103 research outputs found
Unconventional aspects of electronic transport in delafossite oxides
The electronic transport properties of the delafossite oxides ABO are
usually understood in terms of two well separated entities, namely, the
triangular A and (BO) layers. Here we review several cases among
this extensive family of materials where the transport depends on the
interlayer coupling and displays unconventional properties. We review the doped
thermoelectrics based on CuRhO and CuCrO, which show a high-temperature
recovery of Fermi-liquid transport exponents, as well as the highly anisotropic
metals PdCoO, PtCoO and PdCrO where the sheer simplicity of the
Fermi surface leads to unconventional transport. We present some of the
theoretical tools that have been used to investigate these transport properties
and review what can and cannot be learned from the extensive set of electronic
structure calculations that have been performed.Comment: 35 pages, 19 figure
Electronic structure and thermoelectric properties of CuRh(1-x)MgxO2
Electronic structure calculations using the augmented spherical wave method
have been performed for CuRhO2. For this semiconductor crystallizing in the
delafossite structure, it is found that the valence band maximum is mainly due
to the 4d t2g orbitals of Rh^{3+}. The structural characterizations of
CuRh(1-x)MgxO2 show a broad range of Mg^{2+} substitution for Rh^{3+} in this
series, up to about 12%. Measurements of the resistivity and thermopower of the
doped systems show a Fermi liquid-like behavior for temperatures up to about
1000K, resulting in a large weakly temperature dependent power factor. The
thermopower is discussed both within the Boltzmann equation approach as based
on the electronic structure calculations and the temperature independent
correlation functions ratio approximation as based on the Kubo formalism.Comment: 9 pages, 12 figures, more information at
http://www.physik.uni-augsburg.de/~eyert
Long-range magnetic order and spin-lattice coupling in the delafossite CuFeO2
The electronic and magnetic properties of the delafossite CuFeO2 are
investigated by means of electronic structure calculations. They are performed
using density functional theory in the generalized gradient approximation as
well as the new full-potential augmented spherical wave method. The
calculations reveal three different spin states at the iron sites. Taking into
account the correct crystal structure, we find long-range antiferromagnetic
ordering in agreement with experiment. Contrasting previous work, our
calculations show that non-local exchange interactions lead to a semiconducting
ground state.Comment: 5 pages, 5 figures, more information at
http:www.physik.uni-augsburg.de/~eyert
The antiferromagnetic insulator Ca3FeRhO6: characterization and electronic structure calculations
We investigate the antiferromagnetic insulating nature of Ca3FeRhO6 both
experimentally and theoretically. Susceptibility measurements reveal a Neel
temperature T_N = 20 K, and a magnetic moment of 5.3 muB/f. u., while
Moessbauer spectroscopy strongly suggests that the Fe ions, located in trigonal
prismatic sites, are in a 3+ high spin state. Transport measurements display a
simple Arrhenius law, with an activation energy of 0.2 eV. The experimental
results are interpreted with LSDA band structure calculations, which confirm
the Fe 3+ state, the high-spin/low-spin scenario, the antiferromagnetic
ordering, and the value for the activation energy.Comment: 5 pages, 6 figure
The Origin of Magnetic Interactions in Ca3Co2O6
We investigate the microscopic origin of the ferromagnetic and
antiferromagnetic spin exchange couplings in the quasi one-dimensional cobalt
compound Ca3Co2O6. In particular, we establish a local model which stabilizes a
ferromagnetic alignment of the S=2 spins on the cobalt sites with trigonal
prismatic symmetry, for a sufficiently strong Hund's rule coupling on the
cobalt ions. The exchange is mediated through a S=0 cobalt ion at the
octahedral sites of the chain structure. We present a strong coupling
evaluation of the Heisenberg coupling between the S=2 Co spins on a separate
chain. The chains are coupled antiferromagnetically through super-superexchange
via short O-O bonds.Comment: 5 Pages, 3 Figures; added anisotropy term in eq. 9; extended
discussion of phase transitio
Fermi surface of MoO2 studied by angle-resolved photoemission spectroscopy, de Haas-van Alphen measurements, and electronic structure calculations
A comprehensive study of the electronic properties of monoclinic MoO2 from
both an experimental and a theoretical point of view is presented. We focus on
the investigation of the Fermi body and the band structure using angle resolved
photoemission spectroscopy, de Haas-van Alphen measurements, and electronic
structure calculations. For the latter, the new full-potential augmented
spherical wave (ASW) method has been applied. Very good agreement between the
experimental and theoretical results is found. In particular, all Fermi surface
sheets are correctly identified by all three approaches. Previous controversies
concerning additional hole-like surfaces centered around the Z- and B-point
could be resolved; these surfaces were an artefact of the atomic-sphere
approximation used in the old calculations. Our results underline the
importance of electronic structure calculations for the understanding of MoO2
and the neighbouring rutile-type early transition-metal dioxides. This includes
the low-temperature insulating phases of VO2 and NbO2, which have crystal
structures very similar to that of molybdenum dioxide and display the
well-known prominent metal-insulator transitions.Comment: 17 pages, 21 figures, more information at
http://www.physik.uni-augsburg.de/~eyert
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