6,015 research outputs found
Investigations of the effect of nonmagnetic Ca substitution for magnetic Dy on spin-freezing in Dy2Ti2O7
Physical properties of partially Ca substituted hole-doped Dy2Ti2O7 have been
investigated by ac magnetic susceptibility \chi_ac(T), dc magnetic
susceptibility \chi(T), isothermal magnetization M(H) and heat capacity C_p(T)
measurements on Dy1.8Ca0.2Ti2O7. The spin-ice system Dy2Ti2O7 exhibits a
spin-glass type freezing behavior near 16 K. Our frequency dependent \chi_ac(T)
data of Dy1.8Ca0.2Ti2O7 show that the spin-freezing behavior is significantly
influenced by Ca substitution. The effect of partial nonmagnetic Ca2+
substitution for magnetic Dy3+ is similar to the previous study on nonmagnetic
isovalent Y3+ substituted Dy2-xYxTi2O7 (for low levels of dilution), however
the suppression of spin-freezing behavior is substantially stronger for Ca than
Y. The Cole-Cole plot analysis reveals semicircular character and a single
relaxation mode in Dy1.8Ca0.2Ti2O7 as for Dy2Ti2O7. No noticeable change in the
insulating behavior of Dy2Ti2O7 results from the holes produced by 10% Ca2+
substitution for Dy3+ ions.Comment: 9 pages, 7 figures, 1 tabl
Gravitational Collapse of Dust with a Cosmological Constant
The recent analysis of Markovic and Shapiro on the effect of a cosmological
constant on the evolution of a spherically symmetric homogeneous dust ball is
extended to include the inhomogeneous and degenerate cases. The histories are
shown by way of effective potential and Penrose-Carter diagrams.Comment: 2 pages, 2 figures (png), revtex. To appear in Phys. Rev.
Galactic Potentials
The information contained in galactic rotation curves is examined under a
minimal set of assumptions. If emission occurs from stable circular geodesic
orbits of a static spherically symmetric field, with information propagated to
us along null geodesics, observed rotation curves determine galactic potentials
without specific reference to any metric theory of gravity. Given the
potential, the gravitational mass can be obtained by way of an anisotropy
function of this field. The gravitational mass and anisotropy function can be
solved for simultaneously in a Newtonian limit without specifying any specific
source. This procedure, based on a minimal set of assumptions, puts very strong
constraints on any model of the "dark matter".Comment: A somewhat longer form of the final version to appear in Physical
Review Letters.Clarification and further reference
Magnetic correlations of the quasi-one-dimensional half-integer spin-chain antiferromagnets SrVO ( = Co, Mn)
Magnetic correlations of two iso-structural quasi-one-dimensional (1D)
antiferromagnetic spin-chain compounds SrVO ( = Co, Mn) have
been investigated by magnetization and powder neutron diffraction. Two
different collinear antiferromagnetic (AFM) structures, characterized by the
propagation vectors, = (0 0 1) and = (0 0 0), have been found below
5.2 K and 42.2 K for the Co- and Mn-compounds, respectively. For
the Mn-compound, AFM chains (along the axis) order ferromagnetically within
the plane, whereas, for the Co-compound, AFM chains order
ferro-/antiferromagnetically along the direction. The critical exponent
study confirms that the Co- and Mn-compounds belong to the Ising and Heisenberg
universality classes, respectively. For both compounds, short-range spin-spin
correlations are present over a wide temperature range above . The reduced
ordered moments at base temperature (1.5 K) indicate the presence of quantum
fluctuations in both compounds due to the quasi-1D magnetic interactions.Comment: 14 pages, 10 figures, 9 table
Kaluza-Klein solitons reexamined
In (4 + 1) gravity the assumption that the five-dimensional metric is
independent of the fifth coordinate authorizes the extra dimension to be either
spacelike or timelike. As a consequence of this, the time coordinate and the
extra coordinate are interchangeable, which in turn allows the conception of
different scenarios in 4D from a single solution in 5D. In this paper, we make
a thorough investigation of all possible 4D scenarios, associated with this
interchange, for the well-known Kramer-Gross-Perry-Davidson-Owen set of
solutions. We show that there are {\it three} families of solutions with very
distinct geometrical and physical properties. They correspond to different sets
of values of the parameters which characterize the solutions in 5D. The
solutions of physical interest are identified on the basis of physical
requirements on the induced-matter in 4D. We find that only one family
satisfies these requirements; the other two violate the positivity of
mass-energy density. The "physical" solutions possess a lightlike singularity
which coincides with the horizon. The Schwarzschild black string solution as
well as the zero moment dipole solution of Gross and Perry are obtained in
different limits. These are analyzed in the context of Lake's geometrical
approach. We demonstrate that the parameters of the solutions in 5D are not
