140 research outputs found
Quantitative characterization of short-range orthorhombic fluctuations in FeSe through pair distribution function analysis
Neutron and x-ray total scattering measurements have been performed on powder
samples of the iron chalcogenide superconductor FeSe. Using pair distribution
function (PDF) analysis of the total scattering data to investigate short-range
atomic correlations, we establish the existence of an instantaneous, local
orthorhombic structural distortion attributable to nematic fluctuations that
persists well into the high-temperature tetragonal phase, at least up to 300 K
and likely to significantly higher temperatures. This short-range orthorhombic
distortion is correlated over a length scale of about 1 nm at 300 K and grows
to several nm as the temperature is lowered toward the long-range structural
transition temperature. In the low-temperature nematic state, the local
instantaneous structure exhibits an enhanced orthorhombic distortion relative
to the average structure with a typical relaxation length of 3 nm. The
quantitative characterization of these orthorhombic fluctuations sheds light on
nematicity in this canonical iron-based superconductor
Emergent phases in iron pnictides: Double-Q antiferromagnetism, charge order and enhanced nematic correlations
Electron correlations produce a rich phase diagram in the iron pnictides.
Earlier theoretical studies on the correlation effect demonstrated how quantum
fluctuations weaken and concurrently suppress a -symmetric single-Q
antiferromagnetic order and a nematic order. Here we examine the emergent
phases near the quantum phase transition. For a -symmetric collinear
double-Q antiferromagnetic order, we show that it is accompanied by both a
charge order and an enhanced nematic susceptibility. Our results provide
understanding for several intriguing recent experiments in hole-doped iron
arsenides, and bring out common physics that underlies the different magnetic
phases of various iron-based superconductors.Comment: 5+6 pages, 2 figures; (v2) issues with cross-referencing between the
main text and supplementary material are fixe
Identification of OSSO as a near-UV absorber in the Venusian atmosphere
The planet Venus exhibits atmospheric absorption in the 320–400 nm wavelength range produced by unknown chemistry. We investigate electronic transitions in molecules that may exist in the atmosphere of Venus. We identify two different S_2O_2 isomers, cis-OSSO and trans-OSSO, which are formed in significant amounts and are removed predominantly by near-UV photolysis. We estimate the rate of photolysis of cis- and trans-OSSO in the Venusian atmosphere and find that they are good candidates to explain the enigmatic 320–400 nm near-UV absorption. Between 58 and 70 km, the calculated OSSO concentrations are similar to those of sulfur monoxide (SO), generally thought to be the second most abundant sulfur oxide on Venus
Giant spontaneous magnetostriction in MnTe driven by a novel magnetostructural coupling mechanism
We present a comprehensive x-ray scattering study of spontaneous
magnetostriction in hexagonal MnTe, an antiferromagnetic semiconductor with a
Neel temperature of K. We observe the largest
spontaneous magnetovolume effect known for an antiferromagnet, reaching a
volume contraction of . This can be justified
semiquantitatively by considering bulk material properties, the spatial
dependence of the superexchange interaction, and the geometrical arrangement of
magnetic moments in MnTe. The highly unusual linear scaling of the
magnetovolume effect with the short-range magnetic correlations, beginning in
the paramagnetic state well above , points to a novel physical
mechanism, which we explain in terms of a trilinear coupling of the elastic
strain with superposed distinct domains of the antiferromagnetic order
parameter. This novel mechanism for coupling lattice strain to robust
short-range magnetic order casts new light on magnetostrictive phenomena and
also provides a template by which the exceptional magnetostrictive properties
of MnTe might be realized in a wide range of other functional materials.Comment: Submitted May 11, 202
Local atomic and magnetic structure of dilute magnetic semiconductor (Ba,K)(Zn,Mn)As
We have studied the atomic and magnetic structure of the dilute ferromagnetic
semiconductor system (Ba,K)(Zn,Mn)As through atomic and magnetic pair
distribution function analysis of temperature-dependent x-ray and neutron total
scattering data. We detected a change in curvature of the temperature-dependent
unit cell volume of the average tetragonal crystallographic structure at a
