Skip to main content

Thermoelectric detector based on superconductor-ferromagnet heterostructures

Periodic Reporting for period 2 - SUPERTED (Thermoelectric detector based on superconductor-ferromagnet heterostructures)

Reporting period: 2019-09-01 to 2021-02-28

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 second reporting period included severe restrictions of work related with coronavirus lockdown measures that affected especially laboratory intensive work, but also made traveling between the partners very hard. However, we managed to progress in several items towards our final goals: the CSIC partner has now established a process to grow the samples that exhibit both spin splitting and spin polarization in the superconducting state (as measured by CNR), both prerequisites for the thermoelectric radiation detection. First based on samples from our partner (J. Moodera) at MIT and later with samples from CSIC, CNR has been able to demonstrate explicitly the non-reciprocal transport properties of these samples. Namely, in addition to the thermoelectric effect, we found that these devices can act as diodes and rectifiers, which provides another mode of biasless radiation detection. CNRS and JYU have almost completed their setups for thermoelectric detection of sub-THz and X-ray detection, respectively, and we expect to be able to see first radiation detector measurements with true radiation inputs within the next few months. All these processes have been backed up by theory efforts, providing simulation tools for various parts of the project, such as the magnetic proximity effect from a magnet to the superconductor, the detailed tunneling current-voltage curves depending not only on superconductivity and spin-splitting fields, but also on various relaxation effects, simulations of heat balance for given power absorbed in the device, and finally noise simulations to estimate and optimize the sensitivity of the devices. The pandemic took its toll as our company partner BIHUR filed for bankruptcy. Luckily, we could find another company, ADVA, to take its tasks.

The project has resulted so far into 48 scientific papers, 9 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 have followed our project proposal very carefully, but the delays caused by the pandemic seriously 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 the goal of demonstrating the world's first superconducting thermoelectric detection of electromagnetic radiation has moved to the next few months. We believe that such a demonstration will be feasible both for sub-THz and X-ray radiation by the fall 2021. We will request an extension to the project such that the remaining of the project can then be used to optimize the detectors and aim at proving their benefits compared to the present-day superconducting detectors. If successful, this progress 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.