F. Paolucci, G. Germanese, A. Braggio, F. Giazotto
J Low Temp Phys 214, 86–91 (2024). https://doi.org/10.1007/s10909-023-03011-y

Bipolar thermoelectricity in tunnel junctions between superconductors of different energy gap has been recently predicted and experimentally demonstrated. This effect showed thermovoltages up to ±150μV at milliKelvin temperatures. Thus, superconducting tunnel junctions can be exploited to realize a passive single-photon thermoelectric detector TED operating in the broadband range 15 GHz – 50 PHz. In particular, this detector is expected to show a signal-to-noise ratio of about 15 down to ν=50 GHz and a operating window of more than 4 decades. Therefore, the TED might find applications in quantum computing, telecommunications, optoelectronics, spectroscopy and astro-particle physics.

S. Battisti, J. Koch, A. Paghi, L. Ruf, A. Gulian, S. Teknowijoyo, C. Cirillo, Z. Makhdoumi Kakhaki, C. Attanasio, E. Scheer, A. Di Bernardo,G. De Simoni, and F. Giazotto
Appl. Phys. Lett. 22 April 2024; 124 (17): 172601. https://doi.org/10.1063/5.0200257

Here, we demonstrate superconducting Dayem-bridge weak-links made of different stoichiometric compositions of NbRe. Our devices possess a relatively high critical temperature, normal-state resistance, and kinetic inductance. In particular, the high kinetic inductance makes this material a good alternative to more conventional niobium-based superconductors (e.g., NbN or NbTiN) for the realization of superinductors and high-quality factor resonators, whereas the high normal-state resistance yields a large output voltage in superconducting switches and logic elements realized upon this compound. Moreover, out-of-plane critical magnetic fields exceeding 2 T ensure that possible applications requiring high magnetic fields can also be envisaged. Altogether, these features make this material appealing for a number of applications in the framework of quantum technologies.

Alessandro Braggio, Matteo Carrega, Björn Sothmann, and Rafael Sánchez
Phys. Rev. Research 6, L012049 – Published 4 March 2024

Nonlocal thermoelectricity is proposed as a direct probe of interactions, nonthermal states, and the effect of correlations in the nonequilibrium heat transport between 1D quantum channels. In copropagating quantum Hall edge states contacted at different temperatures, the nonlocal thermoelectrical response is only expected if the electron-electron interaction mediates the heat exchange directly measuring the interaction strength. Considering the low-energy limit of zero-range interactions, we analytically solve the charge and energy currents of a nonequilibrium interacting system, determining the universal scaling law in terms of an interaction-dependent energy-relaxation length. Further, a setup with two controllable quantum point contacts allows thermoelectricity to monitor the thermalization of an interacting system as well as the fundamental role of cross-correlations in the heat exchange at intermediate length scales.

A. Paghi, G. Trupiano, C. Puglia, H. Burgaud, G. De Simoni, A. Greco, F. Giazotto
IEEE Transactions on Instrumentation and Measurement, vol. 73, pp. 1-7, 2024 – doi: 10.1109/TIM.2024.3372217

Ad-hoc interface PCBs are today the standard connection between cryogenic cabling and quantum chips. Besides low-loss and low-temperature-dependent-dielectric-permittivity materials, FR4 provides a low-cost solution for fabrication of cryogenic PCBs. Here, we report on an effective way to evaluate the dielectric performance of a FR4 laminate used as substrate for cryogenic microwave PCBs. We designed a coplanar waveguide {\lambda}/2 open-circuit series resonator and we fabricated the PCB using a low-cost manufacturing process. Such a geometry allows to exploit the resonance peak of the resonator to measure the variation of the complex dielectric permittivity as a function of the temperature. Resonance peak frequency and magnitude were used as sensing parameters for the real part of dielectric permittivity and dielectric loss tangent, respectively. We estimated a 9 % reduction of the real part of the dielectric permittivity and a 70 % reduction of the dielectric loss tangent in the temperature range from 300 to 4 K. The proposed approach can be immediately extended to the detection of cryogenic temperature-dependent dielectric performance of any kind on substrate.

