178 research outputs found

### Fermi Surface of the 2D Hubbard Model at Weak Coupling

We calculate the interaction-induced deformation of the Fermi surface in the
two-dimensional Hubbard model within second order perturbation theory. Close to
half-filling, interactions enhance anisotropies of the Fermi surface, but they
never modify the topology of the Fermi surface in the weak coupling regime.Comment: 4 pages, LaTeX2e, 5 embedded EPS figures, accepted to be published in
Z. Phys.

### Theory of the in-plane anisotropy of magnetic excitations in YBa_{2}Cu_{3}O_{6+y}

A pronounced xy-anisotropy was observed in recent neutron scattering
experiments for magnetic excitations in untwinned YBa_{2}Cu_{3}O_{6+y} (YBCO).
The small anisotropy of the bare band structure due to the orthorhombic crystal
symmetry seems to be enhanced by correlation effects. A natural possibility is
that the system is close to a Pomeranchuk instability associated with a d-wave
Fermi surface deformation (dFSD). We investigate this possibility in the
bilayer t-J model within a self-consistent slave-boson mean-field theory. We
show that the dFSD correlations drive a pronounced xy-anisotropy of magnetic
excitations at low doping and at relatively high temperatures, providing a
scenario for the observed xy-anisotropy in optimally doped as well as
underdoped YBCO, including the pseudogap phase.Comment: magnetic excitations in the even channel for YBCO are presented; the
proceedings of the M2S-HTSC VIII conferenc

### Ferromagnetism and triplet superconductivity in the two-dimensional Hubbard model

We review magnetic and superconducting instabilities in the t-t' Hubbard
model on the two-dimensional square lattice as obtained with functional
one-loop renormalization group techniques. Special emphasis is put on
ferromagnetic and triplet superconducting tendencies that could be relevant to
the triplet superconductor Sr2RuO4.Comment: 4.5 pages, 3 figures; Proceedings of M2S-Rio, Rio de Janiero 2003,
submitted to Physica

### d-wave superconductivity and Pomeranchuk instability in the two-dimensional Hubbard model

We present a systematic stability analysis for the two-dimensional Hubbard
model, which is based on a new renormalization group method for interacting
Fermi systems. The flow of effective interactions and susceptibilities confirms
the expected existence of a d-wave pairing instability driven by
antiferromagnetic spin fluctuations. More unexpectedly, we find that strong
forward scattering interactions develop which may lead to a Pomeranchuk
instability breaking the tetragonal symmetry of the Fermi surface.Comment: 4 pages (RevTeX), 4 eps figure

### Exact integral equation for the renormalized Fermi surface

The true Fermi surface of a fermionic many-body system can be viewed as a
fixed point manifold of the renormalization group (RG). Within the framework of
the exact functional RG we show that the fixed point condition implies an exact
integral equation for the counterterm which is needed for a self-consistent
calculation of the Fermi surface. In the simplest approximation, our integral
equation reduces to the self-consistent Hartree-Fock equation for the
counterterm.Comment: 5 pages, 1 figur

### Magnetic and superconducting correlations in the two-dimensional Hubbard model

The interplay and competition of magnetic and superconducting correlations in
the weakly interacting two-dimensional Hubbard Model is investigated by means
of the functional renormalization group. At zero temperature the flow of
interactions in one-loop approximation evolves into a strong coupling regime at
low energy scales, signalling the possible onset of spontaneous symmetry
breaking. This is further analyzed by a mean-field treatment of the strong
renormalized interactions which takes into account magnetic and superconducting
order simultaneously. The effect of strong correlations on single-particle
properties in the normal phase is studied by calculating the flow of the
self-energy.Comment: 16 pages, 10 figure

### Renormalized perturbation theory for Fermi systems: Fermi surface deformation and superconductivity in the two-dimensional Hubbard model

