26,674 research outputs found
Spin fluctuations with two-dimensional XY behavior in a frustrated S = 1/2 square-lattice ferromagnet
The spin dynamics of the layered square-lattice vanadate Pb2VO(PO4)2 is
investigated by electron spin resonance at various magnetic fields and at
temperatures above magnetic ordering. The linewidth divergence towards low
temperatures seems to agree with isotropic Heisenberg-type spin exchange
suggesting that the spin relaxation in this quasi-two dimensional compound is
governed by low-dimensional quantum fluctuations. However, a weak easy- plane
anisotropy of the g factor points to the presence of a planar XY type of
exchange. Indeed, we found that the linewidth divergence is described best by
XY-like spin fluctuations which requires a single parameter only. Therefore,
ESR-probed spin dynamics could establish Pb2VO(PO4)2 as the first frustrated
square lattice system with XY-inherent spin topological fluctuations.Comment: 5 pages, 3 figure
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Are Low Interest Rates Deflationary? A Paradox of Perfect-Foresight Analysis
A prolonged period of extremely low nominal interest rates has not resulted in high inflation. This has led to increased interest in the âNeo-Fisherianâ proposition according to which low nominal interest rates may themselves cause inflation to be lower. The fact that standard models of the effects of monetary policy have the property that perfect foresight equilibria in which the nominal interest rate remains low forever necessarily involve low inflation (at least eventually) might seem to support such a view. Here, however, we argue that such a conclusion depends on a misunderstanding of the circumstances under which it makes sense to predict the effects of a monetary policy commitment by calculating the perfect foresight equilibrium consistent with the policy. We propose an explicit cognitive process by which agents may form their expectations of future endogenous variables. Under some circumstances, such as a commitment to follow a Taylor rule, a perfect foresight equilibrium (PFE) can arise as a limiting case of our more general concept of reflective equilibrium, when the process of reflection is pursued sufficiently far. But we show that an announced intention to fix the nominal interest rate for a long enough period of time creates a situation in which reflective equilibrium need not resemble any PFE. In our view, this makes PFE predictions not plausible outcomes in the case of policies of the latter sort. According to the alternative approach that we recommend, a commitment to maintain a low nominal interest rate for longer should always be expansionary and inflationary, rather than causing deflation; but the effects of such âforward guidanceâ are likely, in the case of a long-horizon commitment, to be much less expansionary or inflationary than the usual PFE analysis would imply
SWIFT J1749.4-2807: A neutron or quark star?
We investigate an unique accreting millisecond pulsar with X-ray eclipses,
SWIFT J1749.42807 (hereafter J1749), and try to limit the binary system by
various methods including that of the Roche lobe, the mass-radius relations of
both a main sequence (MS) and a white dwarf (WD) companion stars, as well as
the measured mass function of the pulsar. The calculations are based on the
assumption that the radius of the companion star has reached its Roche radius
(or at 90%), but the pulsar's mass has not been assumed to be a certain value.
Our results are as follows. The companion star should be a MS. For the case
that the radius equals to its Roche one, we have a companion star with mass
and radius , and the
inclination angle is ; for the case that the radius
reaches 90% of its Roche one, we have , and . We also obtain the mass of J1749,
, and conclude that the pulsar could be a quark star
if the ratio of the critical frequency of rotation-mode instability to the
Keplerian one is higher than . The relatively low pulsar mass (about
) may also challenge the conventional recycling scenario for the
origin and evolution of millisecond pulsars. The results presented in this
paper are expected to be tested by future observations.Comment: RAA accepte
The Rad4TopBP1 ATR-Activation domain functions in G1/S phase in a chromatin-dependent manner
DNA damage checkpoint activation can be subdivided in two steps: initial activation and signal amplification. The events
distinguishing these two phases and their genetic determinants remain obscure. TopBP1, a mediator protein containing
multiple BRCT domains, binds to and activates the ATR/ATRIP complex through its ATR-Activation Domain (AAD). We show
that Schizosaccharomyces pombe Rad4TopBP1 AADâdefective strains are DNA damage sensitive during G1/S-phase, but not
during G2. Using lacO-LacI tethering, we developed a DNA damageâindependent assay for checkpoint activation that is
Rad4TopBP1 AADâdependent. In this assay, checkpoint activation requires histone H2A phosphorylation, the interaction
between TopBP1 and the 9-1-1 complex, and is mediated by the phospho-binding activity of Crb253BP1. Consistent with a
model where Rad4TopBP1 AADâdependent checkpoint activation is ssDNA/RPAâindependent and functions to amplify
otherwise weak checkpoint signals, we demonstrate that the Rad4TopBP1 AAD is important for Chk1 phosphorylation when
resection is limited in G2 by ablation of the resecting nuclease, Exo1. We also show that the Rad4TopBP1 AAD acts additively
with a Rad9 AAD in G1/S phase but not G2. We propose that AADâdependent Rad3ATR checkpoint amplification is
particularly important when DNA resection is limiting. In S. pombe, this manifests in G1/S phase and relies on proteinâ
chromatin interactions
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