  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. To our
knowledge, this is the first well-described case of a condensed matter system
whose phase diagram is drastically altered by dimensional reduction.
