676 research outputs found
Magnetic moment suppression in Ba3CoRu2O9: hybridization effect
An unusual orbital state was recently proposed to explain the magnetic and
transport properties of BaCoRuO [Phys. Rev. B. {\bf 85}, 041201
(2012)]. We show that this state contradicts to the first Hund's rule and does
not realize in the system under consideration because of a too small
crystal-field splitting in the shell. A strong suppression of the
local magnetic moment in BaCoRuO is attributed to a strong
hybridization between the Ru 4 and O 2 states.Comment: 5 pages, 5 figure
Electronic Raman scattering in metals: effects of electron-phonon coupling
We report the first systematic measurements of the Raman scattering by
electrons in elemental metals of Al, Mo, Nb, Os, Pb, Re, Ta, Ti, V, W and
metallic compound La. Experimental spectra are modelled on the base of the
band structures, calculated within the density functional theory, taking
properly into account the effects of electron-phonon scattering. The agreement
between our measured and calculated spectra is excellent for the variety of
metals, thus providing estimates for the electron-phonon coupling constants and
temperature-dependent relaxation rates. The method can be applied for other
metallic materials to evaluate an electron-phonon coupling as an alternative to
the transport and optical measurements
Jahn-Teller distortions and charge, orbital and magnetic orders in NaMn7O12
With the use of the band structure calculations we demonstrate that
previously reported [Nat. Materials {\bf 3}, 48 (2004)] experimental crystal
and magnetic structures for NaMnO are inconsistent with each other.
The optimization of the crystal lattice allows us to predict a new crystal
structure for the low temperature phase, which is qualitatively different from
the one presented before. The AFM-CE type of the magnetic order stabilizes the
structure with the elongated, not compressed MnO octahedra,
striking NaMnO out of the list of the anomalous Jahn-Teller systems.
The orbital correlations were shown to exist even in the cubic phase, while the
charge order appears only in the low temperature distorted phase.Comment: 5 page
Covalent bonds against magnetism in transition metal compounds
Magnetism in transition metal compounds is usually considered starting from a
description of isolated ions, as exact as possible, and treating their
(exchange) interaction at a later stage. We show that this standard approach
may break down in many cases, especially in and compounds. We argue
that there is an important intersite effect -- an orbital-selective formation
of covalent metal-metal bonds, which leads to an "exclusion" of corresponding
electrons from the magnetic subsystem, and thus strongly affects magnetic
properties of the system. This effect is especially prominent for noninteger
electron number, when it results in suppression of the famous double exchange,
the main mechanism of ferromagnetism in transition metal compounds. We study
this novel mechanism analytically and numerically and show that it explains
magnetic properties of not only several materials, including
NbOF and BaAlIrO, but can also be operative in
transition metal oxides, e.g. in CrO under pressure. We also discuss the
role of spin-orbit coupling on the competition between covalency and magnetism.
Our results demonstrate that strong intersite coupling may invalidate the
standard single-site starting point for considering magnetism, and can lead to
a qualitatively new behaviour
Ab initio investigation of the exchange interactions in BiFeO: The Cairo pentagonal lattice compound
We present the \emph{ab initio} calculation of the electronic structure and
magnetic properties of BiFeO. This compound crystallizes in the
orthorhombic crystal structure with the Fe ions forming the Cairo
pentagonal lattice implying strong geometric frustration. The neutron
diffraction measurements reveal nearly orthogonal magnetic configuration, which
at first sight is rather unexpected since it does not minimize the total energy
of the pair of magnetic ions coupled by the Heisenberg exchange interaction.
Here we calculate the electronic structure and exchange integrals of Bi2Fe4O9
within the LSDA+U method. We obtain three different in-plane (J3=36 K, J4=73 K,
J5=23 K) and two interplane (J1=10 K, J2=12 K) exchange parameters. The derived
set of exchange integrals shows that the realistic description of Bi2Fe4O9
needs a more complicated model than the ideal Cairo pentagonal lattice with
only two exchange parameters in the plane. However, if one takes into account
only two largest exchange integrals, then according to the ratio x\equiv
J3/J4=0.49<\sqrt{2} (a critical parameter for the ideal Cairo pentagonal
lattice, see. Ref.~1) the ground state should be the orthogonal magnetic
configuration in agreement with experiment. The microscopic origin of different
exchange interactions is also discussed.Comment: 6 pages, 4 figure
How does an interacting many-body system tunnel through a potential barrier to open space?
The tunneling process in a many-body system is a phenomenon which lies at the
very heart of quantum mechanics. It appears in nature in the form of
alpha-decay, fusion and fission in nuclear physics, photoassociation and
photodissociation in biology and chemistry. A detailed theoretical description
of the decay process in these systems is a very cumbersome problem, either
because of very complicated or even unknown interparticle interactions or due
to a large number of constitutent particles. In this work, we theoretically
study the phenomenon of quantum many-body tunneling in a more transparent and
controllable physical system, in an ultracold atomic gas. We analyze a full,
numerically exact many-body solution of the Schr\"odinger equation of a
one-dimensional system with repulsive interactions tunneling to open space. We
show how the emitted particles dissociate or fragment from the trapped and
coherent source of bosons: the overall many-particle decay process is a quantum
interference of single-particle tunneling processes emerging from sources with
different particle numbers taking place simultaneously. The close relation to
atom lasers and ionization processes allows us to unveil the great relevance of
many-body correlations between the emitted and trapped fractions of the
wavefunction in the respective processes.Comment: 18 pages, 4 figures (7 pages, 2 figures supplementary information
Localized itinerant electrons and unique magnetic properties of SrRu2O6
SrRu2O6 has unique magnetic properties. It is characterized by a very high
N\'eel temperature, despite its quasi-two-dimensional structure, and has a
magnetic moment more than twice reduced compared to the formal ionic count.
First principles calculations show that only an ideal Neel ordering in the Ru
plane is possible, with no other metastable magnetic solutions, and, highly
unusually, yield dielectric gaps for both antiferromagnetic and nonmagnetic
states. We demonstrate that this strange behavior is the result of the
formation of very specific electronic objects, recently suggested for a
geometrically similar Na2IrO3 compound, whereby each electron is well localized
on a particular Ru6 hexagon, and completely delocalized over the corresponding
six Ru sites, thus making the compound strongly localized and highly
itinerant
Suppression of magnetism in Ba5AlIr2O11: interplay of Hund's coupling, molecular orbitals and spin-orbit interaction
The electronic and magnetic properties of BaAlIrO containing
Ir-Ir dimers are investigated using the GGA and GGA+SOC calculations. We found
that strong suppression of the magnetic moment in this compound recently found
in [J. Terzic {\it et al.}, Phys. Rev. B {\bf 91}, 235147 (2015)] is not due to
charge-ordering, but is related to the joint effect of the spin-orbit
interaction and strong covalency, resulting in the formation of metal-metal
bonds. They conspire and act against the intra-atomic Hund's rule exchange
interaction to reduce total magnetic moment of the dimer. We argue that the
same mechanism could be relevant for other and dimerized transition
metal compounds
Theoretical prediction of Jahn-Teller distortions and orbital ordering in Cs2CuCl2Br2
With the use of the density function calculations we show that the actual
crystal structure of CsCuClBr should contain elongated in the
plane CuClBr octahedra, in contrast to the experimentally observed
compression in direction. We also predict that the spins on Cu ions
should be ferromagnetically ordered in plane, while the exchange
interaction along direction is small and its sign is uncertain.Comment: 4 pages, 3 figure
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