free, as previously considered. Instead, they are totally determined by
measurements in 4D. Namely, by the surface gravitational potential of the
astrophysical phenomena, like the Sun or other stars, modeled in Kaluza-Klein
theory. This is an important result which may help in observations for an
experimental/observational test of the theory.Comment: In V2 we include an Appendix, where we examine the conformal
approach. Minor changes at the beginning of section 2. In V3 more references
are added. Minor editorial changes in the Introduction and Conclusions
section
Consumption of submerged aquatic macrophytes by rudd (scardinius erythrophthalmus L.) in New Zealand
In experiments in New Zealand, rudd (Scardinius erythrophthalmus L.) of 108â277mm fork length (FL) ate a wide range of native and introduced submerged aquatic macrophytes in captivity and in the field. Rudd consumed the native charophytes Chara globularis Thuill., Chara fibrosa Ag. ex Bruz., and Nitella spp., the native macrophytes Potamogeton ochreatus Raoul. and Myriophyllum propinquum A. Cunn., and the introduced macrophytes Elodea canadensis Michx., Egeria densa Planch., Lagarosiphon major L., and Ceratophyllum demersum L. Rudd consistently consumed the Nitella spp. and Potamogeton ochreatus before Ceratophyllum demersum. From the results of experiments in tanks and in the field, we found the order of highest to lowest palatability was: Nitella spp. > Potamogeton ochreatus > Elodea canadensis> Chara globularis = Chara fibrosa> Egeria densa = Lagarosiphon major > Myriophyllum propinquum > Ceratophyllum demersum. The order of consumption was subject to some variation with season, especially for Egeria densa, Lagarosiphon major, and Myriophyllum propinquum. Rudd
consumed up to 20% of their body weight per day of Egeria densa in spring, and 22% of their body weight per day of Nitella spp. in summer. Consumption rates were considerably lower in winter than in summer. The results of our field trial suggested that the order of consumption also applies in the field and that rudd are having a profound impact on vulnerable native aquatic plant communities in New Zealand. Nitella spp. and Potamogeton ochreatus are likely to be selectively eaten, and herbivory by rudd might prevent the re-establishment of these species in
restoration efforts
Optimisation of two-dimensional ion trap arrays for quantum simulation
The optimisation of two-dimensional (2D) lattice ion trap geometries for
trapped ion quantum simulation is investigated. The geometry is optimised for
the highest ratio of ion-ion interaction rate to decoherence rate. To calculate
the electric field of such array geometries a numerical simulation based on a
"Biot-Savart like law" method is used. In this article we will focus on square,
hexagonal and centre rectangular lattices for optimisation. A method for
maximising the homogeneity of trapping site properties over an array is
presented for arrays of a range of sizes. We show how both the polygon radii
and separations scale to optimise the ratio between the interaction and
decoherence rate. The optimal polygon radius and separation for a 2D lattice is
found to be a function of the ratio between rf voltage and drive frequency
applied to the array. We then provide a case study for 171Yb+ ions to show how
a two-dimensional quantum simulator array could be designed
Building Machines That Learn and Think Like People
Recent progress in artificial intelligence (AI) has renewed interest in
building systems that learn and think like people. Many advances have come from
using deep neural networks trained end-to-end in tasks such as object
recognition, video games, and board games, achieving performance that equals or
even beats humans in some respects. Despite their biological inspiration and
performance achievements, these systems differ from human intelligence in
crucial ways. We review progress in cognitive science suggesting that truly
human-like learning and thinking machines will have to reach beyond current
engineering trends in both what they learn, and how they learn it.
Specifically, we argue that these machines should (a) build causal models of
the world that support explanation and understanding, rather than merely
solving pattern recognition problems; (b) ground learning in intuitive theories
of physics and psychology, to support and enrich the knowledge that is learned;
and (c) harness compositionality and learning-to-learn to rapidly acquire and
generalize knowledge to new tasks and situations. We suggest concrete
challenges and promising routes towards these goals that can combine the
strengths of recent neural network advances with more structured cognitive
models.Comment: In press at Behavioral and Brain Sciences. Open call for commentary
proposals (until Nov. 22, 2016).
https://www.cambridge.org/core/journals/behavioral-and-brain-sciences/information/calls-for-commentary/open-calls-for-commentar
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