38 research outputs found
Excitonic transverse and amplitude fluctuations in the noncollinear and charge-ordered RbFeFeF
RbFeFeF is an example of an antiferromagnet with charge
ordering of the octahedrally coordinated Fe and Fe ions. As well
as different spin values, Fe () and Fe ()
possess differing orbital ground states with Fe having an orbital
degeneracy with an effective orbital angular momentum of . The resulting
low temperature magnetic structure is non collinear with the spins aligned
perpendicular to nearest neighbors (S. W. Kim \textit{et al.} Chem. Sci.
{\bf{3}}, 741 (2012)). The combination of an orbital degeneracy and non
collinear spin arrangements introduces the possibility for unusual types of
excitations such as amplitude modes of the order parameter. In this paper we
investigate this by applying a multi-level analysis to model neutron
spectroscopy data (M. Songvilay \textit{et al.} Phys. Rev. Lett. {\bf{121}},
087201 (2018)). In particular, we discuss the possible origins of the momentum
and energy broadened continuum scattering observed in terms of amplitude
fluctuations allowed through the presence of an orbital degree of freedom on
the Fe site. We extend previous spin-orbit exciton models based on a
collinear spin structure to understand the measured low-energy excitations and
also to predict and discuss possible amplitude mode scattering in
RbFeFeF.Comment: 17 pages and 7 figure
Broadband critical dynamics in disordered lead-based perovskites*
Materials based on the cubic perovskite unit cell continue to provide the
basis for technologically important materials with two notable recent examples
being lead-based relaxor piezoelectrics and lead-based organic-inorganic halide
photovoltaics. These materials carry considerable disorder, arising from site
substitution in relaxors and molecular vibrations in the organic-inorganics,
yet much of our understanding of these systems derives from the initial classic
work of Prof. Roger A. Cowley, who applied both theory and neutron scattering
methods while at Chalk River Laboratories to the study of lattice vibrations in
SrTiO. Neutron scattering continues to play a vital role in
characterizing lattice vibrations in perovskites owing to the simple cross
section and the wide range of energy resolutions achievable with current
neutron instrumentation. We discuss the dynamics that drive the phase
transitions in the relaxors and organic-inorganic lead-halides in terms of
neutron scattering and compare them to those in phase transitions associated
with a ``central peak" and also a soft mode. We review some of the past
experimental work on these materials and present new data from high-resolution
time-of-flight backscattering spectroscopy taken on organic-inorganic
perovskites. We will show that the structural transitions in disordered
lead-based perovskites are driven by a broad frequency band of excitations.Comment: 23 pages, 9 figures, submitted as part of a special issue of JPCM on
X-ray and Neutron Scattering and dedicated to the memory of Prof. R. A.
Cowle
Magnetic properties of the honeycomb oxide NaCoTeO
We have studied the magnetic properties of NaCoTeO, which
features a honeycomb lattice of magnetic Co ions, through macroscopic
characterization and neutron diffraction on a powder sample. We have shown that
this material orders in a zig-zag antiferromagnetic structure. In addition to
allowing a linear magnetoelectric coupling, this magnetic arrangement displays
very peculiar spatial magnetic correlations, larger in the honeycomb planes
than between the planes, which do not evolve with the temperature. We have
investigated this behavior by Monte Carlo calculations using the
-- model on a honeycomb lattice with a small interplane
interaction. Our model reproduces the experimental neutron structure factor,
although its absence of temperature evolution must be due to additional
ingredients, such as chemical disorder or quantum fluctuations enhanced by the
proximity to a phase boundary.Comment: 9 pages, 13 figure
Decoupled molecular and inorganic framework dynamics in CH3NH3PbCl3
The organic-inorganic lead halide perovskites are composed of organic
molecules imbedded in an inorganic framework. The compounds with general
formula CHNHPbX (MAPbX) display large photovoltaic
efficiencies for halogens =Cl, Br, and I in a wide variety of sample
geometries and preparation methods. The organic cation and inorganic framework
