Second sound has been observed in solid He and He, and in some dielectric solids such as Bi in the temperature range of 1.2 to 4.0 K with a velocity of 780 ± 50 m/s, or solid sodium fluoride (NaF) around 10 to 20 K. In 2021 this effect was observed in a BKT superfluid as well as in a germanium semiconductor