Superfluid Glass of He-4

With a recently developed worm algorithm, we simulate 800 atoms of He-4 at the density corresponding to the zero-$T$ melting point of the hcp crystal. We find that low-$T$ properties of the system--- in our case, at $T=0.2$K---crucially depend on the initial state and cooling protocol. An ideal hcp crystal is a clear-cut insulator. But if we start with a high-$T$ liquid state and quench-cool the sample down to $T=0.2$K, we end up with a new state of matter, 'superglass',---a metastable amorphous solid that features off-diagonal long-range order and superfluidity.