While there is considerable evidence that liquid He II can exist in metastable states produced by the isothermal reduction of the pressure below the vapor pressure P v (T) (tensile strength measurements),1’2 the complementary process of superheating (isobaric increase of temperature beyond the vaporization temperature) has not been observed. Several recent papers suggest that metastable superheating in fact does not occur.3 This is difficult to reconcile with the finite tensile strength, unless one considers the extreme sensitivity of the tensile strength to mechanical vibrations. A very small mechanical shock is sufficient to nucleate vapor phase in the super-stressed liquid. Our observations of superheating are on small samples of He II which are highly vibration-isolated.
KeywordsTensile Strength Lower Branch Mechanical Shock Mutual Friction Sample Reservoir
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