text: 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