4,876 research outputs found
Comment on `Formation of a Dodecagonal Quasicrystalline Phase in a Simple Monatomic Liquid'
In a recent paper M. Dzugutov, Phys. Rev. Lett. 70 2924 (1993), describes a
molecular dynamics cooling simulation where he obtained a large monatomic
dodecagonal quasicrystal from a melt. The structure was stabilized by a special
potential [Phys. Rev. A46 R2984 (1992)] designed to prevent the nucleation of
simple dense crystal structures. In this comment we will give evidence that the
ground state structure for Dzugutov's potential is an ordinary bcc crystal
Total scattering descriptions of local and cooperative distortions in the oxide spinel (Mg,Cu)Cr2O4 with dilute Jahn-Teller ions
The normal spinel oxide MgCr2O4 is cubic at room temperature while the normal
spinel CuCr2O4 is tetragonal as a consequence of the Jahn-Teller nature of Cu2+
on the tetrahedral sites. Despite different end-member structures, complete
solid solutions of Mg_{1-x}Cu_xCr2O4 can be prepared that display a first-order
structural transition with composition x = 0.43 at room temperature. Reverse
Monte Carlo analysis of total neutron scattering on data acquired between 300 K
and 15 K on samples with x = 0.10, 0.20, and 0.43 provides unbiased local and
average structure descriptions of the samples, including an understanding of
the transition from local Jahn-Teller distortions in the cubic phase to
cooperative distortions that result in a tetragonal structure. Distributions of
continuous symmetry measures help to understand and distinguish distorted and
undistorted coordination around the tetrahedral site in the solid solutions.
Magnetic exchange bias is observed in field-cooled hysteresis loops of samples
with dilute Cu2+ concentration and in samples with tetragonal--cubic phase
coexistence around 300 K.Comment: 10 pages, 14 figure
Exchange biasing of single-domain Ni nanoparticles spontaneously grown in an antiferromagnetic MnO matrix
Exchange biased composites of ferromagnetic single-domain Ni nanoparticles
embedded within large grains of MnO have been prepared by reduction of
NiMnO phases in flowing hydrogen. The Ni precipitates are 15-30
nm in extent, and the majority are completely encased within the MnO matrix.
The manner in which the Ni nanoparticles are spontaneously formed imparts a
high ferromagnetic- antiferromagnetic interface/volume ratio, which results in
substantial exchange bias effects. Exchange bias fields of up to 100 Oe are
observed, in cases where the starting Ni content in the precursor
NiMnO phase is small. For particles of approximately the same
size, the exchange bias leads to significant hardening of the magnetization,
with the coercive field scaling nearly linearly with the exchange bias field.Comment: 6 pages PDFLaTeX with 9 figure
Recommended from our members
Effects of a Magazine’s Move to Online-only: Post-print Audience Attention and Readership Retention Revisited
For financial reasons, newspapers and magazines are increasingly going online-only. By doing so, some have returned to profitability, but with what consequences for their audiences? To expand the scant evidence base, we conducted a case study of the UK’s New Musical Express (NME) magazine. By analyzing quantitative audience data from official industry sources, we estimate total time spent with the NME by its British audience fell dramatically post-print—by 72%. This fall mirrors that suffered by The Independent newspaper, which went online-only two years earlier. We also report that the NME’s official net weekly and monthly readership increased post-print, although these results are difficult to compare with The Independent’s because the two titles differed in their print publication frequencies. We conclude that the attention periodicals attract via their print editions is unlikely to immediately transfer to their online editions should they go online-only. Building a fuller theory of print platform cessation, however—one that also encompasses changes in readership/reach—requires more comparable data. This case study provides further evidence to suggest that though, for newspapers and magazines, a post-print existence may be less costly, it is also more constrained, with much of the attention they formerly enjoyed simply stripped away
Frequency noise and intensity noise of next-generation gravitational-wave detectors with RF/DC readout schemes
The sensitivity of next-generation gravitational-wave detectors such as
Advanced LIGO and LCGT should be limited mostly by quantum noise with an
expected technical progress to reduce seismic noise and thermal noise. Those
detectors will employ the optical configuration of resonant-sideband-extraction
that can be realized with a signal-recycling mirror added to the Fabry-Perot
Michelson interferometer. While this configuration can reduce quantum noise of
the detector, it can possibly increase laser frequency noise and intensity
noise. The analysis of laser noise in the interferometer with the conventional
configuration has been done in several papers, and we shall extend the analysis
to the resonant-sideband-extraction configuration with the radiation pressure
effect included. We shall also refer to laser noise in the case we employ the
so-called DC readout scheme.Comment: An error in Fig. 10 in the published version in PRD has been
corrected in this version; an erratum has been submitted to PRD. After
correction, this figure reflects a significant difference in the ways RF and
DC readout schemes are susceptible to laser noise. In addition, the levels of
mirror loss imbalances and input laser amplitude noise have also been updated
