Phonons, electronic charge response and electron–phonon interaction in the high‐temperature superconductors

Abstract

AbstractWe investigate the complete phonon dispersion, the phonon induced electronic charge response and the corresponding self‐consistent change of the crystal potential an electron feels as a direct measure of the electron–phonon interaction in the high‐temperature superconductors within a microscopic model in the framework of linear response theory. Moreover, dielectric and infrared properties are calculated. The experimentally observed strong renormalization of the in‐plane oxygen bond‐stretching modes which appears upon doping in the high‐temperature superconductors is discussed. It is shown that the characteristic softening, indicating a strong nonlocal electron–phonon interaction, is most likely a generic effect of the CuO plane and is driven by a nonlocal coupling of the displaced ions to the localized charge‐fluctuations at the Cu and the Oxy ions. At hand of the oxygen bond‐stretching modes it is illustrated how lattice‐, charge‐ and spin‐degrees of freedom may act synergetically for anisotropic pairing in the high‐temperature superconductors. The different behaviour of these modes during the insulator‐metal transition via the underdoped phase is calculated and from a comparison of these generic modes in the different phases conclusions about the electronic state are drawn. For the non‐cuprate potassium doped high‐temperature superconductor Ba–Bi–O also a very strong and anisotropic renormalization of the oxygen bond‐stretching modes is predicted. In another investigation c‐axis polarized infrared‐ and Raman‐active modes of the HTSC's are calculated in terms of charge fluctuations and anisotropic dipole‐fluctuations. Mode assignments discussed controversially in the literature are proposed. Finally, interlayer phonons propagating along the c‐axis and their accompanying charge response are investigated. Depending on the strength of the interlayer coupling calculations are performed ranging from the static, adiabatic response regime to the non‐adiabatic regime where dynamical screening of the bare Coulomb interaction and phonon–plasmon coupling becomes relevant within a certain region around the direction of the c‐axis. A comparison with the experimental situation is given. Both, the oxygen bond‐stretching modes calculated in adiabatic approximation and the non‐adiabatic coupled c‐axis phonon–plasmon modes are found to be important for pairing via lattice‐ and charge‐degrees of freedom. Favouring aspects to achieve high‐temperature superconductivity are also discussed. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)