Angular dependence of the upper critical field in $\mathbf{\rm Bi_2Sr_2CuO_{6+\delta }}$

The angular dependence of the upper critical magnetic field has been investigated in a wide range of temperatures in very high-quality Bi$_{2}$Sr$ _{2}$CuO$_{6+\delta }$ single crystals with critical temperature $T_{c}$ (midpoint) $\simeq9$ K in magnetic fields up to 28 T. Although the typical value of the normal state resistivity ratio $\rho _{c}/\rho _{ab}\approx 10^{4}$, the anisotropy ratio $H_{c2\parallel ab}$ $/$ $H_{c2\perp ab}$ of the upper critical fields is much smaller and shows an unexpected temperature dependence. A model is proposed based on a strong anisotropy and a small transparency between superconducting layers.