Supersolidity and the Supersolid to Normal Solid Transition

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Technical Abstract.The observation of superfluid-like behavior in bulk solid helium and solid helium confined in porous media in the P.I.'s laboratory has been replicated in a dozen other laboratories. Quantum Monte Carlo simulations suggest superfluidity is not possible in a perfect crystal but dislocation lines may be superfluid. Shear modulus measurements indicate that superfluidity in solid helium or supersolidity is the consequence of a stiffened network of interconnected dislocation lines. This model however, cannot explain the results in porous media since it is difficult to envision dislocation lines inside a pore of a few nm. Three questions will be addressed in this project. One set of experiments will determine if supersolidity is possible in a perfect crystal with little or no dislocation lines. Secondly, heat capacity, thermal conductivity measurements will be carried out to determine how supersolidity in porous media is different from that in bulk solid. Lastly torsional oscillator measurements will be made to determine the spatial extent of quantum phase coherence in a supersolid system. The experiments are exceptionally challenging hence providing an excellent training ground for the post-doctoral, graduate and undergraduate students in the project. The P.I. has a very strong and sustained record in training students for successful scientific careers in academia and in industrial and government laboratoriesNon-technical AbstractWhen liquid helium is cooled below -271 Celsius, or 2 K above absolute zero, it enters the superfluid state with zero viscosity allowing it to flow relative to the walls of a container with no friction. What this means is that if a container is being oscillated back and forth, the superfluid inside will sit perfectly still. Like superconductivity, superfluidity is a manifestation of quantum mechanics at macroscopic scales. In 2004, the P.I. and one of his former student found evidence that below 0.2K, a small fraction (on the order of 1%) of a solid helium sample would remain stationary when its container is being oscillated. This experiment, since replicated in a dozen laboratories worldwide, indicates the existence of superfluidity in a solid, more conveniently called supersolidity. The discovery of this new state of matter has created considerable excitement in the physics community and beyond. An outstanding question of the phenomenon is whether supersolidity is possible in a perfect helium crystal or it requires the presence of defects such as dislocation lines in the crystal. The experiments proposed in the project are designed to resolve this question and will be carried out by post-doctoral, graduate and undergraduate students. The P.I. has a very strong and sustained record in training students for successful scientific careers in academia, industrial and government laboratories.