Abstract
By directly measuring the difference in velocity of 30 and 90 MHz sound waves in liquid below 100 mK, we have determined that the elementary excitation spectrum cannot be described by an expression which has a value for that is greater than 0.01 Å.
- Received 25 May 1972
DOI:https://doi.org/10.1103/PhysRevLett.29.32
©1972 American Physical Society