Anisotropy dependence of triangular lattice formation of Josephson vortices inBi2Sr2CaCu2O8+y

Abstract

To study the anisotropy dependence of the Josephson-vortex phase diagram, we measured the Josephson-vortex flow resistance as a function of magnetic field parallel to the $ab$ plane in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+y}$ (Bi-2212) single crystals. The anisotropy of samples was controlled over a wide range by annealing. Since the Josephson-vortex flow resistance oscillates periodically as a function of the magnetic field when a dense triangular lattice is formed, the lower boundary field of the triangular lattice phase is assigned from the magnetic field where the oscillations start. We observed that the lower boundary field of the triangular lattice phase decreases systematically with increasing anisotropy parameter $\ensuremath{\gamma}$ of Bi-2212 at all measured temperatures. This behavior is consistent with theories which imply that the boundary field is inversely proportional to $\ensuremath{\gamma}$.