Studies of Condensed Matter at Low Temperatures by Ultrasonic and other Mechanical Spectroscopies

In the second half of the twentieth century and in the first decade of the twenty first century, many new phenomena came to light in the fields of condensed matter and of materials properties’ at low temperatures. A few examples of these phenomena are: the plasticity and the behavior of dislocations in solid helium-4 (a quantum solid), “high” temperature superconductivity, occurrence of superfluid flow in solid helium (“supersolid”), and, Bose-Einstein condensation of cold atoms. In this presentation descriptions and some discussions are given on the role played in these studies by ultrasonic and other forms of mechanical spectroscopy.