Sebastiano Battisti, Giorgio De Simoni, Luca Chirolli, Alessandro Braggio, and Francesco Giazotto
Phys. Rev. Research 6, L012022 – Published 25 January 2024

Thermoelectric effects in normal metals and superconductors are usually very small due to the presence of electron-hole symmetry. Here, we show that superconducting junctions brought out of equilibrium manifest a sizable bipolar thermoelectric effect that stems from a strong violation of the detailed balance determined by the crucial role of the interactions at the mean-field level. To fully control the effect, we consider a thermally biased SIS′IS junction where the capacitance of the central region is small enough to establish a Coulomb blockade regime. By exploiting charging effects we are able to tune the Seebeck voltage, the thermocurrent, and thereby the power output of this structure, via an external gate voltage. We then analyze the main figures of merit of bipolar thermoelectricity and we prospect for possible applications.

Daniel Margineda, Alessandro Crippa, Elia Strambini, Yuri Fukaya, Maria Teresa Mercaldo, Mario Cuoco & Francesco Giazotto
Commun Phys 6, 343 (2023) | https://doi.org/10.1038/s42005-023-01458-9

Supercurrent diodes are nonreciprocal electronic elements whose switching current depends on their flow direction. Recently, a variety of composite systems combining different materials and engineered asymmetric superconducting devices have been proposed. Yet, ease of fabrication and tunable sign of supercurrent rectification joined to large efficiency have not been assessed in a single platform so far. We demonstrate that all-metallic superconducting Dayem nanobridges naturally exhibit nonreciprocal supercurrents under an external magnetic field, with a rectification efficiency up to ~ 27%. Our niobium nanostructures are tailored so that the diode polarity can be tuned by varying the amplitude of an out-of-plane magnetic field or the temperature in a regime without magnetic screening. We show that sign reversal of the diode effect may arise from the high-harmonic content of the current phase relation in combination with vortex phase windings present in the bridge or an anomalous phase shift compatible with anisotropic spin-orbit interactions.

Luca Chirolli, Matteo Carrega, and Francesco Giazotto
Quantum 7, 1193 (2023). | https://doi.org/10.22331/q-2023-12-04-1193

The quasicharge superconducting qubit realizes the dual of the transmon and shows strong robustness to flux and charge fluctuations thanks to a very large inductance closed on a Josephson junction. At the same time, a weak anharmonicity of the spectrum is inherited from the parent transmon, that introduces leakage errors and is prone to frequency crowding in multi-qubit setups. We propose a novel design that employs a quartic superinductor and confers a good degree of anharmonicity to the spectrum. The quartic regime is achieved through a properly designed chain of Josephson junction loops that shows minimal quantum fluctuations without introducing a severe dependence on the external fluxes.

Guarcello C., Citro R., Giazotto F., Braggio A.
Applied Physics Letters (123); 152601 (2023) 10.1063/5.0169267

We theoretically study the quasiparticle current behavior of a thermally biased bipolar thermoelectrical superconducting quantum interferenceproximity transistor, formed by a normal metal wire embedded in a superconducting ring and tunnel-coupled to a superconducting probe. In this configuration, the superconducting gap of the wire can be modified through an applied magnetic flux. We analyze the thermoelectric response as a function of magnetic flux, at fixed temperatures, in the case of a device made of the same superconductor. We demonstrate magnetically controllable, bipolar thermoelectric behavior and discuss optimal working conditions by looking at the thermoelectric power and other figures of merit of the device.

Greco A., Pichard Q., Giazotto F.;
Applied Physics Letters (123); 092601 (2023) 10.1063/5.0165259

It was recently experimentally proved that the superconducting counterpart of a diode, i.e., a device that realizes nonreciprocal Cooper pairs transport, can be realized by breaking the spatial and time-reversal symmetry of a system simultaneously. Here we report the theory, fabrication, and operation of a monolithic dc superconducting quantum interference device (dcSQUID) that embedding three-dimensional (3D) Dayem nanobridges as weak links realizes an efficient and magnetic flux-tunable supercurrent diode. The device is entirely realized in Al and achieves a maximum rectification efficiency of ∼ 20%, which stems from the high harmonic content of its current-to-phase relation only without the need of any sizable screening current caused by a finite loop inductance. Our interferometer can be easily integrated with state-ofthe-art superconducting electronics, and since it does not require a finite loop inductance to provide large rectification its downsizing is not limited by the geometrical constraints of the superconducting ring.

