Effective strong-coupling Hamiltonians for bipolaron centers and magnetic impurities with on-site electron-phonon coupling

Journal Article · Tue Nov 01 00:00:00 EST 1988 · Phys. Rev. B: Condens. Matter; (United States)

DOI:https://doi.org/10.1103/PhysRevB.38.9063· OSTI ID:6737616

Schuettler, H; Fedro, A J

We have studied a model of bipolaron (negative-U) centers and magnetic impurities which includes a repulsive on-site Coulomb interaction as well as a coupling of the electronic on-site energy to the displacement amplitude of a local phonon mode. By means of a generalized Schrieffer-Wolff transformation, we derive an effective strong-coupling (or weak-hybridization) Hamiltonian for the low-lying states of the system. In the case of a bipolaron center (where the phonon-mediated on-site attraction dominates over the Coulomb repulsion), the effective Hamiltonian describes conduction-electron scattering and electron-pair tunneling processes between the localized impurity orbital and the conduction band of the metallic host. In the limit of very strong electron-phonon coupling, the pair tunneling matrix elements become exponentially small, whereas conduction electron scattering is suppressed only algebraically. The impurity orbital occupation number is then almost ''frozen in'' and the bipolaron center becomes inefficient in enhancing the superconducting pairing correlations of the host. In the case of a magnetic impurity (with a predominant repulsive Coulomb interaction), the effective Hamiltonian reduces to the form of an ordinary Kondo spin-exchange model. The spin-exchange coupling constants are not affected by the presence of the electron-phonon interaction.

• Research Organization: Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439
• OSTI ID: 6737616
• Journal Information: Phys. Rev. B: Condens. Matter; (United States), Vol. 38:13
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
SUPERCONDUCTIVITY
ELECTRON-PHONON COUPLING
MATHEMATICAL MODELS
TUNNEL EFFECT
BAND THEORY
COOPER PAIRS
COULOMB FIELD
HAMILTONIANS
IMPURITIES
POLARONS
TRANSITION TEMPERATURE
ELECTRIC CONDUCTIVITY
ELECTRIC FIELDS
ELECTRICAL PROPERTIES
MATHEMATICAL OPERATORS
PHYSICAL PROPERTIES
QUANTUM OPERATORS
QUASI PARTICLES
THERMODYNAMIC PROPERTIES
656100* - Condensed Matter Physics- Superconductivity