Microwave and millimeter wave spectroscopy in the slightly hole-doped ladders of Sr$_{14}$Cu$_{24}$O$_{41}$

Abstract

We have measured the temperature- and frequency dependence of the microwave and millimeter wave conductivity $σ_1(T,ω)$ along both the ladder (c-axis) and the leg (a-axis) directions in Sr$_{14}$Cu$_{24}$O$_{41}$. Below a temperature $T^*$($\sim$170 K), we observed a stronger frequency dependence in $σ_1^c(T,ω)$ than that in $σ_1^a(T,ω)$, forming a small resonance peak developed between 30 GHz and 100 GHz. We also observed nonlinear dc conduction along the c-axis at rather low electric fields below $T^*$. These results suggest some collective excitation contributes to the c-axis charge dynamics of the slightly hole-doped ladders of Sr$_{14}$Cu$_{24}$O$_{41}$ below $T^*$.