Claudio Guarcello, Alessandro Braggio, Francesco Giazotto, and Roberta Citro;
Phys. Rev. B 108, L100511 – Published 29 September 2023 | https://doi.org/10.1103/PhysRevB.108.L100511

Thermoelectrical properties are frequently used to characterize materials and endow the free energy from wasted heat with useful purposes. Here, we show that linear thermoelectric effects in tunnel junctions with Fe-based superconductors not only address the dominance between particle and hole states but also even provide information about the superconducting order-parameter symmetry. In particular, we observe that nodal order parameters present a maximal thermoelectric effect at lower temperatures than for nodeless cases. Furthermore, we show also that superconducting tunnel junctions between Fe-based and BCS superconductors could provide a thermoelectric efficiency ZT exceeding 6 with a linear Seebeck coefficient around 𝑆≈800µV/K at a few kelvins. These results pave the way to developing novel thermoelectric machines based on multiband superconductors.

Paolucci F., Germanese G., Braggio A., Giazotto F.
Applied Physics Letters (122); 173503 (2023) 10.1063/5.0145544

We propose a passive single-photon detector based on the bipolar thermoelectric effect occurring in tunnel junctions between two different superconductors thanks to spontaneous electron–hole symmetry breaking. Our superconducting thermoelectric detector (STED) converts a finite temperature difference caused by the absorption of a single photon into an open circuit thermovoltage. Designed with feasible parameters, our STED is able to reveal single photons of frequency ranging from ∼15 GHz to ∼150 PHz depending on the chosen design and materials. In particular, this detector is expected to show values of the signal-to-noise ratio SNR ∼ 15 at ν = 50 GHz when operated at a temperature of 10 mK. Interestingly, this device can be viewed as a digital single-photon detector, since it generates an almost constant voltage VS for the full operation energies. Our STED can reveal single photons in a frequency range wider than four decades with the possibility to discern the energy of the incident photon by measuring the time persistence of the generated thermovoltage. Its broadband operation suggests that our STED could find practical applications in several fields of quantum science and technology, such as quantum computing, telecommunications, optoelectronics, THz spectroscopy, and astro-particle physics.

Gianmichele Blasi, Géraldine Haack, Vittorio Giovannetti, Fabio Taddei, and Alessandro Braggio
Phys. Rev. Research 5, 033142 – Published 30 August 2023 | https://doi.org/10.1103/PhysRevResearch.5.033142

Robust and tunable topological Josephson junctions (TJJs) are highly desirable platforms for investigating the anomalous Josephson effect and topological quantum computation applications. Experimental demonstrations have been done in hybrid superconducting-two dimensional topological insulator (2DTI) platforms, sensitive to magnetic disorder and interactions with phonons and other electrons. In this work, we propose a robust and electrostatically tunable TJJ by combining the physics of the integer quantum Hall (IQH) regime and of superconductors. We provide analytical insights about the corresponding Andreev bound state spectrum, the Josephson current and the anomalous current. We demonstrate the existence of protected zero-energy crossings, that can be controlled through electrostatic external gates. This electrostatic tunability has a direct advantage to compensate for non-ideal interfaces and undesirable reflections that may occur in any realistic samples. TJJs in the IQH regime could be realized in graphene and other 2D materials. They are of particular relevance towards scalable and robust Andreev-qubit platforms, and also for efficient phase batteries.

A. Hijano, F.S. Bergeret, F. Giazotto, and A. Braggio
Phys. Rev. Applied 19, 044024 – Published 7 April 2023 – DOI:10.1103/PhysRevApplied.19.044024

Asymmetric superconducting tunnel junctions with gaps
Δ1>Δ2 have been proven to show a peculiar nonlinear bipolar thermoelectric effect. This arises due to the spontaneous breaking of electron-hole symmetry in the system, and it is maximized at the matching-peak bias |V|=Vp=(Δ1−Δ2)/e. In this paper, we investigate the interplay of photon-assisted tunneling (PAT) and bipolar thermoelectric generation. In particular, we show how thermoelectricity, at the matching peak, is supported by photon absorption and emission processes at the frequency-shifted sidebands V=±Vp+nℏω,n∈Z.This represents a sort of microwave-assisted thermoelectricity. We show the existence of multiple stable solutions, being associated with different photon sidebands, when a load is connected to the junction. Finally, we discuss how the nonlinear cooling effects are modified by the PAT. The proposed device can detect millimeter-wavelength signals by converting a temperature gradient into a thermoelectric current or voltage.

