Anisotropy of $H_{c2\mathrm{}}$ in a cesium fluoroxide tungsten bronze

The $T_c$ and $H_{c2\mathrm{}}$ of single crystals of nominal composition ${\mathrm{Cs}}_{0.1}$W${\mathrm{O}}_{2.9}$${\mathrm{F}}_{0.1}$ were measured. The crystals studied grew in two forms, hexagonal needles and platelets. The Cs concentration, as measured by x-ray fluorescence analysis, was 0.1 ± 0.01 for the needles and 0.06 ± 0.006 for the platelets. $T_c$ ranged from 4.87 to 4.94 K for both forms, with a transition width of 0.05 K. Upper critical field $H_{c2\mathrm{}}$ rotation data were taken using a standard four-lead contact method. $H_{c2\mathrm{}}$ data in the plane perpendicular to the crystal $c$ axis showed an anisotropy having 60° periodicity, reflecting the hexagonal symmetry of the crystal. The anisotropy ratios ranged from 0.113 to 0.313 in this plane. Rotation in a plane containing the $c$ axis showed 180° symmetry and anisotropy ratios ranging from 1.67 to 1.91, nearly as large as those found in layered structures. The temperature dependence of $H_{c2\mathrm{}}$ along three crystallographic axes is also reported.