68,238 research outputs found
Gravitationally enhanced depolarization of ultracold neutrons in magnetic-field gradients
Trapped ultracold neutrons (UCN) have for many years been the mainstay of experiments to search for the electric dipole moment (EDM) of the neutron, a critical parameter in constraining scenarios of new physics beyond the Standard Model. Because their energies are so low, UCN preferentially populate the lower region of their physical enclosure, and do not sample uniformly the ambient magnetic field throughout the storage volume. This leads to a substantial increase in the rate of depolarization, as well as to shifts in the measured frequency of the stored neutrons. Consequences for EDM measurements are discussed
Unveiling Palomar 2: The Most Obscure Globular Cluster in the Outer Halo
We present the first color-magnitude study for Palomar 2, a distant and
heavily obscured globular cluster near the Galactic anticenter. Our (V,V-I)
color-magnitude diagram (CMD), obtained with the UH8K camera at the CFHT,
reaches V(lim) = 24 and clearly shows the principal sequences of the cluster,
though with substantial overall foreground absorption and differential
reddening. The CMD morphology shows a well populated red horizontal branch with
a sparser extension to the blue, similar to clusters such as NGC 1261, 1851, or
6229 with metallicities near [Fe/H] = -1.3, placing it about 34 kpc
from the Galactic center. We use starcounts of the bright stars to measure the
core radius, half-mass radius, and central concentration of the cluster. Its
integrated luminosity is M_V = -7.9, making it clearly brighter and more
massive than most other clusters in the outer halo.Comment: 25 pages, aastex, with 8 postscript figures; accepted for publication
in AJ, September 1997. Also available by e-mail from
[email protected]. Please consult Harris directly for (big)
postscript files of Figures 1a,b (the images of the cluster
Magnetic susceptibility of diluted pyrochlore and SCGO antiferromagnets
We investigate the magnetic susceptibility of the classical Heisenberg
antiferromagnet with nearest-neighbour interactions on the geometrically
frustrated pyrochlore lattice, for a pure system and in the presence of
dilution with nonmagnetic ions. Using the fact that the correlation length in
this system for small dilution is always short, we obtain an approximate but
accurate expression for the magnetic susceptibility at all temperatures. We
extend this theory to the compound SrCr_{9-9x}Ga_{3+9x}O_{19} (SCGO) and
provide an explanation of the phenomenological model recently proposed by
Schiffer and Daruka [Phys. Rev. B56, 13712 (1997)].Comment: 4 pages, Latex, 4 postscript figures automatically include
The Anomalous Hall Effect in YBaCuO
The temperature dependence of the normal state Hall effect and
magnetoresistance in YBaCuO is investigated using the Nearly
Antiferromagnetic Fermi Liquid description of planar quasiparticles. We find
that highly anisotropic scattering at different regions of the Fermi surface
gives rise to the measured anomalous temperature dependence of the resistivity
and Hall coefficient while yielding the universal temperature dependence of the
Hall angle observed for both clean and dirty samples. This universality is
shown to arise from the limited momentum transfers available for the anomalous,
spin fluctuation scattering and is preserved for any system with strong
antiferromagnetic correlations.Comment: REVTeX, 10 pages + 4 figures in a single (compressed/uuencoded)
PostScript fil
Proposal for a [111] Magnetization Plateau in the Spin Liquid State of Tb2Ti2O7
Despite a Curie-Weiss temperature K, the Tb2Ti2O7
pyrochlore magnetic material lacks long range magnetic order down to at least
mK. It has recently been proposed that the low temperature
collective paramagnetic or spin liquid regime of this material may be akin to a
spin ice state subject to both thermal and quantum fluctuations a {\it
quantum spin ice} (QSI) of sorts. Here we explore the effect of a magnetic
field along the direction on the QSI state. To do so, we
investigate the magnetic properties of a microscopic model of Tb2Ti2O7 in an
independent tetrahedron approximation in a finite along . Such
a model describes semi-quantitatively the collective paramagnetic regime where
nontrivial spin correlations start to develop at the shortest lengthscale, that
is over a single tetrahedron, but where no long range order is yet present. Our
results show that a magnetization plateau develops at low temperatures as the
system develops ferromagnetic spin-ice-like "two-in/two-out"
correlations at the shortest lengthscale. From these results, we are led to
propose that the observation of such a [111] magnetization plateau in Tb2Ti2O7
would provide compelling evidence for a QSI at in this material and
help guide the development of a theory for the origin of its spin liquid state.Comment: 6 pages, 3 figure
Loneliness Across the Life Span
Most people have experienced loneliness and have been able to overcome it to reconnect with other people. In the current review, we provide a life-span perspective on one component of the evolutionary theory of loneliness—a component we refer to as the reaffiliation motive (RAM). The RAM represents the motivation to reconnect with others that is triggered by perceived social isolation. Loneliness is often a transient experience because the RAM leads to reconnection, but sometimes this motivation can fail, leading to prolonged loneliness. We review evidence of how aspects of the RAM change across development and how these aspects can fail for different reasons across the life span. We conclude with a discussion of age-appropriate interventions that may help to alleviate prolonged lonelines
Dipolar Interactions and Origin of Spin Ice in Ising Pyrochlore Magnets
Recent experiments suggest that the Ising pyrochlore magnets and display qualitative
properties of the spin ice model proposed by Harris {\it et al.} \prl {\bf 79},
2554 (1997). We discuss the dipolar energy scale present in both these
materials and consider how they can display spin ice behavior {\it despite} the
presence of long range interactions. Specifically, we present numerical
simulations and a mean field analysis of pyrochlore Ising systems in the
presence of nearest neighbor exchange and long range dipolar interactions. We
find that two possible phases can occur, a long range ordered antiferromagnetic
one and the other dominated by spin ice features. Our quantitative theory is in
very good agreement with experimental data on both
and . We suggest that the nearest neighbor exchange in
is {\it antiferromagnetic} and that spin ice behavior
is induced by long range dipolar interactions.Comment: 4 postscript figures included. Submitted to Physical Review Letters
Contact: [email protected]
Critical Dynamics of the Contact Process with Quenched Disorder
We study critical spreading dynamics in the two-dimensional contact process
(CP) with quenched disorder in the form of random dilution. In the pure model,
spreading from a single particle at the critical point is
characterized by the critical exponents of directed percolation: in
dimensions, , , and . Disorder causes a
dramatic change in the critical exponents, to , , and . These exponents govern spreading following
a long crossover period. The usual hyperscaling relation, , is violated. Our results support the conjecture by Bramson, Durrett, and
Schonmann [Ann. Prob. {\bf 19}, 960 (1991)], that in two or more dimensions the
disordered CP has only a single phase transition.Comment: 11 pages, REVTeX, four figures available on reques
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