378 research outputs found
Two-dimensional ferromagnetic spin-orbital excitations in honeycomb VI3
VI is a ferromagnet with planar honeycomb sheets of bonded V
ions held together by van der Waals forces. We apply neutron spectroscopy to
measure the two dimensional () magnetic excitations in the
ferromagnetic phase, finding two energetically gapped ( 55 K) and dispersive excitations. We apply a multi-level spin
wave formalism to describe the spectra in terms of two coexisting domains
hosting differing V orbital ground states built from contrasting
distorted octahedral environments. This analysis fits a common nearest neighbor
in-plane exchange coupling (=-8.6 0.3 meV) between V sites. The
distorted local crystalline electric field combined with spin-orbit coupling
provides the needed magnetic anisotropy for spatially long-ranged
two-dimensional ferromagnetism in VI.Comment: (main text - 7 pages, 4 figures; supplementary information - 13
pages, 9 figures, to be published in Phys. Rev. B
Spin-orbit excitons in CoO
CoO has an odd number of electrons in its unit cell, and therefore is
expected to be metallic. Yet, CoO is strongly insulating owing to significant
electronic correlations, thus classifying it as a Mott insulator. We
investigate the magnetic fluctuations in CoO using neutron spectroscopy. The
strong and spatially far-reaching exchange constants reported in [Sarte et al.
Phys. Rev. B 98 024415 (2018)], combined with the single-ion spin-orbit
coupling of similar magnitude [Cowley et al. Phys. Rev. B 88, 205117 (2013)]
results in significant mixing between spin-orbit levels in the low
temperature magnetically ordered phase. The high degree of entanglement,
combined with the structural domains originating from the Jahn-Teller
structural distortion at 300 K, make the magnetic excitation spectrum
highly structured in both energy and momentum. We extend previous theoretical
work on PrTl [Buyers et al. Phys. Rev. B 11, 266 (1975)] to construct a
mean-field and multi-level spin exciton model employing the aforementioned spin
exchange and spin-orbit coupling parameters for coupled Co ions on a
rocksalt lattice. This parameterization, based on a tetragonally distorted
type-II antiferromagnetic unit cell, captures both the sharp low energy
excitations at the magnetic zone center, and the energy broadened peaks at the
zone boundary. However, the model fails to describe the momentum dependence of
the excitations at high energy transfers, where the neutron response decays
faster with momentum than the Co form factor. We discuss such a failure
in terms of a possible breakdown of localized spin-orbit excitons at high
energy transfers.Comment: (main text - 21 pages, 12 figures; supplementary information - 15
pages, 3 figures, to be published in Phys. Rev. B
Metastable and localized Ising magnetism in α−CoV2O6 magnetization plateaus
-CoVO consists of Ising
spins located on an anisotropic triangular motif with magnetization plateaus in
an applied field. We combine neutron diffraction with low temperature
magnetization to investigate the magnetic periodicity in the vicinity of these
plateaus. We find these steps to be characterized by metastable and spatially
short-range ( 10 ) magnetic correlations with antiphase
boundaries defining a local periodicity of $\langle \hat{T}^{2} \rangle =\
\uparrow \downarrow\langle \hat{T}^{3} \rangle =\ \uparrow \uparrow
\downarrow\langle \hat{T}^{4} \rangle=\ \uparrow \uparrow \downarrow
\downarrow\uparrow \uparrow \uparrow \downarrow$ spin arrangements. This
shows the presence of spatially short range and metastable/hysteretic,
commensurate magnetism in Ising magnetization steps.Comment: 9 pages, 6 figures, to be published in Phys. Rev.
