84 research outputs found
Assessment of radiographic screen-film systems: a comparison between the use of a microdensitometer and a drum film digitiser
A high-end drum film digitiser (Tango, Germany) and a calibrated linear microdensitometer developed by PTB were used to assess the modulation transfer function (MTF) and the noise power spectra (NPS) of 3 mammographic screen film systems at optical density levels of 0.8, 1.5 and 2.5. The use of a drum scanner to assess MTF and NPS data appears to be adequate but requires an appropriate characterisation of the scanner to verify its internal noise level and its MTF. It is further necessary to calibrate the scanner output in terms of visual diffuse optical densities. Processing of two-dimensional digital data of grating images need to be more strictly defined for accurate MTF measurements of screen-film systems. Nevertheless, even now it seems to be feasible to use commercially available high-end and well calibrated scanners to assess screen film systems. This is especially important for quality assurance purposes because important parameters of screen film systems such like MTF and NPS can now be determined without using sophisticated microdensitometers which are not commercially availabl
The Verwey structure of a natural magnetite
Complex charge and orbital molecule order observed in natural magnetite comparable to meteoritic samples is the most complex electronic order known to occur naturally.</p
Graphene Transport at High Carrier Densities using a Polymer Electrolyte Gate
We report the study of graphene devices in Hall-bar geometry, gated with a
polymer electrolyte. High densities of 6 are
consistently reached, significantly higher than with conventional back-gating.
The mobility follows an inverse dependence on density, which can be correlated
to a dominant scattering from weak scatterers. Furthermore, our measurements
show a Bloch-Gr\"uneisen regime until 100 K (at 6.2 ),
consistent with an increase of the density. Ubiquitous in our experiments is a
small upturn in resistivity around 3 , whose origin is
discussed. We identify two potential causes for the upturn: the renormalization
of Fermi velocity and an electrochemically-enhanced scattering rate.Comment: 13 pages, 4 figures, Published Versio
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-Wave Spectrum in `Single-Domain' Magnetic Ground State of Triangular Lattice Antiferromagnet CuFeO2
By means of neutron scattering measurements, we have investigated spin-wave
excitation in a collinear four-sublattice (4SL) magnetic ground state of a
triangular lattice antiferromagnet CuFeO2, which has been of recent interest as
a strongly frustrated magnet, a spin-lattice coupled system and a multiferroic.
To avoid mixing of spin-wave spectrum from magnetic domains having three
different orientations reflecting trigonal symmetry of the crystal structure,
we have applied uniaxial pressure on [1-10] direction of a single crystal
CuFeO2. By elastic neutron scattering measurements, we have found that only 10
MPa of the uniaxial pressure results in almost 'single domain' state in the 4SL
phase. We have thus performed inelastic neutron scattering measurements using
the single domain sample, and have identified two distinct spin- wave branches.
The dispersion relation of the upper spin-wave branch cannot be explained by
the previous theoretical model [R. S. Fishman: J. Appl. Phys. 103 (2008)
07B109]. This implies the importance of the lattice degree of freedom in the
spin-wave excitation in this system, because the previous calculation neglected
the effect of the spin-driven lattice distortion in the 4SL phase. We have also
discussed relationship between the present results and the recently discovered
"electromagnon" excitation.Comment: 5 pages, 3 figures, accepted for publication in J. Phys. Soc. Jp
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.
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
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