99 research outputs found
Towards analytic description of a transition from weak to strong coupling regime in correlated electron systems. I. Systematic diagrammatic theory with two-particle Green functions
We analyze behavior of correlated electrons described by Hubbard-like models
at intermediate and strong coupling. We show that with increasing interaction a
pole in a generic two-particle Green function is approached. The pole signals
metal-insulator transition at half filling and gives rise to a new vanishing
``Kondo'' scale causing breakdown of weak-coupling perturbation theory. To
describe the critical behavior at the metal-insulator transition a novel,
self-consistent diagrammatic technique with two-particle Green functions is
developed. The theory is based on the linked-cluster expansion for the
thermodynamic potential with electron-electron interaction as propagator.
Parquet diagrams with a generating functional are derived. Numerical
instabilities due to the metal-insulator transition are demonstrated on
simplifications of the parquet algebra with ring and ladder series only. A
stable numerical solution in the critical region is reached by factorization of
singular terms via a low-frequency expansion in the vertex function. We stress
the necessity for dynamical vertex renormalizations, missing in the simple
approximations, in order to describe the critical, strong-coupling behavior
correctly. We propose a simplification of the full parquet approximation by
keeping only most divergent terms in the asymptotic strong-coupling region. A
qualitatively new, feasible approximation suitable for the description of a
transition from weak to strong coupling is obtained.Comment: 17 pages, 4 figures, REVTe
Influence of Spin Wave Excitations on the Ferromagnetic Phase Diagram in the Hubbard-Model
The subject of the present paper is the theoretical description of collective
electronic excitations, i.e. spin waves, in the Hubbard-model. Starting with
the widely used Random-Phase-Approximation, which combines Hartree-Fock theory
with the summation of the two-particle ladder, we extend the theory to a more
sophisticated single particle approximation, namely the
Spectral-Density-Ansatz. Doing so we have to introduce a `screened`
Coulomb-interaction rather than the bare Hubbard-interaction in order to obtain
physically reasonable spinwave dispersions. The discussion following the
technical procedure shows that comparison of standard RPA with our new
approximation reduces the occurrence of a ferromagnetic phase further with
respect to the phase-diagrams delivered by the single particle theories.Comment: 8 pages, 9 figures, RevTex4, accepted for publication in Phys. Rev.
Density of states of a layered S/N d-wave superconductor
We calculate the density of states of a layered superconductor in which there
are two layers per unit cell. One of the layers contains a d-wave pairing
interaction while the other is a normal metal. The goal of this article is to
understand how the d-wave behaviour of the system is modified by the coupling
between the layer-types. This coupling takes the form of coherent, single
particle tunneling along the c-axis. We find that there are two physically
different limits of behaviour, which depend on the relative locations of the
Fermi surfaces of the two layer-types. We also discuss the interference between
the interlayer coupling and pairing interaction and we find that this
interference leads to features in the density of states.Comment: 33 pages and 11 PostScript figure
The Hubbard model within the equations of motion approach
The Hubbard model has a special role in Condensed Matter Theory as it is
considered as the simplest Hamiltonian model one can write in order to describe
anomalous physical properties of some class of real materials. Unfortunately,
this model is not exactly solved except for some limits and therefore one
should resort to analytical methods, like the Equations of Motion Approach, or
to numerical techniques in order to attain a description of its relevant
features in the whole range of physical parameters (interaction, filling and
temperature). In this manuscript, the Composite Operator Method, which exploits
the above mentioned analytical technique, is presented and systematically
applied in order to get information about the behavior of all relevant
properties of the model (local, thermodynamic, single- and two- particle ones)
in comparison with many other analytical techniques, the above cited known
limits and numerical simulations. Within this approach, the Hubbard model is
shown to be also capable to describe some anomalous behaviors of the cuprate
superconductors.Comment: 232 pages, more than 300 figures, more than 500 reference
Downregulation of pyrophosphate: d-fructose-6-phosphate 1-phosphotransferase activity in sugarcane culms enhances sucrose accumulation due to elevated hexose-phosphate levels
Analyses of transgenic sugarcane clones with 45â95% reduced cytosolic pyrophosphate: d-fructose-6-phosphate 1-phosphotransferase (PFP, EC 2.7.1.90) activity displayed no visual phenotypical change, but significant changes were evident in in vivo metabolite levels and fluxes during internode development. In three independent transgenic lines, sucrose concentrations increased between three- and sixfold in immature internodes, compared to the levels in the wildtype control. There was an eightfold increase in the hexose-phosphate:triose-phosphate ratio in immature internodes, a significant restriction in the triose phosphate to hexose phosphate cycle and significant increase in sucrose cycling as monitored by 13C nuclear magnetic resonance. This suggests that an increase in the hexose-phosphate concentrations resulting from a restriction in the conversion of hexose phosphates to triose phosphates drive sucrose synthesis in the young internodes. These effects became less pronounced as the tissue matured. Decreased expression of PFP also resulted in an increase of the ATP/ADP and UTP/UDP ratios, and an increase of the total uridine nucleotide and, at a later stage, the total adenine nucleotide pool, revealing strong interactions between PPi metabolism and general energy metabolism. Finally, decreased PFP leads to a reduction of PPi levels in older internodes indicating that in these developmental stages PFP acts in the gluconeogenic direction. The lowered PPi levels might also contribute to the absence of increases in sucrose contents in the more mature tissues of transgenic sugarcane with reduced PFP activity
Charged domain-wall dynamics in doped antiferromagnets and spin fluctuations in cuprate superconductors
Theoretical Physic
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