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

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 * ( ∼ 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 *.