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

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

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.

Dynamical local lattice instabilitiy triggered high tc superconductivity

High $T_c$ cuprate superconductors are characterized by two robust features: their strong electronic correlations and their intrinsic dynamical local lattice instabilities. Focusing on exclusively that latter, we picture their parent state in form of a quantum vacuum representing an electronic magma in which bound diamagnetic spin-singlet pairs pop in and out of existence in a Fermi sea of itinerant electrons. The mechanism behind that resides in the structural incompatibility of two stereo-chemical configurations Cu$^{\rm II}$O$_4$ and Cu$^{\rm III}$O$_4$ which compose the CuO$_2$ planes. It leads to spontaneously fluctuating Cu - O - Cu valence bonds which establish a local Feshbach resonance exchange coupling between bound and unbound electron pairs. The coupling, being the only free parameter in this scenario, the hole doping of the parent state is monitored by varying the total number of unpaired and paired electrons, in chemical equilibrium with each other. Upon lowering the temperature to below a certain $T^*$, bound and unbound electron pairs lock together in a local quantum superposition, generating transient localized bound electron pairs and a concomitant opening of a pseudo-gap in the single-particle density of states. At low temperature, this pseudo-gap state transits via a first order hole doping induced phase transition into a superconducting state in which the localized transient bound electron pairs get spatially phase correlated. The mechanism driving that transition is a phase separation between two phases having different relative densities of bound and unbound electron pairs, which is reminiscent of the physics of $^4$He - $^3$He mixtures.Comment: 9 pages 9 figure

Thermodynamics and tunneling spectroscopy in the pseudogap regime of the boson fermion model

Motivated by the STM experimental data on Bi_2 Sr_2 CaCU_2 O_{8+x} which indicate the tunneling conductance asymmetry sigma(-V) not equal sigma(V), we report that such a behavior can be explained in terms of the boson fermion model. It has been shown in the recent studies, based on various selfconsistent techniques to capture the many-body effects, that the low energy spectrum of the boson fermion model is featured by an appearance of the pseudogap at T^* > T_c. We argue that the pseudogap structure has to exhibit a particle-hole asymmetry. This asymmetry may eventually depend on the boson concentration.Comment: 4 pages, 2 figures. submitted to Physica

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