34 research outputs found
Anomalous expansion and phonon damping due to the Co spin-state transition in RCoO_3 with R = La, Pr, Nd and Eu
We present a combined study of the thermal expansion and the thermal
conductivity of the perovskite series RCoO_3 with R = La, Nd, Pr and Eu. The
well-known spin-state transition in LaCoO_3 is strongly affected by the
exchange of the R ions due to their different ionic radii, i.e. chemical
pressure. This can be monitored in detail by measurements of the thermal
expansion, which is a highly sensitive probe for detecting spin-state
transitions. The Co ions in the higher spin state act as additional scattering
centers for phonons, therefore suppressing the phonon thermal conductivity.
Based on the analysis of the interplay between spin-state transition and heat
transport, we present a quantitative model of the thermal conductivity for the
entire series. In PrCoO_3, an additional scattering effect is active at low
temperatures. This effect arises from the crystal field splitting of the 4f
multiplet, which allows for resonant scattering of phonons between the various
4f levels.Comment: 15 pages including 5 figure
Magnetothermal transport in the spin-1/2 chains of copper pyrazine dinitrate
We present experiments on the thermal transport in the spin-1/2 chain
compound copper pyrazine dinitrate Cu(C_4 H_4 N_2)(NO_3)_2. The heat
conductivity shows a surprisingly strong dependence on the applied magnetic
field B, characterized at low temperatures by two main features. The first one
appearing at low B is a characteristic dip located at mu_B B ~ k_B T, that may
arise from Umklapp scattering. The second one is a plateau-like feature in the
quantum critical regime, mu_B |B-B_c| < k_B T, where B_c is the saturation
field at T=0. The latter feature clearly points towards a momentum and field
independent mean free path of the spin excitations, contrary to theoretical
expectations.Comment: 4 pages, 4 figure
Towards predicting Pedestrian Evacuation Time and Density from Floorplans using a Vision Transformer
Conventional pedestrian simulators are inevitable tools in the design process
of a building, as they enable project engineers to prevent overcrowding
situations and plan escape routes for evacuation. However, simulation runtime
and the multiple cumbersome steps in generating simulation results are
potential bottlenecks during the building design process. Data-driven
approaches have demonstrated their capability to outperform conventional
methods in speed while delivering similar or even better results across many
disciplines. In this work, we present a deep learning-based approach based on a
Vision Transformer to predict density heatmaps over time and total evacuation
time from a given floorplan. Specifically, due to limited availability of
public datasets, we implement a parametric data generation pipeline including a
conventional simulator. This enables us to build a large synthetic dataset that
we use to train our architecture. Furthermore, we seamlessly integrate our
model into a BIM-authoring tool to generate simulation results instantly and
automatically
Uniaxial pressure dependencies of the phase transitions in GdMnO
GdMnO shows an incommensurate antiferromagnetic order below
K, transforms into a canted A-type antiferromagnet below K, and for
finite magnetic fields along the b axis ferroelectric order occurs below
K. From high-resolution thermal expansion measurements along all
three principal axes, we determine the uniaxial pressure dependencies of the
various transition temperatures and discuss their correlation to changes of the
magnetic exchange couplings in MnO ().Comment: 2 pages, 3 figures, submitted to JMMM (Proceedings of ICM'06, Kyoto
Thermal Conductivity, Thermopower, and Figure of Merit of La_{1-x}Sr_xCoO_3
We present a study of the thermal conductivity k and the thermopower S of
single crystals of La_{1-x}Sr_xCoO_3 with 0<= x <= 0.3. For all Sr
concentrations La_{1-x}Sr_xCoO_3 has rather low k values, whereas S strongly
changes as a function of x. We discuss the influence of the temperature- and
the doping-induced spin-state transitions of the Co ions on both, S and k. From
S, k, and the electrical resistivity rho we derive the thermoelectric figure of
merit Z=S^2/(k*rho). For intermediate Sr concentrations we find notably large
values of Z indicating that Co-based materials could be promising candidates
for thermoelectric cooling.Comment: 7 pages, 5 figures included, submitted to Phys. Rev.
Thermal conductivity of R2CuO4, with R = La, Pr and Gd
We present measurements of the in-plane kappa_ab and out-of-plane kappa_c
thermal conductivity of Pr2CuO4 and Gd2CuO4 single crystals. The anisotropy
gives strong evidence for a large contribution of magnetic excitations to
kappa_ab i.e. for a heat current within the CuO2 planes. However, the absolute
values of kappa_mag are lower than previous results on La2CuO4. These
differences probably arise from deviations from the nominal oxygen
stoichiometry. This has a drastic influence on kappa_mag, which is shown by an
investigation of a La2CuO4+delta polycrystal.Comment: 2 pages, 1 figure; presented at SCES200
Magnetoresistance, specific heat and magnetocaloric effect of equiatomic rare-earth transition-metal magnesium compounds
We present a study of the magnetoresistance, the specific heat and the
magnetocaloric effect of equiatomic Mg intermetallics with , Eu, Gd, Yb and , Au and of GdAuIn. Depending on the
composition these compounds are paramagnetic (, Yb) or they
order either ferro- or antiferromagnetically with transition temperatures
ranging from about 13 to 81 K. All of them are metallic, but the resistivity
varies over 3 orders of magnitude. The magnetic order causes a strong decrease
of the resistivity and around the ordering temperature we find pronounced
magnetoresistance effects. The magnetic ordering also leads to well-defined
anomalies in the specific heat. An analysis of the entropy change leads to the
conclusions that generally the magnetic transition can be described by an
ordering of localized moments arising from the half-filled
shells of Eu or Gd. However, for GdAgMg we find clear evidence
for two phase transitions indicating that the magnetic ordering sets in
partially below about 125 K and is completed via an almost first-order
transition at 39 K. The magnetocaloric effect is weak for the antiferromagnets
and rather pronounced for the ferromagnets for low magnetic fields around the
zero-field Curie temperature.Comment: 12 pages, 7 figures include
Evidence for a large magnetic heat current in insulating cuprates
The in-plane thermal conductivity of the two-dimensional
antiferromagnetic monolayer cuprate SrCuOCl is studied. Analysis of
the unusual temperature dependence of reveals that at low temperatures the
heat is carried by phonons, whereas at high temperatures magnetic excitations
contribute significantly. Comparison with other insulating layered cuprates
suggests that a large magnetic contribution to the thermal conductivity is an
intrinsic property of these materials.Comment: 4 pages, 2 figures included, submitted to Phys. Rev.
Anomalous thermal expansion and strong damping of the thermal conductivity of NdMnO and TbMnO due to 4f crystal-field excitations
We present measurements of the thermal conductivity and the thermal
expansion of NdMnO and TbMnO. In both compounds a splitting of
the multiplet of the ion causes Schottky contributions to
. In TbMnO this contribution arises from a crystal-field splitting,
while in NdMnO it is due to the Nd-Mn exchange coupling. Another
consequence of this coupling is a strongly enhanced canting of the Mn moments.
The thermal conductivity is greatly suppressed in both compounds. The main
scattering process at low temperatures is resonant scattering of phonons
between different energy levels of the multiplets, whereas the complex 3d
magnetism of the Mn ions is of minor importance.Comment: 9 pages including 6 figure