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

TL;DR

This study investigates how superconductivity and charge order influence the sub-terahertz reflectivity in La-based cuprate crystals, revealing signatures of charge order and Josephson plasma resonance through low-frequency optical measurements.

Contribution

It provides direct optical evidence linking charge ordering and superconductivity to specific features in the reflectivity spectra of La$_{1.875}$Ba$_{0.125-y}$Sr$_{y}$CuO$_4$.

Findings

Detection of Josephson Plasma Resonance at low temperatures.

Observation of a far-infrared peak associated with charge order.

Linear scaling of peak frequency with charge ordering temperature.

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.