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

– We have measured the temperatureand frequency dependence of the microwave and millimeter wave conductivity σ1(T, ω) along both the ladder (c-axis) and the leg (a-axis) directions in Sr14Cu24O41. Below a temperature T ∗(∼170 K), we observed a stronger frequency dependence in σc 1(T, ω) than that in σ a 1 (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 Sr14Cu24O41 below T ∗. Following a theoretical prediction [1] and the experimental discovery [2] of superconductivity in the two-leg ladder systems, the study of Sr14−xCaxCu24O41 compounds [3] with both Cu2O3 ladders and CuO2 chains has attracted the attention of many physicists. Self-doped holes already exist in this compound, since the average valence of Cu is +2.25. An early optical measurement revealed that the isovalent substitution of Ca for Sr made the self-doped holes redistribute from the CuO2 chains to the Cu2O3 ladders, which effectively enables the hole doping on the two-leg ladders [4]. They also suggested that the low-energy charge excitation (.1 eV) was mainly dominated by holes on the Cu2O3 ladders rather than on the CuO2 chains, because the Cu3d-O2p transfer integral of the ladders was larger than that of the chains due to the difference of the angle of the Cu-O-Cu bonds. Thus, systematic study of the low-energy charge dynamics of Sr14−xCaxCu24O41 compounds is expected to reveal rich physics from the holes on the two-leg ladders, which may also provide useful information for understanding high-Tc superconductivity in cuprates. It is particularly important to study the charge dynamics of the parent material Sr14Cu24O41 (estimated to have 0.07 holes per ladder Cu site [4]), because it provides a starting point to