157 research outputs found
Electrical and magnetic properties of the complete solid solution series between SrRuO3 and LaRhO3: Filling t2g versus tilting
A complete solid solution series between the t2g^4 perovskite ferromagnet
SrRuO3 and the diamagnetic t2g^6 perovskite LaRhO3 has been prepared. The
evolution with composition x in (SrRuO3)(1-x)(LaRhO3)(x) of the crystal
structure and electrical and magnetic properties has been studied and is
reported here. As x increases, the octahedral tilt angle gradually increases,
along with the pseudocubic lattice parameter and unit cell volume. Electrical
resistivity measurements reveal a compositionally driven metal to insulator
transition between x = 0.1 and 0.2. Ferromagnetic ordering gives over to glassy
magnetism for x > 0.3 and no magnetic ordering is found above 2 K for x > 0.5.
M_sat and Theta_CW decrease with increasing x and remain constant after x =
0.5. The magnetism appears poised between localized and itinerant behavior, and
becomes more localized with increasing x as evidenced by the evolution of the
Rhodes-Wohlfarth ratio. mu_eff per Ru is equal to the quenched spin-only S
value across the entire solid solution. Comparisons with Sr(1-x)Ca(x)RuO3
reinforce the important role of structural distortions in determining magnetic
ground state. It is suggested that electrical transport and magnetic properties
are not strongly coupled in this system
Does the bonding rule break down in AsSe glass?
The local coordination numbers of AsSe glass were determined by a
combination of anomalous x-ray scattering experiments, reverse Monte Carlo
calculations, and {\it ab initio} molecular dynamics simulations. The
well-known `8- bonding rule' proposed by Mott breaks down around the As
atoms, exceeding the rule by 7--26%. An experimental prediction based on
mean-field theory agrees with the present experimental and theoretical results.
The fourfold coordinated As atoms likely form As-As wrong bond chains rather
than ethan-like configurations, which is identified as the origin for the
breakdown of the `8- bonding rule'.Comment: 6 pages, 6figures, 1table, submitted to Europhysics Letter
Total scattering descriptions of local and cooperative distortions in the oxide spinel (Mg,Cu)Cr2O4 with dilute Jahn-Teller ions
The normal spinel oxide MgCr2O4 is cubic at room temperature while the normal
spinel CuCr2O4 is tetragonal as a consequence of the Jahn-Teller nature of Cu2+
on the tetrahedral sites. Despite different end-member structures, complete
solid solutions of Mg_{1-x}Cu_xCr2O4 can be prepared that display a first-order
structural transition with composition x = 0.43 at room temperature. Reverse
Monte Carlo analysis of total neutron scattering on data acquired between 300 K
and 15 K on samples with x = 0.10, 0.20, and 0.43 provides unbiased local and
average structure descriptions of the samples, including an understanding of
the transition from local Jahn-Teller distortions in the cubic phase to
cooperative distortions that result in a tetragonal structure. Distributions of
continuous symmetry measures help to understand and distinguish distorted and
undistorted coordination around the tetrahedral site in the solid solutions.
Magnetic exchange bias is observed in field-cooled hysteresis loops of samples
with dilute Cu2+ concentration and in samples with tetragonal--cubic phase
coexistence around 300 K.Comment: 10 pages, 14 figure
Theoretical Guarantees for Bridging Metric Measure Embedding and Optimal Transport
We propose a novel approach for comparing distributions whose supports do not
necessarily lie on the same metric space. Unlike Gromov-Wasserstein (GW)
distance which compares pairwise distances of elements from each distribution,
we consider a method allowing to embed the metric measure spaces in a common
Euclidean space and compute an optimal transport (OT) on the embedded
distributions. This leads to what we call a sub-embedding robust Wasserstein
(SERW) distance. Under some conditions, SERW is a distance that considers an OT
distance of the (low-distorted) embedded distributions using a common metric.
In addition to this novel proposal that generalizes several recent OT works,
our contributions stand on several theoretical analyses: (i) we characterize
the embedding spaces to define SERW distance for distribution alignment; (ii)
we prove that SERW mimics almost the same properties of GW distance, and we
give a cost relation between GW and SERW. The paper also provides some
numerical illustrations of how SERW behaves on matching problems
Exchange biasing of single-domain Ni nanoparticles spontaneously grown in an antiferromagnetic MnO matrix
Exchange biased composites of ferromagnetic single-domain Ni nanoparticles
embedded within large grains of MnO have been prepared by reduction of
NiMnO phases in flowing hydrogen. The Ni precipitates are 15-30
nm in extent, and the majority are completely encased within the MnO matrix.
