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Equation of State of overpressurized liquid ^4He at zero temperature

In recent experiments, Balibar and collaborators have been able to produce metastable superfluid $^4$He up to approximately 200 bar, a pressure much higher than the freezing point (25 bar). Interpretation of some of their experimental data requires the knowledge of the equation of state of bulk $^4$He in this high-pressure regime. As this data is not experimentally accessible, Maris and collaborators have proposed to use the data extrapolated from analytical fits to the well known equation of state in the stable regime. Moreover, combining these extrapolations with density functional estimations of the dependence of the roton energy on the pressure, Maris and co. predicted the existence of a maximum liquid pressure ($\sim$ 200 bar) due to the achievement of a zero-energy roton. Using the same theoretical analysis that allowed in the past to reproduce accurately the equation of state of liquid $^4$He in the stable domain, we present now extended results for this equation of state up to $\sim$ 280 bar. Our calculations, based on the diffusion Monte Carlo method, show some significant differences with the proposed extrapolations, which could be relevant for future experiments. Also, preliminary data show a rather slow decrease of the roton energy with increasing pressure, which locates the possible instability much higher in pressure than Maris's predictions.

metastable superfluid; overpressure; equation of state; roton

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