Superheating in He II and successive phase transitions in metastable states.

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We measure the superheating in He II and He I over a significant range of pressures with particular attention paid to the superheating limits. He II is found to undergo successive phase transitions, He II..-->..superheated He II..-->../sup T//sub s/lambda superheated He I..-->../sup T//sub s/vapor, with an isobaric increase in temperature. The results for T/sub s/lambda show that He II can persist in a superheated metastable state to the extended lambda line. The experimental results for T/sub s/ are compared with predictions from conventional homogeneous nucleation theory and satisfactory agreement is found. Analysis of the T/sub s/ data with respect to the scaled pressure and temperature confirms quantum effects in the superheating limit.