50 research outputs found
Fractional Chern insulator on a triangular lattice of strongly correlated electrons
We discuss the low-energy limit of three-orbital Kondo-lattice and Hubbard
models describing orbitals on a triangular lattice near half-filling.
We analyze how very flat bands with non-trivial topological character, a Chern
number C=1, arise both in the limit of infinite on-site interactions as well as
in more realistic regimes. Exact diagonalization is then used to investigate
fractional filling of an effective one-band spinless-fermion model including
nearest-neighbor interaction ; it reveals signatures of fractional Chern
insulators (FCIs) for several filling fractions. In addition to indications
based on energies, e.g. flux insertion and fractional statistics of quasiholes,
Chern numbers are obtained. It is shown that FCIs are robust against disorder
in the underlying magnetic texture that defines the topological character of
the band. We also investigate competition between FCI states and a charge
density wave (CDW) and discuss particle-hole asymmetry as well as Fermi-surface
nesting. FCI states turn out to be rather robust and do not require very flat
bands, but can also arise when filling or an absence of Fermi-surface nesting
disfavor the competing CDW. Nevertheless, very flat bands allow FCI states to
be induced by weaker interactions than those needed for more dispersive bands.Comment: 14 pages, 13 figure
Half-Metallic Ferromagnetism and the spin polarization in CrO
We present electronic structure calculations in combination with local and
non-local many-body correlation effects for the half-metallic ferromagnet
CrO. Finite-temperature Dynamical Mean Field Theory results show the
existence of non-quasiparticle states, which were recently observed as almost
currentless minority spin states near the Fermi energy in resonant scattering
experients. At zero temperatures, Variational Cluster Approach calculations
support the half-metallic nature of CrO as seen in superconducting point
contact spectroscopy. The combination of these two techniques allowed us to
qualitatively describe the spin-polarization in CrO.Comment: 5 pages, 3 figure
Macroscopic Degeneracy and Emergent Frustration in a Honeycomb Lattice Magnet
Using a hybrid method based on fermionic diagonalization and classical Monte
Carlo, we investigate the interplay between itinerant and localized spins, with
competing double- and super-exchange interactions, on a honeycomb lattice. For
moderate superexchange, a geometrically frustrated triangular lattice of
hexagons forms spontaneously. For slightly larger superexchange a dimerized
groundstate is stable that has macroscopic degeneracy. The presence of these
states on a non-frustrated honeycomb lattice highlights a novel phenomenon in
this itinerant electron system: emergent geometrical frustration and
degeneracy.Comment: 4+ pages, 4 figures; published versio
Properties of a two orbital model for oxypnictide superconductors: Magnetic order, B_2g spin-singlet pairing channel, and its nodal structure
A two orbital model for the new Fe-based superconductors is studied using the
Lanczos method as well as pairing mean-field approximations. Our main goals are
(i) to provide a comprehensive analysis of this model using numerical
techniques with focus on half-filling and on the state with two more electrons
than half-filling and (ii) to investigate the nodal structure of the mean-field
superconducting state and compare the results with angle-resolved photoemission
data. In particular, we provide evidence that at half-filling spin 'stripes',
as observed experimentally, dominate over competing states.
