103 research outputs found
Thermally assisted ordering in Mott insulators
Ginzburg-Landau theory describes phase transitions as the competition between
energy and entropy: The ordered phase has lower energy, while the disordered
phase has larger entropy. When heating the system, ordering is reduced
entropically until it vanishes at the critical temperature. This established
picture implicitly assumes that the energy difference between ordered and
disordered phase does not change with temperature. We show that for the Mott
insulator KCuF3 this assumption is strongly violated: thermal expansion
energetically stabilizes the orbitally-ordered phase to such and extent that no
phase transition is observed. This new mechanism explains not only the absence
of a phase transition in KCuF3 but even suggests the possibility of an inverted
transition in closed-shell systems, where the ordered phase emerges only at
high temperatures.Comment: 5 pages, 5 figure
Mechanism of structural phase transitions in KCrF3
We study the origin of the cubic to tetragonal and tetragonal to monoclinic
structural transitions in KCrF3, and the associated change in orbital order,
paying particular attention to the relevance of super-exchange in both phases.
We show that super-exchange is not the main mechanism driving these
transitions. Specifically, it is not strong enough to be responsible for the
high-temperature cubic to tetragonal transition and does not yield the type of
orbital order observed in the monoclinic phase. The energy difference between
the tetragonal and the monoclinic structure is tiny, and most likely results
from the interplay between volume, covalency, and localization effects. The
transition is rather driven by Slater exchange than super-exchange.
Nevertheless, once the monoclinic distortions are present, super-exchange helps
in stabilizing the low symmetry structure. The orbital order we obtain for this
monoclinic phase is consistent with the magnetic transition at 80 K.Comment: 8 pages, 6 figure
Multiplet effects in orbital and spin ordering phenomena: A hybridization-expansion quantum impurity solver study
Orbital and spin ordering phenomena in strongly correlated systems are
commonly studied using the local-density approximation + dynamical mean-field
theory approach. Typically, however, such simulations are restricted to
simplified models (density-density Coulomb interactions, high symmetry
couplings and few-band models). In this work we implement an efficient general
hybridization-expansion continuous-time quantum Monte Carlo impurity solver
(Krylov approach) which allows us to investigate orbital and spin ordering in a
more realistic setting, including interactions that are often neglected (e.g.,
spin-flip and pair-hopping terms), enlarged basis sets (full d versus eg),
low-symmetry distortions, and reaching the very low-temperature (experimental)
regime. We use this solver to study ordering phenomena in a selection of
exemplary low-symmetry transition-metal oxides: LaMnO3 and rare-earth
manganites as well as the perovskites CaVO3 and YTiO3. We show that spin-flip
and pair hopping terms do not affect the Kugel-Khomskii orbital-order melting
transition in rare-earth manganites, or the suppression of orbital fluctuations
driven by crystal field and Coulomb repulsion. For the Mott insulator YTiO3 we
find a ferromagnetic transition temperature 50 K, in remarkably good agreement
with experiments. For LaMnO3 we show that the classical t2g-spin approximation,
commonly adopted for studying manganites, yields indeed an occupied eg orbital
in very good agreement with that obtained for the full d 5-orbital Hubbard
model, while the spin-spin e_g-t_{2g} correlation function calculated from the
full d model is 0.74, very close to the value expected for aligned eg and t2g
spins; the eg spectral function matrix is also well reproduced. Finally, we
show that the t2g screening reduces the eg-eg Coulomb repulsion by about 10%Comment: 9 pages, 5 figure
Importance of exchange-anisotropy and superexchange for the spin-state transitions in LnCoO3 (Ln=La,Y,RE) cobaltates
Spin-state transitions are the hallmark of rare-earth cobaltates. In order to
understand them, it is essential to identify all relevant parameters which
shift the energy balance between spin states, and determine their trends. We
find that \Delta, the eg-t2g crystal-field splitting, increases by ~250 meV
when increasing pressure to 8 GPa and by about 150 meV when cooling from 1000K
to 5K. It changes, however, by less than 100 meV when La is substituted with
another rare earth. Also the Hund's rule coupling J_avg is about the same in
systems with very different spin-state transition temperature, like LaCoO3 and
EuCoO3. Consequently, in addition to \Delta and J_avg, the Coulomb-exchange
anisotropy \Delta J_ avg and the super-exchange energy-gain \Delta E_SE play a
crucial role, and are comparable with spin-state dependent relaxation effects
due to covalency. We show that in the LnCoO3 series, with Ln=Y or a rare earth
(RE), super-exchange progressively stabilizes a low-spin ground state as the
Ln^{3+} ionic radius decreases. We give a simple model to describe spin-state
transitions and show that, at low temperature, the formation of isolated
high-spin/low-spin pairs is favored, while in the high-temperature phase, the
most likely homogeneous state is high-spin, rather than intermediate spin. An
orbital-selective Mott state could be a fingerprint of such a state.Comment: 10 pages, 9 figure
Dynamical mean-field theory of correlated electrons: lecture notes of the Autumn School on Correlated Electrons 2022; Forschungszentrum Jülich, 4-7 October 2022
Origin of Jahn-Teller distortion and orbital-order in LaMnO3
The origin of the cooperative Jahn-Teller distortion and orbital-order in
LaMnO3 is central to the physics of the manganites. The question is complicated
by the simultaneous presence of tetragonal and GdFeO3-type distortions and the
strong Hund's rule coupling between e_g and t_2g electrons. To clarify the
situation we calculate the transition temperature for the Kugel-Khomskii
superexchange mechanism by using the local density approximation+dynamical
mean-field method, and disentangle the effects of super-exchange from those of
lattice distortions. We find that super-exchange alone would yield T_KK=650 K.
The tetragonal and GdFeO3-type distortions, however, reduce T_KK to 550 K. Thus
electron-phonon coupling is essential to explain the persistence of local
Jahn-Teller distortions to at least 1150 K and to reproduce the occupied
orbital deduced from neutron scattering.Comment: 4 pages, 3 figures; published version (minor changes
Fermi Surface of SrRuO: Spin-Orbit and Anisotropic Coulomb Interaction Effects
The topology of the Fermi surface of Sr2RuO4 is well described by local-density approximation calculations with spin-orbit interaction, but the relative size of its different sheets is not. By accounting for many-body effects via dynamical mean-field theory, we show that the standard isotropic Coulomb interaction alone worsens or does not correct this discrepancy. In order to reproduce experiments, it is essential to account for the Coulomb anisotropy. The latter is small but has strong effects; it competes with the Coulomb-enhanced spin-orbit coupling and the isotropic Coulomb term in determining the Fermi surface shape. Its effects are likely sizable in other correlated multiorbital systems. In addition, we find that the low-energy self-energy matrix—responsible for the reshaping of the Fermi surface—sizably differs from the static Hartree-Fock limit. Finally, we find a strong spin-orbital entanglement; this supports the view that the conventional description of Cooper pairs via factorized spin and orbital part might not apply to Sr2RuO4
Orbital-order melting in rare-earth manganites: the role of super-exchange
We study the mechanism of orbital-order melting observed at temperature T_OO
in the series of rare-earth manganites. We find that many-body super-exchange
yields a transition-temperature T_KK that decreases with decreasing rare-earth
radius, and increases with pressure, opposite to the experimental T_OO. We show
that the tetragonal crystal-field splitting reduces T_KK further increasing the
discrepancies with experiments. This proves that super-exchange effects,
although very efficient, in the light of the experimentally observed trends,
play a minor role for the melting of orbital ordering in rare-earth manganites.Comment: 4 pages, 5 figure
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