are bound by hydrogen bonds that tether the molecules to the halide anions, and
this has been suggested to be important to the optoelectronic properties. We
have studied the effects of this bonding using time-of-flight neutron
spectroscopy to measure the molecular dynamics in CHNHPbCl
(MAPbCl). Low-energy/high-resolution neutron backscattering reveals
thermally-activated molecular dynamics with a characteristic temperature of
95\,K. At this same temperature, higher-energy neutron spectroscopy
indicates the presence of an anomalous broadening in energy (reduced lifetime)
associated with the molecular vibrations. By contrast, neutron powder
diffraction shows that a spatially long-range structural phase transitions
occurs at 178\,K (cubic tetragonal) and 173\,K (tetragonal
orthorhombic). The large difference between these two temperature
scales suggests that the molecular and inorganic lattice dynamics in MAPbCl
are actually decoupled. With the assumption that underlying physical mechanisms
do not change with differing halogens in the organic-inorganic perovskites, we
speculate that the energy scale most relevant to the photovoltaic properties of
the lead-halogen perovskites is set by the lead-halide bond, not by the
hydrogen bond.Comment: (10 pages, 5 figures, to be published in Physical Review Materials
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
Magnetic surface reconstruction in the van der Waals antiferromagnet Fe1+xTe
We acknowledge financial support from the EPSRC (EP/R031924/1 and EP/R032130/1) and NIST Center for Neutron Research. C.H. acknowledges support by the Austrian Science Fund (FWF) Project No. P32144-N36 and the VSC4 of the Vienna University of TechnologyFe1+xTe is a two-dimensional van der Waals antiferromagnet that becomes superconducting on anion substitution on the Te site. The properties of the parent phase of Fe1+xTe are sensitive to the amount of interstitial iron situated between the iron-tellurium layers. Fe1+xTe displays collinear magnetic order coexisting with low-temperature metallic resistivity for small concentrations of interstitial iron x and helical magnetic order for large values of x. While this phase diagram has been established through scattering [see, for example, E. E. Rodriguez et al., Phys. Rev. B 84, 064403 (2011); S. Rossler et al., ibid. 84, 174506 (2011)], recent scanning tunneling microscopy measurements [C. Trainer et al., Sci. Adv. 5, eaav3478 (2019)] have observed a different magnetic structure for small interstitial iron concentrations x with a significant canting of the magnetic moments along the crystallographic c axis of θ = 28° ± 3°. In this paper, we revisit themagnetic structure of Fe1.09Te using spherical neutron polarimetry and scanning tunneling microscopy to search for this canting in the bulk phase, and we compare surface and bulk magnetism. The results show that the bulk magnetic structure of Fe1.09Te is consistent with collinear in-plane order (θ= 0 with an error of ∼ 5°). Comparison with scanning tunneling microscopy on a series of Fe1+xTe samples reveals that the surface exhibits a magnetic surface reconstruction with a canting angle of the spins of θ = 29.8°. We suggest that this is a consequence of structural relaxation of the surface layer resulting in an out-of-plane magnetocrystalline anisotropy. The magnetism in Fe1+xTe displays different properties at the surface when the symmetry constraints of the bulk are removed.Publisher PDFPeer reviewe
From one- to two-magnon excitations in the S=3/2 magnet β-CaCr2O4
We apply neutron spectroscopy to measure the magnetic dynamics in the S=3/2
magnet -CaCrO (T=21 K). The low-energy fluctuations, in the
ordered state, resemble large-S linear spin-waves from the incommensurate
ground state. However, at higher energy transfers, these semi-classical and
harmonic dynamics are replaced by an energy and momentum broadened continuum of
excitations. Applying kinematic constraints required for energy and momentum
conservation, sum rules of neutron scattering, and comparison against exact
diagonalization calculations, we show that the dynamics at high-energy
transfers resemble low-S one-dimensional quantum fluctuations.
-CaCrO represents an example of a magnet at the border between
classical N\'eel and quantum phases, displaying dual characteristics
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