Depending on parameters, the state with two more electrons added to half
filling is either triplet or singlet. Since experiments suggest spin singlet
pairs, our focus is on this state. Under rotation, it transforms as the B_2g
representation of the D_4h group. We also show that the s+/- pairing operator
transforms as A_1g and becomes dominant only in an unphysical regime of the
model where the undoped state is an insulator. For robust values of the
effective electronic attraction producing the Cooper pairs, assumption
compatible with recent angle-resolved photoemission (ARPES) results that
suggesting small Cooper-pair size, the nodes of the two-orbital model are found
to be located only at the electron pockets. Since recent ARPES efforts have
searched for nodes at the hole pockets or only in a few directions at the
electron pockets, our results for the nodal distribution may help to guide
future experiments. More in general, the investigations reported here aim to
establish several of the properties of the two orbital model. Only a detailed
comparison with experiments will clarify how far this simple model present a
valid description of the Fe pnictides
Absence of Hole Confinement in Transition Metal Oxides with Orbital Degeneracy
We investigate the spectral properties of a hole moving in a two-dimensional
Hubbard model for strongly correlated t_2g electrons. Although superexchange
interactions are Ising-like, a quasi-one-dimensional coherent hole motion
arises due to effective three-site terms. This mechanism is fundamentally
different from the hole motion via quantum fluctuations in the conventional
spin model with SU(2) symmetry. The orbital model describes also propagation of
a hole in some e_g compounds, and we argue that orbital degeneracy alone does
not lead to hole self-localization.Comment: 4 pages, 5 figure
Orbital polarons versus itinerant e_g electrons in doped manganites
We study an effective one-dimensional (1D) orbital t-J model derived for
strongly correlated e_g electrons in doped manganites. The ferromagnetic spin
order at half filling is supported by orbital superexchange prop. to J which
stabilizes orbital order with alternating x^2-y^2 and 3z^2-r^2 orbitals. In a
doped system it competes with the kinetic energy prop. to t. When a single hole
is doped to a half-filled chain, its motion is hindered and a localized orbital
polaron is formed. An increasing doping generates either separated polarons or
phase separation into hole-rich and hole-poor regions, and eventually polarizes
the orbitals and gives a it metallic phase with occupied 3z^2-r^2 orbitals.
This crossover, investigated by exact diagonalization at zero temperature, is
demonstrated both by the behavior of correlation functions and by spectral
properties, showing that the orbital chain with Ising superexchange is more
classical and thus radically different from the 1D spin t-J model. At finite
temperature we derive and investigate an effective 1D orbital model using a
combination of exact diagonalization with classical Monte-Carlo for spin
correlations. A competition between the antiferromagnetic and ferromagnetic
spin order was established at half filling, and localized polarons were found
for antiferromagnetic interactions at low hole doping. Finally, we clarify that
the Jahn-Teller alternating potential stabilizes the orbital order with
staggered orbitals, inducing the ferromagnetic spin order and enhancing the
localized features in the excitation spectra. Implications of these findings
for colossal magnetoresistance manganites are discussed.Comment: 19 pages, 20 figure
Doping dependence of spin and orbital correlations in layered manganites
We investigate the interplay between spin and orbital correlations in
monolayer and bilayer manganites using an effective spin-orbital t-J model
which treats explicitly the e_g orbital degrees of freedom coupled to classical
t_{2g} spins. Using finite clusters with periodic boundary conditions, the
orbital many-body problem is solved by exact diagonalization, either by
optimizing spin configuration at zero temperature, or by using classical
Monte-Carlo for the spin subsystem at finite temperature. In undoped
two-dimensional clusters, a complementary behavior of orbital and spin
correlations is found - the ferromagnetic spin order coexists with alternating
orbital order, while the antiferromagnetic spin order, triggered by t_{2g} spin
superexchange, coexists with ferro-orbital order. With finite crystal field
term, we introduce a realistic model for La_{1-x}Sr_{1+x}MnO_4, describing a
gradual change from predominantly out-of-plane 3z^2-r^2 to in-plane x^2-y^2
orbital occupation under increasing doping. The present electronic model is
sufficient to explain the stability of the CE phase in monolayer manganites at
doping x=0.5, and also yields the C-type antiferromagnetic phase found in
Nd_{1-x}Sr_{1+x}MnO_4 at high doping. Also in bilayer manganites magnetic
phases and the accompanying orbital order change with increasing doping. Here
the model predicts C-AF and G-AF phases at high doping x>0.75, as found
experimentally in La_{2-2x}Sr_{1+2x}Mn_2O_7.Comment: 23 pages, 21 figure
Aspects of the FM Kondo Model: From Unbiased MC Simulations to Back-of-an-Envelope Explanations
Effective models are derived from the ferromagnetic Kondo lattice model with
classical corespins, which greatly reduce the numerical effort. Results for
these models are presented. They indicate that double exchange gives the
correct order of magnitude and the correct doping dependence of the Curie
temperature. Furthermore, we find that the jump in the particle density
previously interpreted as phase separation is rather explained by ferromagnetic
polarons.Comment: Proceedings of Wandlitz Days of Magnetism 200