Second sound measurements very near the lambda point

High-resolution measurements of the second sound velocity near T-lambda in superfluid He-4 at the vapor pressure are reported. Experiments were perfor med using the resonant technique in cavities 13, 4 and 2 mm in height. A di sc of paramagnetic salt, copper ammonium bromide, in direct contact with th e helium sample and coupled to a SQUID magnetometer, was used as the sensor . The sound was generated by wire-wound heaters embedded in the end opposit e the sensor in each cavity. The superfluid density was determined from sec ond sound measurements and the critical exponent nu was obtained from fits to the data. The results for the exponent were found to be very sensitive t o the treatment of systematic effects in the data. (C) 2000 Elsevier Scienc e B.V. All rights reserved.