Limited local electron-lattice coupling in manganites
(Pr,Ca)MnO3 is the archetypal charge-ordered manganite, but in
Pr0.48Ca0.52MnO3 we find (using convergent-beam electron diffraction and
dark-field images) that the superlattice period is locally incommensurate with
respect to the parent lattice, and that the superlattice orientation possesses
significant local variations. This suggests that local electron-lattice
coupling never overwhelmingly dominates the rich physics of manganites, even in
the most extreme scenarios that produce the largest colossal magnetoresistance
effects.Comment: 9 pages, 4 figures; accepted in Physical Review
Identification and classification of high risk groups for Coal Workers' Pneumoconiosis using an artificial neural network based on occupational histories: a retrospective cohort study
<p>Abstract</p> <p>Background</p> <p>Coal workers' pneumoconiosis (CWP) is a preventable, but not fully curable occupational lung disease. More and more coal miners are likely to be at risk of developing CWP owing to an increase in coal production and utilization, especially in developing countries. Coal miners with different occupational categories and durations of dust exposure may be at different levels of risk for CWP. It is necessary to identify and classify different levels of risk for CWP in coal miners with different work histories. In this way, we can recommend different intervals for medical examinations according to different levels of risk for CWP. Our findings may provide a basis for further emending the measures of CWP prevention and control.</p> <p>Methods</p> <p>The study was performed using longitudinal retrospective data in the Tiefa Colliery in China. A three-layer artificial neural network with 6 input variables, 15 neurons in the hidden layer, and 1 output neuron was developed in conjunction with coal miners' occupational exposure data. Sensitivity and ROC analyses were adapted to explain the importance of input variables and the performance of the neural network. The occupational characteristics and the probability values predicted were used to categorize coal miners for their levels of risk for CWP.</p> <p>Results</p> <p>The sensitivity analysis showed that influence of the duration of dust exposure and occupational category on CWP was 65% and 67%, respectively. The area under the ROC in 3 sets was 0.981, 0.969, and 0.992. There were 7959 coal miners with a probability value < 0.001. The average duration of dust exposure was 15.35 years. The average duration of ex-dust exposure was 0.69 years. Of the coal miners, 79.27% worked in helping and mining. Most of the coal miners were born after 1950 and were first exposed to dust after 1970. One hundred forty-four coal miners had a probability value ≥0.1. The average durations of dust exposure and ex-dust exposure were 25.70 and 16.30 years, respectively. Most of the coal miners were born before 1950 and began to be exposed to dust before 1980. Of the coal miners, 90.28% worked in tunneling.</p> <p>Conclusion</p> <p>The duration of dust exposure and occupational category were the two most important factors for CWP. Coal miners at different levels of risk for CWP could be classified by the three-layer neural network analysis based on occupational history.</p
The nanoscale phase separation in hole-doped manganites
A macroscopic phase separation, in which ferromagnetic clusters are observed
in an insulating matrix, is sometimes observed, and believed to be essential to
the colossal magnetoresistive (CMR) properties of manganese oxides. The
application of a magnetic field may indeed trigger large magnetoresistance
effects due to the percolation between clusters allowing the movement of the
charge carriers. However, this macroscopic phase separation is mainly related
to extrinsic defects or impurities, which hinder the long-ranged charge-orbital
order of the system. We show in the present article that rather than the
macroscopic phase separation, an homogeneous short-ranged charge-orbital order
accompanied by a spin glass state occurs, as an intrinsic result of the
uniformity of the random potential perturbation induced by the solid solution
of the cations on the -sites of the structure of these materials. Hence the
phase separation does occur, but in a more subtle and interesting nanoscopic
form, here referred as ``homogeneous''. Remarkably, this ``nanoscale phase
separation'' alone is able to bring forth the colossal magnetoresistance in the
perovskite manganites, and is potentially relevant to a wide variety of other
magnetic and/or electrical properties of manganites, as well as many other
transition metal oxides, in bulk or thin film form as we exemplify throughout
the article.Comment: jpsj2 TeX style (J. Phys. Soc. Jpn); 18 pages, 7 figure
Reprobing the mechanism of negative thermal expansion in siliceous faujasite
Combined Rietveld refinement and pair distribution function analysis of total neutron scattering data unveils the finer details of the negative thermal expansion mechanism of siliceous faujasite.</p
Long range electronic phase separation in CaFe3O5
Electronic phase separation is an important feature of many correlated perovskite compounds but hasn’t been seen in other complex oxides with similar physical behaviour such as magnetite. Hong et al. find phase separation between a magnetite-like charge ordered phase and a charge averaged phase in CaFe3O5
Absence of moment fragmentation in the mixed B-site pyrochlore Nd<sub>2</sub>GaSbO<sub>7</sub>
Nd-based pyrochlore oxides of the form NdO have garnered a
significant amount of interest owing to the moment fragmentation physics
observed in NdZrO and speculated in NdHfO.
Notably this phenomenon is not ubiquitous in this family, as it is absent in
NdSnO, which features a smaller ionic radius on the -site.
Here, we explore the necessary conditions for moment fragmentation in the Nd
pyrochlore family through a detailed study of the mixed -site pyrochlore
NdGaSbO. The -site of this system is characterized by
significant disorder and an extremely small average ionic radius. Similarly to
NdSnO, we find no evidence for moment fragmentation through
our bulk characterization and neutron scattering experiments, indicating that
chemical pressure (and not necessarily the -site disorder) plays a key role
in the presence or absence of this phenomenon in this material family.
Surprisingly, the presence of significant -site disorder in
NdGaSbO does not generate a spin glass ground state and instead the
same all-in-all-out magnetic order identified in other Nd pyrochlores is found
here.Comment: 11 pages, 8 figure
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