Electric and thermal properties of superconducting Josephson systems and Applications

The conversion of the whole heat into work is impossible according to the second law of thermodynamics. For this reason, heat is commonly considered the most degraded form of energy. It could represent a source of noise for most physical processes, leading to degradation of device performance. Massive heat production at the nanoscale places limitations on the operation of nanodevices. For this reason, it is important to control this quantity in nanocircuits at low temperatures, which form the basis of quantum technology. Therefore, the investigation of heat transport and the control of heat dissipated in electrical circuits have attracted great interest. Over the years, great efforts have been made to improve heat containment strategies, develop on-chip coolers, and explore the possibility of employing unwanted waste energy in thermal quantum machines. In this direction, thermoelectric elements play a crucial role in their capability of converting a temperature difference into electrical power (Seebeck effect) and vice versa(Peltier effect). Since the discovery of thermoelectricity in 1821, researchers have tried to understand and control this phenomenon in order to use it to recycle waste energy. Research into new thermoelectric materials with high electrical conductivity and low thermal conductivity has helped improve thermoelectric efficiency (ZT). The advent of semiconductors (1960s) and nanotechnology (1990s) has increased thermoelectric performance. Superconductors are a special case. At first glance, thermoelectric effects and the superconducting state are competing for different reasons. First, a strong contribution to thermoelectric effects is associated with deviations from the particle-hole (PH) symmetry, which characterizes normal metals and superconductors. Indeed, an asymmetry between the transport of electrons and holes is necessary to produce thermoelectricity. Second, the thermoelectric currents associated with single-particle excitations, called quasiparticles, are usually screened by the dissipationless motion of Cooper pairs. Therefore, the thermoelectric voltage would be either vanishingly small or undetectable in bulk superconductors, as noted earlier. Despite these limitations, superconductors contain some prerequisites for the development of enormous thermoelectric effects, such as their energy-dependent quasiparticle density of states (DoS) according to the Bardeen-Cooper-Schrieffer (BCS) theory [9]. This potential has been confirmed in hybrid superconducting-ferromagnetic insulator tunnel junctions, where the combination of spin filtering and spin splitting of the BCS superconducting DoS explicitly breaks the PH symmetry, leading to strong thermoelectricity [10]. Only a few years ago, a bipolar thermoelectric effect was theoretically predicted in a Josephson junction involving two superconductors with different zero-temperature energy gaps and vanishing Josephson coupling [11, 12]. In such a structure, large temperature gradients and a large voltage bias (nonlinear regime) determine the condition for spontaneous breaking of PH symmetry, which characterizes a bipolar thermoelectric effect, where both polarities of thermoelectric voltage are possible for a fixed thermal and electronic configuration. The present work is placed in this context. The experimental demonstration of bipolar nonlinear thermoelectricity in Josephson junctions has led to the realization of the first Bipolar Thermoelectric Josephson Engine (BTJE) capable of generating a thermoelectric voltage across a load resistor. Potential applications to exploit the nonlinear thermoelectric effect have been conceived and investigated: thermoelectric memories capable of converting waste heat into reusable thermopower, and single-photon detectors that generate a thermoelectric response passively, i.e., without a power supply. Finally, heat confinement in one-dimensional Josephson junctions has been explored to improve the performance of active superconducting sensors capable of detecting electromagnetic radiation by exploiting a change in its resistance due to a rise in the temperature of the sensing element. A nano-Transition Edge Sensor (nano-TES) and a Josephson Escape Sensor (JES) have been realized to operate as bolometers and calorimeters for astroparticle physics, terahertz imaging, and quantum technologies.