233 research outputs found
Anisotropic susceptibility of the geometrically frustrated spin-chain compound Ca3Co2O6
Ca3Co2O6 is a system exhibiting a series of fascinating properties, including
magnetization plateaus and remarkably slow dynamics at low-T. These properties
are intimately related to the geometrical frustration, which results from a
particular combination of features: (i) the chains are arranged on a triangular
lattice; (ii) there is a large uniaxial anisotropy; (iii) the intrachain and
interchain couplings are ferromagnetic and antiferromagnetic, respectively.
The uniaxial anisotropy is thus an issue of crucial importance for the
analysis of the physical properties of Ca3Co2O6. However, it turns out that no
precise investigation of this magnetic anisotropy has been performed so far. On
the basis of susceptibility data directly recorded on single crystals, the
present study reports on quantitative information about the anisotropy of
Ca3Co2O6.Comment: 10 pages, 5 figure
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
Intrinsic effects of substitution and intercalation on thermal transport in two-dimensional TiS single crystals
The promising thermoelectric material TiS can be easily chemically doped
and intercalated. We present here studies of single crystals that are
intercalated with excess Ti or Co, or substituted with Ta. We demonstrate the
intrinsic impact of these dopants on the thermal transport in the absence of
grain boundary scattering. We show that Ta doping has the greatest impact on
the thermal scattering rate per ion added, leading to a five-fold reduction in
the lattice thermal conductivity as compared to stoichiometric single crystals.Comment: 5 pages, 2 figure
Cellular Automata and Kan Extensions
In this paper, we formalize precisely the sense in which the application of a cellular automaton to partial configurations is a natural extension of its local transition function through the categorical notion of Kan extension. In fact, the two possible ways to do such an extension and the ingredients involved in their definition are related through Kan extensions in many ways. These relations provide additional links between computer science and category theory, and also give a new point of view on the famous Curtis-Hedlund theorem of cellular automata from the extended topological point of view provided by category theory. These links also allow to relatively easily generalize concepts pioneered by cellular automata to arbitrary kinds of possibly evolving spaces. No prior knowledge of category theory is assumed
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
Magnetization reversal in mixed ferrite-chromite perovskites with non magnetic cation on the A-site
In this work, we have performed Monte Carlo simulations in a classical model
for RFeCrO with R=Y and Lu, comparing the numerical simulations
with experiments and mean field calculations. In the analyzed compounds, the
antisymmetric exchange or Dzyaloshinskii-Moriya (DM) interaction induced a weak
ferromagnetism due to a canting of the antiferromagnetically ordered spins.
This model is able to reproduce the magnetization reversal (MR) observed
experimentally in a field cooling process for intermediate values and the
dependence with of the critical temperatures. We also analyzed the
conditions for the existence of MR in terms of the strength of DM interactions
between Fe and Cr ions with the x values variations.Comment: 8 pages, 7 figure
Large anisotropic thermal conductivity of intrinsically two-dimensional metallic oxide PdCoO
The highly conductive layered metallic oxide \pdcoo{} is a near-perfect
analogue to an alkali metal in two dimensions. It is distinguished from other
two-dimensional electron systems where the Fermi surface does not reach the
Brillouin zone boundary by a high planar electron density exceeding
cm. The simple single-band quasi-2D electronic structure results in
strongly anisotropic transport properties and limits the effectiveness of
electron-phonon scattering. Measurements on single crystals in the temperature
range from 10-300K show that the thermal conductivity is much more weakly
anisotropic than the electrical resistivity, as a result of significant phonon
heat transport. The in-plane thermoelectric power is linear in temperature at
300\,K and displays a purity-dependent peak around 50K. Given the extreme
simplicity of the band-structure, it is possible to identify this peak with
phonon drag driven by normal electron-phonon scattering processes.Comment: 3 figure
CuFe2S3 as electrode material for Li-ion batteries
Electrochemical performances of the isocubanite CuFe2S3 tested as electrode material for Li-ion batteries have been investigated. A first discharge capacity of 860 mA h g(-1) shows a conversion process leading to Li2S, copper and iron nanoparticles. Interestingly, a reversible capacity of 560 mA h g(-1) at 1.5 V is demonstrated with good cyclability up to 30 cycles
Structure and electronic properties of the quasi-one-dimensional BaâCoâââZnâSâ series
This work focuses on the structure and physical properties of the solid solution BaâCoâââZnâSâ (0 †x †1), a family of quasi-one-dimensional sulfides with end members BaâCoSâ and BaâZnSâ. The structure of selected compounds with increasing ZnÂČâș content has been analysed using, neutron diffraction, TEM and EXAFS and the physical properties via magnetic susceptibility and resistivity measurements. The progressive substitution of the non-magnetic ZnÂČâș cation for CoÂČâș rapidly destroys the antiferromagnetic transition present at 46 K in the quasi one-dimensional BaâCoSâ, leading to paramagnetic behaviour down to the lowest investigated temperature (5K) for compounds with x > 0.25. For compounds with x â„ 0.4, a pure CW regime is recovered around 300 K, yielding effective moments consistent with the g factor of the tetrahedrally coordinated CoÂČâș previously determined for BaâCoSâ. The ZnÂČâș/CoÂČâș substitution also removes the metallic-like behaviour of BaâCoSâ causing an increase in the value of the resistivity with all the BaâCoâââZnâSâ compounds showing semiconducting behaviour. The negative magnetoresistance of BaâCoSâ is improved by the ZnÂČâș/CoÂČâș substitution, with values of â 6% for BaâCoâ.ââ
Znâ.ââ
Sâ, â 9% for BaâCoâ.â
Znâ.â
Sâ and â 8% for BaâCoâ.ââ
Znâ.ââ
Sâ. However, there does not seem to be a correlation between the values of the resistivity and the magnetoresistance and the content of ZnÂČâș, leading to the hypothesis that transport properties may be linked more closely to extrinsic properties
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
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