162,109 research outputs found
The pseudogap phenomenon in high-Tc superconductors
No full textWe review the possible mechanisms leading up to the phenomenon of a pseudogap which characterizes the normal-state properties of high-T superconductors. We suggest that this pseudogap is not due to superconducting phase fluctuations and hence is not related to a crossover between a BCS state of Cooper pairs and a Bose-Einstein condensation of local pairs. We rather argue that it is due to uncorrelated pairing which is already manifest in the local electronic structure and accessible by photoemission and tunneling experiments
Local dynamical lattice instabilities: Prerequisites for resonant pairing superconductivity
No full textFluctuating local diamagnetic pairs of electrons, embedded in a Fermi sea, are candidates for non-phononmediated superconductors without the stringent conditions on Tc which arise in phonon-mediated BCS classical low-Tc superconductors. The local accumulations of charge, from which such diamagnetic fluctuations originate, are irrevocably coupled to local dynamical lattice instabilities and form composite charge-lattice excitations of the system. For a superconducting phase to be realized, such excitations must be itinerant spatially phase-coherent modes. This can be achieved by resonant pair tunneling in and out of polaronic cation-ligand sites. Materials in which superconductivity driven by such local lattice instability can be expected have a Tc which is controlled by the phase stiffness rather than the amplitude of the diamagnetic pair fluctuations. Above Tc, a pseudogap phase will be maintained up to T*, at which this pairing amplitude disappears. We discuss the characteristic local charge and lattice properties which characterize this pseudogap phase and which form the prerequisites for establishing a phase-coherent macroscopic superconducting state
Hall coefficient and angle-resolved photoemission in systems with strong pair fluctuations
No full textInterrelation between the pseudogap and the incoherent quasi-particle features of high-Tc superconduc
No full textResonating bipolarons
No full textElectrons coupled to local lattice deformations end up in
self-trapped localized molecular states involving their binding
into bipolarons when the coupling is stronger than a certain
critical value. Below that value they exist as essentially
itinerant electrons. We propose that the abrupt crossover between
the two regimes can be described by resonant pairing similar to
the Feshbach resonance in binary atomic collision processes. Given
the intrinsically local nature of the exchange of pairs of
itinerant electrons and localized bipolarons, we demonstrate the
occurrence of such a resonance on a finite-size cluster made out
of metallic atoms surrounding a polaronic ligand center
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