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

The reflectivity R(ω) of both the ab plane and the c axis of two single crystals of La1.875Ba0.125−ySryCuO4 has been measured down to 5cm−1, using coherent synchrotron radiation below 30cm−1. For y=0.085, a Josephson plasma resonance is detected at T⪡Tc=31K in the out-of-plane Rc(ω), and a far-infrared peak (FIP) appears in the in-plane optical conductivity σab(ω) below 50K, where nonstatic charge ordering (CO) is reported by x-ray scattering. For y=0.05 (Tc=10K), below the ordering temperature TCO, a FIP is again observed in σab(ω). 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.