Thermoelectric Effects in Superconductor-Ferromagnet Hybrids

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Non-technical abstract: Thermoelectric systems convert heat to electricity and vice versa. They have applications in refrigeration, waste heat recovery, and thermometry. With no gas or liquid components and no moving parts, thermoelectric devices are small, portable, and sturdy. However, the energy conversion efficiency of thermoelectric materials decreases with decreasing temperature. Therefore, to date, there is no efficient thermoelectric systems operating below ~ 200 K. The thermoelectric system this project is focused on fabricating and measuring, is predicted to be efficient at elevated temperatures. From a technology perspective, this will enable applications that were previously thought impossible such as thermoelectric cooling for cryogenic devices and space telescopes. This project trains a new generation of students with expertise in quantum systems, synthesis, characterization, cryogenics, low-noise electronics, and computation. Undergraduate research and recruitment of underrepresented groups is facilitated through Colorado School of Mines' bridge program with community colleges. Additionally, modules are being developed, introducing the science behind this project, for use in Mines' outreach programs to Denver Public Schools, Vertical Skills Academy (which is a school for dyslexic children in Evergreen, CO), and the community.Technical abstract: Thermoelectric effects, which are normally negligible in superconductors, are expected to increase dramatically in superconductor (S) - ferromagnet (F) hybrids. Although exciting theoretical predictions abound, there are hardly any experimental studies in this field. Moreover, there are no studies, experimental or theoretical, on the role of dimensionality and the nature of superconductivity (singlet vs. triplet). This is a tremendous missed opportunity for several reasons. First, if the predictions of high thermoelectric figure of merit at cryogenic temperatures are correct, S-F systems will be the only efficient thermoelectrics in this temperature range. Second, there is fascinating Physics at S-F interfaces, including triplet superconductivity, seen in charge and spin transport measurements. Thermoelectric effects offer a novel probe into this Physics. Third, the effects of reduced dimensionality on quasiparticles in a superconductor, which are relevant for thermal effects, has not been explored. Finally, there are a number of fascinating predictions for thermal effects in S-F systems that have not been realized because the 'groundwork' experiments are missing. This project fills this gap by systematically investigating thermoelectric effects in S-F hybrids using low-temperature, charge, spin and thermal transport measurements. The results test existing theoretical predictions, inform future studies and explore heralded applications including nanoscale cooling. These efforts build upon the PI's expertise in nanoscale fabrication and low temperature transport experiments.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.