Origin of the hump structure in the in-plane optical conductivity of high-TC cuprates based on a SU(2) slave-boson theory

An improved version of the SU(2) slave-boson approach is applied to study the in-plane optical conductivity of the two dimensional systems of high-TC cuprates. We investigate the role of fluctuations of both the phase and amplitude of order parameters on the (Drude) peak-dip-hump structure in the in-plane conductivity as a function of hole doping concentration and temperature. The mid-infrared (MIR) hump in the in-plane optical conductivity is shown to originate from the antiferromagnetic spin fluctuations of short range (the amplitude fluctuations of spin singlet pairing order parameters), which is consistent with our previous U(1) study. However the inclusion of both the phase and amplitude fluctuations is shown to substantially improve the qualitative feature of the optical conductivity by showing substantially reduced Drude peak widths for the entire doping range. Both the shift of the hump position to lower frequency and the growth of the hump peak height with increasing hole concentration is shown to be consistent with observations.