EXPERIMENTAL INVESTIGATION OF THE LAMBDA TRANSITION OF LIQUID HELIUM USING ACOUSTIC TECHNIQUES.
CALIFORNIA UNIV LOS ANGELES DEPT OF PHYSICS
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The velocity and attenuation of first sound have been measured simultaneously to within microdegrees of the lambda transition of liquid helium. Measurements were made in the frequency range of 20-100 kc. With the high Q modes of the resonator, relative sound velocity measurements of one part in 100,000 could be obtained. Above the transition the velocity of sound had the temperature dependence predicted by the Pippard Theory out to 5 millidegrees from the lambda point. The velocity of sound below the transition does not agree with the Pippard Theory except within a fraction of a millidegree of the transition. The added temperature dependence beyond that expected from the Pippard theory can be fit to a series expansion in delta T. This additional dependence is potulated to be due to the effects of the order parameter. The minimum of the sound velocity at the lambda point was found to be finite and equal to 217.05 ms. Using the convective properties of the resonator, the position of the density maximum was found to be approximately 6.2 millidegrees above the lambda point. Author