Divergencies appearing in perturbation expansions of interacting many-body
systems can often be removed by expanding around a suitably chosen renormalized
(instead of the non-interacting) Hamiltonian. We describe such a renormalized
perturbation expansion for interacting Fermi systems, which treats Fermi
surface shifts and superconductivity with an arbitrary gap function via
additive counterterms. The expansion is formulated explicitly for the Hubbard
model to second order in the interaction. Numerical soutions of the
self-consistency condition determining the Fermi surface and the gap function
are calculated for the two-dimensional case. For the repulsive Hubbard model
close to half-filling we find a superconducting state with d-wave symmetry, as
expected. For Fermi levels close to the van Hove singularity a Pomeranchuk
instability leads to Fermi surfaces with broken square lattice symmetry, whose
topology can be closed or open. For the attractive Hubbard model the second
order calculation yeilds s-wave superconductivity with a weakly momentum
dependent gap, whose size is reduced compared to the mean-field result.Comment: 18 pages incl. 6 figure

### Deformation of the Fermi surface in the extended Hubbard model

The deformation of the Fermi surface induced by Coulomb interactions is
investigated in the t-t'-Hubbard model. The interplay of the local U and
extended V interactions is analyzed. It is found that exchange interactions V
enhance small anisotropies producing deformations of the Fermi surface which
break the point group symmetry of the square lattice at the Van Hove filling.
This Pomeranchuck instability competes with ferromagnetism and is suppressed at
a critical value of U(V). The interaction V renormalizes the t' parameter to
smaller values what favours nesting. It also induces changes on the topology of
the Fermi surface which can go from hole to electron-like what may explain
recent ARPES experiments.Comment: 5 pages, 4 ps figure

### Uso da violĂŞncia domĂ©stica como prĂˇtica educativa: conhecendo a realidade em Diamantina â€“ MG/ Brasil = Domestic violence as an educative practice: knowing the reality in Diamantina-MG-Brazil

O presente artigo objetiva analisar o uso da violĂŞncia domĂ©stica como prĂˇtica educativa em Diamantina MG. Nesse sentido, foi utilizado um questionĂˇrio aberto para avaliar o uso de disciplina nĂŁo violenta, violĂŞncia psicolĂłgica e violĂŞncia fĂsica, esta classificada como leve, moderada e grave. A amostra estudada se constituiu de 90 responsĂˇveis por crianĂ§as, escolhidas aleatoriamente a partir do universo de 7. 455 crianĂ§as matriculadas nas creches, prĂ©-escolas e ensino fundamental, pĂşblicos e privados, na sede do municĂpio, periferia e tambĂ©m zona rural. Alguns dos resultados mostraram que todos os entrevistados adotam a disciplina nĂŁo violenta, 95,56% a ViolĂŞncia psicolĂłgica e 94,44% a violĂŞncia fĂsica, que, nas formas moderada e grave, Ă© praticada por 74,44% dos entrevistados. O ensino de novas prĂˇticas educacionais surge, dessa forma, como uma necessidade urgent

### Antiferromagnetism of the 2D Hubbard Model at Half Filling: Analytic Ground State at Weak Coupling

We introduce a local formalism to deal with the Hubbard model on a N times N
square lattice (for even N) in terms of eigenstates of number operators, having
well defined point symmetry. For U -> 0, the low lying shells of the kinetic
energy are filled in the ground state. At half filling, using the 2N-2 one-body
states of the partially occupied shell S_{hf}, we build a set of (2N-2 N-1)^{2}
degenerate unperturbed ground states with S_{z}=0 which are then resolved by
the Hubbard interaction \hat{W}=U\sum_{r}\hat{n}_{r\ua}\hat{n}_{r\da}. In
S_{hf} we study the many-body eigenstates of the kinetic energy with vanishing
eigenvalue of the Hubbard repulsion (W=0 states). In the S_{z}=0 sector, this
is a N times degenerate multiplet. From the singlet component one obtains the
ground state of the Hubbard model for U=0^{+}, which is unique in agreement
with a theorem by Lieb. The wave function demonstrates an antiferromagnetic
order, a lattice step translation being equivalent to a spin flip. We show that
the total momentum vanishes, while the point symmetry is s or d for even or odd
N/2, respectively.Comment: 13 pages, no figure

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