Normal-State Optical Conductivity of the Cuprates from Tightly Bound Pre-Formed Pairs

We demonstrate that the main low-energy structures of the normal-state optical conductivity of the cuprates can be described naturally from the picture of tightly bound pre-formed pairs. Specifically, we show that the observed onset at ~100meV (and its long tail extending toward zero energy) originate from inter-band excitations near the van Hove singularities of the pre-formed pairs. Without using any free parameter, our results reproduce not only the correct energy of the onset, but also the non-monotonic trends of its temperature and doping dependence. 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.