Determination of the optical properties of La2-xBaxCuO4 for several dopings, including the anomalous x=1/8 phase

The optical properties of single crystals of the high-temperature superconductor La2-xBaxCuO4 have been measured over a wide frequency and temperature range for light polarized in the a-b planes and along the c axis. Three different Ba concentrations have been examined, x = 0.095 with a critical temperature T-c = 32 K, x = 0.125 where the superconductivity is dramatically weakened with T-c similar or equal to 2.4K, and x = 0.145 with T-c similar or equal to 24 K. The in-plane behavior of the optical conductivity for these materials at high temperature is described by a Drude-like response with a scattering rate that decreases with temperature. Below T-c in the x = 0.095 and 0.145 materials there is a clear signature of the formation of a superconducting state in the optical properties allowing the superfluid density (rho(s0)) and the penetration depth to be determined. In the anomalous 1/8 phase, some spectral weight shifts from lower to higher frequency (greater than or similar to 300 cm(-1)) on cooling below the spin-ordering temperature T-so similar or equal to 42 K, associated with the onset of spin-stripe order; we discuss alternative interpretations in terms of a conventional density-wave gap versus the response to pair-density-wave superconductivity. The two dopings for which a superconducting response is observed both fall on the universal scaling line rho(s0)/8 similar or equal to 4.4 sigma T-dc(c), which is consistent with the observation of strong dissipation within the a-b planes. The optical properties for light polarized along the c axis reveal an insulating character dominated by lattice vibrations, superimposed on a weak electronic background. No Josephson plasma edge is observed in the low-frequency reflectance along the c axis for x = 1/8; however, sharp plasma edges are observed for x = 0.095 and 0.145 below T-c.