205 research outputs found
Is Quantum Einstein Gravity Nonperturbatively Renormalizable?
We find considerable evidence supporting the conjecture that four-dimensional
Quantum Einstein Gravity is ``asymptotically safe'' in Weinberg's sense. This
would mean that the theory is likely to be nonperturbatively renormalizable and
thus could be considered a fundamental (rather than merely effective) theory
which is mathematically consistent and predictive down to arbitrarily small
length scales. For a truncated version of the exact flow equation of the
effective average action we establish the existence of a non-Gaussian
renormalization group fixed point which is suitable for the construction of a
nonperturbative infinite cutoff-limit. The truncation ansatz includes the
Einstein-Hilbert action and a higher derivative term.Comment: 18 pages, latex, 3 figure
Yard-Sale exchange on networks: Wealth sharing and wealth appropriation
Yard-Sale (YS) is a stochastic multiplicative wealth-exchange model with two
phases: a stable one where wealth is shared, and an unstable one where wealth
condenses onto one agent. YS is here studied numerically on 1d rings, 2d square
lattices, and random graphs with variable average coordination, comparing its
properties with those in mean field (MF). Equilibrium properties in the stable
phase are almost unaffected by the introduction of a network. Measurement of
decorrelation times in the stable phase allow us to determine the critical
interface with very good precision, and it turns out to be the same, for all
networks analyzed, as the one that can be analytically derived in MF. In the
unstable phase, on the other hand, dynamical as well as asymptotic properties
are strongly network-dependent. Wealth no longer condenses on a single agent,
as in MF, but onto an extensive set of agents, the properties of which depend
on the network. Connections with previous studies of coalescence of immobile
reactants are discussed, and their analytic predictions are successfully
compared with our numerical results.Comment: 10 pages, 7 figures. Submitted to JSTA
Observation of Fermi-surface-dependent nodeless superconducting gaps in Ba0.6K0.4Fe2As2
The recent discovery of superconductivity in iron-arsenic compounds below a
transition temperature (Tc) as high as 55K ended the monopoly of copper oxides
(cuprates) in the family of high-Tc superconductors. A critical issue in
understanding this new superconductor, as in the case of cuprates, is the
nature, in particular the symmetry and orbital dependence, of the
superconducting gap. There are conflicting experimental results, mostly from
indirect measurements of the low energy excitation gap, ranging from one gap to
two gaps, from line nodes to nodeless gap function in momentum space. Here we
report a direct observation of the superconducting gap, including its momentum,
temperature, and Fermi surface (FS) dependence in Ba0.6K0.4Fe2As2 (Tc = 37 K)
using angle-resolved photoelectron spectroscopy. We find two superconducting
gaps with different values: a large gap (~ 12 meV) on the two small hole-like
and electron-like FS sheets, and a small gap (~ 6 meV) on the large hole-like
FS. Both gaps, closing simultaneously at the bulk Tc, are nodeless and nearly
isotropic around their respective FS sheets. The isotropic pairing interactions
are strongly orbital dependent, as the ratio 2Delta/kBTc switches from weak to
strong coupling on different bands. The same and surprisingly large
superconducting gap due to strong pairing on the two small FS, which are
connected by the (pi, 0) spin-density-wave vector in the parent compound,
strongly suggests that the pairing mechanism originates from the inter-band
interactions between these two nested FS sheets.Comment: 4 figure
Superconducting gap symmetry of Ba0.6K0.4Fe2As2 studied by angle-resolved photoemission spectroscopy
We have performed high-resolution angle-resolved photoemission spectroscopy
on the optimally-doped BaKFeAs compound and determined
the accurate momentum dependence of the superconducting (SC) gap in four
Fermi-surface sheets including a newly discovered outer electron pocket at the
M point. The SC gap on this pocket is nearly isotropic and its magnitude is
comparable ( 11 meV) to that of the inner electron and hole
pockets (12 meV), although it is substantially larger than that of the
outer hole pocket (6 meV). The Fermi-surface dependence of the SC gap
value is basically consistent with () = coscos
formula expected for the extended s-wave symmetry. The observed finite
deviation from the simple formula suggests the importance of multi-orbital
effects.Comment: 4 pages, 3 figures, 1 tabl
