574 research outputs found
Magnetic incommensurability and fluctuating charge density waves in the repulsive Hubbard model
Magnetic and charge susceptibilities of the two-dimensional repulsive Hubbard
model are investigated applying a strong coupling diagram technique in which
the expansion in powers of the hopping constants is used. For small lattices
and high temperatures results are in agreement with Monte Carlo simulations.
With the departure from half-filling the low-frequency magnetic
susceptibility becomes incommensurate and the incommensurability parameter
grows with . The incommensurability, its dependence on frequency and on
resemble experimental results in lanthanum cuprates. Also for finite sharp
maxima appear in the static charge susceptibility. The maxima are finite which
points to the absence of the long-range charge ordering (static stripes).
However, for the maxima are located near the momenta
, . In this case an interaction of carriers with
tetragonal distortions can stabilize stripes with the wavelength of four
lattice spacings, as observed in the low-temperature tetragonal phase of
cuprates. As follows from the obtained results, the magnetic incommensurability
is not a consequence of the stripes.Comment: 4 pages, 3 figures, manuscript for proceefings of LT2
A planar diagram approach to the correlation problem
We transpose an idea of 't Hooft from its context of Yang and Mills' theory
of strongly interacting quarks to that of strongly correlated electrons in
transition metal oxides and show that a Hubbard model of N interacting electron
species reduces, to leading orders in N, to a sum of almost planar diagrams.
The resulting generating functional and integral equations are very similar to
those of the FLEX approximation of Bickers and Scalapino. This adds the Hubbard
model at large N to the list of solvable models of strongly correlated
electrons.
PACS Numbers: 71.27.+a 71.10.-w 71.10.FdComment: revtex, 5 pages, with 3 eps figure
Toward a systematic 1/d expansion: Two particle properties
We present a procedure to calculate 1/d corrections to the two-particle
properties around the infinite dimensional dynamical mean field limit. Our
method is based on a modified version of the scheme of Ref.
onlinecite{SchillerIngersent}}. To test our method we study the Hubbard model
at half filling within the fluctuation exchange approximation (FLEX), a
selfconsistent generalization of iterative perturbation theory. Apart from the
inherent unstabilities of FLEX, our method is stable and results in causal
solutions. We find that 1/d corrections to the local approximation are
relatively small in the Hubbard model.Comment: 4 pages, 4 eps figures, REVTe
Fluctuation Exchange Analysis of Superconductivity in the Standard Three-Band CuO2 Model
The fluctuation exchange, or FLEX, approximation for interacting electrons is
applied to study instabilities in the standard three-band model for CuO2 layers
in the high-temperature superconductors. Both intra-orbital and near-neigbor
Coulomb interactions are retained. The filling dependence of the d(x2-y2)
transition temperature is studied in both the "hole-doped" and "electron-doped"
regimes using parameters derived from constrained-occupancy density-functional
theory for La2CuO4. The agreement with experiment on the overdoped hole side of
the phase diagram is remarkably good, i.e., transitions emerge in the 40 K
range with no free parameters. In addition the importance of the "orbital
antiferromagnetic," or flux phase, charge density channel is emphasized for an
understanding of the underdoped regime.Comment: REVTex and PostScript, 31 pages, 26 figures; to appear in Phys. Rev.
B (1998); only revised EPS figures 3, 4, 6a, 6b, 6c, 7 and 8 to correct
disappearance of some labels due to technical problem
Self-consistent Green function approach for calculations of electronic structure in transition metals
We present an approach for self-consistent calculations of the many-body
Green function in transition metals. The distinguishing feature of our approach
is the use of the one-site approximation and the self-consistent quasiparticle
wave function basis set, obtained from the solution of the Schrodinger equation
with a nonlocal potential. We analyze several sets of skeleton diagrams as
generating functionals for the Green function self-energy, including GW and
fluctuating exchange sets. Their relative contribution to the electronic
structure in 3d-metals was identified. Calculations for Fe and Ni revealed
stronger energy dependence of the effective interaction and self-energy of the
d-electrons near the Fermi level compared to s and p electron states.
Reasonable agreement with experimental results is obtained
A new approach to strongly correlated fermion systems: the spin-particle-hole coherent-state path integral
We describe a new path integral approach to strongly correlated fermion
systems, considering the Hubbard model as a specific example. Our approach is
based on the introduction of spin-particle-hole coherent states which
generalize the spin-1/2 coherent states by allowing the creation of a hole or
an additional particle. The action of the fermion system
can be expressed as a function of two
Grassmann variables (,) describing
particles propagating in the lower and upper Hubbard bands, and a unit vector
field whose dynamics arises from spin fluctuations. In the strong
correlation limit, can be truncated to quartic
order in the fermionic fields and used as the starting point of a
strong-coupling diagrammatic expansion in ( being the intersite
hopping amplitude and the on-site Coulomb repulsion). We discuss possible
applications of this formalism and its connection to the t-J model and the
spin-fermion model.Comment: 20 pages RevTex, 10 figure
The Superconducting Instabilities of the non half-filled Hubbard Model in Two Dimensions
The problem of weakly correlated electrons on a square lattice is formulated
in terms of one-loop renormalization group. Starting from the action for the
entire Brillouin zone (and not with a low-energy effective action) we reduce
successively the cutoff about the Fermi surface and follow the
renormalization of the coupling as a function of three energy-momenta. We
calculate the intrinsic scale where the renormalization group flow
crosses over from the regime () where the electron-electron
(e-e) and electron-hole (e-h) terms are equally important to the regime
() where only the e-e term plays a role. In the low energy
regime only the pairing interaction is marginally relevant, containing
contributions from all renormalization group steps of the regime . After diagonalization of , we identify its most
attractive eigenvalue . At low filling,
corresponds to the representation ( symmetry), while near half
filling the strongest attraction occurs in the representation
( symmetry). In the direction of the van Hove singularities, the
order parameter shows peaks with increasing strength as one approaches half
filling. Using the form of pairing and the structure of the renormalization
group equations in the low energy regime, we give our interpretation of ARPES
experiments trying to determine the symmetry of the order parameter in the
Bi2212 high- compound.Comment: 24 pages (RevTeX) + 11 figures (the tex file appeared incomplete
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
Self-Consistent Perturbation Theory for Thermodynamics of Magnetic Impurity Systems
Integral equations for thermodynamic quantities are derived in the framework
of the non-crossing approximation (NCA). Entropy and specific heat of 4f
contribution are calculated without numerical differentiations of thermodynamic
potential. The formulation is applied to systems such as PrFe4P12 with
singlet-triplet crystalline electric field (CEF) levels.Comment: 3 pages, 2 figures, proc. ASR-WYP-2005 (JAERI
Operator projection method applied to the single-particle Green's function in the Hubbard model
A new non-perturbative framework for many-body correlated systems is
formulated by extending the operator projection method (OPM). This method
offers a systematic expansion which enables us to project into the low-energy
structure after extracting the higher-energy hierarchy. This method also opens
a way to systematically take into account the effects of collective
excitations. The Mott-Hubbard metal-insulator transition in the Hubbard model
is studied by means of this projection beyond the second order by taking into
account magnetic and charge fluctuations in the presence of the high-energy
Mott-Hubbard structure. At half filling, the Mott-Hubbard gap is correctly
eproduced between the separated two bands. Near half filling, a strongly
renormalized low-energy single-particle excitations coexisting with the
Mott-Hubbard bands are shown to appear. Signifcance of momentum-dependent
self-energy in the results is stressed.Comment: 6 pages, final version to appear in J. Phys. Soc. Jp
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