Determination of the optical properties of La_(2_x)Ba_xCuO_4 for several dopings, including the anomalous x = 1/8 phase

摘要

The optical properties of single crystals of the high-temperature superconductor La_(1-x)Ba_xCuO_4 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 ∽_- 2.4K,and x = 0.145with T_c ∽_- 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 (ρ_(s0)) and the penetration depth to be determined. In the anomalous 1/8 phase, some spectral weight shifts from lower to higher frequency (>_∽300 cm~(-1)) on cooling below the spin-ordering temperature T-(so) ∽_- 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 ρ_(s0)/8 ∽_— 4.4σ_(dc)T_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.