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Microwave dielectric losses at millikelvin temperatures of substrates suitable for High critical Temperature Superconductors epitaxial growth

We have investigated both the temperature and the power dependence of microwave losses for various dielectrics commonly used as substrates for the growth of High critical Temperature Superconductor thin films. We present measurement of niobium superconducting $λ/2$ coplanar waveguide resonators, fabricated on $La_{0.3}Sr_{0.7}Al_{0.65}Ta_{0.35}O_3$ (LSAT), $MgO$ (MgO) and $LaAlO_3$ (LAO), at the millikelvin temperature range and at low input power. By comparing our results with the Two-Level System (TLS) model, we have discriminated among different dominant loss mechanisms. LSAT has shown the best results as regards the dielectric losses in the investigated regimes.