We investigate the physical origin of unconventional low-energy excitations
in cuprate superconductors by considering the effect of coexisting competing
orders (CO) and superconductivity (SC) and of quantum fluctuations and other
bosonic modes on the low-energy charge excitation spectra. By incorporating
both SC and CO in the bare Green's function and quantum phase fluctuations in
the self-energy, we can consistently account for various empirical findings in
both the hole- and electron-type cuprates, including the excess subgap
quasiparticle density of states, ``dichotomy'' in the fluctuation-renormalized
quasiparticle spectral density in momentum space, and the occurrence and
magnitude of a low-energy pseudogap being dependent on the relative gap
strength of CO and SC. Comparing these calculated results with experiments of
ours and others, we suggest that there are two energy scales associated with
the pseudogap phenomena, with the high-energy pseudogap probably of magnetic
origin and the low-energy pseudogap associated with competing orders.Comment: 10 pages, 5 figures. Invited paper for the 2006 Taiwan International
Conference on Superconductivity. Correspondence author: Nai-Chang Yeh
(e-mail: [email protected]
