Evidence of an anisotropic charge-excitation gap in stripe-ordered \boldmath La$_{2-x}$Ba$_x$CuO$_4$ with $x=1/8$ \unboldmath

The {\it ab}-plane optical properties of a cleaved single crystal of La$_{2-x}$Ba$_x$CuO$_4$ for $x=1/8$ ($T_c \simeq 2.4$~K) have been measured over a wide frequency and temperature range. The low-frequency conductivity is Drude-like and shows a metallic response with decreasing temperature. However, below $\simeq 60$~K, corresponding to the onset of charge-stripe order, there is a rapid loss of spectral weight below about 40~meV, resulting in a major reduction in the number of free carriers. This suggests a partial gapping of the Fermi surface. Surprisingly, the sample is still metallic and becomes a superconductor at low temperature. This material is a striking example of how charge and spin stripe order, metallic behavior and superconductivity can coexist.