Shadow bands in single-layered Bi_2Sr_2CuO_6 studied by angle-resolved photoemission spectroscopy
We have performed systematic angle-resolved photoemission spectroscopy
(ARPES) on single-layered cuprate superconductor Bi2Sr2CuO6 to elucidate the
origin of shadow band. We found that the shadow band is exactly the c(2x2)
replica of the main band irrespective of the carrier concentration and its
intensity is invariable with respect to temperature, doping, and substitution
constituents of block layers. This result rules out the possibility of
antiferromagnetic correlation and supports the structural origin of shadow
band. ARPES experiments on optimally doped La1.85Sr0.15CuO4 also clarified the
existence of the c(2x2) shadow band, demonstrating that the shadow band is not
a unique feature of Bi-based cuprates. We conclude that the shadow band is
related to the orthorhombic distortion at the crystal surface.Comment: 6 pages, 4figure
Breakdown of the mean-field approximation in a wealth distribution model
One of the key socioeconomic phenomena to explain is the distribution of
wealth. Bouchaud and M\'ezard have proposed an interesting model of economy
[Bouchaud and M\'ezard (2000)] based on trade and investments of agents. In the
mean-field approximation, the model produces a stationary wealth distribution
with a power-law tail. In this paper we examine characteristic time scales of
the model and show that for any finite number of agents, the validity of the
mean-field result is time-limited and the model in fact has no stationary
wealth distribution. Further analysis suggests that for heterogeneous agents,
the limitations are even stronger. We conclude with general implications of the
presented results.Comment: 11 pages, 3 figure
Correlation Structures of Correlated Binomial Models and Implied Default Distribution
We show how to analyze and interpret the correlation structures, the
conditional expectation values and correlation coefficients of exchangeable
Bernoulli random variables. We study implied default distributions for the
iTraxx-CJ tranches and some popular probabilistic models, including the
Gaussian copula model, Beta binomial distribution model and long-range Ising
model. We interpret the differences in their profiles in terms of the
correlation structures. The implied default distribution has singular
correlation structures, reflecting the credit market implications. We point out
two possible origins of the singular behavior.Comment: 16 pages, 7 figure
Small-scale anisotropy of cosmic rays above 10^19eV observed with the Akeno Giant Air Shower Array
With the Akeno Giant Air Shower Array (AGASA), 581 cosmic rays above 10^19eV,
47 above 4 x 10^19eV, and 7 above 10^20eV are observed until August 1998.
Arrival direction distribution of these extremely high energy cosmic rays has
been studied. While no significant large-scale anisotropy is found on the
celestial sphere, some interesting clusters of cosmic rays are observed. Above
4 x 10^19eV, there are one triplet and three doublets within separation angle
of 2.5^o and the probability of observing these clusters by a chance
coincidence under an isotropic distribution is smaller than 1 %. Especially the
triplet is observed against expected 0.05 events. The cos(\theta_GC)
distribution expected from the Dark Matter Halo model fits the data as well as
an isotropic distribution above 2 x 10^19eV and 4 x 10^19eV, but is a poorer
fit than isotropy above 10^19eV. Arrival direction distribution of seven
10^20eV cosmic rays is consistent with that of lower energy cosmic rays and is
uniform. Three of seven are members of doublets above about 4 x 10^19eV.Comment: 40 pages, 12 figure, AASTeX *** Authors found a typo on Table 2 --
Energy of event 94/07/06 **
Infectious Default Model with Recovery and Continuous Limit
We introduce an infectious default and recovery model for N obligors.
Obligors are assumed to be exchangeable and their states are described by N
Bernoulli random variables S_{i} (i=1,...,N). They are expressed by multiplying
independent Bernoulli variables X_{i},Y_{ij},Y'_{ij}, and default and recovery
infections are described by Y_{ij} and Y'_{ij}. We obtain the default
probability function P(k) for k defaults. Taking its continuous limit, we find
two nontrivial probability distributions with the reflection symmetry of S_{i}
\leftrightarrow 1-S_{i}. Their profiles are singular and oscillating and we
understand it theoretically. We also compare P(k) with an implied default
distribution function inferred from the quotes of iTraxx-CJ. In order to
explain the behavior of the implied distribution, the recovery effect may be
necessary.Comment: 13 pages, 7 figure
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