A. Hijano, F.S. Bergeret, F. Giazotto, and A. Braggio
Applied Physics Letters (122); 242603-1 (2023) 10.1063/5.0152705

Recent studies have shown the potential for bipolar thermoelectricity in superconducting tunnel junctions with asymmetric energy gaps. The thermoelectric performance of these systems is significantly impacted by the inverse proximity effects present in the normal-superconducting bilayer, which is utilized to adjust the gap asymmetry in the junction. Here, we identify the most effective bilayer configurations, and we find that directly tunnel-coupling the normal metal side of the cold bilayer with the hot superconductor is more advantageous compared to the scheme used in experiments. By utilizing quasiclassical equations, we examined the nonlinear thermoelectric junction performance as a function of the normal metal film thickness and the quality of the normal-superconducting interface within the bilayer, thereby determining the optimal design to observe and maximize this nonequilibrium effect. Our results offer a roadmap to achieve improved thermoelectric performance in superconducting tunnel junctions, with promising implications for a number of applications.

Gaia Germanese, Federico Paolucci, Giampiero Marchegiani, Alessandro Braggio, and Francesco Giazotto
Phys. Rev. Applied 19, 014074 – Published 31 January 2023

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.

J. H. Mateos, L. Tosi, A. Braggio, F. Taddei, L. Arrachea
Preprint

We study nonlocal thermoelectricity in a superconducting wire subject to spin-orbit coupling and a magnetic field with a relative orientation θ between them. We calculate the current flowing in a normal probe attached to the bulk of a superconducting wire, as a result of a temperature difference applied at the ends of the wire. We find that the thermoelectric response occurs in ranges of the angles θ which correspond to the emergence of Bogoliubov-Fermi points in the energy spectrum of the superconducting wire.

Alessandro Paghi, Giacomo Trupiano, Giorgio De Simoni, Omer Arif, Lucia Sorba, Francesco Giazotto
Preprint

Superconducting circuits based on hybrid InAs Josephson Junctions (JJs) play a starring role in the design of fast and ultra-low power consumption solid-state quantum electronics and exploring novel physical phenomena. Conventionally, 3D substrates, 2D quantum wells (QWs), and 1D nanowires (NWs) made of InAs are employed to create superconductive circuits with hybrid JJs. Each platform has its advantages and disadvantages. Here, we proposed the InAs-on-insulator (InAsOI) as a groundbreaking platform for developing superconducting electronics. An epilayer of semiconductive InAs with different electron densities was grown onto an InAlAs metamorphic buffer layer, efficiently used as a cryogenic insulator to decouple adjacent devices electrically. JJs with various lengths and widths were fabricated employing Al as a superconductor and InAs with different electron densities. We achieved a switching current density of 7.3 uA/um, a critical voltage of 50-to-80 uV, and a critical temperature equal to that of the superconductor used. For all the JJs, the switching current follows a characteristic Fraunhofer pattern with an out-of-plane magnetic field. These achievements enable the use of InAsOI to design and fabricate surface-exposed Josephson Field Effect Transistors with high critical current densities and superior gating properties.

Alok Nath Singh, Bibek Bhandari, Alessandro Braggio, Francesco Giazotto, Andrew N. Jordan
Preprint

Thermoelectric devices that operate on quantum principles have been under extensive investigation in the past decades. These devices are at the fundamental limits of miniaturized heat engines and refrigerators, advancing the field of quantum thermodynamics. Most research in this area concerns the use of conduction electrons and holes as charge and heat carriers, and only very recently have superconductors been considered as thermal engines and thermoelectric devices. Here, we investigate the thermoelectric response of an Abrikosov vortex in type-II superconductors in the deep quantum limit. We consider two thermoelectric geometries, a type-II SIN junction and a local Scanning Tunneling Microscope (STM)-tip normal metal probe over the superconductor. We exploit the strong breaking of particle-hole symmetry in bound states at sub-gap energies within the superconducting vortex to realize a giant thermoelectric response in the presence of fluxons. We predict a thermovoltage of a few mV/K at sub-Kelvin temperatures using both semi-analytic and numerical self-consistent solutions of the Bogoliubov-de Gennes equations. Relevant thermoelectric coefficients and figures of merit are found within our model, both in linear and nonlinear regimes. The ZT of the SIN junction is around 1, rising to above 3 for the STM junction centered at the vortex core. We also discuss how this system can be used as a sensitive thermocouple, diode, or localized bolometer to detect low-energy single photons.

