16 research outputs found
Boson-Fermion Duality and Metastability in Cuprate Superconductors
The intrinsic structural metastability in cuprate high T materials,
evidenced in a checker-board domain structure of the CuO 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
Bogoliubov shadow bands in the normal state of superconducting systems with strong pair fluctuations
On the basis of a scenario where electron pairing is induced by resonant
two-particle scattering (the Boson Fermion model), we show how precursors of
the superconducting state - in form of overdamped Bogoliubov modes - emerge in
the normal state upon approaching the transition temperature from above. This
result is obtained by a renormalization technique based on continuous unitary
transformations (the flow equations), projecting out the coherent contributions
in the electron spectral function from an incoherent background.Comment: 4 pages, 2 figure
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 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
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
Interrelation between the pseudogap and the incoherent quasi-particle features of high-Tc superconductors
Using a scenario of a hybridized mixture of localized bipolarons and
conduction electrons, we demonstrate for the latter the simultaneous appearance
of a pseudogap and of strong incoherent contributions to their quasi-particle
spectrum which arise from phonon shake-off effects. This can be traced back to
temporarily fluctuating local lattice deformations, giving rise to a
double-peak structure in the pair distribution function, which should be a key
feature in testing the origin of these incoherent contributions, recently seen
in angle-resolved photoemission spectroscopy (ARPES).Comment: 4 pages, 3 figures, to be published in Phys. Rev. Let