Spectroscopic Studies of Quasiparticle Low-Energy Excitations in Cuprate and Iron-Based High-Temperature Superconductors

Recent development in the physics of high-temperature superconductivity (SC) is reviewed, with special emphasis on the studies of the low-energy excitations of cuprate and iron-based superconductors. For cuprate superconductors, a phenomenology based on coexisting competing orders with superconductivity in the ground state of these doped Mott insulators is shown to provide a consistent account for a wide range of experimental findings. In the case of iron-based superconductors, studies of the low-energy excitations reveal interesting similarities and differences when compared with cuprate superconductors. In contrast to the single-band cuprate superconductivity with an insulating parent state, the ferrous superconductors are multi-band materials with a semi-metallic parent state and exhibit two-gap superconductivity when doped. On the other hand, both systems exhibit strong antiferromagnetic correlation and Fermi-surface distortion, leading to unconventional pairing symmetries with sign-changing order parameters on different parts of the Fermi surface. These findings suggest that the pairing potentials in both the cuprate and the ferrous superconductors are generally repulsive, thus favoring a pairing mechanism that is electronically driven and a pairing strength that is closely related to the electronic correlation. The physical implications of the unified phenomenology based on antiferromagnetic correlations and remaining open issues associated with the cuprate and ferrous superconductivity are discussed