More than seven years has passed since the momentous paper by Chan and Kim has drawn attention yet again to the exciting subject of supersolidity.* With the benefit of hindsight I will discuss what happened in this fields in the last seven years.
Chan and Kim have observed the mechanical anomaly of the torsional oscillator subjected to the constant oscillating torque. The anomaly in itself is small and sets in at quite low temperatures below about 160mK. Related to this mechanical anomaly is observed anomaly in the specific heat of solid He4.
In this general talk I will give an overview of the field. I will give arguments for and against supersolidity.* While there are suggestive clues pointing to supersolid as an explanation, substantial theoretical and experimental difficulties call for an alternative explanation. I will argue that the glass scenario offers an alternative interpretation of the torsional oscillator experiments in contrast to the supersolid scenario of nonclassical rotational inertia. Work of Los Alamos group suggests that the low-temperature specific heat anomaly is due to a glassy state that develops at low temperatures and is caused by a distribution of tunneling systems in the crystal. We propose that dislocation related defects produce those tunneling systems. Further, we argue (Phys. Rev. B 76 (2007) 014530) that the reported putative mass decoupling, that means, an increase in the oscillator frequency, is consistent with a glass-like transition.
*Supersolid phase is one of the enigmatic states of matter whereby the very same atoms exhibit simultaneously crystalline order, with long range order in density modulation and at the same time the off diagonal ling range order, best characterized as some sort of superfluid phase stiffness that would allow atoms flow without resistance in response to smooth changes of the superfluid phase. The richness of quantum mechanics allows the very same atoms to be partially locked in the lattice and yet to partially flow in response to the small pressure gradients.
