INTERSTITIALS, VACANCIES, AND SUPERSOLID ORDER IN VORTEX CRYSTALS

Interstitials and vacancies in the Abrikosov phase of clean type-11 su perconductors are line imperfections, which cannot extend across macro scopic equilibrated samples at low temperatures. We argue that the ent ropy associated with line wandering nevertheless can cause these defec ts to proliferate at a sharp transition which will exist if this occur s below the temperature at which the crystal actually melts. Vortices are both entangled and crystalline in the resulting ''supersolid'' pha se, which in a dual ''boson''-analog system is closely related to a tw o-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 t o behave two dimensionally. Numerical calculations show that interstit ials, rather than vacancies, are the preferred defect for B much great er than phi0lambda1(2), and allow us to estimate whether proliferation also occurs for B less than or similar to B(x). The implications of t he supersolid phase for transport measurements, dislocation configurat ions, and neutron diffraction are discussed.