1,665 research outputs found
Orbital ordering promotes weakly-interacting S=1/2 dimers in the triangular lattice compound Sr3Cr2O8
The weakly interacting S=1/2 dimers system Sr3Cr2O8 has been investigated by
powder neutron diffraction and inelastic neutron scattering. Our data reveal a
structural phase transition below room temperature corresponding to an
antiferro-orbital ordering with nearly 90 degrees arrangement of the occupied
3z^2-r^2 d-orbital. This configuration leads to a drastic reduction of the
inter-dimer exchange energies with respect to the high temperature
orbital-disorder state, as shown by a spin-dimer analysis of the
super-superexchange interactions performed using the Extended Huckel Tight
Binding method. Inelastic neutron scattering reveals the presence of a quasi
non-dispersive magnetic excitation at 5.4 meV, in agreement with the picture of
weakly-interacting dimers
What did I leave out? Omitted variables in regression and qualitative comparative analysis
This is the author accepted manuscript. The final version is available from Palgrave Macmillan for European Consortium for Political Research via the DOI in this record.Social scientists often face a fundamental problem: Did I leave something causally important
out of my explanation? How do I diagnose this? Where do I look for solutions to this
problem? We build bridges between regression models and qualitative comparative analysis
(QCA) by comparing diagnostics and solutions to the problem of omitted variables and
conditions. We then discuss various approaches and tackle the theoretical issues around
causality which must be addressed before attending to technical fixes. In the conclusions we
reflect on the bridges built between the two traditions and draw more general lessons about
the logic of social science research.Research for this project was funded by the ERC project 694632 Protego –
Procedural Tools for Effective Governance and by the Goethe University of Frankfur
First-principles study of multiferroic RbFe(MoO)
We have investigated the magnetic structure and ferroelectricity in
RbFe(MoO) via first-principles calculations. Phenomenological analyses
have shown that ferroelectricity may arise due to both the triangular chirality
of the magnetic structure, and through coupling between the magnetic helicity
and the ferroaxial structural distortion. Indeed, it was recently proposed that
the structural distortion plays a key role in stabilising the chiral magnetic
structure itself. We have determined the relative contribution of the two
mechanisms via \emph{ab-initio} calculations. Whilst the structural axiality
does induce the magnetic helix by modulating the symmetric exchange
interactions, the electric polarization is largely due to the in-plane spin
triangular chirality, with both electronic and ionic contributions being of
relativistic origin. At the microscopic level, we interpret the polarization as
a secondary steric consequence of the inverse Dzyaloshinskii-Moriya mechanism
and accordingly explain why the ferroaxial component of the electric
polarization must be small
Crisis, learning and policy change in the European Union
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.What is the causal relationship between crisis, learning and change? How does causality works in the current responses to the sovereign debt crisis provided by the European Union? We question the conventional identification of the cause-and-effect relationship provided by theories of crisis management, integration and policy learning. Drawing on models of contingent learning developed within psychology and behavioural and evolutionary economics, we theorise that surprise produces behavioural change via a fast-paced associative cue-outcome mechanism and that policy learning follows change. We then run our exercise in causal identification through a plausibility probe. We show that our argument passes the plausibility probe. Our conclusions on cognition and situational effects on learning suggest a new research agenda, more sensitive to how individuals behave in the real world and more robust in its micro-foundations
Non-collinear long-range magnetic ordering in HgCr2S4
The low-temperature magnetic structure of \HG has been studied by
high-resolution powder neutron diffraction. Long-range incommensurate magnetic
order sets in at T22K with propagation vector
\textbf{k}=(0,0,0.18). On cooling below T, the propagation vector
increases and saturates at the commensurate value \textbf{k}=(0,0,0.25). The
magnetic structure below T consists of ferromagnetic layers in the
\textit{ab}-plane stacked in a spiral arrangement along the \textit{c}-axis.
