Origin of peak-dip-hump structure in the in-plane optical conductivity of the high-TC cuprates: Role of antiferromagnetic spin fluctuations of short-range order

An improved U(1) slave-boson approach is applied to study the optical conductivity of the two-dimensional systems of antiferromagnetically correlated electrons over a wide range of hole doping and temperature. Interplay between the spin and charge degrees of freedom is discussed to explain the origin of the peak-dip-hump structure in the in-plane conductivity of high-TC cuprates. The role of spin fluctuations of short-range order (spin singlet pair) is investigated. It is shown that the spin fluctuations of the short-range order can cause the midinfrared hump, by exhibiting a linear increase of the hump frequency with the antiferromagnetic Heisenberg coupling strength.