96 research outputs found
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
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
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
Dynamic scaling in the vicinity of the Luttinger liquid fixed point
We calculate the single-particle spectral function A (k, omega) of a
one-dimensional Luttinger liquid by means of a functional renormalization group
(RG) approach. Given an infrared energy cutoff Lambda = Lambda_0 e^{- l}, our
approach yields the spectral function in the scaling form, A_{\Lambda} (k_F +
p, omega) = tau Z_l tilde{A}_l (p xi, omega tau), where k_F is the Fermi
momentum, Z_l is the wave-function renormalization factor, tau = 1 / \Lambda is
the time scale and xi = v_F / \Lambda is the length scale associated with
Lambda. At the Luttinger liquid fixed point (l rightarrow infty) our RG result
for A (k, omega) exhibits the correct anomalous scaling properties, and for k =
\pm k_F agrees exactly with the well-known bosonization result at weak
coupling. Our calculation demonstrates that the field rescaling is essential
for obtaining the crossover from Fermi liquid behavior to Luttinger liquid
behavior from a truncation of the hierarchy of exact RG flow equations as the
infrared cutoff is reduced.Comment: 15 pages, 5 figure
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
Functional renormalization group for d-wave superconductivity in Hubbard type models
The temperature dependence of d-wave superconducting order for two
dimensional fermions with d-wave attraction is investigated by means of the
functional renormalization group with partial bosonization. Below the critical
temperature T_c we find superconductivity, a gap in the electron propagator and
a temperature dependent anomalous dimension. At T_c the renormalized
"superfluid density" jumps and the approach to T_c from above is characterized
by essential scaling. These features are characteristic for a phase transition
of the Kosterlitz-Thouless (KT) type.Comment: 5 pages, 4 figures, references added, discussion improve
Magnetic and superconducting instabilities of the Hubbard model at the van Hove filling
We use a novel temperature-flow renormalization group technique to analyze
magnetic and superconducting instabilities in the two-dimensional t-t' Hubbard
model for particle densities close to the van Hove filling as a function of the
next-nearest neighbor hopping t'. In the one-loop flow at the van Hove filling,
the characteristic temperature for the flow to strong coupling is suppressed
drastically around t'_c approx. -0.33t, suggesting a quantum critical point
between d-wave pairing at moderate t'>t'_c and ferromagnetism for t'<t'_c. Upon
increasing the particle density in the latter regime the leading instability
occurs in the triplet pairing channel.Comment: 4 pages, to appear in Physical Review Letter
Superconducting and pseudogap phases from scaling near a Van Hove singularity
We study the quantum corrections to the Fermi energy of a two-dimensional
electron system, showing that it is attracted towards the Van Hove singularity
for a certain range of doping levels. The scaling of the Fermi level allows to
cure the infrared singularities left in the BCS channel after renormalization
of the leading logarithm near the divergent density of states. A phase of
d-wave superconductivity arises beyond the point of optimal doping
corresponding to the peak of the superconducting instability. For lower doping
levels, the condensation of particle-hole pairs due to the nesting of the
saddle points takes over, leading to the opening of a gap for quasiparticles in
the neighborhood of the singular points.Comment: 4 pages, 6 Postscript figures, the physical discussion of the results
has been clarifie
Fermi surface renormalization in Hubbard ladders
We derive the one-loop renormalization equations for the shift in the
Fermi-wavevectors for one-dimensional interacting models with four Fermi-points
(two left and two right movers) and two Fermi velocities v_1 and v_2. We find
the shift to be proportional to (v_1-v_2)U^2, where U is the Hubbard-U. Our
results apply to the Hubbard ladder and to the t_1-t_2 Hubbard model. The
Fermi-sea with fewer particles tends to empty. The stability of a saddle point
due to shifts of the Fermi-energy and the shift of the Fermi-wavevector at the
Mott-Hubbard transition are discussed.Comment: 5 pages, 4 Postscript figure
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