Comments On Supersolidity Institutional Repository Document uri icon

abstract

  • Assuming that the well-confirmed non-classical rotational inertia (NCRI) effect in solid $^4$He, suggested by Leggett, indicates supersolid behavior, we make a number of remarks about both theory and experiment. (1) The long-wavelength, low-frequency ("hydrodynamic") part of the theory of Andreev and Lifshitz has nine variables, and thus must have nine modes. We find a new mode associated with lattice point diffusion (and thus vacancy diffusion); it may explain the absence of supersolid behavior in low-frequency pressure-driven flow. (2) The observed upper limit for the NCRI fraction (NCRIf) of about 20%, in disordered samples, is more-or-less the same as the already predicted upper limit for the superfluid fraction of a well-ordered crystal; we argue that this may not be a coincidence. (3) The negative experimental evidence for a second propagating hydrodynamic mode (expected to be fourth sound-like) may be due to the long relaxation times $ au$ at low temperature $T$; only for frequencies satisfying $omega aull1$ does the hydrodynamic theory apply. (4) The fundamental principles of quantum mechanics imply that Bose-Einstein condensation is not necessary to define a quantum-mechanical phase; therefore the absence of a finite condensate fraction $f_{0}$ does not necessarily imply the absence of superfluidity. (5) Just as vortices should avoid occupied lattice sites to provide a vortex-lattice interaction, the lattice should interact with the vortices to provide a lattice-vortex interaction; thus dislocations should interact with vortices, whose motion is affected by rotation. We discuss some experimental implications for the vortex liquid model, shear response, hysteresis, and relaxation.

author list (cited authors)

  • Saslow, W. M.

complete list of authors

  • Saslow, WM

publication date

  • June 2009