Use of coherent synchrotron radiation in a study on cuprates exhibiting superconductivity and charge order

The reflectivity R(ω) of both the ab plane and the c axis of single crystals of La1.875Ba0.125−ySryCuO4, with 0.05 ⩽ y < 0.125 has been measured down to 5 cm−1, using coherent synchrotron radiation below 30 cm−1. This extension to very low infrared frequencies allows one to detect far-infrared peaks (FIP) in the optical conductivity, which are related to charge ordering, and which can be hardly observed by using conventional sources. In the superconducting phase, one can also observe a Josephson plasma resonance in the c-axis reflectivity, and its shift with temperature. At higher frequencies, the effect of charge ordering is monitored through a depletion in the optical conductivity which extends up to 4000 cm−1.