710 research outputs found

### Magnetic-field and chemical-potential effects on the low-energy separation

We show that in the presence of a magnetic field the usual low-energy
separation of the Hubbard chain is replaced by a ``$c$'' and ``$s$''
separation. Here $c$ and $s$ refer to small-momentum and low-energy independent
excitation modes which couple both to charge and spin. Importantly, we find the
exact generators of these excitations both in the electronic and pseudoparticle
basis. In the limit of zero magnetic field these generators become the usual
charge and spin fluctuation operators. The $c$ and $s$ elementary excitations
are associated with the $c$ and $s$ pseudoparticles, respectively. We also
study the separate pseudoparticle left and right conservation laws. In the
presence of the magnetic field the small-momentum and low-energy excitations
can be bosonized. However, the suitable bosonization corresponds to the $c$ and
$s$ pseudoparticle modes and not to the usual charge and spin fluctuations. We
evaluate exactly the commutator between the electronic-density operators. Its
spin-dependent factor is in general non diagonal and depends on the
interaction. The associate bosonic commutation relations characterize the
present unconventional low-energy separation.Comment: 29 pages, latex, submitted to Phys. Rev.

### Instabilities of the Hubbard chain in a magnetic field

We find and characterize the instabilities of the repulsive Hubbard chain in
a magnetic field by studing all response functions at low frequency \omega and
arbitrary momentum. The instabilities occur at momenta which are simple
combinations of the (U=0) \sigma =\uparrow ,\downarrow Fermi points, \pm
k_{F\sigma}. For finite values of the on-site repulsion U the instabilities
occur for single \sigma electron adding or removing at momenta \pm k_{F\sigma},
for transverse spin-density wave (SDW) at momenta \pm 2k_F (where
2k_F=k_{F\uparrow}+k_{F\downarrow}), and for charge-density wave (CDW) and SDW
at momenta \pm 2k_{F\uparrow} and \pm 2k_{F\downarrow}. While at zero magnetic
field removing or adding single electrons is dominant, the presence of that
field brings about a dominance for the transverse \pm 2k_F SDW over all the
remaining instabilities for a large domain of $U$ and density n values. We go
beyond conformal-field theory and study divergences which occur at finite
frequency in the one-electron Green function at half filling and in the
transverse-spin response function in the fully-polarized ferromagnetic phase.Comment: LaTeX file, 15 pages plus 9 figures. Accepted for publication in
Phys. Rev. B. The figures can be obtained upon request from Pedro Sacramento
at [email protected]

### The TTF finite-energy spectral features in photoemission of TTF-TCNQ: The Hubbard-chain description

A dynamical theory which accounts for all microscopic one-electron processes
is used to study the spectral function of the 1D Hubbard model for the whole
$(k, \omega)$-plane, beyond previous studies which focused on the weight
distribution in the vicinity of the singular branch lines only. While our
predictions agree with those of the latter studies concerning the
tetracyanoquinodimethane (TCNQ) related singular features in photoemission of
the organic compound tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ)
metallic phase, the generalized theory also leads to quantitative agreement
concerning the tetrathiafulvalene (TTF) related finite-energy spectral
features, which are found to correspond to a value of the on-site repulsion $U$
larger than for TCNQ. Our study reveals the microscopic mechanisms behind the
unusual spectral features of TTF-TCNQ and provides a good overall description
of those features for the whole $(k, \omega)$-plane.Comment: To appear in Journal of Physics: Condensed Matte

### Spectral microscopic mechanisms and quantum phase transitions in a 1D correlated problem

In this paper we study the dominant microscopic processes that generate
nearly the whole one-electron removal and addition spectral weight of the
one-dimensional Hubbard model for all values of the on-site repulsion $U$. We
find that for the doped Mott-Hubbard insulator there is a competition between
the microscopic processes that generate the one-electron upper-Hubbard band
spectral-weight distributions of the Mott-Hubbard insulating phase and
finite-doping-concentration metallic phase, respectively. The spectral-weight
distributions generated by the non-perturbative processes studied here are
shown elsewhere to agree quantitatively for the whole momentum and energy
bandwidth with the peak dispersions observed by angle-resolved photoelectron
spectroscopy in quasi-one-dimensional compounds.Comment: 18 pages, 2 figure

### Scattering mechanisms and spectral properties of the one-dimensional Hubbard model

It is found that the finite-energy spectral properties of the one-dimensional
Hubbard model are controlled by the scattering of charged $\eta$-spin-zero
$2\nu$-holon composite objects, spin-zero $2\nu$-spinon composite objects, and
charged $\eta$-spin-less and spin-less objects, rather than by the scattering
of independent $\eta$-spin 1/2 holons and spin 1/2 spinons. Here $\nu
=1,2,...$. The corresponding $S$ matrix is calculated and its relation to the
spectral properties is clarified.Comment: 8 pages, no figure

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