The
ground state of Fe2+ (S = 2) in
α- and β-FeMoO4 is investigated by experiments
including X-ray diffraction, Raman scattering, and 57Fe–Mössbauer
spectroscopy below 300 K and evaluated by theoretical modeling. Both
modifications crystallize in the space group C2/m with the same set of Wyckoff positions. The structural
feature of α- and β-FeMoO4 is a tetramer of
the so-called butterfly motif. Two iron-sites (Fe2) form an antiferromagnetically
coupled dimer whereas two Fe1 establish an antiferromagnetic intertetramer
coupling. The effective magnetic exchange of the two magnetic sublattices
is based on dominating Dzyaloshinskii–Moriya interaction due
to the rare situation of canceling Heisenberg exchange interactions.
According to our investigations, the ground states of the two polymorphs
differ in terms of their Fe-site specific electric field gradients V
ii
. Contrary to the α-phase,
a degenerate set of V
zz
and V
yy
for both iron
sites in β-FeMoO4 is extracted from density functional
theory calculations. In the vicinity of the phase transition (β
→ α), the degeneracy of the β-phase is lifted.
Correspondingly, we observe a softening of the ν(Mo–O)
phonon modes. Detailed Mössbauer spectra confirm the crosslike
90° antiferromagnetic structure for both modifications and solve
the origin of the longstanding issue of disparate quadrupole splittings
in α- and β-FeMoO4.
Comparative Raman scattering studies of single crystals GdBaCo 2 O 5+ with ~ 0.35 ÷ 0.5, and polycrystalline GdBaCo 2 O 5.5±0.05 in the temperature range 5 -295 K have been made. 8A g + 3B 1g modes in backscattering XX, and XY configuration from polycrystalline and single crystal samples with ~ 0.5 have been observed. The Pmmm space group symmetry of GdBaCo 2 O 5.5 has been used as a basic structure. Phonon frequencies have been estimated using a modified Thomas-Fermi electron density calculations and a frozen phonon approach. Intensities and frequency of some A g and B 1g modes show a nonuniform temperature dependence in the temperature range of 100-250 K. This correlates with changes of the magnetic ground states and magnetization previously observed in GdBaCo 2 O 5.5 .
We present an experimental study of the quasi-elastic Raman scattering (QES) of plane-wave and twisted light by liquid crystals. Depending on their temperature, these crystals can exhibit isotropic, nematic and chiral nematic phases. The question is addressed of how the phase of a crystal and the state of incident light can affect the quasi–elastic energy spectra of the scattered radiation, whose shape is usually described by the combination of Lorentzian and Gaussian components. Special attention is paid to the chiral phase, for which the Raman QES spectrum is dominated by a Lorentzian with reduced linewidth, pointing to diminished disorder and configurational entropy. Moreover, this phase is also known for a regime of iridescence (selective backscattering) which arises when the wavelength of incident light becomes comparable with the chiral pitch length. Detailed measurements, performed in this resonant regime and by employing twisted light, carrying various projections of the orbital angular momentum (OAM), have indicated a low-energy scattering surplus depending on OAM. We argue that this observation might indicate a transfer of angular momentum between light and liquid crystal.
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