Effects of Superconductivity and Charge Order on the sub-Terahertz reflectivity of La${}_{1.875}$Ba${}_{0.125-y}$Sr${}_y$CuO${}_4$

Abstract

The reflectivity $R(\omega )$ of both the $ab$ plane and the c axis of two single crystals of La${}_{1.875}$Ba${}_{0.125-y}$Sr${}_y$CuO${}_4$ 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 $T\ll T_c$ = 31 K in $R_c(\omega )$, and a far-infrared peak (FIP) appears in the optical conductivity below 50 K, where non-static charge ordering (CO) is reported by X-ray scattering. For $y$ = 0.05 ($T_c$ = 10 K), a FIP is observed in the low-temperature tetragonal phase below the ordering temperature $T_{CO}$. At 1/8 doping the peak frequency scales linearly with $T_{CO}$, confirming that the FIP is an infrared signature of CO, either static or fluctuating.

v2: longer version, 9 pages, 6 color figures