Journal of Statistical Physics, Vol. .57, Nos. 3/4, 1989, p. 511-530.
A theory of the entangled flux liquids which arise in the new high-T_c superconductors is reviewed. New physics appears because of the weak interplanar couplings and high critical temperatures in these materials. Flux line wandering melts the conventional Abrikosov flux lattice, and leads to an entangled vortex state whose statistical mechanics is closely related to the physics of interacting bosons in two dimensions. The phase diagram as a function of magnetic field and temperature is discussed, and it is argued that an entangled vortex liquid appears just above H_cl at all nonzero temperatures. The decay of vortex line correlations in the entangled liquid state is controlled by the superfluid excitation spectrum of the bosons. Line wandering produces drastic changes in the B(H) constitutive relation near H_c1.