INTERSTITIALS, VACANCIES, AND SUPERSOLID ORDER IN VORTEX CRYSTALS

Interstitials and vacancies in the Abrikosov phase of clean type-11 superconductors are line imperfections, which cannot extend across macroscopic equilibrated samples at low temperatures. We argue that the entropy associated with line wandering nevertheless can cause these defects to proliferate at a sharp transition which will exist if this occurs below the temperature at which the crystal actually melts. Vortices are both entangled and crystalline in the resulting ''supersolid'' phase, which in a dual ''boson''-analog system is closely related to a two-dimensional quantum crystal of He-4 with interstitials or vacancies in its ground state. The supersolid must occur for B much greater than B(x), where B(x) is the decoupling field above which vortices begin to behave two dimensionally. Numerical calculations show that interstitials, rather than vacancies, are the preferred defect for B much greater than phi0lambda1(2), and allow us to estimate whether proliferation also occurs for B less than or similar to B(x). The implications of the supersolid phase for transport measurements, dislocation configurations, and neutron diffraction are discussed.