Phase Control of Bipolar Thermoelectricity in Josephson Tunnel Junctions

Advanced research in superconductivity and low-temperature superconductors has enabled the design of high-performance superconducting circuits with application prospects in fast electronics, quantum computers, quantum sensing, heat management and energy harvesting. These applications leverage the nonlinear current-phase relation of the Josephson effect and the temperature dependence of the superconducting properties. Such effects are common in weak-link structures connecting two superconductors.

Thermoelectricity in superconductors refers to the ability of a superconducting material to generate an electrical current when there is a temperature gradient across the material. This is known as the thermoelectric effect or the Seebeck effect. The thermoelectric effect arises because of the way in which charge carriers in a material respond to temperature. When a temperature gradient is applied to a material, the charges on one end of the material gain more energy than the charges on the other end. This creates a voltage difference between the two ends of the material, which thereby can be used to generate an electrical current. In superconductors, usually the thermoelectric effects are suppressed, in some measure, by the superconducting electrical shortening induced by the Cooper pair dissipationless motion and by the particle-hole (PH) symmetry of the gapped density of states. However, intriguingly one may speculate that the gap opening may potentially determine a strong thermoelectric effect due to the strong energy dependence of the density of states. However, the competition between the superconducting state and thermoelectric effects have been recently overcome since linear thermoelectric properties of superconductors have been explored in hybrid superconducting-ferromagnetic insulator/metals tunnel junctions. In such cases, the spin polarization effectively breaks the PH symmetry of a spin-split superconductor. So, for many years, it was thought that thermoelectricity in superconductors could only be attained by explicitly breaking the PH symmetry (extrinsically).

However, things can be particularly odd in the nonlinear regime when a large temperature gradient exceeding the threshold value is applied to the junction. Interestingly, recent studies[1] have established that superconducting tunnel junctions, where the Josephson coupling is sufficiently suppressed, can develop unique bipolar thermoelectric phenomena in the presence of a large thermal gradient. [1,2,3] This is mainly attributed to nonequilibrium spontaneous PH symmetry breaking. Unfortunately, there is still limited experimental validation of the thermoelectric Josephson phenomenon.[4]

Herein, Italian scientists: Dr. Gaia Germanese, Dr.  Federico Paolucci, Dr. Alessandro Braggio and Dr. Francesco Giazotto from NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore in collaboration with Dr. Giampiero Marchegiani from Technology Innovation Institute Abu Dhabi investigated the bipolar thermoelectric effect in Josephson junctions. In particular, they focused on phase control and the effect of Josephson contribution on the thermoelectricity modulation of Cooper pair’s transport. Their research work is currently published in the peer-reviewed journal, Physical Review Applied.

In their approach, the thermoelectric performance was phase controlled by a double-loop superconducting quantum interference device (DSQUID). The authors discussed in detail the interplay of three main contributions. They include quasiparticle current responsible for sustaining thermoelectricity, intrinsically reactive Cooper pairs tunneling and breaking and recombining Cooper pairs. The quasiparticle current subtracted by the Josephson contribution was analyzed to clearly establish the absolute negative conductance at zero bias. This result further demonstrated the spontaneous symmetry-breaking of the PH symmetry even in linear-in-bias regime. Finally, a thermoelectric engine was developed to investigate the metastable state further introduced by the Josephson component.

The authors observed that thermoelectricity could be achieved in the presence of both Josephson coupling until its complete screening/suppression. Indeed, the thermoelectric generation induced by the pure quasiparticle transport was phase independent upon removal of the Josephson contribution from the measured net current. Moreover, a phase-coherent control of the engine performance was even achieved by phase tuning the thermoelectric effect.[4,5] So, it was also possible to tune the thermoelectric current and output power by altering the magnetic flux piercing the interferometer loops.

In summary, the researchers demonstrated successfully the generation of thermoelectricity in a structure consisting of Josephson tunnel junctions arranged in parallel configuration in the presence of a large thermal gradient. Josephson contribution was found to induce an extra metastable state at V ≈ 0, which affected the hysteretic loop, thereby extending the application domain of the device. In a statement to Advances in Engineering, Dr. Francesco Giazotto and Dr. Alessandro Braggio explained that their study would provide a better understanding of nonlinear thermoelectric effects in different solid-state systems shining further light on the novelty of bipolar thermoelectricity and its potential in applications.