  A muonium hydride molecule is a bound state of muonium and hydrogen atoms. It
has half the mass of a parahydrogen molecule and very similar electronic
properties in its ground state. The phase diagram of an assembly of such
particles is investigated by first principle quantum simulations. In the bulk
limit, the low-temperature equilibrium phase is a crystal of extraordinarily
low density, lower than that of any other known atomic or molecular crystal.
Despite the low density and particle mass, the melting temperature is
surprisingly high (close to 9 K). No (metastable) supersolid phase is observed.
We investigated the physical properties of nanoscale clusters (up to 200
particles) of muonium hydride and found the superfluid response to be greatly
enhanced compared to that of parahydrogen clusters. The possible experimental
realization of these systems is discussed.
