Jiri Maly

Pseudogap effects induced by resonant pair scattering

We demonstrate how resonant pair scattering of correlated electrons above T_c can give rise to pseudogap behavior. This resonance in the scattering T-matrix appears for superconducting interactions of intermediate strength, within the framework of a simple fermionic model. It is associated with a splitting of the single peak in the spectral function into a pair of peaks separated by an energy gap. Our physical picture is contrasted with that derived from other T-matrix schemes, with superconducting fluctuation effects, and with preformed pair (boson-fermion) models. Implications for photoemission and tunneling experiments in the cuprates are discussed.

Numerical studies of the s-wave pseudogap state and related Tc: the pairing approximation theory

Abstract We investigate the pseudogap onset temperature T *, the superconducting transition temperature T c and the general nature of the pseudogap phase using a diagrammatic BCS/Bose–Einstein cross-over theory. This scheme is based on the “pairing approximation” of Kadanoff and Martin, further extended by Patton (KMP). Our consideration of the KMP “pairing approximation” is driven by the objective to obtain BCS like behavior in the weak coupling limit. Two coupled equations, corresponding to those for the single particle and pair propagators, must be solved numerically, along with the number equation constraint. The variation of small to large coupling constant g is explored, whereby the system is found to cross-over from BCS to Bose–Einstein behavior. Our numerical calculations proceed in two stages: first, we investigate the “lowest order theory”, which is appropriate at temperatures well above T c . We use this theory to determine where the Fermi liquid state first breaks down. This breakdown, which occurs at T * and is associated with intermediate values of the coupling, corresponds to a splitting of the single peaked (Fermi liquid) electronic spectral function into two peaks well separated by a gap, as might be expected for the pseudogap phase. Indeed, our calculations provide physical insight into the pseudogap state which is characterized by the presence of metastable pairs or “resonances”, which occupy states around the Fermi energy; in this way, they effectively reduce the single partile density of states. The superconducting instability T c is evaluated in the second stage of our calculations. Here, we introduce “mode coupling” effects, in which the long lived pairs are affected by the single particle pseudogap states and vice versa. Our T c equations, which turn out to be rather simple as a result of the KMP scheme, reveal a rich structure as a function of g in which the pseudogap is found to compete with superconductivity. Our results are compared with alternate theories in the literature.

Superconducting order parameter in bilayer cuprates: Occurrence of [pi] phase shifts in corner junctions

We study the order-parameter symmetry in bilayer cuprates such as Y-Ba-Cu-O, where interesting [pi] phase shifts have been observed in Josephson junctions. Taking models that represent the measured spin-fluctuation spectra of this cuprate, as well as more general models of Coulomb-correlation effects, we classify the allowed symmetries and determine their associated physical properties. [pi] phase shifts are shown to be a consequence of repulsive interactions, independent of whether a magnetic mechanism is operative. While it is known to occur in [ital d] states, this behavior can also be associated with (orthorhombic) [ital s] symmetry when the two subband gaps have opposite phase. Implications for the magnitude of [ital T][sub [ital c]] are discussed.

Coulomb correlations and pseudogap effects in a preformed pair model for the cuprates.

We extend previous work on pre-formed pair models of superconductivity to incorporate Coulomb correlation effects. For neutral systems, these models have provided a useful scheme which interpolates between BCS and Bose Einstein condensation with increasing coupling and thereby describes some aspects of pseudo-gap phenomena. However, charge fluctuations (via the plasmon,

THE CUPRATE PSEUDOGAP : PRECURSOR SUPERCONDUCTIVITY WITHOUT PREFORMED PAIRS

\omega_p

π Phase shifts in Bi-layer s-wave superconductors

) significantly modify the collective modes and therefore the interpolation behavior. We discuss the resulting behavior of the pseudo-gap and thermodynamic quantities such as

π Junctions in s-wave superconductors: Bi-layer effects

T_c

Superconductivity from a pseudogapped normal state: A mode-coupling approach to precursor superconductivity

,

SUPERCONDUCTING ORDER PARAMETER SYMMETRY IN MULTILAYER CUPRATES

\chi

the ''pairing approximation'' theory

and