Quantum phenomena in rotating solid helium

Solid helium-4 is an example of a quantum crystal due to the presence of large quantum fluctuations (zero point motion). It provides an ideal testbed for investigating the quantum behaviour of crystalline matter and structural defects such as dislocations (crystallographic line defects). Experiments by Kim and Chan, showed that a small component of solid helium appeared to decouple from the oscillatory motion of their container (a torsional oscillator), sparking a huge amount of controversy regarding whether so-called supersolidity (i.e. the paradoxical superfluid flow of a solid) occurs in solid helium. The most recent research points towards the effect been largely due to the anomalous quantum plasticity of solid helium due to the motion of dislocations and their subsequent freezing due to pinning by helium-3 impurities at very low temperatures.

However, there have been several other very recent observations involving four different research groups of an effect due to steady DC rotation that may be independent of the changes in elasticity and instead due to some exotic quantum behaviour and perhaps supersolidity. The correct interpretation of these experiments is unclear and controversial. In addition, the group of Hallock has observed direct DC mass flow through solid helium which has been interpreted as being due to superflow along the cores of dislocations. There is thus a need for new experiments to search for unambiguous evidence of quantum coherence (such as supersolidity) in solid helium.

Rotation has been successfully used to probe the helium superfluids in many different experiments, perhaps most remarkably in the demonstration of persistent mass currents (due to flow with no dissipation) - the "smoking gun" of superfluidity. We will use our new rotating dilution refrigerator to search for novel quantum phenomena when solid helium is rotated. Firstly, we will look for effects due to steady rotation on a disk of solid helium housed in a torsional oscillator that are independent of elastic effects, which will be measured simultaneously using shear plates. Secondly, we will conduct a direct search for persistent mass currents in solid helium by measuring the angular momentum of flow generated by rotation in an annular channel using a high-sensitivity gyroscope. Observation of persistent currents would constitute direct evidence of supersolid behaviour. Plastic effects and relaxation due to mechanical agitation, such as rapid changes in rotation will also be investigated.

The beneficiaries of this proposed research fall into five different categories:

Evidence of new states of matter, such as some form of supersolidity or other quantum effect, will be a major discovery and will stimulate future theoretical, computational and experimental research.

Materials science underpins many areas of engineering. Understanding the plasticity of solids (due to the gliding of dislocations) is of fundamental importance and recent research [e.g. L. Proville et al. Nat. Phys. 11, 845 (2012).] has suggested that quantum effects play an important role in the behaviour of defects in materials in general. Solid helium is an ideal model system that allows the quantum physics of structural defects, including flow due to plasticity and perhaps due to other quantum effects, to be probed in detail. This programme of research will thus advance our knowledge of materials in general.

Direct observation of supersolidity (such as persistent mass flow) would be an exciting discovery. The counter-intuitive and paradoxical behaviour of solid helium will be used to enthuse the next generation of scientists through thought-provoking public engagement activities.

The planned experiments are technically challenging and the implementation will lead to advances in techniques and instrumentation that may have wider applicability in other areas of science and engineering.

Impact will be achieved through the training of researchers. The researchers involved with this project will receive a thorough training in experimental low temperature physics and related computational techniques combined with transferable skills training.