767 research outputs found
Segregation during directional melting and its implications on seeded crystal growth: A theoretical analysis
Directional melting of binary systems, as encountered during seeding in melt growth, is analyzed for concurrent compositional changes at the crystal-melt interface. It is shown that steady state conditions cannot normally be reached during seeding and that the growth interface temperature at the initial stages of seeded growth is a function of backmelt conditions. The theoretical treatment is numerically applied to Hg1-xCdXTe and Ga-doped Ge
Neutron-Diffraction Measurements of an Antiferromagnetic Semiconducting Phase in the Vicinity of the High-Temperature Superconducting State of KFeSe
The recently discovered K-Fe-Se high temperature superconductor has caused
heated debate regarding the nature of its parent compound. Transport,
angle-resolved photoemission spectroscopy, and STM measurements have suggested
that its parent compound could be insulating, semiconducting or even metallic
[M. H. Fang, H.-D. Wang, C.-H. Dong, Z.-J. Li, C.-M. Feng, J. Chen, and H. Q.
Yuan, Europhys. Lett. 94, 27009 (2011); F. Chen et al. Phys. Rev. X 1, 021020
(2011); and W. Li et al.,Phys. Rev. Lett. 109, 057003 (2012)]. Because the
magnetic ground states associated with these different phases have not yet been
identified and the relationship between magnetism and superconductivity is not
fully understood, the real parent compound of this system remains elusive.
Here, we report neutron-diffraction experiments that reveal a semiconducting
antiferromagnetic (AFM) phase with rhombus iron vacancy order. The magnetic
order of the semiconducting phase is the same as the stripe AFM order of the
iron pnictide parent compounds. Moreover, while the root5*root5 block AFM phase
coexists with superconductivity, the stripe AFM order is suppressed by it. This
leads us to conjecture that the new semiconducting magnetic ordered phase is
the true parent phase of this superconductor.Comment: 1 table, 4 figures,5 page
Time Response of Water-based Liquid Scintillator from X-ray Excitation
Water-based liquid scintillators (WbLS) present an attractive target medium
for large-scale detectors with the ability to enhance the separation of
Cherenkov and scintillation signals from a single target. This work
characterizes the scintillation properties of WbLS samples based on LAB/PPO
liquid scintillator (LS). X-ray luminescence spectra, decay profiles, and
relative light yields are measured for WbLS of varying LS concentration as well
as for pure LS with a range of PPO concentrations up to 90 g/L. The
scintillation properties of the WbLS are related to the precursor LAB/PPO:
starting from 90 g/L PPO in LAB before synthesis, the resulting WbLS have
spectroscopic properties that instead match 10 g/L PPO in LAB. This could
indicate that the concentration of active PPO in the WbLS samples depends on
their processing.Comment: 6 pages, 7 figures, 2 tables. Submitted to Materials Advances, a
journal of the Royal Society of Chemistr
Gas Transport in Porous Media: Simulations and Experiments on Partially Densified Aerogels
The experimental density dependence of gas (argon and nitrogen) permeability
of partially densified silica aerogels in the Knudsen regime is quantitatively
accounted for by a computer model. The model simulates both the structure of
the sintered material and the random ballistic motion of a point particle
inside its voids. The same model is also able to account for the densit y
dependence of the specific pore surface as measured from nitrogen adsorption
experiments.Comment: RevTex, 11 pages + 5 postscript figures appended using "uufiles".
Published in Europhys. Lett. 29, p. 567 (1995
Universal magnetic and structural behaviors in the iron arsenides
Commonalities among the order parameters of the ubiquitous antiferromagnetism
present in the parent compounds of the iron arsenide high temperature
superconductors are explored. Additionally, comparison is made between the well
established two-dimensional Heisenberg-Ising magnet, KNiF and iron
arsenide systems residing at a critical point whose structural and magnetic
phase transitions coincide. In particular, analysis is presented regarding two
distinct classes of phase transition behavior reflected in the development of
antiferromagnetic and structural order in the three main classes of iron
arsenide superconductors. Two distinct universality classes are mirrored in
their magnetic phase transitions which empirically are determined by the
proximity of the coupled structural and magnetic phase transitions in these
materials.Comment: 6 pages, 4 figure
Electronic bulk and domain wall properties in B-site doped hexagonal ErMnO
Acceptor and donor doping is a standard for tailoring semiconductors. More
recently, doping was adapted to optimize the behavior at ferroelectric domain
walls. In contrast to more than a century of research on semiconductors, the
impact of chemical substitutions on the local electronic response at domain
walls is largely unexplored. Here, the hexagonal manganite ErMnO is donor
doped with Ti. Density functional theory calculations show that
Ti goes to the B-site, replacing Mn. Scanning probe microscopy
measurements confirm the robustness of the ferroelectric domain template. The
electronic transport at both macro- and nanoscopic length scales is
characterized. The measurements demonstrate the intrinsic nature of emergent
domain wall currents and point towards Poole-Frenkel conductance as the
dominant transport mechanism. Aside from the new insight into the electronic
properties of hexagonal manganites, B-site doping adds an additional degree of
freedom for tuning the domain wall functionality
Europium-doped barium bromide iodide
Single crystals of Ba0.96Eu0.04BrI (barium europium bromide iodide) were grown by the Bridgman technique. The title compound adopts the ordered PbCl2 structure [Braekken (1932 â–¶). Z. Kristallogr.
83, 222–282]. All atoms occupy the fourfold special positions (4c, site symmetry m) of the space group Pnma with a statistical distribution of Ba and Eu. They lie on the mirror planes, perpendicular to the b axis at y = ±0.25. Each cation is coordinated by nine anions in a tricapped trigonal prismatic arrangement
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