345 research outputs found
Orbital fluctuations in the different phases of LaVO3 and YVO3
We investigate the importance of quantum orbital fluctuations in the
orthorhombic and monoclinic phases of the Mott insulators LaVO3 and YVO3.
First, we construct ab-initio material-specific t2g Hubbard models. Then, by
using dynamical mean-field theory, we calculate the spectral matrix as a
function of temperature. Our Hubbard bands and Mott gaps are in very good
agreement with spectroscopy. We show that in orthorhombic LaVO3, quantum
orbital fluctuations are strong and that they are suppressed only in the
monoclinic 140 K phase. In YVO3 the suppression happens already at 300 K. We
show that Jahn-Teller and GdFeO3-type distortions are both crucial in
determining the type of orbital and magnetic order in the low temperature
phases.Comment: 4 pages, 3 figures, final version. To appear in PR
Many-body models for molecular nanomagnets
We present a flexible and effective ab-initio scheme to build many-body
models for molecular nanomagnets, and to calculate magnetic exchange couplings
and zero-field splittings. It is based on using localized Foster-Boys orbitals
as one-electron basis. We apply this scheme to three paradigmatic systems, the
antiferromagnetic rings Cr8 and Cr7Ni and the single molecule magnet Fe4. In
all cases we identify the essential magnetic interactions and find excellent
agreement with experiments.Comment: 5 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
Nature of the Mott transition in Ca2RuO4
We study the origin of the temperature-induced Mott transition in Ca2RuO4. As
a method we use the local-density approximation+dynamical mean-field theory. We
show the following. (i) The Mott transition is driven by the change in
structure from long to short c-axis layered perovskite (L-Pbca to S-Pbca); it
occurs together with orbital order, which follows, rather than produces, the
structural transition. (ii) In the metallic L-Pbca phase the orbital
polarization is ~0. (iii) In the insulating S-Pbca phase the lower energy
orbital, ~xy, is full. (iv) The spin-flip and pair-hopping Coulomb terms reduce
the effective masses in the metallic phase. Our results indicate that a similar
scenario applies to Ca_{2-x}Sr_xRuO_4 (x<0.2). In the metallic x< 0.5
structures electrons are progressively transferred to the xz/yz bands with
increasing x, however we find no orbital-selective Mott transition down to ~300
K.Comment: 4 pages, 3 figures; published versio
Electronic Structure Calculations with LDA+DMFT
The LDA+DMFT method is a very powerful tool for gaining insight into the
physics of strongly correlated materials. It combines traditional ab-initio
density-functional techniques with the dynamical mean-field theory. The core
aspects of the method are (i) building material-specific Hubbard-like many-body
models and (ii) solving them in the dynamical mean-field approximation. Step
(i) requires the construction of a localized one-electron basis, typically a
set of Wannier functions. It also involves a number of approximations, such as
the choice of the degrees of freedom for which many-body effects are explicitly
taken into account, the scheme to account for screening effects, or the form of
the double-counting correction. Step (ii) requires the dynamical mean-field
solution of multi-orbital generalized Hubbard models. Here central is the
quantum-impurity solver, which is also the computationally most demanding part
of the full LDA+DMFT approach. In this chapter I will introduce the core
aspects of the LDA+DMFT method and present a prototypical application.Comment: 21 pages, 7 figures. Chapter of "Many-Electron Approaches in Physics,
Chemistry and Mathematics: A Multidisciplinary View", eds. V. Bach and L.
Delle Site, Springer 201
Band structure and optical properties of opal photonic crystals
A theoretical approach for the interpretation of reflectance spectra of opal
photonic crystals with fcc structure and (111) surface orientation is
presented. It is based on the calculation of photonic bands and density of
states corresponding to a specified angle of incidence in air. The results
yield a clear distinction between diffraction in the direction of light
propagation by (111) family planes (leading to the formation of a stop band)
and diffraction in other directions by higher-order planes (corresponding to
the excitation of photonic modes in the crystal). Reflectance measurements on
artificial opals made of self-assembled polystyrene spheres are analyzed
according to the theoretical scheme and give evidence of diffraction by
higher-order crystalline planes in the photonic structure.Comment: to appear in PR
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
Frustration driven structural distortion in VOMoO4
Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR),
magnetization measurements and electronic structure calculations in VOMoO4 are
presented. It is found that VOMoO4 is a frustrated two-dimensional
antiferromagnet on a square lattice with competing exchange interactions along
the side J1 and the diagonal J2 of the square. From magnetization measurements
J1+J2 is estimated around 155 K, in satisfactory agreement with the values
derived from electronic structure calculations. Around 100 K a structural
distortion, possibly driven by the frustration, is evidenced. This distortion
induces significant modifications in the NMR and EPR spectra which can be
accounted for by valence fluctuations. The analysis of the spectra suggests
that the size of the domains where the lattice is distorted progressively grows
as the temperature approaches the transition to the magnetic ground state at
Tc=42 K
The electronic structures and magnetic properties of perovskite ruthenates from constrained orbital hybridization calculations
We introduce a method to analyze the effect of hybridization by shifting
corresponding atomic levels using external potentials. Based on this approach,
we study perovskite ruthenates,\ and unambiguously identify that the covalency
between the \textit{A}-site cation and O ion will modify the Ru-O hybridization
and change the density of state at Fermi level, consequently affect the
magnetic properties significantly. We also study the effect of pressure and
reveal that hydrostatic pressure has a small effect on the Ru-O-Ru bond angle
of SrRuO, while it will decrease the Ru-O length and increase the band
width significantly. Therefore, the magnetic ordering temperature will decrease
monotonically with pressure
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