Pseudogap and collective mode in the optical conductivity spectra of hole-doped ladders in Sr14-xCaxCu24O41

Anisotropic optical conductivity spectra have been investigated for the hol e-doped two-leg ladder system Sr14-xCaxCu24O41. As in the case of the under doped high-T-c cuprates, upon cooling a pseudogap opens in the spectrum for the transverse (a-axis) polarization, and concurrently a peak develops in the far infrared region of the longitudinal (c-axis) spectrum. A possible i nterpretation for these results is that the pseudogap signals a formation o f hole pairs on the ladders and the peak might correspond to a pinned colle ctive mode of a regular array of hole pairs. [S0031-9007(99)08340-9].