Thursday 27.01.11, 15:15-16:15 HS P


Roles of zeros of Green's function in the pseudogap state in high-Tc cuprates

Shiro Sakai

Institute for Solid State Physics, Vienna University of Technology

Various spectral anomalies observed by angle-resolved photoemission spectroscopy (ARPES) in the pseudogap state of high-Tc cuprates hold a key not only to understanding the mechanism of the superconductivity but also to a possible manifestation of a novel metallic phase distinguished from the Fermi liquid.
We propose a unified understanding of these anomalies, based on the electronic structure calculated by the cluster dynamical mean-field theory (CDMFT) for hole- or electron-doped Mott insulators in the two-dimensional Hubbard model. The key feature in the calculated electronic structure is that it contains two different types of singularities at low energy: One is poles of the single-particle Green's function G, and the other is poles of the self-energy, i.e., zeros of G. The former is usual in metals, constituting the Fermi surface at the Fermi level, while the latter is unusual: The zeros of G exist in the gap in the Mott insulator, but they still persist in the slightly doped region.[1]
The surface of zeros around the Fermi level characterizes the pseudogap, which emerges as a direct consequence of the proximity to the Mott insulator and requires no symmetry breaking. Interferences of the pole and zero surfaces naturally explain various spectral anomalies, such as electron-hole asymmetry, Fermi arc, pocket Fermi surfaces, back-bending dispersion and high-energy kink observed by ARPES in hole- or electron-doped cuprates.[2]
Moreover, the pseudogap is found to have an unanticipated s-wave like structure. It is distinct from the previous theories assuming d-wave pseudogap, but still consistent with ARPES data because the location of the gap depends on momentum: The gap exists above the Fermi level in the nodal direction while around it in the antinodal region, resulting in a seemingly d-wave gap below the Fermi level. We confirm the s-wave structure by means of CDMFT + continuous-time Monte Carlo method up to 16 sites. The result imposes a strong constraint on the interpretation of the pseudogap.
[1] T. D. Stanescu and G. Kotliar, Phys. Rev. B 74, 125110 (2006).
[2] S. Sakai, Y. Motome, and M. Imada, Phys. Rev. Lett. 102, 056404 (2009); Physica B 404, 3183 (2009); Phys. Rev. B 82, 134505 (2010).