Optical spectroscopy of La$_{2-x}$Ba$_{x}$CuO$_{4}$ single crystals: influence of stripe order

The $ab$-plane and $c$-axis reflectance spectra of ten La$_{2-x}$Ba$_{x}$CuO$_{4}$ single crystals, with $x$ ranging from undoped to optimally doped, have been measured over a wide frequency range and at temperatures from 10 to 300 K. The influence of stripe order around $x=0.125$ appears in the spectra below $T = 50$~K, observed both as a reduction in the free-carrier (normal state) and superfluid (superconducting state) density and by the appearance of a relatively narrow conductivity band near 25 meV. The superfluid density is estimated from the real part of the dielectric function and the $f$-sum rule. The $c$-axis spectra are those of an insulator or very bad metal, with very little doping or temperature dependence. The Josephson plasma edge is not observed in any of these spectra.