561 research outputs found
Nematic state of the FeSe superconductor
We study the crystal structure of the tetragonal iron selenide FeSe and its nematic phase transition to the low-temperature orthorhombic structure using synchrotron x-ray and neutron scattering analyzed in both real space and reciprocal space. We show that in the local structure the orthorhombic distortion associated with the electronically driven nematic order is more pronounced at short length scales. It also survives to temperatures above 90 K, where reciprocal-space analysis suggests tetragonal symmetry. Additionally, the real-space pair distribution function analysis of the synchrotron x-ray diffraction data reveals a tiny broadening of the peaks corresponding to the nearest Fe-Fe, nearest Fe-Se, and next-nearest Fe-Se bond distances as well as the tetrahedral torsion angles at a short length scale of 20 Å. This broadening appears below 20 K and is attributed to a pseudogap. However, we did not observe any further reduction in local symmetry below orthorhombic down to 3 K. Our results suggest that the superconducting gap anisotropy in FeSe is not associated with any symmetry-lowering short-range structural correlations
Staircase baker's map generates flaring-type time series
The baker’s map, invented by Eberhard Hopf in 1937, is an intuitively accesible, two-dimensional chaos-generating discrete dynamical system. This map, which describes the transformation of an idealized two-dimensional dough by stretching, cutting and piling, is non-dissipative. Nevertheless the “x” variable is identical with the dissipative, one-dimensional Bernoulli-shift-generating map. The generalization proposed here takes up ideas of Yaacov Sinai in a modified form. It has a staircase-like shape, with every next step half as high as the preceding one. Each pair of neighboring elements exchanges an equal volume (area) during every iteration step in a scaled manner. Since the density of iterated points is constant, the thin tail (to the right, say) is visited only exponentially rarely. This observation already explains the map's main qualitative behavior: The “x” variable shows “flares”. The time series of this variable is closely analogous to that of a flaring-type dissipative dynamical system – like those recently described in an abstract economic model. An initial point starting its journey in the tale (or “antenna”, if we tilt the map upwards by 90 degrees) is predictably attracted by the broad left hand (bottom) part, in order to only very rarely venture out again to the tip. Yet whenever it does so, it thereby creates, with the top of a flare, a new “far-from-equilibrium” initial condition, in this reversible system. The system therefore qualifies as a discrete analogue to a far-from-equilibrium multiparticle Hamiltonian system. The height of the flare hereby corresponds to the momentary height of the H function of a gas. An observable which is even more closely related to the momentary negative entropy was recently described. Dependent on the numerical accuracy chosen, “Poincaré cycles” of two different types (periodic and nonperiodic) can be observed for the first time
The electronic structure of the doped one-dimensional transition metal oxide Y1-xCaxBaNiO5 studied using x-ray absorption
A strong anisotropic distribution of the holes in Ni 3d and O 2p orbitals is
observed in the polarization dependent O1s and Ni2p3/2 x-ray absorption
spectroscopy of the linear-chain nickelate Y1-xCaxBaNiO5 (x = 0, 0.05, 0.1,
0.2), which demonstrates the one-dimensional nature of the electronic state in
these compounds. Furthermore, the additional holes introduced by Ca-doping
occupy both O 2p and Ni 3d orbitals along the NiO5 chains. By comparing the
experimental Ni 2p3/2 absorption spectra of Y1-xCaxBaNiO5 to those from charge
transfer multiplet calculations we can derive the orbital character of the
additional holes to be of ca. 60% O2p and ca. 40% Ni 3d.Comment: pdf only. Submitted to PR
Fractional Generalization of Gradient Systems
We consider a fractional generalization of gradient systems. We use
differential forms and exterior derivatives of fractional orders. Examples of
fractional gradient systems are considered. We describe the stationary states
of these systems.Comment: 11 pages, LaTe
Random Field and Random Anisotropy Effects in Defect-Free Three-Dimensional XY Models
Monte Carlo simulations have been used to study a vortex-free XY ferromagnet
with a random field or a random anisotropy on simple cubic lattices. In the
random field case, which can be related to a charge-density wave pinned by
random point defects, it is found that long-range order is destroyed even for
weak randomness. In the random anisotropy case, which can be related to a
randomly pinned spin-density wave, the long-range order is not destroyed and
the correlation length is finite. In both cases there are many local minima of
the free energy separated by high entropy barriers. Our results for the random
field case are consistent with the existence of a Bragg glass phase of the type
discussed by Emig, Bogner and Nattermann.Comment: 10 pages, including 2 figures, extensively revise
First principles study of strain/electronic interplay in ZnO; Stress and temperature dependence of the piezoelectric constants
We present a first-principles study of the relationship between stress,
temperature and electronic properties in piezoelectric ZnO. Our method is a
plane wave pseudopotential implementation of density functional theory and
density functional linear response within the local density approximation. We
observe marked changes in the piezoelectric and dielectric constants when the
material is distorted. This stress dependence is the result of strong, bond
length dependent, hybridization between the O and Zn electrons. Our
results indicate that fine tuning of the piezoelectric properties for specific
device applications can be achieved by control of the ZnO lattice constant, for
example by epitaxial growth on an appropriate substrate.Comment: accepted for publication in Phys. Rev.
