Effects of superconductivity and charge order on the subterahertz reflectivity of La1.875Ba0.125-ySryCuO4

The reflectivity R(omega) of both the ab plane and the c axis of two single crystals of La1.875Ba0.125-ySryCuO4 has been measured down to 5 cm(-1), using coherent synchrotron radiation below 30 cm(-1). For y=0.085, a Josephson plasma resonance is detected at T < T-c=31 K in the out-of-plane R-c(omega), and a far-infrared peak (FIP) appears in the in-plane optical conductivity sigma(ab)(omega) below 50 K, where nonstatic charge ordering (CO) is reported by x-ray scattering. For y=0.05 (T-c=10 K), below the ordering temperature T-CO, a FIP is again observed in sigma(ab)(omega). The FIP then appears to be an infrared signature of CO, either static or fluctuating, as reported in previous work on the La-Sr cuprates.