Translational and bond-orientational order in the vortex lattice of the high-Tc superconductor Bi2.1Sr1.9Ca0.9Cu2O8+δ

Abstract

We have investigated the microscopic ordering of the flux lattice in the mixed state of ${\mathrm{Bi}}_{2.1}$ ${\mathrm{Sr}}_{1.9}$ ${\mathrm{Ca}}_{0.9}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ near ${\mathit{H}}_{\mathit{c}1}$ by image analysis of Bitter-decorated samples. We observe a dramatic transition from strong isotropic disorder to hexatic order with increasing magnetic field. This transition occurs at a lower applied magnetic field for samples which are annealed in oxygen.