaking time-reversal symmetry with a finite magnetic field and its consequence on thermoelectric performances.

Ultimo aggiornamento: 01/07/2025