Phase diagram of muonium hydride: the significant effect of dimensionality

We carry out a theoretical investigation of the low-temperature phase diagram of muonium hydride in two dimensions, using numerical simulations. It is shown that the phase diagram of this substance is qualitatively different in two and three dimensions. Specifically, while in three dimensions it has been shown to be essentially identical to that of parahydrogen, i.e., only displaying a single (crystalline) phase, in two dimensions it is very similar to that of He-4, with an equilibrium liquid phase that turns superfluid at a temperature as high as ~ 2.2 K, and that crystallizes under applied pressure. This is a well-described case of a continuous-space condensed matter system whose ground state equilibrium phase is qualitatively altered by dimensional reduction.