Yuriy Yerin, Stefan-Ludwig Drechsler, A. A. Varlamov, Mario Cuoco, Francesco Giazotto
Preprint

We consider nonreciprocal supercurrent effects in Josephson junctions based on multiband superconductors with a pairing structure that can break time-reversal symmetry. We demonstrate that a nonreciprocal supercurrent can be generally achieved by the cooperation of interband superconducting phase mismatch and interband scattering as well as by multiband phase frustration. The effect of interband impurity scattering indicates that the amplitude and sign of the nonreciprocal supercurrent are sensitive to the interband phase relation. For the case of a three-band superconductor, due to phase frustration, we show that the profile of the supercurrent rectification is marked by a hexagonal pattern of nodal lines with vanishing amplitude. Remarkably, around the nodal lines, the supercurrent rectification amplitude exhibits three-fold structures with an alternating sign. We show that the hexagonal pattern and the three-fold structure in the interband phase space turn out to be dependent on the tunneling amplitude of each band. These findings provide hallmarks of the supercurrent rectification which can be potentially employed to unveil the occurrence of spin-singlet multiband superconductivity with time-reversal symmetry breaking.

A. Greco, Q. Pichard, E. Strambini, F. Giazotto
Preprint

The development of superconducting electronics requires careful characterization of the components that make up electronic circuits. Superconducting weak links are the building blocks of most superconducting electronics components and are characterized by highly nonlinear current-to-phase relations (CPR), which are often not perfectly known. Recent research has found that the Josephson diode effect (JDE) can be related to the high harmonic content of the current-to-phase relation of the weak links embedded in superconducting interferometers. This makes the JDE a natural tool for exploring the harmonic content of weak links beyond single-harmonic CPR. In this study, we present the theoretical model and experimental characterization of a double-loop superconducting quantum interference device (DL-SQUID) that embeds all-metallic superconductor-normal metal-superconductor junctions. The proposed device exhibits the JDE due to the interference of the supercurrents of three weak links in parallel, and this feature can be adjusted through two magnetic fluxes, which act as experimental knobs. We carry out a theoretical study of the device in terms of the relative weight of the interferometer arms and the experimental characterization concerning flux tunability and temperature.

F. Antola, A. Braggio, G. De Simoni, F. Giazotto
Preprint

Efficient heat management at cryogenic temperatures is crucial for superconducting quantum technologies. In this study, we demonstrate the heat diode performance of a gap asymmetric superconducting tunnel junction. Our results show that the mechanism of bipolar thermoelectricity, which occurs when the hot side has the larger gap, boosts heat rectification. This improvement is consistent for a broad range of thermal biases and for different values of the superconducting gaps. Additionally, we demonstrate that bipolar thermoelectricity can act as a heat pipe, reducing heat losses towards cold terminals and increasing overall efficiency up to 65%. Finally, we show that by adjusting the electrical load, it is possible to tune the heat pipe and diode performances.

Yuri Fukaya, Maria Teresa Mercaldo, Daniel Margineda, Alessandro Crippa, Elia Strambini, Francesco Giazotto, Carmine Ortix, Mario Cuoco
Preprint

We investigate supercurrent nonreciprocal effects in a superconducting weak-link hosting distinct types of vortices. We demonstrate how the winding number of the vortex, its spatial configuration and the shape of the superconducting lead can steer the sign and amplitude of the supercurrent rectification. We find a general criterion for the vortex pattern to maximize the rectification amplitude of the supercurrent. The underlying strategy is the search of specific vortex core position yielding a vanishing amplitude of the supercurrent first harmonic. We also prove that supercurrent nonreciprocal effects can be used to diagnose high-winding vortex and to distinguish between different types of vorticity. Our results thus provide a toolkit to control the supercurrent rectification by means of vortex phase textures and nonreciprocal signatures to detect vortex states with nonstandard phase patterns.

Giorgio De Simoni, Francesco Giazotto
Preprint

We suggest using a device called the Bootstrap Superconducting Quantum Interference Device (BS-SQUID) to break the reciprocity in charge transport. This device uses magnetic flux back-action to create a nonreciprocal current-voltage characteristic, which results in a supercurrent rectification coefficient of up to approximately 95\%. The BS-SQUID works as a quasi-ideal supercurrent diode (SD) and maintains its efficiency up to about 40\% of its critical temperature. The external magnetic flux can be used to adjust or reverse the rectification polarity. Finally, we discuss the finite-voltage operation regime of the SD and present a possible application of our device as a half- and full-wave signal rectifier in the microwave regime.

