The Spin Gap in the Context of the Boson-Fermion Model for High TcT_c Superconductivity

The issue of the spin gap in the magnetic susceptibility $\chi''(q,\omega)$ in high T_c superconductors is discussed within a scenario of a mixture of localized tightly bound electron pairs in singlet states (bi-polarons) and itinerant electrons. Due to a local exchange between the two species of charge carriers, antiferromagnetic correlations are induced amongst the itinerant electrons in the vicinity of the sites containing the bound electron pairs. As the temperature is lowered these exchange processes become spatially correlated leading to a spin wave-like spectrum in the subsystem of the itinerant electrons. The onset of such coherence is accompanied by the opening of a pseudo gap in the density of states of the electron subsystem whose temperature dependence is reflected in that of $\chi'' (q,\omega)$ near $q =(\pi,\pi)$ where a ``spin gap'' is observed by inelastic neutron scattering and NMR.Comment: 9 pages Latex, 3 figures available upon request. To appear in Physica

The Atomic Limit of the Boson-Fermion Model

The Boson-Fermion model, describing a mixture of hybridized localized Bosons and itinerant Fermions on a lattice, is known to exhibit spectral properties for the Fermions which upon lowering the temperature develop into a three pole structure in the vicinity of the Fermi level. These spectral features go hand in hand with the opening of a pseudogap in the density of states upon approaching the critical temperature Tc when superconductivity sets in. In the present work we study this model, in the atomic limit where the three pole structure arises naturally from the local bonding, anti-bonding and non-bonding states between the Bosons and Fermions.Comment: revtex, 9 pages and 6 eps figures. Submitted to Europhysics Letter

Breakdown of Landau Fermi liquid properties in the 2D2D Boson-Fermion model

We study the normal state spectral properties of the fermionic excitations in the Boson-Fermion model. The fermionic single particle excitations show a flattening of the dispersion as the Fermi vector ${\bf k}_{_F}$ is approached from below, forshadowing a Bogoliubov spectrum of a superconducting ground state. The width of the quasiparticle excitations near ${\bf k}_{_F}$ increases monotonically as the temperature is lowered. In the fermionic distribution function this temperature dependence is manifest in a strong modification of $n({\bf k})$ in a small region below ${\bf k}_{_F}$, but a nearly $T$ independant $n({\bf k}_{_F})$.Comment: 10 pages, RevTeX 3.

Metal-insulator crossover in the Boson-Fermion model in infinite dimensions

The Boson-Fermion model, describing a mixture of tightly bound electron pairs and quasi-free electrons hybridized with each other via a charge exchange term, is studied in the limit of infinite dimensions, using the Non-Crossing Approximation within the Dynamical Mean Field Theory. It is shown that a metal-insulator crossover, driven by strong pair fluctuations, takes place as the temperature is lowered. It manifests itself in the opening of a pseudogap in the electron density of states, accompanied by a corresponding effect in the optical and dc conductivity.Comment: 4 pages, 3 figures, to be published in Phys. Rev. Let

Remnant superfluid collective phase oscillations in the normal state of systems with resonant pairing

The signature of superfluidity in bosonic systems is a sound wave-like spectrum of the single particle excitations which in the case of strong interactions is roughly temperature independent. In fermionic systems, where fermion pairing arises as a resonance phenomenon between free fermions and paired fermionic states (examples are: the atomic gases of lithium or potassium controlled by a Feshbach resonance, polaronic systems in the intermediary coupling regime, d-wave hole pairing in the strongly correlated Hubbard system), remnants of such superfluid characteristics are expected to be visible in the normal state. The single particle excitations maintain there a sound wave like structure for wave vectors above a certain q_{min}(T) where they practically coincide there with the spectrum of the superfluid phase for T<T_{c}. Upon approaching the transition from above this region in q-space extends down to small momenta, except for a narrow region around q=0 where such modes change into damped free particleComment: 5 pages, 3 figures; to appear in Phys Rev

The pseudogap in underdoped high Tc superconductors in the framework of the Boson Fermion model

The question of whether the pseudogap in high $T_c$ cuprates is related to super conducting precursor effects or to the existence of extrinsic bosonic massive excitations is investigated on the basis of the Boson-Fermion model. The characteristic three peak structure of the electronic spectral function and the temperature dependent Fermi vector derived here are signatures for a two component scenario which can be tested by ARPES and BIS experiments.Comment: revtex version with 3 eps figures. Revised version to appear in Phys. Rev. B. 4 c programs adde

Boson-Fermion Duality and Metastability in Cuprate Superconductors

The intrinsic structural metastability in cuprate high T$_c$ materials, evidenced in a checker-board domain structure of the CuO$_2$ planes, locally breaks translational and rotational symmetry. Dynamical charge - deformation fluctuations of such nano-size unidirectional domains, involving Cu-O-Cu molecular bonds, result in resonantly fluctuating diamagnetic pairs embedded in a correlated Fermi liquid. As a consequence, the single-particle spectral properties acquire simultaneously (i) fermionic low energy Bogoliubov branches for propagating Cooper pairs and (ii) bosonic localized glassy structures for tightly bound states of them at high energies. The partial localization of the single-particle excitations results in a fractionation of the Fermi surface as the strength of the exchange coupling between itinerant fermions and partially localized fermion pairs increases upon moving from the nodal to the anti-nodal point. This is also the reason why, upon hole doping, bound fermion pairs predominantly accumulate near the anti-nodal points and ultimately condense in an anisotropic fashion, tracking the gap in the single particle spectrum.Comment: 11 pages 5figure