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 $\ddot{I}{ƒ}_1(T,\ddot{I}‰)$ along both the ladder (c-axis) and the leg (a-axis) directions in Sr${}_{14}$Cu${}_{24}$O${}_{41}$. Below a temperature $T^{\ast }$($\sim$170 K), we observed a stronger frequency dependence in $\ddot{I}{ƒ}_1^c(T,\ddot{I}‰)$ than that in $\ddot{I}{ƒ}_1^a(T,\ddot{I}‰)$, 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 $T^{\ast }$. 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 $T^{\ast }$.

7 pages, 4 figure, to be published in Europhysics Letters