Observability of a Projected New State of Matter: A Metallic Superfluid

Dissipationless quantum states, such as superconductivity and superfluidity, have attracted interest for almost a century. A variety of systems exhibit these macroscopic quantum phenomena, ranging from superconducting electrons in metals to superfluid liquids, atomic vapors, and even large nuclei. It was recently suggested that liquid metallic hydrogen could form two new and unusual dissipationless quantum states, namely, the metallic superfluid and the superconducting superfluid. Liquid metallic hydrogen is projected to occur only at an extremely high pressure of about 400 GPa, with pressures on hydrogen of 320 GPa having already been reported. The issue to be addressed is whether this state could be experimentally observable in principle. We propose four experimental probes for detecting it.

Figure 1

Liquid metallic hydrogen in a high-pressure diamond anvil cell. The red insertion makes the sample multiply connected. Thus, a thermal quench should produce $2\pi \times [\mathrm{\text{integer}}]$ winding of the phase of the protonic condensate. This will result in a detectable fractional magnetic flux passing through the diamond anvil cell. The fraction of the flux quantum will depend on the ratios of superfluid densities of electronic and protonic condensates.