Understanding the origin of electron incoherence is believed to be the first step toward the resolution of the mysteries of the high-T
c cuprate superconductors. Such electron incoherence manifests itself most evidently in the non-Drude form of the optical absorption spectrum of the system. The spectral weight transfer related to such dissipative response, which is absent in conventional Fermi liquid metal, has direct consequence on the dc transport property of the system. However, a theoretical study of the optical conductivity of a strongly correlated model is a formidable task. Here we present an exact formula for the optical conductivity of the 2D Hubbard model from the low energy effective theory perspective. We show that the optical conductivity in Matsubara frequency of the 2D Hubbard model can be represented as the ensemble average of the optical conductivity of non-interacting systems in the background of fluctuating local moment. We find that such an ensemble average can be done exactly with a sign-problem-free Monte Carlo simulation if we assume the widely adopted Millis-Monien-Pines spin susceptibility for the fluctuating local moment. For thermal fluctuation of the local moment, our formula can be used to calculate directly the optical conductivity in real frequency which can be compared with the result of optical measurements in the cuprate superconductors.
The kernel polynomial method, as an approach to deriving highly-accurate spectral properties of large sparse matrices, is extended to investigate the effects of disorder in two-dimensional inhomogeneous superconductors. Being able to solve the Bogoliubov-de Gennes equations for quite large square lattices self-consistently, we gain new microscopic insights and make intuitive observations of the localization of low-energy quasiparticles on the nanometer scale. We also compare the behaviors of the optical conductivities of superconductors with different pairing symmetries. We find that the Drude weight turns out to be finite only for the weakly-disordered d -wave superconductor while obvious suppression of the optical conductivity in the infrared range can be observed in the disordered s -wave superconductor.