Observation of Hexagonally Correlated Flux Quanta In Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

The high-resolution Bitter pattern technique has been used to reveal the magnetic structure of single-crystal samples of high-$T_c$ superconductor Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ at 4.2 K. Typical patterns consist of hexagonally correlated, singly quantized vortices of flux $\frac{\mathrm{hc}}{2e}$. That is, the structures are comparable to those that would be observed in conventional type-II superconductors under similar conditions.