Puzzling "bad metal" resistivity and optical conductivity of normal-state cuprates: an emergent Bose liquid perspective

We investigate anomalous normal-state in-plane resistivity and optical conductivity of cuprates by using multi-orbital bose-liquid model. Specifically, we assume that the charge carriers in cuprate are tightly bound pre-formed pairs. We show the low-energy optical spectrum involving both intra- and inter-band excitation. While inter-band excitation explain the continuous spectrum and its contribution around the van-Hove singularities of the pre-formed pairs can reproduce the experimental observed onset near 1000cm^-1, the intra-band excitation will mainly contributes to low frequency conductivity and gives rise to linear resistivity without saturation. Instead of considering about the mean free path, we demonstrate this universal bad metal behavior originate from the continuously decreasing height of low-frequency peak since its weight transfers to high energy region with temperature.Our study reveals the bosonic nature of the low-energy carriers and provides a strong support for the picture of tightly bound preformed pairs in describing the high-temperature superconductivity and other low-energy physics of the cuprates.