Federico Paolucci, Federica Bianco, Francesco Giazotto, Stefano Roddaro
Preprint

In the emergent field of quantum technology, the ability to manage heat at the nanoscale and in cryogenic conditions is crucial for enhancing device performance in terms of noise, coherence, and sensitivity. Here, we demonstrate the active cooling and refrigeration of the electron gas in a graphene thermal transistor, by taking advantage of nanoscale superconductive tunnel contacts able to pump or extract heat directly from the electrons in the device. Our prototypes achieved a top cooling of electrons in graphene of about 15 mK at a bath temperature of about 450 mK, demonstrating the viability of the proposed device architecture. Our experimental findings are backed by a detailed thermal model that accurately replicated the observed device behavior. Alternative cooling schemes and perspectives are discussed in light of the reported results. Finally, our graphene thermal transistor could find application in superconducting hybrid quantum technologies.

Luca Chirolli, Alessandro Braggio, Francesco Giazotto
Preprint

Quantum design of Cooper quartets in a double quantum dot system coupled to ordinary superconducting leads is presented as a novel platform for the study of an elusive many-body state of matter, that is at the basis of the phenomenon of charge-4e superconductivity. A fundamentally novel, maximally correlated ground state, in the form of a superposition of vacuum |0⟩ and four-electron state |4e⟩, emerges as a narrow resonance and it is promoted by an attractive interdot interaction. A novel phenomenology in the dissipationless transport regime is elucidated, that yields typical flux quantization in units of h/4e and manifests in non-local multi-terminal coherence and in two-Cooper pair transport properties mediated by the quartet ground state. The results open the way to the exploration of correlation effects and non-local coherence in hybrid superconducting devices, parity-protected quantum computing schemes and more generally, the work poses the basis for the design and simulation of novel correlated states of matter starting from ordinary ingredients available in a quantum solid state lab.

Daniel Margineda, Alessandro Crippa, Elia Strambini, Yuri Fukaya, Maria Teresa Mercaldo, Carmine Ortix, Mario Cuoco, Francesco Giazotto
Preprint

Back-action refers to a response that retro-acts on a system to tailor its properties with respect to an external stimulus. This self-induced effect generally belongs to both the natural and technological realm, ranging from neural networks to optics and electronic circuitry. In electronics, back-action mechanisms are at the heart of many classes of devices such as amplifiers, oscillators, and sensors. Here, we demonstrate that back-action can be successfully exploited to achieve non-reciprocal transport in superconducting circuits. Our device realizes a supercurrent diode, since the dissipationless current flows in one direction whereas dissipative transport occurs in the opposite direction. Supercurrent diodes presented so far rely on magnetic elements or vortices to mediate charge transport or external magnetic fields to break time-reversal symmetry. In our implementation, back-action solely turns a conventional reciprocal superconducting weak link with no asymmetry between the current bias directions into a diode, where the critical current amplitude depends on the bias sign. The self-interaction of the supercurrent with the device stems from the gate tunability of the critical current, which uniquely promotes up to ∼88% of magnetic field-free signal rectification and diode functionality with selectable polarity. The concept we introduce is very general and can be applied directly to a large variety of devices, thereby opening novel functionalities in superconducting electronics.

Clodoaldo I. L. de Araujo, Pauli Virtanen, Maria Spies, Carmen González-Orellana, Samuel Kerschbaumer, Maxim Ilyn, Celia Rogero, Tero T. Heikkilä, Francesco Giazotto, E. Strambini
Preprint

Heat engines are key devices that convert thermal energy into usable energy. Strong thermoelectricity, at the basis of electrical heat engines, is present in superconducting spin tunnel barriers at cryogenic temperatures where conventional semiconducting or metallic technologies cease to work. Here we realize a superconducting spintronic heat engine consisting of a ferromagnetic insulator/superconductor/insulator/ferromagnet tunnel junction (EuS/Al/AlOx/Co). The efficiency of the engine is quantified for bath temperatures ranging from 25 mK up to 800 mK, and at different load resistances. Moreover, we show that the sign of the generated thermoelectric voltage can be inverted according to the parallel or anti-parallel orientation of the two ferromagnetic layers, EuS and Co. This realizes a thermoelectric spin valve controlling the sign and strength of the Seebeck coefficient, thereby implementing a thermoelectric memory cell. We propose a theoretical model that allows describing the experimental data and predicts the engine efficiency for different device parameters.