Local dynamical lattice instabilities: Prerequisites for resonant pairing superconductivity

Abstract

Fluctuating 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

Phonon anomalies and dynamic stripes

“…[68,69,70] The idea that electronic correlations are responsible for the enhanced electron-phonon coupling is further reinforced on the qualitative level by theoretical investigations based on the Hubbard-Holstein model and similar models, which find an enhancement of phonon renormalization by electronic correlations not included in LDA. [71] Strong renormalization of the bond stretching phonons has been taken as evidence for a soft collective charge mode [72,73] or an incipient instability [74] with respect to the formation of either polarons, biporarons [75,76,77], charge density wave order [78], phase separation [79,80,81], valence bond order [82], or other inhomogeneity [83]. These may or may not be related to the mechanism of stripe formation.…”

Anomalous S-like dispersion of in-gap single-particle excitations in the pseudogap state of cuprate superconductors

“…Its microscopic origin may be diverse. It can derive from either a polaronic mechanism in strongly coupled electron-lattice systems showing intrinsic dynamical lattice instabilities, 14 or alternatively from strong electronic correlations leading to RVB physics. 12 In this case singlet pairs on plaquette clusters get exchanged with pairs of uncorrelated holes in their immediate neighborhood, 10 forming bound states whereby a hole accompanies itinerant singlet electron pairs.…”

“…The intrinsic metastability of the cuprate crystal structure 13 involves dynamically fluctuating molecular Cu-O-Cu clusters, [14][15][16] which, on a finite space-time scale either capture charge carriers in form of bound singlet pairs or accommodate them as itinerant particles while passing through them. This breaks crystalline symmetry on a local level 17 and causes the fermionic charge carriers to be simultaneously itinerant and localized, as observed in scanning tunneling microscope imaging studies ͑see Ref.…”

“…10 In a polaronic scenario, such plaquettes represent clusters involving stereochemical C II -O-C II configurations housing two electrons, which upon hole doping change into C III -O-C III bonds. [14][15][16] The essential physics of resonant pairing is manifest on the level of individual molecular clusters at effective lattice sites i, encoded in the spectral properties of the atomic limit of Eq. ͑1͒, H at = lim t→0 H. In order to keep the algebra as simple as possible, we shall restrict ourselves here to the case where the bosonic level coincides with the center of the fermionic band, i.e., ⌬ B =2 0 .…”

COOPERATIVE LOCALIZATION-DELOCALIZATION IN THE HIGH Tc CUPRATES

“…The strength of the boson-fermion exchange coupling in our effective Hamiltonian can be estimated from our study of the single cluster problem and is given by F pair exch , which is related to the electron-lattice coupling α and the bare phonon frequency ω 0 , as discussed in detail in Ref. 37. In order to get an insight into the inter-related amplitude and phase fluctuations in hole doped High T c cuprates, we transform this Hamiltonian by a succession of unitary transformations into a block diagonal form, composed of exclusively (i) renormalized fermionic particles and (ii) renormalized bosonic particles.…”