3,521 research outputs found
Mott transition and suppression of orbital fluctuations in orthorhombic 3 perovskites
Using Wannier-functions, a low-energy Hamiltonian is derived for
orthorhombic transition-metal oxides. Electronic correlations are
treated with a new implementation of dynamical mean-field theory for non-cubic
systems. Good agreement with photoemission data is obtained. The interplay of
correlation effects and cation covalency (GdFeO-type distortions) is
found to suppress orbital fluctuations in LaTiO and even more in
YTiO, and to favor the transition to the insulating state.Comment: 4 pages, 3 figures; revised manuscrip
Local magnetic moments in iron and nickel at ambient and Earth's core conditions
Some Bravais lattices have a particular geometry that can slow down the
motion of Bloch electrons by pre-localization due to the band-structure
properties. Another known source of electronic localization in solids is the
Coulomb repulsion in partially filled d- or f-orbitals, which leads to the
formation of local magnetic moments. The combination of these two effects is
usually considered of little relevance to strongly correlated materials. Here
we show that it represents, instead, the underlying physical mechanism in two
of the most important ferromagnets: nickel and iron. In nickel, the van Hove
singularity has an unexpected impact on the magnetism. As a result, the
electron-electron scattering rate is linear in temperature, in violation of the
conventional Landau theory of metals. This is true even at Earth's core
pressures, at which iron is instead a good Fermi liquid. The importance of
nickel in models of geomagnetism may have therefore to be reconsidered.Comment: Supplementary Information available at
https://www.nature.com/articles/ncomms16062#supplementary-informatio
Correlation effects on the electronic structure of TiOCl: a NMTO+DMFT study
Using the recently developed N-th order muffin-tin orbital-based downfolding
technique in combination with the Dynamical Mean Field theory, we investigate
the electronic properties of the much discussed Mott insulator TiOCl in the
undimerized phase. Inclusion of correlation effects through this approach
provides a description of the spectral function into an upper and a lower
Hubbard band with broad valence states formed out of the orbitally polarized,
lower Hubbard band. We find that these results are in good agreement with
recent photo-emission spectra.Comment: 4 pages, 3 figure
On the mechanism for orbital-ordering in KCuF3
The Mott insulating perovskite KCuF3 is considered the archetype of an
orbitally-ordered system. By using the LDA+dynamical mean-field theory (DMFT)
method, we investigate the mechanism for orbital-ordering (OO) in this
material. We show that the purely electronic Kugel-Khomskii super-exchange
mechanism (KK) alone leads to a remarkably large transition temperature of T_KK
about 350 K. However, orbital-order is experimentally believed to persist to at
least 800 K. Thus Jahn-Teller distortions are essential for stabilizing
orbital-order at such high temperatures.Comment: 4 pages, 5 figure
Spin-density fluctuations and the fluctuation-dissipation theorem in 3d ferromagnetic metals
Spatial and time scales of spin density fluctuations (SDF) were analyzed in
3d ferromagnets using ab initio linear response calculations of complete
wavevector and energy dependence of the dynamic spin susceptibility tensor. We
demonstrate that SDF are spread continuously over the entire Brillouin zone and
while majority of them reside within the 3d bandwidth, a significant amount
comes from much higher energies. A validity of the adiabatic approximation in
spin dynamics is discussed. The SDF spectrum is shown to have two main
constituents: a minor low-energy spin wave contribution and a much larger
high-energy component from more localized excitations. Using the
fluctuation-dissipation theorem (FDT), the on-site spin correlator (SC) and the
related effective fluctuating moment were properly evaluated and their
universal dependence on the 3d band population is further discussed
Diagrammatic routes to nonlocal correlations beyond dynamical mean field theory
Strong electronic correlations pose one of the biggest challenges to solid
state theory. We review recently developed methods that address this problem by
starting with the local, eminently important correlations of dynamical mean
field theory (DMFT). On top of this, non-local correlations on all length
scales are generated through Feynman diagrams, with a local two-particle vertex
instead of the bare Coulomb interaction as a building block. With these
diagrammatic extensions of DMFT long-range charge-, magnetic-, and
superconducting fluctuations as well as (quantum) criticality can be addressed
in strongly correlated electron systems. We provide an overview of the
successes and results achieved---hitherto mainly for model Hamiltonians---and
outline future prospects for realistic material calculations.Comment: 60 pages, 42 figures, replaced by the version to be published in Rev.
Mod. Phys. 201
Functional renormalization group study of an eight-band model for the iron arsenides
We investigate the superconducting pairing instabilities of eight-band models
for the iron arsenides. Using a functional renormalization group treatment, we
determine how the critical energy scale for superconductivity depends on the
electronic band structure. Most importantly, if we vary the parameters from
values corresponding to LaFeAsO to SmFeAsO, the pairing scale is strongly
enhanced, in accordance with the experimental observation. We analyze the
reasons for this trend and compare the results of the eight-band approach to
those found using five-band models.Comment: 11 pages, 10 figure
Magnetic susceptibility, exchange interactions and spin-wave spectra in the local spin density approximation
Starting from exact expression for the dynamical spin susceptibility in the
time-dependent density functional theory a controversial issue about exchange
interaction parameters and spin-wave excitation spectra of itinerant electron
ferromagnets is reconsidered. It is shown that the original expressions for
exchange integrals based on the magnetic force theorem (J. Phys. F14 L125
(1984)) are optimal for the calculations of the magnon spectrum whereas static
response function is better described by the ``renormalized'' magnetic force
theorem by P. Bruno (Phys. Rev. Lett. 90, 087205 (2003)). This conclusion is
confirmed by the {\it ab initio} calculations for Fe and Ni.Comment: 12 pages, 2 figures, submitted to JPC
Non-perturbative model and ferromagnetism in dilute magnets
We calculate magnetic couplings in the model for dilute magnets, in
order both to identify the relevant parameters which control ferromagnetism and
also to bridge the gap between first principle calculations and model
approaches. The magnetic exchange interactions are calculated
non-perturbatively and disorder in the configuration of impurities is treated
exacly, allowing us to test the validity of effective medium theories.
Results differ qualitatively from those of weak coupling. In contrast to mean
field theory, increasing may not favor high Curie temperatures:
scales primarily with the bandwidth. High temperature ferromagnetism at small
dilutions is associated with resonant structure in the p-band. Comparison to
diluted magnetic semiconductors indicate that Ga(Mn)As has such a resonant
structure and thus this material is already close to optimality.Comment: 4 pages, 4 Figure
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