1,683 research outputs found
On (not) deriving the entropy of barocaloric phase transitions from crystallography and neutron spectroscopy
We review well-known signatures of disorder in crystallographic and inelastic
neutron scattering data. We show that these can arise from different types of
disorder, corresponding to different values of the system entropy. Correlating
the entropy of a material with its atomistic structure and dynamics is in
general a difficult problem that requires correlating information between
multiple experimental techniques including crystallography, spectroscopy, and
calorimetry. These comments are illustrated with particular reference to
barocalorics, but are relevant to a broad range of calorics and other
disordered crystalline materials.Comment: 11 pages, 3 figure
Studying the spatial distribution of interstellar dust
The spacial distribution of interstellar dust reflects both interstellar dynamics and the processes which form and destroy dust in the interstellar medium (ISM). The IRAS survey, because of its high sensitivity to thermal emission from dust in the IR, provides new approaches to determining the spatial distribution of dust. The initial results are reported of an attempt to use the IRAS data to probe the spatial distribution of dust - by searching for thermal emission from dust in the vicinity of bright stars. These results show that this technique (which relies on finding IR emission associated with randomly selected stars) can ultimately be used to study the distribution of dust in the ISM. The density of the cloud producing the IR emission may be derived by assuming that the dust is at its projected distance from the star and that the heating is due to the star's (known) radiation field. The heating radiation is folded into a grain model, and the number of emitting grains adjusted to reproduce the observed energy distribution. It is noted that this technique is capable in principle of detecting dust densities much lower than those typical of the cirrus clouds
A high resolution, three-dimensional view of the D-28 calving event from Amery Ice Shelf with ICESat-2 and satellite imagery
Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 48(3), (2021): e2020GL091200, doi:10.1029/2020GL091200.Tabular calving events occur from Antarctica's large ice shelves only every few decades, and are preceded by rift propagation. We used high-resolution imagery and ICESat-2 data to determine the propagation rates for the three active rifts on Amery Ice Shelf (AIS; T1, T2, and E3) and observe the calving of D-28 on September 25, 2019 along T1. AIS front advance accelerated downstream of T1 in the years before calving, possibly increasing stress at the rift tip. T1 experienced significant acceleration for 12 days before calving, coinciding with a jump in propagation of E3. ICESat-2's high resolution and repeat acquisitions every 91 days allowed for analysis of the ice front before and after calving, and rift detection where it was not visible in imagery as a ∼1 m surface depression, suggesting that it propagates as a basal fracture. Our results show that ICESat-2 can provide process-scale information about iceberg calving.We received funding from the following sources: NASA NNX15AC80G and NSF grant 1443677 (Fricker and Becker) and NASA 80NSSC20K0960 (Walker). We are grateful to Mike Cloutier, Polar Geospatial Center for assistance with WorldView imagery. Geospatial support was provided by Polar Geospatial Center under NSF-OPP awards 1043681 and 1559691.2021-07-1
Spin dynamics of coupled spin ladders near quantum criticality in Ba2CuTeO6
We report inelastic neutron scattering measurements of the magnetic
excitations in Ba2CuTeO6, proposed by ab initio calculations to magnetically
realize weakly coupled antiferromagnetic two-leg spin-1/2 ladders. Isolated
ladders are expected to have a singlet ground state protected by a spin gap.
