4,638 research outputs found
Magnetic interaction at an interface between manganite and other transition metal oxides
A general consideration is presented for the magnetic interaction at an
interface between a perovskite manganite and other transition metal oxides. The
latter is specified by the electron number in the level as
. Based on the molecular orbitals formed at the interface and
the generalized Hund's rule, the sign of the magnetic interaction is rather
uniquely determined. The exception is when the orbital is
stabilized in the interfacial manganite layer neighboring to a
or system. In this case, the magnetic
interaction is sensitive to the occupancy of the Mn orbital. It
is also shown that the magnetic interaction between the interfacial Mn layer
and the bulk region can be changed. Manganite-based heterostructures thus show
a rich magnetic behavior. We also present how to generalize the argument
including orbitals.Comment: 7pages, 4 figures, 1 tabl
Ferromagnetism and orbital order in a topological ferroelectric
We explore via density functional calculations the magnetic doping of a
topological ferroelectric as an unconventional route to multiferroicity.
Vanadium doping of the layered perovskite LaTiO largely
preserves electric polarization and produces robust ferromagnetic order, hence
proper multiferroicity. The marked tendency of dopants to cluster into chains
results in an insulating character at generic doping. Ferromagnetism stems from
the symmetry breaking of the multi-orbital V system via an unusual
"antiferro"-orbital order, and from the host's low-symmetry layered structure.Comment: 4 pages, 3 figures; Physical Review Letters 109, in print (2012
Electron Confinement, Orbital Ordering, and Orbital Moments in - Oxide Heterostructures
The (SrTiO)/(SrVO) multilayer system is studied
with first principles methods through the observed insulator-to-metal
transition with increasing thickness of the SrVO layer. When correlation
effects with reasonable magnitude are included, crystal field splittings from
the structural relaxations together with spin-orbit coupling (SOC) determines
the behavior of the electronic and magnetic structures. These confined slabs of
SrVO prefer =() orbital ordering of and
() orbitals within the plane, accompanied by
=(0,0) spin order (ferromagnetic alignment). The result is a
SOC-driven ferromagnetic Mott insulator. The orbital moment of 0.75
strongly compensates the spin moment on the sublattice. The
insulator-metal transition for (occurring between =4 and
=5) is reproduced. Unlike in the isoelectronic TiO/VO
(rutile structure) system and in spite of some similarities in orbital
ordering, no semi-Dirac point [{\it Phys. Rev. Lett.} {\bf 102}, 166803 (2009)]
is encountered, but the insulator-to-metal transition occurs through a
different type of unusual phase. For n=5 this system is very near (or at) a
unique semimetallic state in which the Fermi energy is topologically determined
and the Fermi surface consists of identical electron and hole Fermi circles
centered at =0. The dispersion consists of what can be regarded as a
continuum of radially-directed Dirac points, forming a "Dirac circle".Comment: 9 pages, 8 figure
Phase Competition in Ln0.5a0.5mno3 Perovskites
Single crystals of the systems Pr0.5(Ca1-xSrx)0.5MnO3,
(Pr1-yYy)0.5(Ca1-xSrx)0.5MnO3, and Sm0.5Sr0.5MnO3 were grown to provide a
series of samples with fixed ratio Mn(III)/Mn(IV)=1 having geometric tolerance
factors that span the transition from localized to itinerant electronic
behavior of the MnO3 array. A unique ferromagnetic phase appears at the
critical tolerance factor tc= 0.975 that separates charge ordering and
localized-electron behavior for t<tc from itinerant or molecular-orbital
behavior for t>tc. This ferromagnetic phase, which has to be distinguished from
the ferromagnetic metallic phase stabilized at tolerance factors t>tc,
separates two distinguishable Type-CE antiferromagnetic phases that are
metamagnetic. Measurements of the transport properties under hydrostatic
pressure were carried out on a compositions t a little below tc in order to
compare the effects of chemical vs. hydrostatic pressure on the phases that
compete with one another near t=tc.Comment: 10 pages. To be publised in Phys. Rev.
No Indications of Axion-Like Particles From Fermi
As very high energy (~100 GeV) gamma rays travel over cosmological distances,
their flux is attenuated through interactions with the extragalactic background
light. Observations of distant gamma ray sources at energies between ~200 GeV
and a few TeV by ground-based gamma ray telescopes such as HESS, however,
suggest that the universe is more transparent to very high energy photons than
had been anticipated. One possible explanation for this is the existence of
axion-like-particles (ALPs) which gamma rays can efficiently oscillate into,
enabling them to travel cosmological distances without attenuation. In this
article, we use data from the Fermi Gamma Ray Space Telescope to calculate the
spectra at 1-100 GeV of two gamma ray sources, 1ES1101-232 at redshift z=0.186
and H2356-309 at z=0.165, and use this in conjunction with the measurements of
ground-based telescopes to test the ALP hypothesis. We find that the
observations can be well-fit by an intrinsic power-law source spectrum with
indices of -1.72 and -2.1 for 1ES1101-232 and H2356-309, respectively, and that
no ALPs or other exotic physics is necessary to explain the observed degree of
attenuation.Comment: 7 pages, 4 figures. v3: Matches published version, the analysis of
H2356-309 is revised, no change in conclusion
Exchange parameters from approximate self-interaction correction scheme
The approximate atomic self-interaction corrections (ASIC) method to density
functional theory is put to the test by calculating the exchange interaction
for a number of prototypical materials, critical to local exchange and
correlation functionals. ASIC total energy calculations are mapped onto an
Heisenberg pair-wise interaction and the exchange constants J are compared to
those obtained with other methods. In general the ASIC scheme drastically
improves the bandstructure, which for almost all the cases investigated
resemble closely available photo-emission data. In contrast the results for the
exchange parameters are less satisfactory. Although ASIC performs reasonably
well for systems where the magnetism originates from half-filled bands, it
suffers from similar problems than those of LDA for other situations. In
particular the exchange constants are still overestimated. This reflects a
subtle interplay between exchange and correlation energy, not captured by the
ASIC.Comment: 10 page
First-principles study of ferroelectric domain walls in multiferroic bismuth ferrite
We present a first-principles density functional study of the structural,
electronic and magnetic properties of the ferroelectric domain walls in
multiferroic BiFeO3. We find that domain walls in which the rotations of the
oxygen octahedra do not change their phase when the polarization reorients are
the most favorable, and of these the 109 degree domain wall centered around the
BiO plane has the lowest energy. The 109 degree and 180 degree walls have a
significant change in the component of their polarization perpendicular to the
wall; the corresponding step in the electrostatic potential is consistent with
a recent report of electrical conductivity at the domain walls. Finally, we
show that changes in the Fe-O-Fe bond angles at the domain walls cause changes
in the canting of the Fe magnetic moments which can enhance the local
magnetization at the domain walls.Comment: 9 pages, 20 figure
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