The manner in which the Ni nanoparticles are spontaneously formed imparts a
high ferromagnetic- antiferromagnetic interface/volume ratio, which results in
substantial exchange bias effects. Exchange bias fields of up to 100 Oe are
observed, in cases where the starting Ni content in the precursor
NiMnO phase is small. For particles of approximately the same
size, the exchange bias leads to significant hardening of the magnetization,
with the coercive field scaling nearly linearly with the exchange bias field.Comment: 6 pages PDFLaTeX with 9 figure
Phase relations in K_xFe_{2-y}Se_2 and the structure of superconducting K_xFe_2Se_2 via high-resolution synchrotron diffraction
Superconductivity in iron selenides has experienced a rapid growth, but not
without major inconsistencies in the reported properties. For
alkali-intercalated iron selenides, even the structure of the superconducting
phase is a subject of debate, in part because the onset of superconductivity is
affected much more delicately by stoichiometry and preparation than in cuprate
or pnictide superconductors. If high-quality, pure, superconducting
intercalated iron selenides are ever to be made, the intertwined physics and
chemistry must be explained by systematic studies of how these materials form
and by and identifying the many coexisting phases. To that end, we prepared
pure K_2Fe_4Se_5 powder and superconductors in the K_xFe_{2-y}Se_2 system, and
examined differences in their structures by high-resolution synchrotron and
single-crystal x-ray diffraction. We found four distinct phases: semiconducting
K_2Fe_4Se_5, a metallic superconducting phase K_xFe_2Se_2 with x ranging from
0.38 to 0.58, an insulator KFe_{1.6}Se_2 with no vacancy ordering, and an
oxidized phase K_{0.51(5)}Fe_{0.70(2)}Se that forms the PbClF structure upon
exposure to moisture. We find that the vacancy-ordered phase K_2Fe_4Se_5 does
not become superconducting by doping, but the distinct iron-rich minority phase
K_xFe_2Se_2 precipitates from single crystals upon cooling from above the
vacancy ordering temperature. This coexistence of metallic and semiconducting
phases explains a broad maximum in resistivity around 100 K. Further studies to
understand the solubility of excess Fe in the K_xFe_{2-y}Se_2 structure will
shed light on the maximum fraction of superconducting K_xFe_2Se_2 that can be
obtained by solid state synthesis.Comment: 12 pages, 16 figures, supplemental materia
Tuning magnetic frustration on the diamond lattice of the A-site magnetic spinels CoAlGaO: Lattice expansion and site disorder
The spinels CoBO with magnetic Co ions on the diamond lattice
A site can be frustrated because of competing near-neighbor () and
next-near neighbor () interactions. Here we describe attempts to tune the
relative strengths of these interactions by substitution on the non-magnetic
B-site. The system we employ is CoAlGaO, where Al is
systematically replaced by the larger Ga, ostensibly on the B site. As
expected, Ga substitution expands the lattice, resulting in Co atoms on the
A-site being pushed further from one other and thereby weakening magnetic
interactions. In addition, Ga distributes between the B and the A site in a
concentration dependent manner displacing an increasing amount of Co from the A
site with increasing . This increased inversion, which is confirmed by
neutron diffraction studies carried out at room temperature, affects magnetic
ordering very significantly, and changes the nature of the ground state.
Modeling of the magnetic coupling illustrates the complexity that arises from
the cation site disorder.Comment: 9 pages, 10 figure
In-situ high-pressure powder X-ray diffraction study of α -zirconium phosphate
The high-pressure structural chemistry of -zirconium phosphate, -Zr(HPO4) 2H2O, was studied using in-situ high-pressure diffraction and synchrotron radiation. The layered phosphate was studied under both hydrostatic and non-hydrostatic conditions and Rietveld refinement carried out on the resulting diffraction patterns. It was found that under hydrostatic conditions no uptake of additional water molecules from the pressure-transmitting medium occurred, contrary to what had previously been observed with some zeolite materials and a layered titanium phosphate. Under hydrostatic conditions the sample remained crystalline up to 10 GPa, but under non-hydrostatic conditions the sample amorphized between 7.3 and 9.5 GPa. The calculated bulk modulus, K 0 = 15.2 GPa, showed the material to be very compressible with the weak linkages in the structure of the type ZrâOâP
Technical report: Graph Neural Networks go Grammatical
This paper proposes a framework to formally link a fragment of an algebraic
language to a Graph Neural Network (GNN). It relies on Context Free Grammars
(CFG) to organise algebraic operations into generative rules that can be
translated into a GNN layer model. Since the rules and variables of a CFG
directly derived from a language contain redundancies, a grammar reduction
scheme is presented making tractable the translation into a GNN layer. Applying
this strategy, a grammar compliant with the third-order Weisfeiler-Lehman
(3-WL) test is defined from MATLANG. From this 3-WL CFG, we derive a provably
3-WL GNN model called GN. Moreover, this grammatical approach allows us
to provide algebraic formulas to count the cycles of length up to six and
chordal cycles at the edge level, which enlightens the counting power of 3-WL.
Several experiments illustrate that GN efficiently outperforms other
3-WL GNNs on many downstream tasks.Comment: 27 pages, 7 figure
Magnetodielectric coupling in Mn3O4
We have investigated the dielectric anomalies associated with spin ordering
transitions in the tetragonal spinel MnO, using thermodynamic,
magnetic, and dielectric measurements. We find that two of the three magnetic
ordering transitions in MnO lead to decreases in the temperature
dependent dielectric constant at zero applied field. Applying a magnetic field
to the polycrystalline sample leaves these two dielectric anomalies practically
unchanged, but leads to an increase in the dielectric constant at the
intermediate spin-ordering transition. We discuss possible origins for this
magnetodielectric behavior in terms of spin-phonon coupling. Band structure
calculations suggest that in its ferrimagnetic state, MnO corresponds
to a semiconductor with no orbital degeneracy due to strong Jahn-Teller
distortion.Comment: 6 pages, 7 figure
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