4 research outputs found
Multiorbital effects in the functional renormalization group: A weak-coupling study of the Emery model
We perform an instability analysis of the Emery three-band model at hole
doping and weak coupling within a channel-decomposed functional renormalization
group flow proposed in Phys. Rev. B 79, 195125 (2009). In our approach,
momentum dependences are taken into account with improved precision compared to
previous studies of related models. Around a generic parameter set, we find a
strong competition of antiferromagnetic and d-wave Cooper instabilities with a
smooth behavior under a variation of doping and additional hopping parameters.
For increasingly incommensurate ordering tendencies in the magnetic channel,
the d-wave pairing gap is deformed at its maxima. Comparing our results for the
Emery model to those obtained for the two-dimensional one-band Hubbard model
with effective parameters, we find that, despite considerable qualitative
agreement, multi-orbital effects have a significant impact on a quantitative
level.Comment: revised version: 22 pages, 11 figure
An alternative functional renormalization group approach to the single impurity Anderson model
We present an alternative functional renormalization group (fRG) approach to
the single-impurity Anderson model at finite temperatures. Starting with the
exact self-energy and interaction vertex of a small system ('core') containing
a correlated site, we switch on the hybridization with a non-interacting bath
in the fRG-flow and calculate spectra of the correlated site. Different
truncations of the RG-flow-equations and choices of the core are compared and
discussed. Furthermore we calculate the linear conductance and the magnetic
susceptibility as functions of temperature and interaction strength. The
signatures of Kondo physics arising in the flow are compared with numerical
renormalization group results.Comment: 16 page
Effective low-energy Hamiltonians for interacting nanostructures
We present a functional renormalization group (fRG) treatment of trigonal
graphene nanodiscs and composites thereof, modeled by finite-size Hubbard-like
Hamiltonians with honeycomb lattice structure. At half filling, the
noninteracting spectrum of these structures contains a certain number of
half-filled states at the Fermi level. For the case of trigonal nanodiscs,
including interactions between these degenerate states was argued to lead to a
large ground state spin with potential spintronics applications. Here we
perform a systematic fRG flow where the excited single-particle states are
integrated out with a decreasing energy cutoff, yielding a renormalized
low-energy Hamiltonian for the zero-energy states that includes effects of the
excited levels. The numerical implementation corroborates the results obtained
with a simpler Hartree-Fock treatment of the interaction effects within the
zero-energy states only. In particular, for trigonal nanodiscs the degeneracy
of the one-particle-states with zero-energy turns out to be very robust against
influences of the higher levels. As an explanation, we give a general argument
that within this fRG scheme the zero-energy degeneracy remains unsplit under
quite general conditions and for any size of the trigonal nanodisc. We
furthermore discuss the differences in the effective Hamiltonian and their
ground states of single nanodiscs and composite bow-tie-shaped systems.Comment: 13 page