Orientational anisotropy of the upper critical field in single-crystal Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{Bi}}_{2.2}\mathrm{Ca}{\mathrm{Sr}}_{1.9}{\mathrm{Cu}}_2{\mathrm{O}}_{8+x}$

We have measured the dependence of the upper critical magnetic field upon orientation for single-crystal samples of Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ and ${\mathrm{Bi}}_{2.2}\mathrm{Ca}{\mathrm{Sr}}_{1.9}{\mathrm{Cu}}_2{\mathrm{O}}_{8+x}$ via four-probe resistance. We find continuous behavior of $H_{c2\mathrm{}}\left(\theta \right)$ for small deviations from parallel orientation for the Y sample, as expected for an anisotropic, three-dimensional superconductor. For the Bi sample, however, our data are more consistent with a thin-film model, exhibiting a cusp at parallel orientation, suggesting that ${\mathrm{Bi}}_{2.2}\mathrm{Ca}{\mathrm{Sr}}_{1.9}{\mathrm{Cu}}_2{\mathrm{O}}_{8+x}$ is a quasi-two-dimensional system.