369 research outputs found
Insulating state and the importance of the spin-orbit coupling in CaCoRhO
We have carried out a comparative theoretical study of the electronic
structure of the novel one-dimensional CaCoRhO and CaFeRhO
systems. The insulating antiferromagnetic state for the CaFeRhO can be
well explained by band structure calculations with the closed shell high-spin
(Fe) and low-spin (Rh) configurations. We
found for the CaCoRhO that the Co has a strong tendency to be
(Co) rather than (Co), and that there is an orbital
degeneracy in the local Co electronic structure. We argue that it is the
spin-orbit coupling which will lift this degeneracy thereby enabling local spin
density approximation + Hubbard U (LSDA+U) band structure calculations to
generate the band gap. We predict that the orbital contribution to the magnetic
moment in CaCoRhO is substantial, i.e. significantly larger than 1
per formula unit. Moreover, we propose a model for the contrasting
intra-chain magnetism in both materials.Comment: 7 pages, 4 figures, and 1 tabl
Nature of magnetism in CaCoO
We find using LSDA+U band structure calculations that the novel
one-dimensional cobaltate CaCoO is not a ferromagnetic half-metal
but a Mott insulator. Both the octahedral and the trigonal Co ions are formally
trivalent, with the octahedral being in the low-spin and the trigonal in the
high-spin state. The inclusion of the spin-orbit coupling leads to the
occupation of the minority-spin orbital for the unusually coordinated
trigonal Co, producing a giant orbital moment (1.57 ). It also results
in an anomalously large magnetocrystalline anisotropy (of order 70 meV),
elucidating why the magnetism is highly Ising-like. The role of the oxygen
holes, carrying an induced magnetic moment of 0.13 per oxygen, for
the exchange interactions is discussed.Comment: 5 pages, 4 figures, and 1 tabl
Field-controlled phase separation at the impurity-induced magnetic ordering in the spin-Peierls magnet CuGeO3
The fraction of the paramagnetic phase surviving at the impurity-induced
antiferromagnetic order transition of the doped spin-Peierls magnet
Cu(1-x)Mg(x)GeO3 (x < 5%) is found to increase with an external magnetic field.
This effect is qualitatively explained by the competition of Zeeman energy and
exchange interaction between local antiferromagnetic clustersComment: 4 pages 4 figure
Transmission phase lapses in quantum dots: the role of dot-lead coupling asymmetry
Lapses of transmission phase in transport through quantum dots are ubiquitous
already in the absence of interaction, in which case their precise location is
determined by the signs and magnitudes of the tunnelling matrix elements.
However, actual measurements for a quantum dot embedded in an Aharonov-Bohm
interferometer show systematic sequences of phase lapses separated by Coulomb
peaks -- an issue that attracted much attention and generated controversy.
Using a two-level quantum dot as an example we show that this phenomenon can be
accounted for by the combined effect of asymmetric dot-lead couplings (left
lead/right lead asymmetry as well as different level broadening for different
levels) and interaction-induced "population switching" of the levels, rendering
this behaviour generic. We construct and analyse a mean field scheme for an
interacting quantum dot, and investigate the properties of the mean field
solution, paying special attention to the character of its dependence
(continuous vs. discontinuous) on the chemical potential or gate voltage.Comment: 34 LaTeX pages in IOP format, 9 figures; misprints correcte
Shifts of the nuclear resonance in the vortex lattice in YBaCuO
The NMR and NQR spectra of Cu in the CuO plane of
YBaCuO in the superconducting state are discussed in terms of the
phenomenological theory of Ginzburg-Landau type extended to lower temperatures.
We show that the observed spectra, Kumagai {\em et al.}, PRB {\bf 63}, 144502
(2001), can be explained by a standard theory of the Bernoulli potential with
the charge transfer between CuO planes and CuO chains assumed.Comment: 11 pages 7 figure
Phase diagram of disordered spin-Peierls systems
We study the competition between the spin-Peierls and the antiferromagnetic
ordering in disordered quasi-one-dimensional spin systems. We obtain the
temperature vs disorder-strength phase diagram, which qualitatively agrees with
recent experiments on doped CuGeO_3.Comment: 4 pages, revtex, epsf, 2 Postscript figure
Na2IrO3 as a molecular orbital crystal
Contrary to previous studies that classify Na2IrO3 as a realization of the
Heisenberg-Kitaev model with dominant spin-orbit coupling, we show that this
system represents a highly unusual case in which the electronic structure is
dominated by the formation of quasi-molecular orbitals (QMOs), with substantial
quenching of the orbital moments. The QMOs consist of six atomic orbitals on an
Ir hexagon, but each Ir atom belongs to three different QMOs. The concept of
such QMOs in solids invokes very different physics compared to the models
considered previously. Employing density functional theory calculations and
model considerations we find that both the insulating behavior and the
experimentally observed zigzag antiferromagnetism in Na2IrO3 naturally follow
from the QMO model.Comment: Final version, accepted by PR
Orbital-quenching-induced magnetism in Ba_2NaOsO_6
The double perovskite \bnoo with heptavalent Os () is observed to remain
in the ideal cubic structure ({\it i.e.} without orbital ordering) despite
single occupation of the orbitals, even in the ferromagnetically
ordered phase below 6.8 K. Analysis based on the {\it ab initio} dispersion
expressed in terms of an Os -based Wannier function picture, spin-orbit
coupling, Hund's coupling, and strong Coulomb repulsion shows that the magnetic
OsO cluster is near a moment-less condition due to spin and orbital
compensation. Quenching (hybridization) then drives the emergence of the small
moment. This compensation, unprecedented in transition metals, arises in a
unified picture that accounts for the observed Mott insulating behavior.Comment: in press at Europhysics Letter
Orbital-assisted metal-insulator transition in VO
We found direct experimental evidence for an orbital switching in the V 3d
states across the metal-insulator transition in VO. We have used
soft-x-ray absorption spectroscopy at the V edges as a sensitive
local probe, and have determined quantitatively the orbital polarizations.
These results strongly suggest that, in going from the metallic to the
insulating state, the orbital occupation changes in a manner that charge
fluctuations and effective band widths are reduced, that the system becomes
more 1-dimensional and more susceptible to a Peierls-like transition, and that
the required massive orbital switching can only be made if the system is close
to a Mott insulating regime
Orbitally driven spin-singlet dimerization in =1 LaRuO
Using x-ray absorption spectroscopy at the Ru- edge we reveal that
the Ru ions remain in the =1 spin state across the rare 4d-orbital
ordering transition and spin-gap formation. We find using local spin density
approximation + Hubbard U (LSDA+U) band structure calculations that the crystal
fields in the low temperature phase are not strong enough to stabilize the
=0 state. Instead, we identify a distinct orbital ordering with a
significant anisotropy of the antiferromagnetic exchange couplings. We conclude
that LaRuO appears to be a novel material in which the
orbital physics drives the formation of spin-singlet dimers in a quasi
2-dimensional =1 system.Comment: 5 pages, 4 figures, and 1 tabl
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