182 research outputs found
Single particle spectrum of the flux phase in the FM Kondo Model
We investigate the 2D ferromagnetic Kondo lattice model for manganites with
classical corespins at Hund's rule coupling J_H=6, with antiferromagnetic
superexchange 0.03 < J' < 0.05. We employ canonical and grand canonical
unbiased Monte Carlo simulations and find paramagnetism, weak ferromagnetism
and the Flux phase, depending on doping and on J'. The observed single particle
spectrum in the flux phase differs from the idealized infinite lattice case,
but agrees well with an idealized finite lattice case with thermal
fluctuations.Comment: contribution to the SCES04 conferenc
Spin-orbital physics for p orbitals in alkali RO_2 hyperoxides --- generalization of the Goodenough-Kanamori rules
We derive a realistic spin-orbital model at finite Hund's exchange for alkali
hyperoxides. We find that, due to the geometric frustration of the oxygen
lattice, spin and orbital waves destabilize both spin and p-orbital order in
almost all potential ground states. We show that the orbital order induced by
the lattice overrules the one favoured by superexchange and that this, together
with the large interorbital hopping, leads to generalized Goodenough-Kanamori
rules. They (i) lift the geometric frustration of the lattice, and (ii) explain
the observed layered C-type antiferromagnetic order in alkali hyperoxides. This
is confirmed by a spin-wave dispersion with no soft-mode behavior presented
here as a prediction for future experiments.Comment: 7 pages, 5 figures; accepted in EP
Spectral properties of orbital polarons in Mott insulators
We address the spectral properties of Mott insulators with orbital degrees of
freedom, and investigate cases where the orbital symmetry leads to Ising-like
superexchange in the orbital sector. The paradigm of a hole propagating by its
coupling to quantum fluctuations, known from the spin t-J model, then no longer
applies. We find instead that when one of the two orbital flavors is immobile,
as in the Falicov-Kimball model, trapped orbital polarons coexist with free
hole propagation emerging from the effective three-site hopping in the regime
of large on-site Coulomb interaction U. The spectral functions are found
analytically in this case within the retraceable path approximation in one and
two dimensions. On the contrary, when both of the orbitals are active, as in
the model for electrons in two dimensions, we find propagating
polarons with incoherent scattering dressing the moving hole and renormalizing
the quasiparticle dispersion. Here, the spectral functions, calculated using
the self-consistent Born approximation, are anisotropic and depend on the
orbital flavor. Unbiased conclusions concerning the spectral properties are
established by comparing the above results for the orbital t-J models with
those obtained using the variational cluster approximation or exact
diagonalization for the corresponding Hubbard models. The present work makes
predictions concerning the essential features of photoemission spectra of
certain fluorides and vanadates.Comment: 26 pages, 16 figures; to appear in Physical Review
Magnetism of one-dimensional Wigner lattices and its impact on charge order
The magnetic phase diagram of the quarter-filled generalized Wigner lattice
with nearest- and next-nearest-neighbor hopping t_1 and t_2 is explored. We
find a region at negative t_2 with fully saturated ferromagnetic ground states
that we attribute to kinetic exchange. Such interaction disfavors
antiferromagnetism at t_2 <0 and stems from virtual excitations across the
charge gap of the Wigner lattice, which is much smaller than the Mott-Hubbard
gap proportional to U. Remarkably, we find a strong dependence of the charge
structure factor on magnetism even in the limit U to infinity, in contrast to
the expectation that charge ordering in the Wigner lattice regime should be
well described by spinless fermions. Our results, obtained using the
density-matrix renormalization group and exact diagonalization, can be
transparently explained by means of an effective low-energy Hamiltonian
Temperature effects in a spin-orbital model for manganites
We study a two-dimensional effective orbital superexchange model derived for
strongly correlated e_g electrons coupled to t_{2g} core spins in layered
manganites. One finds that the ferromagnetic and antiferromagnetic correlations
closely compete, and small changes of parameters can switch the type of
magnetic order. For the same reason, spin order is easily destroyed with rising
temperature, while alternating orbital correlations can persist to temperatures
where FM order has already melted. A scenario for the AF phase observed in
LaSrMnO_4 is presented.Comment: 4 pages, 3 figures, contribution to the PM05-meeting in Poznan,
Polan
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