Extension of the λ curve of 4He into the region of the metastable state of liquid helium
Institute of Thermal Physics, Ural Branch of the Russian Academy of Sciences, Pervomaiskaya str. 91, Ekaterinburg, 620219, Russian Federation
A possible extension (λ’) of the λ curve of liquid helium beyond the T, p boiling line, i.e., into the region of a metastable liquid state, is examined. The relation between the slope of the λ curve dp/dTλ and the sign of the thermal expansion coefficient of liquid helium ap is discussed. Experimental data for ap are analyzed using thermodynamic equations to reveal that the derivative dp/dTλ changes sign in the metastable region. As a result, the shape of the λ’ extension becomes similar to that of the pressure-shifted Bose-condensation line of an ideal 4He gas. The position of the λ’ curve is not entirely arbitrary due to the spinodal nature of stretched liquid helium.