312 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
Energy scales in 4f1 delafossite magnets: crystal-field splittings larger than the strength of spin-orbit coupling in KCeO2
Ytterbium-based delafossites with effective S=1/2 moments are investigated
intensively as candidates for quantum spin-liquid ground states. While the
synthesis of related cerium compounds has also been reported,many important
details concerning their crystal, electronic, and magnetic structures are
unclear. Here we analyze the S=1/2 system KCeO2, combining complementary
theoretical methods. The lattice geometry was optimized and the band structure
investigated using density functional theory extended to the level of a GGA+U
calculation in order to reproduce the correct insulating behavior. The Ce 4f1
states were then analyzed in more detail with the help of ab initio
wave-function-based computations. Unusually large effective crystal-field
splittings of up to 320 meV are predicted, which puts KCeO2 in the strong field
coupling regime. Our results reveal a subtle interplay between ligand-cage
electrostatics and the trigonal field generated by the extended crystalline
surroundings, relevant in the context of recent studies on tuning the nature of
the ground-state wave function in 4f triangular-lattice and pyrochlore
compounds. It also makes KCeO2 an interesting model system in relation to the
effect of large crystal-field splittings on the anisotropy of intersite
exchange in spin-orbit coupled quantum magnets.Comment: 6 pages, 2 figures, and 3 table
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
The electron-hole liquid in a polar semiconductor: Cubic SiC
The binding energy EB = (17 +/- 3) meV and density n = (9.2 +/- 1.7) x 1018 cm-3 of the EHL in cubic SiC are determined from excitation-dependent spectra. Comparing these values with ground state properties calculated with and without electron-phonon-interaction using newly determined valence band parameters evidence for the importance of e.p.i. in SiC is found.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23743/1/0000715.pd
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.
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
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
Internet-based Self-Assessment after the Tsunami: lessons learned
BACKGROUND: In the aftermath of the Tsunami disaster in 2004, an online psychological self-assessment (ONSET) was developed and made available by the University of Zurich in order to provide an online screening instrument for Tsunami victims to test if they were traumatized and in need of mental health care. The objective of the study was to report the lessons learnt that were made using an Internet-based, self-screening instrument after a large-scale disaster and to discuss its outreach and usefulness.
METHODS: Users of the online self-assessment decided after finishing the procedure whether their dataset could be used for quality control and scientific evaluation Their answers were stored anonymously only if they consented (which was the case in 88% of the sample), stratified analyses according to level of exposure were conducted.
RESULTS: A total of 2,914 adult users gave their consent for analysis of the screenings. Almost three quarter of the sample filled out the ONSET questionnaire within the first four weeks. Forty-one percent of the users reported direct exposure to the Tsunami disaster. Users who were injured by the Tsunami and users who reported dead or injured family members showed the highest degree of PTSD symptoms.
CONCLUSION: ONSET was used by a large number of subjects who thought to be affected by the catastrophe in order to help them decide if they needed to see a mental health professional. Furthermore, men more frequently accessed the instrument than women, indicating that Internet-based testing facilitates reaching out to a different group of people than "ordinary" public mental health strategies
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