Unconventional superconductivity is extensively sought for in contemporary research. Of particular interest are chiral superconductors which possess non-trivial topological properties resulting in superconducting (SC) order parameters (OPs) that may break time-reversal symmetry (TRS). The possibility of applications to topological quantum computation have placed such materials at the forefront of condensed matter research. Recent measurements of the polar Kerr effect (PKE), in which a rotation of polarization is detected for a beam of light reflected from the surface of a superconductor, have emerged as a key experimental probe of TRS breaking. Here we propose the development of a new generation of spectroscopic instrumentation for the PKE spectroscopy in the sub-THz frequency range, the energy scale that is comparable with the SC gap magnitude of unconventional superconductors. The THz range PKE spectroscopy will enable to study the broken symmetries, the origin of unconventional pairing, the in-gap collective modes, and the structures of the SC OPs. We plan to measure the PKE at sub-THz frequencies and with sub-milli-radian angular resolution from a variety of unconventional superconductors that are cooled to 100 mK, deep into SC state. The aim is to understand the basic mechanisms leading to unconventional superconductivity in these systems in order to find answers to the fundamental questions, such as: What is the structure of the SC gap in Sr2RuO4, URu2Si2, and UPt3? Is the TRS broken in (a) the Hidden Order state and in (b) SC state of URu2Si2? Which symmetries are broken at the transition from the HO state into the unconventional SC state? – and to elucidate the microscopic origin of superconductivity in the new families of unconventional superconductors. In a broader view, the project will keep Estonian physics on the forefront of science through new scientific contacts and will promote physics education by engaging students and postdocs in the research.
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