88 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
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.
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
Kitaev interactions in the Co honeycomb antiferromagnets Na3Co2SbO6 and Na2Co2TeO6
Co ions in an octahedral crystal field, stabilise a j = 1/2
ground state with an orbital degree of freedom and have been recently put
forward for realising Kitaev interactions, a prediction we have tested by
investigating spin dynamics in two cobalt honeycomb lattice compounds,
NaCoTeO and NaCoSbO, using inelastic neutron
scattering. We used linear spin wave theory to show that the magnetic spectra
can be reproduced with a spin Hamiltonian including a dominant Kitaev
nearest-neighbour interaction, weaker Heisenberg interactions up to the third
neighbour and bond-dependent off-diagonal exchange interactions. Beyond the
Kitaev interaction that alone would induce a quantum spin liquid state, the
presence of these additional couplings is responsible for the zigzag-type
long-range magnetic ordering observed at low temperature in both compounds.
These results provide evidence for the realization of Kitaev-type coupling in
cobalt-based materials, despite hosting a weaker spin-orbit coupling than their
4d and 5d counterparts
Monte Carlo simulation of a mammographic test phantom
Monte Carlo simulation of a mammographic test phantom A test phantom, including a wide range of mammographic tissue equivalent materials and test details, was imaged on a digital mammographic system. In order to quantify the effect of scatter on the contrast obtained for the test details, calculations of the scatter-to-primary ratio (S/P) have been made using a Monte Carlo simulation of the digital mammographic imaging chain, grid and test phantom. The results show that the S/P values corresponding to the imaging conditions used were in the range 0.0844.126. Calculated and measured pixel values in different regions (if the image were compared as a validation of the model and showed excellent agreement. The results indicate the potential of Monte Carlo methods in the image quality-patient dose process optimisation, especially in the assessment of imaging conditions not available on standard mammographic units
Gd-149:What's confirmed? What's new?
A long run performed with EUROGAM II allowed remeasuring the Gd-149 superdeformed (SD) band 1. The Delta I = 4 bifurcation in band 1 is confirmed and two resolved gamma-ray transitions linking the SD band 1 and the normal deformed states have been observed
Gd-149:What's confirmed? What's new?
A long run performed with EUROGAM II allowed remeasuring the Gd-149 superdeformed (SD) band 1. The Delta I = 4 bifurcation in band 1 is confirmed and two resolved gamma-ray transitions linking the SD band 1 and the normal deformed states have been observed
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