R. Kagiwada

The velocity of second sound near Tλ

Abstract The results of second sound velocity measurements made very near the lambda point with a new type of transducer are in substantial agreement with the hydrodynamic value.

Studies of the Threshold‐of‐Cavitation Noise in Liquid Helium

Further studies of the threshold‐of‐cavitation noise in liquid helium have been made with an ultrasonic plane‐wave system. The behavior of this threshold was found to have a similarity to that of the specific heat in the λ region. Both above and below the λ temperature, the threshold was very insensitive to variations of static pressure and unchanged by addition of solid air or the imposition of electric fields. These results are difficult to explain on the basis of conventional theories of nucleation, but possibly indicate that nucleation involves the aggregation of submicroscopic structures.

Effect of Finite Size on Superfluidity; Third Sound

When the quantum liquid, 4He, is bounded in size and when one or more of its dimensions becomes comparable with a characteristic healing length, the superfluid state is altered. In particular, the superfluid onset temperature is lowered and the superfluid fraction at a given temperature is reduced. Third sound is a surface wave that propagates in liquid‐helium films where only the superfluid component moves. Third‐sound measurements have been made in films several atomic layers thick from which reduced onset temperatures and superfluid fractions have been determined. These results will be discussed in the framework of current theories.

ULTRASONIC ATTENUATION STEPS IN THE MIXED STATE OF NIOBIUM

Abstract Ultrasonic attenuation spikes and steps, coincident with temperature spikes and magnetic flux steps have been observed in an irreversible niobium single crystal.

Ultrasonic Attenuation in Normal‐Conducting and Mixed and Meissner Superconducting States of Niobium

The acoustic attenuation has been measured in a pure single crystal of niobium from about 1.2°–40°K. Longitudinal waves up to a frequency of 225 Mc/sec were propagated in the [100] direction in both normal and superconducting states. In the superconducting state, measurements were made with and without the presence of an external magnetic field. In the presence of a magnetic field, measurements were made with either a fixed temperature and increasing or decreasing field or a fixed field and increasing or decreasing temperature. Ultrasonic‐attenuation data obtained by increasing the magnetic field and holding temperature constant seem to obey a [Hc2(T) − H0]12 dependence near Hc2(T) as Maki has predicted, where Hc2(T) is the upper critical field and H0 is the external applied field. In the absence of an external magnetic field, αs/αn can be fitted below T/Tc=0.6 to the usual bcs expression with 2Δ=3.64±0.05 kTc. [Work supported by the U. S. Office of Naval Research.]