  Possible phases and the B-T phase diagram of interlayer Josephson vortices
induced by a magnetic field parallel to the superconducting layers are
investigated by Monte Carlo simulations based on the anisotropic, frustrated XY
model. While for low magnetic fields and small anisotropy parameters a single
first-order transition is observed similarly to the melting of Abrikosov (or
pancake) vortex lattice, an intermediate phase, characterized by decoupled,
two-dimensional (2D) quasi long-range crystalline order (QLRCO) and
superconductivity, is found at high magnetic fields and large anisotropy
parameters. Combining the simulation results with a symmetry argument, it is
revealed that this intermediate phase is of Kosterlitz-Thouless (KT) type, and
the melting of 2D quasi Josephson vortex lattices and suppression of
superconductivity is a KT transition. Evolution of the intermediate phase to
the low-temperature phase of 3D LRCO is second order and belongs to the 3D XY
universality class. The three phase boundaries merge at a multicritical point.
It is revealed that decoupling of the 3D Josephson vortex lattice into the 2D
phase is triggered by hops of Josephson flux lines across superconducting
layers activated by thermal fluctuations. The equilibrium phase diagram with
the KT phase at high magnetic fields and large anisotropy parameters is
consistent with the peculiar Lorentz-force-independent dissipation observed in
highly anisotropic high-Tc superconductor Bi2Sr2CaCu2O(8+y) by Iye et al.
(Physica 159C, 433 (1989)).
