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Thermoelectric detector based on superconductor-ferromagnet heterostructures

Periodic Reporting for period 3 - SUPERTED (Thermoelectric detector based on superconductor-ferromagnet heterostructures)

Reporting period: 2021-03-01 to 2023-02-28

Superconducting detectors, such as the transition edge sensor and the kinetic inductance detector, are some of the most sensitive detectors of electromagnetic radiation and they have found application in various fields ranging from astrophysical observations to security imaging and materials characterization. The present tendency is to increase the number of sensor pixels to allow for a simultaneous imaging and spectroscopy in the video rate of the measured object. However, increasing the number of pixels is hampered by the technical difficulty of fabricating and controlling the bias lines needed next to each pixel in these types of sensors, along with the heating problem associated with them. In this project, we propose to study a new type of sensor that overcomes this limitation as it is based on the thermoelectric conversion of the radiation signal to electrically measurable one. This approach is based on the newly found giant thermoelectric effect taking place in superconductor/ferromagnet heterostructures. Utilizing this effect, the sensor pixels can be self-powered by the measured radiation, and therefore extra bias lines are not needed (patent pending for the detector concept). Within the project, we aim to establish a proof of concept of this device by (i) fabricating such detector elements, and (ii) characterizing single pixels of thermoelectric detectors for X-ray and THz imaging via approaches that are scalable to large arrays. Within the project, we also actively seek to establish technology transfer to pave the way for the possible commercial application of such sensors.
The SUPERTED project was successful in providing the first demonstration of biasless detection of electromagnetic radiation utilising the thermoelectric effect in superconductor/ferromagnet hybrid structures. In particular, we established detector sites, one devoted to sub-THz radiation and another one for X-ray radiation, with capability of operating such detectors. We also made an experimental proof of concept of an actual radiation detector in the THz domain, therefore reaching a technology readiness level of 3. In addition, we have identified the steps required to realise optimal detectors in both regimes of electromagnetic radiation. These results have required testing a large number (more than 500) samples in four different sites, and a close collaboration between theory and experimental groups to understand the main parameters affecting detector performance. Besides the main goal of thermoelectric radiation detectors, our project realised a European facility to grow superconductor/ferromagnetic insulator hybrid structures with desired properties. Such systems can be used in various purposes, many of them established within the theory efforts as part of the project. As a new and unanticipated finding, we noticed that the hybrid structures studied in the project also act as quasiparticle tunnel diodes, and hence could be used in also other tasks than detectors, such as current delimiters or rectifiers. Our consortium has protected the IPR for both innovations, the detector and the diode, via patenting. We found out that to operate the detectors, a small external magnetic field is needed as the ferromagnetic insulator used in the project, EuS, needs to be magnetised after refrigerating the detectors to the operating temperature. Fortunately the magnetic field required for magnetising the samples is much smaller than would be needed for spin splitting in the absence of magnetism, and we realised the detector setups with Helmholz coils providing the needed fields. Interestingly, such coils in detector setups also allow tuning the detectors in situ to various operating regimes. What is more, the read-out techniques realised in the project are amenable to multiplexing and we have outlined two different strategies of detector multiplexing in large detector arrays. We have already disseminated many of the results in scientific papers, press releases, and conference presentations, and our dissemination efforts continue beyond the project. Besides the two patented innovations which we will advance via a start-up company of one of our partners, the multilayer growth expertise in our consortium has raised interest within another company, and we have started a collaboration to exploit it.

The project has resulted so far into 88 scientific papers, 18 of them in journals with an impact factor larger than 7. In addition, we have told about SUPERTED in various events and press releases.
Before the project, ferromagnetic insulator/superconductor samples exhibiting spin splitting at zero external field were produced by a single group, and even a single researcher (J. Moodera) in the world. This not only slowed down efforts to use this phenomenon for thermoelectricity, but also for example for realizing topological superconductivity in systems containing simultaneously ferromagnetic insulators, superconductors and systems with strong spin-orbit coupling. Now we are able to prepare such samples within the project, allowing for much more versatile planning and optimizing of the samples. We have also found novel functionalities for these samples, which work also as diodes or rectifiers. In addition, we have created a wealth of knowledge related to the physical phenomena necessary to be understood for optimizing the devices, such as on the dependence of the magnetic proximity effect on disorder effects, on the sample thicknesses, or on position or time dependent magnetization. We made some pioneering attempts to use two-dimensional materials as parts of our hybrid structures, especially providing magnetic proximity effect. From this work we identified a new way of realising non-reciprocal superconducting effects in form of Josephson diodes. Before the project the main way of characterising the magnetic proximity effect into superconductors was studying the spin splitting in tunnelling conductance. As a result of the project, we have identified also other ways of characterising it, useful also in regimes where spin splitting is masked by spin relaxation. In particular, for isotropic spin relaxation we suggest using the non-reciprocal tunnelling characteristics, i.e. the diode effect, whereas for anisotropic spin relaxation we can use the diode effect for which our project has provided strong theoretical understanding.

Delays caused by the pandemic have seriously affected our operations due to long lockdown periods during which no experimental work could be done, due to the state of resolvency of one of the partners and the ensuing change of partners, due to the difficulty of traveling between partners, and due to the delays in shipments of relevant technology required for the aims. This is why it took near to the end of the project before we managed to demonstrate our main aim of thermoelectric detection, and we still need to optimise the effect to reach good figures of merit. Nevertheless, upon identification of viable routes to optimizing the detectors, routes depending on the detected wavelength of radiation, we are now in place to take next steps in developing the detectors. Namely, arrays of thermoelectric detectors may open the way to widen the use of superconducting detectors to new regimes of imaging. Such advances are important in a wide range of applications, ranging from astrophysical and security imaging to elemental analysis of materials and bioimaging.
Graphical representation of the SUPERTED sensor.