Plain s -Wave Superconductivity near Magnetic Criticality: Enhancement of Attractive Electron–Boson Coupling Vertex Corrections

• Published in: Journal of the Physical Society of Japan Vol. 86; no. 7; p. 73703
• Main Authors: Tazai, Rina, Yamakawa, Youichi, Tsuchiizu, Masahisa, Kontani, Hiroshi
• Format: Journal Article
• Language: English
• Published: Tokyo The Physical Society of Japan 15-07-2017
• Subjects: Coupling; Electrons; Magnetic permeability; Magnetism; Metals; Quantum physics; Superconductivity; Theorems
• ISSN: 0031-9015; 1347-4073
• DOI: 10.7566/JPSJ.86.073703

Recent experiments revealed that the plain s-wave state without any sign-reversal emerges in various metals near magnetic criticality. To understand this counter-intuitive phenomenon, we study the gap equation for the multiorbital Hubbard-Holstein model, by analyzing the vertex correction (VC) due to the higher-order electron-correlation effects. We find that the phonon-mediated orbital fluctuations are magnified by the VC for the susceptibility (χ-VC). In addition, the charge-channel attractive interaction is enlarged by the VC for the coupling-constant (U-VC), which is significant when the interaction has prominent q-dependences; therefore the Migdal theorem fails. Due to both χ-VC and U-VC, the plain s-wave state is caused by the small electron-phonon interaction near the magnetic criticality against the repulsive Coulomb interaction. We find that the direct Coulomb repulsion for the plain s-wave Cooper pair is strongly reduced by the "multiorbital screening effect".