Observation of a hexatic vortex glass in flux lattices of the high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{Bi}}_{2.1}$${\mathrm{Sr}}_{1.9}$${\mathrm{Ca}}_{0.9}$${\mathrm{Cu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$

We report observation of hexatic order in Abrikosov flux lattices in very clean crystals of the high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{Bi}}_{2.1}$ ${\mathrm{Sr}}_{1.9}$ ${\mathrm{Ca}}_{0.9}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$. Our experiments consist of in situ magnetic decoration of the flux lattice at 4.2 K. Analysis of the decoration images shows that the positional order decays exponentially with a correlation length of a few lattice constants while the orientational order persists for hundreds of lattice constants and decays algebraically with an exponent ${\mathrm{\eta }}_6$=0.06±0.01. Our results confirm recent theoretical speculation that the positional order should be far more sensitive to disorder than the orientational order and that the low-temperature ordered phase of the flux lines in these systems might be a hexatic glass.