Symmetry analysis using corepresentations theory reveals a point group symmetry
in the ordered magnetic phase of 422 (D), which is incompatible with
macroscopic ferroelectricity. This finding indicates that the spontaneous
electric polarization observed experimentally cannot be coupled to the magnetic
order parameter
Electrical switching of magnetic polarity in a multiferroic BiFeO3 device at room temperature
We have directly imaged reversible electrical switching of the cycloidal
rotation direction (magnetic polarity) in a (111)-BiFeO3 epitaxial-film device
at room temperature by non-resonant x-ray magnetic scattering. Consistent with
previous reports, fully relaxed (111)-BiFeO3 epitaxial films consisting of a
single ferroelectric domain were found to comprise a sub-micron-scale mosaic of
magneto-elastic domains, all sharing a common direction of the magnetic
polarity, which was found to switch reversibly upon reversal of the
ferroelectric polarization without any measurable change of the magneto-elastic
domain population. A real-space polarimetry map of our device clearly
distinguished between regions of the sample electrically addressed into the two
magnetic states with a resolution of a few tens of micron. Contrary to the
general belief that the magneto-electric coupling in BiFeO3 is weak, we find
that electrical switching has a dramatic effect on the magnetic structure, with
the magnetic moments rotating on average by 90 degrees at every cycle.Comment: 6 pages, 5 figures; corrected figure
A Gillespie algorithm for efficient simulation of quantum jump trajectories
The jump unravelling of a quantum master equation decomposes the dynamics of
an open quantum system into abrupt jumps, interspersed by periods of coherent
dynamics where no jumps occur. Simulating these jump trajectories is
computationally expensive, as it requires very small time steps to ensure
convergence. This computational challenge is aggravated in regimes where the
coherent, Hamiltonian dynamics are fast compared to the dissipative dynamics
responsible for the jumps. Here, we present a quantum version of the Gillespie
algorithm that bypasses this issue by directly constructing the waiting time
distribution for the next jump to occur. In effect, this avoids the need for
timestep discretisation altogether, instead evolving the system continuously
from one jump to the next. We describe the algorithm in detail and discuss
relevant limiting cases. To illustrate it we include four example applications
of increasing physical complexity. These additionally serve to compare the
performance of the algorithm to alternative approaches -- namely, the
widely-used routines contained in the powerful Python library QuTip. We find
significant gains in efficiency for our algorithm and discuss in which regimes
these are most pronounced. Publicly available implementations of our code are
provided in Julia and Mathematica.Comment: 13 pages, 4 figures. Comments welcom
Temperature and field dependence of the phase separation, structure, and magnetic ordering in LaCaMnO, (, 0.50, and 0.53)
Neutron powder diffraction measurements, combined with magnetization and
resistivity data, have been carried out in the doped perovskite
LaCaMnO (, 0.50, and 0.53) to elucidate the structural,
magnetic, and electronic properties of the system around the composition
corresponding to an equal number of Mn3+ and Mn4+. At room temperature all
three samples are paramagnetic and single phase, with crystallographic symmetry
Pnma. The samples then all become ferromagnetic (FM) at K. At
K, however, a second distinct crystallographic phase (denoted A-II)
begins to form. Initially the intrinsic widths of the peaks are quite large,
but they narrow as the temperature decreases and the phase fraction increases,
indicating microscopic coexistence. The fraction of the sample that exhibits
the A-II phase increases with decreasing temperature and also increases with
increasing Ca doping, but the transition never goes to completion to the lowest
temperatures measured (5 K) and the two phases therefore coexist in this
temperature-composition regime. Phase A-II orders antiferromagnetically (AFM)
below a N\'{e}el temperature K, with the CE-type magnetic
structure. Resistivity measurements show that this phase is a conductor, while
the CE phase is insulating. Application of magnetic fields up to 9 T
progressively inhibits the formation of the A-II phase, but this suppression is
path dependent, being much stronger for example if the sample is field-cooled
compared to zero-field cooling and then applying the field. The H-T phase
diagram obtained from the diffraction measurements is in good agreement with
the results of magnetization and resistivity.Comment: 12 pages, 3 tables, 11 figure
Immunohistochemical localization of constitutive and inducible heat shock protein 70 in carp (Cyprinus carpio) and trout (Oncorhynchus mykiss) exposed to transport stress.
In the present work we investigated by immunohistochemistry the cellular localization of constitutive as well as inducible heat shock protein 70 in several tissues of common carp (Cyprinus carpio) and rainbow trout (Oncorhynchus mykiss) exposed to transport stress. In carp, the constitutive form (HSC70) was detected only in red skeletal muscle of both control and stressed animals. In the same species, the inducible form (HSP70) was evident in the epithelia of renal tubules, gills and skin of stressed animals, whereas in controls only red skeletal muscle exhibited an immunopositivity to HSP70 antibody. In trout, immunostaining to HSC70 antibody was found mainly in the epithelia of intestine, gills and skin of both control and stressed animals although the reactivity was generally higher in animals exposed to transport stress. In the same species immunostaining to HSP70 antibody was observed only in red skeletal muscle and epidermis of control animals
The structure of intercalated water in superconducting NaCoO1.37DO: Implications for the superconducting phase diagram
We have used electron and neutron powder diffraction to elucidate the
structural properties of superconducting \NaD. Our measurements show that our
superconducting sample exhbits a number of supercells ranging from
to , but the most predominant one, observed also in the neutron
data, is a double hexagonal cell with dimensions \dhx. Rietveld analysis
reveals that \deut\space is inserted between CoO sheets as to form a
layered network of NaO triangular prisms. Our model removes the need to
invoke a 5K superconducting point compound and suggests that a solid solution
of Na is possible within a constant amount of water .Comment: 4 pages, 3 figure
- …