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 Ni$_x$Mn$_{1-x}$O$_4$ 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 $x$ in the precursor Ni$_x$Mn$_{1-x}$O$_4$ 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

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