Quantum Fluids & Solids
N. Mulders, J. T. West, M. H. W. Chan, C. N. Kodituwakku, C. A. Burns, and L. B. Lurio, Torsional oscillator and synchrotron X-Ray experiments on solid 4He in aerogel, Phys. Rev. Lett. 101, 165303 (2008). [PDF] S. O. Diallo, J. V. Pearce, R. T. Azuah, O. Kirichek, J. W. Taylor, and H. R. Glyde, Bose-Einstein condensation in solid 4He, Phys. Rev. Lett. 98, 205301 (2007). [PDF] H. R. Glyde, Condensed-matter physics: Defects and perfect flows, Nature 444, 693 (2006). [URL] M. H. W. Chan, N. Mulders, and J. Reppy, Helium in aerogel, Phys. Today 49(8), 30 (1996). [PDF] H. R. Glyde, Excitations in Liquid and Solid Helium (Oxford University Press, Oxford, 1995).
Superconductivity (superflow of charged electrons), superflow of neutral bosons in liquid helium, and superflow of ultracold atoms were first reported in 1911, 1938, and 1995, respectively. Each discovery lead to a Nobel prize in Physics. In these 3D fluids, superflow arises from Bose-Einstein condensation (BEC), which is the macroscopic occupation of a single quantum state at low temperatures.
In 2004, remarkably, a superfluid component in solid helium was reported by E. Kim and M .H. W. Chan, Science 305, 1941 (2004). This is quite unexpected since in solids the atoms are localized around lattice points. The apparent superfluid fraction has been verified in several independent measurements and superflow is believed to be via defects, such as mobile vacancies, or in dislocations, in grain boundaries, in amorphous regions and or along surfaces. The supersolid phase was originally proposed by Andreev and Lifshitz in 1969, suggesting that any crystal having ground-state vacancies - that is, a mismatch at low temperatures between the number of lattice sites and the number of atoms - would form a BEC and have superfluid properties. However,
experiments and computer simulations find that for solid helium such a mismatch is not thermodynamically stable, and thus solid helium in the true ground state should not be a supersolid.
The largest superfluid fraction valus are observed in solids that have a large surface to volume ratio. But the new findings do not resolve the puzzle completely. How do we explain the consistency of the results of Kim and Chan - that is, why do they see the same apparent superflow response in various porous media such as vycor glass, porous gold and bulk helium, and at various pressures? One would expect that the density of grain boundaries would be much greater in porous media. How does the very small amplitude of the oscillator produce such a large angular momentum in the grain boundaries? Could dislocations, responsible for plastic flow in crystals, play a role?
Direct observation of BEC as a macroscopic fraction of particles condensed in the zero momentum state would be convincing evidence of superflow in solid helium - to date, Delaware researcher have failed to confirm superflow in this fashion. Nevertheless, what is emerging is that helium, long used as a testbed for ideas on quantum many-body physics (including Bose and Fermi superfluidity, quantum crystals and magnetism) or even cosmology (phase transitions in helium replicating early stages of the Universe evolution), may also play a role in understanding the quantum mechanics of extended defects.
- superfluid He in porosity aerogels (Mulders)
- solid and liquid He in nanoporous media (Mulders, Glyde)
- superfluid-insulator quantum phase transitions in disordered 3He and 4He (Glyde, Mulders)
- neutron scattering studies of superfluids and supersolids (Glyde).