Ba2CuTeO6 orders magnetically, but with a small Neel temperature relative to
the exchange strength, suggesting that the interladder couplings are relatively
small and only just able to stabilize magnetic order, placing Ba2CuTeO6 close
in parameter space to the critical point separating the gapped phase and Neel
order. Through comparison of the observed spin dynamics with linear spin wave
theory and quantum Monte Carlo calculations, we propose values for all relevant
intra- and interladder exchange parameters, which place the system on the
ordered side of the phase diagram in proximity to the critical point. We also
compare high field magnetization data with quantum Monte Carlo predictions for
the proposed model of coupled ladders.Comment: 14 pages, 12 figure
Intermultiplet transitions and magnetic long-range order in Sm-based pyrochlores
We present bulk and neutron scattering measurements performed on the
isotopically enriched and
samples. Both compounds display sharp heat
capacity anomalies, at 350 mK and 440 mK, respectively. Inelastic neutron
scattering measurements are employed to determine the crystalline electric
field (CEF) level scheme, which includes transitions between the ground-state
and first excited multiplets of the ion. To further
validate those results, the single-ion magnetic susceptibility of the compounds
is calculated and compared with the experimental DC-susceptibility measured in
low applied magnetic fields. It is demonstrated that the inclusion of
intermultiplet transitions in the CEF analysis is fundamental to the
understanding of the intermediate and, more importantly, low temperature
magnetic behaviour of the Sm-based pyrochlores. Finally, the heat capacity
anomaly is shown to correspond to the onset of an all-in-all-out long-range
order in the stannate sample, while in the titanate a dipolar long-range order
can be only indirectly inferred.Comment: 13 pages, 10 Figure
On the role of intrinsic disorder in the structural phase transition of magnetoelectric EuTiO3
Up to now the crystallographic structure of the magnetoelectric perovskite
EuTiO3 was considered to remain cubic down to low temperature. Here we present
high resolution synchrotron X-ray powder diffraction data showing the existence
of a structural phase transition, from cubic Pm-3m to tetragonal I4/mcm,
involving TiO6 octahedra tilting, in analogy to the case of SrTiO3. The
temperature evolution of the tilting angle indicates a second-order phase
transition with an estimated Tc=235K. This critical temperature is well below
the recent anomaly reported by specific heat measurement at TA\sim282K. By
performing atomic pair distribution function analysis on diffraction data we
provide evidence of a mismatch between the local (short-range) and the average
crystallographic structures in this material. Below the estimated Tc, the
average model symmetry is fully compatible with the local environment
distortion but the former is characterized by a reduced value of the tilting
angle compared to the latter. At T=240K data show the presence of local
octahedra tilting identical to the low temperature one, while the average
crystallographic structure remains cubic. On this basis, we propose intrinsic
lattice disorder to be of fundamental importance in the understanding of EuTiO3
properties.Comment: 13 pages, 8 figures, 2 table
Gapless spin-liquid state in the structurally disorder-free triangular antiferromagnet NaYbO
We present the structural characterization and low-temperature magnetism of
the triangular-lattice delafossite NaYbO. Synchrotron x-ray diffraction and
neutron scattering exclude both structural disorder and crystal-electric-field
randomness, whereas heat-capacity measurements and muon spectroscopy reveal the
absence of magnetic order and persistent spin dynamics down to at least 70\,mK.
Continuous magnetic excitations with the low-energy spectral weight
accumulating at the -point of the Brillouin zone indicate the formation of a
novel spin-liquid phase in a triangular antiferromagnet. This phase is gapless
and shows a non-trivial evolution of the low-temperature specific heat. Our
work demonstrates that NaYbO practically gives the most direct experimental
access to the spin-liquid physics of triangular antiferromagnets.Comment: 6 pages, 4figure
Dynamics in the ordered and disordered phases of barocaloric adamantane
High-entropy order-disorder phase transitions can be used for efficient and
eco-friendly barocaloric solid-state cooling. Here the barocaloric effect is
reported in an archetypal plastic crystal, adamantane. Adamantane has a
colossal isothermally reversible entropy change of 106 J K-1 kg-1 . Extremely
low hysteresis means that this can be accessed at pressure differences less
than 200 bar. Configurational entropy can only account for about 40% of the
total entropy change; the remainder is due to vibrational effects. Using
neutron spectroscopy and supercell lattice dynamics calculations, it is found
that this vibrational entropy change is mainly caused by softening in the
high-entropy phase of acoustic modes that correspond to molecular rotations. We
attribute this behaviour to the contrast between an 'interlocked' state in the
low-entropy phase and sphere-like behaviour in the high-entropy phase. Although
adamantane is a simple van der Waals solid with near-spherical molecules, this
approach can be leveraged for the design of more complex barocaloric molecular
crystals. Moreover, this study shows that supercell lattice dynamics
calculations can accurately map the effect of orientational disorder on the
phonon spectrum, paving the way for studying the vibrational entropy, thermal
conductivity, and other thermodynamic effects in more complex materials.Comment: 14 pages, 6 figure
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