The pseudogap in the c-axis optical conductivity in high T_c-cuprates

Abstract
The pseudogap phenomenon is most remarkable in the underdoped region of high-T_c cuprates. Since the superconducting state in the optimally doped high-T_c cuprates is well described by the BCS-like d-wave superconductors, it is natural to assume that the pseudogap arises from the standard superconducting fluctuation. In particular in the layered compunds like high-T_c cuprates Varlamov and his coworkers pointed out the density of states (DOS) correction to the superconducting fluctuation will play the crucial role. However, unfortunately in their analysis d-wave nature of the high-T_c cuprates is ignored. Perhaps more seriously some unnecessary approximations were introduced in their analysis. The present theory gives somewhat different expressions of quasi-particle density of states for B = 0 and B ≠ 0, which can be tested experimentally.