552 research outputs found
Covalent bonding and hybridization effects in the corundum-type transition-metal oxides V2O3 and Ti2O3
The electronic structure of the corundum-type transition-metal oxides V2O3
and Ti2O3 is studied by means of the augmented spherical wave method, based on
density-functional theory and the local density approximation. Comparing the
results for the vanadate and the titanate allows us to understand the peculiar
shape of the metal 3d a_{1g} density of states, which is present in both
compounds. The a_{1g} states are subject to pronounced bonding-antibonding
splitting due to metal-metal overlap along the c-axis of the corundum
structure. However, the corresponding partial density of states is strongly
asymmetric with considerably more weight on the high energy branch. We argue
that this asymmetry is due to an unexpected broadening of the bonding a_{1g}
states, which is caused by hybridization with the e_g^{pi} bands. In contrast,
the antibonding a_{1g} states display no such hybridization and form a sharp
peak. Our results shed new light on the role of the a_{1g} orbitals for the
metal-insulator transitions of V2O3. In particular, due to a_{1g} - e_g^{pi}
hybridization, an interpretation in terms of molecular orbital singlet states
on the metal-metal pairs along the c-axis is not an adequate description.Comment: 7 pages, 3 figures, more information at
http://www.physik.uni-augsburg.de/~eyert
Electronic structure of spinel-type LiV_2O_4
The band structure of the cubic spinel compound LiV_2O_4, which has been
reported recently to show heavy Fermion behavior, has been calculated within
the local-density approximation using a full-potential version of the linear
augmented-plane-wave method. The results show that partially-filled V 3d bands
are located about 1.9 eV above the O 2p bands and the V 3d bands are split into
a lower partially-filled t_{2g} complex and an upper unoccupied e_{g} manifold.
The fact that the conduction electrons originate solely from the t_{2g} bands
suggests that the mechanism for the mass enhancement in this system is
different from that in the 4f heavy Fermion systems, where these effects are
attributed to the hybridization between the localized 4f levels and itinerant
spd bands.Comment: 5 pages, revte
Doping Dependence of the Electronic Structure of Ba_{1-x}K_{x}BiO_{3} Studied by X-Ray Absorption Spectroscopy
We have performed x-ray absorption spectroscopy (XAS) and x-ray photoemission
spectroscopy (XPS) studies of single crystal Ba_{1-x}K_{x}BiO_{3} (BKBO)
covering the whole composition range . Several features in
the oxygen 1\textit{s} core XAS spectra show systematic changes with .
Spectral weight around the absorption threshold increases with hole doping and
shows a finite jump between and 0.40, which signals the
metal-insulator transition. We have compared the obtained results with
band-structure calculations. Comparison with the XAS results of
BaPb_{1-x}Bi_{x}O_{3} has revealed quite different doping dependences between
BKBO and BPBO. We have also observed systematic core-level shifts in the XPS
spectra as well as in the XAS threshold as functions of , which can be
attributed to a chemical potential shift accompanying the hole doping. The
observed chemical potential shift is found to be slower than that predicted by
the rigid band model based on the band-structure calculations.Comment: 8 pages, 8 figures include
On the nature of the magnetic ground-state wave function of V_2O_3
After a brief historical introduction, we dwell on two recent experiments in
the low-temperature, monoclinic phase of V_2O_3: K-edge resonant x-ray
scattering and non-reciprocal linear dichroism, whose interpretations are in
conflict, as they require incompatible magnetic space groups. Such a conflict
is critically reviewed, in the light of the present literature, and new
experimental tests are suggested, in order to determine unambiguously the
magnetic group. We then focus on the correlated, non-local nature of the
ground-state wave function, that is at the basis of some drawbacks of the LDA+U
approach: we singled out the physical mechanism that makes LDA+U unreliable,
and indicate the way out for a possible remedy. Finally we explain, by means of
a symmetry argument related to the molecular wave function, why the magnetic
moment lies in the glide plane, even in the absence of any local symmetry at
vanadium sites.Comment: 7 pages, 1 figur
The Metal-Insulator Transition of the Magneli phase V_4O_7: Implications for V_2O_3
The metal-insulator transition (MIT) of the Magneli phase V_4O_7 is studied
by means of electronic structure calculations using the augmented spherical
wave method. The calculations are based on density functional theory and the
local density approximation. Changes of the electronic structure at the MIT are
discussed in relation to the structural transformations occuring
simultaneously. The analysis is based on a unified point of view of the crystal
structures of all Magneli phase compounds V_nO_2n-1 (3 =< n =< 9) as well as of
VO_2 and V_2O_3. This allows to group the electronic bands into states behaving
similar to the dioxide or the sesquioxide. In addition, the relationship
between the structural and electronic properties near the MIT of these oxides
can be studied on an equal footing. For V_4O_7, a strong influence of
metal-metal bonding across octahedral faces is found for states both parallel
and perpendicular to the hexagonal c_hex axis of V_2O_3. Furthermore, the
structural changes at the MIT cause localization of those states, which mediate
in-plane metal-metal bonding via octahedral edges. This band narrowing opens
the way to an increased influence of electronic correlations, which are
regarded as playing a key role for the MIT of V_2O_3.Comment: 7 pages, 3 figures, more information at
http://www.physik.uni-augsburg.de/~eyert
New magnetic phase in metallic V_{2-y}O_3 close to the metal insulator transition
We have observed two spin density wave (SDW) phases in hole doped metallic
V_{2-y}O_3, one evolves from the other as a function of doping, pressure or
temperature. They differ in their response to an external magnetic field, which
can also induce a transition between them. The phase boundary between these two
states in the temperature-, doping-, and pressure-dependent phase diagram has
been determined by magnetization and magnetotransport measurements. One phase
exists at high doping level and has already been described in the literature.