Diffusion and viscosity in a supercooled polydisperse system
We have carried out extensive molecular dynamics simulations of a supercooled
polydisperse Lennard-Jones liquid with large variations in temperature at a
fixed pressure. The particles in the system are considered to be polydisperse
both in size and mass. The temperature dependence of the dynamical properties
such as the viscosity () and the self-diffusion coefficients () of
different size particles is studied. Both viscosity and diffusion coefficients
show super-Arrhenius temperature dependence and fit well to the well-known
Vogel-Fulcher-Tammann (VFT) equation. Within the temperature range
investigated, the value of the Angell's fragility parameter (D )
classifies the present system into a strongly fragile liquid. The critical
temperature for diffusion () increases with the size of the
particles. The critical temperature for viscosity () is larger than
that for the diffusion and a sizeable deviations appear for the smaller size
particles implying a decoupling of translational diffusion from viscosity in
deeply supercooled liquid. Indeed, the diffusion shows markedly non-Stokesian
behavior at low temperatures where a highly nonlinear dependence on size is
observed. An inspection of the trajectories of the particles shows that at low
temperatures the motions of both the smallest and largest size particles are
discontinuous (jump-type). However, the crossover from continuous Brownian to
large length hopping motion takes place at shorter time scales for the smaller
size particles.Comment: Revtex4, 7 pages, 8 figure
A Tapestry Of Educational Technology Women Leaders In Higher Education: A Qualitative Study
A qualitative study was used to understand the experiences of 12 women, leaders in Education Technology in higher education. Through interviews, women leaders described their environment as well as personal and behavioral aspects of their work. Findings revealed four threads of descriptive concepts including relationships, leadership, persistence, and advice. Relationships were from workplaces and professional networks. Leadership was defined by vision and teamwork. Persistence was addressed as either values-based or relationship-based. The fourth thread in the findings, advice, was divided into three sub-threads: education, family (both personal and work), and managing emotions. A qualitative approach was used to highlight interview responses to demonstrate the experiences of women leaders in Education Technology in Higher Education
Structural and optical properties of high quality zinc-blende/wurtzite GaAs hetero-nanowires
The structural and optical properties of 3 different kinds of GaAs nanowires
with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are
presented. A variety of shorter and longer segments of zinc-blende or wurtzite
crystal phases are observed by transmission electron microscopy in the
nanowires. Sharp photoluminescence lines are observed with emission energies
tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is
increased. The downward shift of the emission peaks can be understood by
carrier confinement at the interfaces, in quantum wells and in random short
period superlattices existent in these nanowires, assuming a staggered
band-offset between wurtzite and zinc-blende GaAs. The latter is confirmed also
by time resolved measurements. The extremely local nature of these optical
transitions is evidenced also by cathodoluminescence measurements. Raman
spectroscopy on single wires shows different strain conditions, depending on
the wurtzite content which affects also the band alignments. Finally, the
occurrence of the two crystallographic phases is discussed in thermodynamic
terms.Comment: 24 page
Participation in medical decision-making across Europe: an international longitudinal multicenter study
Background: The purpose of this paper was to examine national differences in the desire to participate in decision-making of people with severe mental illness in six European countries.
Methods: The data was taken from a European longitudinal observational study (CEDAR; ISRCTN75841675). A sample of 514 patients with severe mental illness from the study centers in Ulm, Germany, London, England, Naples, Italy, Debrecen, Hungary, Aalborg, Denmark and Zurich, Switzerland were assessed as to desire to participate in medical decision-making. Associations between desire for participation in decision-making and center location were analyzed with generalized estimating equations.
Results: We found large cross-national differences in patients’ desire to participate in decision-making, with the center explaining 40% of total variance in the desire for participation (p<0.001). Averaged over time and independent of patient characteristics, London (mean=2.27), Ulm (mean=2.13) and Zurich (mean=2.14) showed significantly higher scores in desire for participation, followed by Aalborg (mean=1.97), where scores were in turn significantly higher than in Debrecen (mean=1.56). The lowest scores were reported in Naples (mean=1.14). Over time, desire for participation in decision-making increased significantly in Zurich (b=0.23) and decreased in Naples (b=-0.14). In all other centers, values remained stable.
Conclusions: This study demonstrates that patients’ desire for participation in decisionmaking varies by location. We suggest that more research attention be focused on identifying specific cultural and social factors in each country to further explain observed differences across Europe
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