Optical study of charge dynamics along thec-axis in high-Tc superconductors

Comparing the optical spectra withEc for typical high-T _c superconducting cuprates, we discuss the charge dynamics along thec-axis. The plasma energy or the mass anisotropic factorm _* ^c //m _* ^b is one of the key parameters determining the spectrum forEc. In Bi₂Sr₂CaCu₂O₈ and La_2–x Sr _x CuO₄ with a largem _* ^c /m _* ^b , the plasma energy is smaller than the superconducting gap energy, and thus the supercurrent along thec-axis is a kind of Josephson current flowing through insulating layers such as the BiO layers. On the other hand, in YBa₂Cu₃O₇ with a smallm _* ^c /m _* ^a , it seems that the coherent supercurrent flows along thec-axis. Although the spectrum forEc strongly depends on samples, presumably due to the difference in the hole concentration, a clear anisotropy between theab- and thec-directions is observed in the characteristics energy scales such as the plasma energy as well as the reflectivity knee energy.