to be more realistic for Advanced LIG
Interatomic distances and atomic valences in NaZn_(13)
The crystal structure of NaZn_(13) and of several homologous compounds AB_(13) was reported by Ketelaar and by Zintl & Hauke to be based on space group O_h^6-Fm3c, with 8 :Na in 8(a): ¼, ¼:, ¼; ... ; 8 Zn_I in 8(b): 0, 0, 0; .... ; and 96 Zn_(II) in 96(i): 0, y, z; ... . Approximate values were reported for the parameters a_0, y, and z; for NaZn_(13) Zintl & Hauke reported 12.27 Å, 0.178, and 0.122 for these three parameters. Each Zn_I is surrounded by twelve Zn_(II) at the vertices of a nearly regular icosahedron, and each Na by twenty-four Zn_(II) at the vertices of a snub cube. Our interest in the structure was largely concerned with the valences of the two different kinds of Zn atoms, it being presumptive that Zn_I has a larger valence than Zn_(II) because its icosahedral coordination requires it to be smaller than Zn_(II). Lines on new powder photographs of NaZn_(13) were measured and the intensities were estimated visually with as much precision as possible. Least-squares treatments were employed in order to obtain the best possible values for the three parameters; the values obtained are a_0 = 12.2836 ± 0.0003Å, y = 0.1806 ± 0.0003, and z = 0.1192 ± 0.0003. The uncertainties given are calculated standard deviations. Analysis of the interatomic distances yields a selfconsistent interpretation in which Zn_I is assumed to be quinquevalent and Zn_(II) quadrivalent, while Na may have a valence of unity or one as high as 1¼, the excess over unity being suggested by the interatomic distances and being, if real, presumably a consequence of electron transfer. A valence electron number of approximately 432 per unit cell is obtained, which is in good agreement with the value 428.48 predicted on the basis of a filled Brillouin polyhedron defined by the forms {444}, {640}, and {800}
Radiation from low-momentum zoom-whirl orbits
We study zoom-whirl behaviour of equal mass, non-spinning black hole binaries
in full general relativity. The magnitude of the linear momentum of the initial
data is fixed to that of a quasi-circular orbit, and its direction is varied.
We find a global maximum in radiated energy for a configuration which completes
roughly one orbit. The radiated energy in this case exceeds the value of a
quasi-circular binary with the same momentum by 15%. The direction parameter
only requires minor tuning for the localization of the maximum. There is
non-trivial dependence of the energy radiated on eccentricity (several local
maxima and minima). Correlations with orbital dynamics shortly before merger
are discussed. While being strongly gauge dependent, these findings are
intuitive from a physical point of view and support basic ideas about the
efficiency of gravitational radiation from a binary system.Comment: 9 pages, 6 figures, Amaldi8 conference proceedings as publishe
Phase relations in K_xFe_{2-y}Se_2 and the structure of superconducting K_xFe_2Se_2 via high-resolution synchrotron diffraction
Superconductivity in iron selenides has experienced a rapid growth, but not
without major inconsistencies in the reported properties. For
alkali-intercalated iron selenides, even the structure of the superconducting
phase is a subject of debate, in part because the onset of superconductivity is
affected much more delicately by stoichiometry and preparation than in cuprate
or pnictide superconductors. If high-quality, pure, superconducting
intercalated iron selenides are ever to be made, the intertwined physics and
chemistry must be explained by systematic studies of how these materials form
and by and identifying the many coexisting phases. To that end, we prepared
pure K_2Fe_4Se_5 powder and superconductors in the K_xFe_{2-y}Se_2 system, and
examined differences in their structures by high-resolution synchrotron and
single-crystal x-ray diffraction. We found four distinct phases: semiconducting
K_2Fe_4Se_5, a metallic superconducting phase K_xFe_2Se_2 with x ranging from
0.38 to 0.58, an insulator KFe_{1.6}Se_2 with no vacancy ordering, and an
oxidized phase K_{0.51(5)}Fe_{0.70(2)}Se that forms the PbClF structure upon
exposure to moisture. We find that the vacancy-ordered phase K_2Fe_4Se_5 does
not become superconducting by doping, but the distinct iron-rich minority phase
K_xFe_2Se_2 precipitates from single crystals upon cooling from above the
vacancy ordering temperature. This coexistence of metallic and semiconducting
phases explains a broad maximum in resistivity around 100 K. Further studies to
understand the solubility of excess Fe in the K_xFe_{2-y}Se_2 structure will
shed light on the maximum fraction of superconducting K_xFe_2Se_2 that can be
obtained by solid state synthesis.Comment: 12 pages, 16 figures, supplemental materia
Impact of densitized lapse slicings on evolutions of a wobbling black hole
We present long-term stable and second-order convergent evolutions of an
excised wobbling black hole. Our results clearly demonstrate that the use of a
densitized lapse function extends the lifetime of simulations dramatically. We
also show the improvement in the stability of single static black holes when an
algebraic densitized lapse condition is applied. In addition, we introduce a
computationally inexpensive approach for tracking the location of the
singularity suitable for mildly distorted black holes. The method is based on
investigating the fall-off behavior and asymmetry of appropriate grid
variables. This simple tracking method allows one to adjust the location of the
excision region to follow the coordinate motion of the singularity.Comment: 10 pages, 8 figure
- …