2,752 research outputs found
Quasiparticle undressing in a dynamic Hubbard model: exact diagonalization study
Dynamic Hubbard models have been proposed as extensions of the conventional
Hubbard model to describe the orbital relaxation that occurs upon double
occupancy of an atomic orbital. These models give rise to pairing of holes and
superconductivity in certain parameter ranges. Here we explore the changes in
carrier effective mass and quasiparticle weight and in one- and two-particle
spectral functions that occur in a dynamic Hubbard model upon pairing, by exact
diagonalization of small systems. It is found that pairing is associated with
lowering of effective mass and increase of quasiparticle weight, manifested in
transfer of spectral weight from high to low frequencies in one- and
two-particle spectral functions. This 'undressing' phenomenology resembles
observations in transport, photoemission and optical experiments in high T_c
cuprates. This behavior is contrasted with that of a conventional electron-hole
symmetric Holstein-like model with attractive on-site interaction, where
pairing is associated with 'dressing' instead of 'undressing'
Electronic dynamic Hubbard model: exact diagonalization study
A model to describe electronic correlations in energy bands is considered.
The model is a generalization of the conventional Hubbard model that allows for
the fact that the wavefunction for two electrons occupying the same Wannier
orbital is different from the product of single electron wavefunctions. We
diagonalize the Hamiltonian exactly on a four-site cluster and study its
properties as function of band filling. The quasiparticle weight is found to
decrease and the quasiparticle effective mass to increase as the electronic
band filling increases, and spectral weight in one- and two-particle spectral
functions is transfered from low to high frequencies as the band filling
increases. Quasiparticles at the Fermi energy are found to be more 'dressed'
when the Fermi level is in the upper half of the band (hole carriers) than when
it is in the lower half of the band (electron carriers). The effective
interaction between carriers is found to be strongly dependent on band filling
becoming less repulsive as the band filling increases, and attractive near the
top of the band in certain parameter ranges. The effective interaction is most
attractive when the single hole carriers are most heavily dressed, and in the
parameter regime where the effective interaction is attractive, hole carriers
are found to 'undress', hence become more like electrons, when they pair. It is
proposed that these are generic properties of electronic energy bands in solids
that reflect a fundamental electron-hole asymmetry of condensed matter. The
relation of these results to the understanding of superconductivity in solids
is discussed.Comment: Small changes following referee's comment
Optimal quantum pump in the presence of a superconducting lead
We investigate the parametric pumping of a hybrid structure consisting of a
normal quantum dot, a normal lead and a superconducting lead. Using the time
dependent scattering matrix theory, we have derived a general expression for
the pumped electric current and heat current. We have also derived the
relationship among the instantaneous pumped heat current, electric current, and
shot noise. This gives a lower bound for the pumped heat current in the hybrid
system similar to that of the normal case obtained by Avron et al
Relaxation Effects in the Transition Temperature of Superconducting HgBa2CuO4+delta
In previous studies on a number of under- and overdoped high temperature
superconductors, including YBa_{2}Cu_{3}O_{7-y} and Tl_{2}Ba_{2}CuO_{6+\delta},
the transition temperature T_c has been found to change with time in a manner
which depends on the sample's detailed temperature and pressure history. This
relaxation behavior in T_c is believed to originate from rearrangements within
the oxygen sublattice. In the present high-pressure studies on
HgBa_{2}CuO_{4+\delta} to 0.8 GPa we find clear evidence for weak relaxation
effects in strongly under- and overdoped samples () with
an activation energy . For overdoped
HgBa_{2}CuO_{4+\delta} E_{A} increases under pressure more rapidly than
previously observed for YBa_{2}Cu_{3}O_{6.41}, yielding an activation volume of
+11 \pm 5 cm^{3}; the dependence of T_c on pressure is markedly nonlinear, an
anomalous result for high-T_c superconductors in the present pressure range,
giving evidence for a change in the electronic and/or structural properties
near 0.4 GPa
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