We discuss the origin of the pseudogap phenomena in the cuprate superconductors by analyzing three recent ARPES observations from the perspective of the spin-Fermion model. Firstly, we show that the recent observation of the vanishing of the pseudogap around (π,0) in the electron-doped cuprate Pr1.3−xLa0.7CexCuO4 is consistent with the AF band folding picture of the pseudogap in the electron-doped cuprates, provided that we assume a strongly momentum dependent quasi-particle scattering rate on the Fermi surface [1,2]. Secondly, we argue that the pseudogap in the hole-doped cuprates is unlikely an AF band folding gap, since the spin fluctuation in the hole-doped cuprates is much more short-ranged and dynamical in nature than that in the electron-doped cuprates. In particular, we show that electron pairing is indispensable to eliminate the Fermi level crossing along (π,0)-(π,π) in a way that is consistent with the ARPES observation in the under-doped Bi-2201 system around T* [3,4]. Nevertheless, we find that the AF spin fluctuation in the hole-doped cuprates is responsible for the emergence of the broad high energy hump structure in the anti-nodal region, and the mismatch between the back-bending momentum of the hump maximum and the underlying Fermi momentum along (π,0)-(π,π), and in particular, the extremely flatness of the anti-nodal quasi-particle dispersion in the superconducting state [5,6]. We argue that the particle-hole symmetry breaking should better be studied in the energy space, rather than in the momentum space . Lastly, we show that the recent observation of the critical behavior at the pseudogap end point is consistent with the spin-Fermion picture [7,8], even though the local spin in the cuprates is far from critical at such a high doping level, if one note the singular AF response of the quasi-particle system at the VHS doping.
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