Colloquium: Identifying the propagating charge modes in doped Mott insulators

High-temperature superconductivity in the copper-oxide ceramics remains an unsolved problem because we do not know what the propagating degrees of freedom are in the normal state. As a result, we do not know what are the weakly interacting degrees of freedom which pair up to form the superconducting condensate. That the electrons are not the propagating degrees of freedom in the cuprates is seen most directly from experiments that show spectral weight redistributions over all energy scales. In the correct low-energy theory, such rearrangements are minimized. This review focuses on the range of experimental consequences such ultraviolet-infrared mixings have on the normal state of the cuprates, such as the pseudogap, midinfrared band, temperature dependence of the Hall number, the superfluid density, and a recent theoretical advance which permits the identification of the propagating degrees of freedom in a doped Mott insulator. Within this theory, a wide range of phenomena which typify the normal state of the cuprates is shown to arise including T linear resistivity.