temperature coinciding with the onset of ferromagnetic order. We also observed
the existence of a well-defined local orthorhombic structure on a short length
scale of \AA, resulting in a rather asymmetrical local environment
of the Mn and As ions. Finally, the magnetic PDF revealed ferromagnetic
alignment of Mn spins along the crystallographic -axis, with robust
nearest-neighbor ferromagnetic correlations that exist even above the
ferromagnetic ordering temperature. We discuss these results in the context of
other experiments and theoretical studies on this system
Real-space investigation of short-range magnetic correlations in fluoride pyrochlores NaCaCoF and NaSrCoF with magnetic pair distribution function analysis
We present time-of-flight neutron total scattering and polarized neutron
scattering measurements of the magnetically frustrated compounds
NaCaCoF and NaSrCoF, which belong to a class of recently
discovered pyrochlore compounds based on transition metals and fluorine. The
magnetic pair distribution function (mPDF) technique is used to analyze and
model the total scattering data in real space. We find that a
previously-proposed model of short-range XY-like correlations with a length
scale of 10-15 \AA, combined with nearest-neighbor collinear antiferromagnetic
correlations, accurately describes the mPDF data at low temperature, confirming
the magnetic ground state in these materials. This model is further verified by
the polarized neutron scattering data. From an analysis of the temperature
dependence of the mPDF and polarized neutron scattering data, we find that
short-range correlations persist on the nearest-neighbor length scale up to 200
K, approximately two orders of magnitude higher than the spin freezing
temperatures of these compounds. These results highlight the opportunity
presented by these new pyrochlore compounds to study the effects of geometric
frustration at relatively high temperatures, while also advancing the mPDF
technique and providing a novel opportunity to investigate a genuinely
short-range-ordered magnetic ground state directly in real space
Local atomic and magnetic structure of multiferroic (Sr,Ba)(Mn,Ti)O
We present a detailed study of the local atomic and magnetic structure of the
type-I multiferroic perovskite system (Sr,Ba)MnO using x-ray and neutron
pair distribution function (PDF) analysis, polarized neutron scattering, and
muon spin relaxation (SR) techniques. The atomic PDF analysis reveals
widespread nanoscale tetragonal distortions of the crystal structure even in
the paraelectric phase with average cubic symmetry, corresponding to incipient
ferroelectricity in the local structure. Magnetic PDF analysis, polarized
neutron scattering, and SR likewise confirm the presence of short-range
antiferromagnetic correlations in the paramagnetic state, which grow in
magnitude as the temperature approaches the magnetic transition. We show that
these short-range magnetic correlations coincide with a reduction of the
tetragonal (i.e. ferroelectric) distortion in the average structure, suggesting
that short-range magnetism can play an important role in magnetoelectric and/or
magnetostructural phenomena even without genuine long-range magnetic order. The
reduction of the tetragonal distortion scales linearly with the local magnetic
order parameter, pointing to spontaneous linear magnetoelectric coupling in
this system. These findings provide greater insight into the multiferroic
properties of (Sr,Ba)(Mn,Ti)O and demonstrate the importance of
investigating the local atomic and magnetic structure to gain a deeper
understanding of the intertwined degrees of freedom in multiferroics
Gradual Enhancement of Stripe-Type Antiferromagnetism in Spin Ladder Material BaFeS Under Pressure
We report pressure-dependent neutron diffraction and muon spin
relaxation/rotation measurements combined with first-principles calculations to
investigate the structural, magnetic, and electronic properties of
BaFeS under pressure. The experimental results reveal a gradual
enhancement of the stripe-type ordering temperature with increasing pressure up
to 2.6 GPa and no observable change in the size of the ordered moment. The ab
initio calculations suggest that the magnetism is highly sensitive to the Fe-S
bond lengths and angles, clarifying discrepancies with previously published
results. In contrast to our experimental observations, the calculations predict
a monotonic reduction of the ordered moment with pressure. We suggest that the
robustness of the stripe-type antiferromagnetism is due to strong electron
correlations not fully considered in the calculations
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