The second phase is found in a small parameter range close to the boundary to
the antiferromagnetic insulating phase (AFI). The quantum phase transitions
between these states as a function of pressure and doping and the respective
metamagnetic behavior observed in these phases are discussed in the light of
structurally induced changes of the band structure.Comment: REVTeX, 8 pages, 12 EPS figures, submitted to PR
Semimetalic antiferromagnetism in the half-Heusler compound CuMnSb
The half-Heusler compound CuMnSb, the first antiferromagnet (AFM) in the
Mn-based class of Heuslers and half-Heuslers that contains several conventional
and half metallic ferromagnets, shows a peculiar stability of its magnetic
order in high magnetic fields. Density functional based studies reveal an
unusual nature of its unstable (and therefore unseen) paramagnetic state, which
for one electron less (CuMnSn, for example) would be a zero gap semiconductor
(accidentally so) between two sets of very narrow, topologically separate bands
of Mn 3d character. The extremely flat Mn 3d bands result from the environment:
Mn has four tetrahedrally coordinated Cu atoms whose 3d states lie well below
the Fermi level, and the other four tetrahedrally coordinated sites are empty,
leaving chemically isolated Mn 3d states. The AFM phase can be pictured
heuristically as a self-doped CuMnSb compensated semimetal
with heavy mass electrons and light mass holes, with magnetic coupling
proceeding through Kondo and/or antiKondo coupling separately through the two
carrier types. The ratio of the linear specific heat coefficient and the
calculated Fermi level density of states indicates a large mass enhancement
, or larger if a correlated band structure is taken as the
reference
Vertex-corrected tunneling inversion in superconductors: Pb
The McMillan-Rowell tunneling inversion program, which extracts the
electron-phonon spectral function and the Coulomb
pseudopotential from experimental tunneling data, is generalized to
include the lowest-order vertex correction. We neglect the momentum dependence
of the electron-phonon matrix elements, which is equivalent to using a local
approximation. The perturbation theory is performed on the imaginary axis and
then an exact analytic continuation is employed to produce the density of
states on the real axis. Comparison is made with the experimental data for Pb.Comment: 14 pages, typeset in ReVTeX, including three encapsulated postscript
figure
On possible superconductivity in the doped ladder compound La_(1-x)Sr_xCuO_2.5
LaCuO_2.5 is a system of coupled, two-chain, cuprate ladders which may be
doped systematically by Sr substitution. Motivated by the recent synthesis of
single crystals, we investigate theoretically the possibility of
superconductivity in this compound. We use a model of spin fluctuation-mediated
superconductivity, where the pairing potential is strongly peaked at \pi in the
ladder direction. We solve the coupled gap equations on the bonding and
antibonding ladder bands to find superconducting solutions across the range of
doping, and discuss their relevance to the real material.Comment: RevTex, 4 pages, 7 figure
Tight-binding parameters and exchange integrals of Ba_2Cu_3O_4Cl_2
Band structure calculations for Ba_2Cu_3O_4Cl_2 within the local density
approximation (LDA) are presented. The investigated compound is similar to the
antiferromagnetic parent compounds of cuprate superconductors but contains
additional Cu_B atoms in the planes. Within the LDA, metallic behavior is found
with two bands crossing the Fermi surface (FS). These bands are built mainly
from Cu 3d_{x^2-y^2} and O 2p_{x,y} orbitals, and a corresponding tight-binding
(TB) model has been parameterized. All orbitals can be subdivided in two sets
corresponding to the A- and B-subsystems, respectively, the coupling between
which is found to be small. To describe the experimentally observed
antiferromagnetic insulating state, we propose an extended Hubbard model with
the derived TB parameters and local correlation terms characteristic for
cuprates. Using the derived parameter set we calculate the exchange integrals
for the Cu_3O_4 plane. The results are in quite reasonable agreement with the
experimental values for the isostructural compound Sr_2Cu_3O_4Cl_2.Comment: 5 pages (2 tables included), 4 ps-figure
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