972 research outputs found
Critical Behavior in Doping-Driven MetalInsulator Transition on Single-Crystalline Organic Mott-FET
We present the carrier transport properties in the vicinity of a
doping-driven Mott transition observed at a field-effect transistor (FET)
channel using a single crystal of the typical two-dimensional organic Mott
insulator -(BEDT-TTF)CuN(CN)Cl (-Cl).The FET shows a
continuous metalinsulator transition (MIT) as electrostatic doping proceeds.
The phase transition appears to involve two-step crossovers, one in Hall
measurement and the other in conductivity measurement. The crossover in
conductivity occurs around the conductance quantum , and hence is not
associated with "bad metal" behavior, which is in stark contrast to the MIT in
half-filled organic Mott insulators or that in doped inorganic Mott insulators.
Through in-depth scaling analysis of the conductivity, it is found that the
above carrier transport properties in the vicinity of the MIT can be described
by a high-temperature Mott quantum critical crossover, which is theoretically
argued to be a ubiquitous feature of various types of Mott transitions. [This
document is the unedited Authors' version of a Submitted Work that was
subsequently accepted for publication in Nano Letters, copyright \copyright
American Chemical Society after peer review. To access the final edited and
published work see http://dx.doi.org/10.1021/acs.nanolett.6b03817]Comment: 40 pages, 16 figures in Nano Letters, ASAP (2017
Variational Bayes via Propositionalization
We propose a unified approach to VB (variational Bayes) in
symbolic-statistical modeling via propositionalization.
By propositionalization we mean, broadly, expressing and
computing probabilistic models such as BNs (Bayesian
networks) and PCFGs (probabilistic context free grammars)
in terms of propositional logic that considers
propositional variables as binary random variables.
Our proposal is motivated by three observations. The
first one is that PPC (propostionalized probability
computation), i.e. probability computation formalized in
a propositional setting, has turned out to be general and
efficient when variable values are sparsely
interdependent. Examples include (discrete) BNs, PCFGs
and more generally PRISM which is a Turing complete logic
programming language with EM learning ability we have been
developing, and computes probabilities using graphically
represented AND/OR boolean formulas. Efficiency of PPC is
classically testified by the Inside-Outside algorithm in
the case of PCFGs and by recent PPC approaches in the case
of BNs such as the one by Darwiche et al. that exploits
probability and CSI (context specific independence).
Dechter et al. also revealed that PPC is a general
computation scheme for BNs by their formulation of AND/OR
search spaces.
Second of all, while VB has been around for sometime as a
practically effective approach to Bayesian modeling, it\u27s
use is still somewhat restricted to simple models such as
BNs and HMMs (hidden Markov models) though its usefulness
is established through a variety of applications from
model selection to prediction. On the other hand it is
already proved that VB can be extended to PCFGs and is
efficiently implementable using dynamic programming. Note
that PCFGs are just one class of PPC and much more general
PPC is realized by PRISM. Accordingly if VB is extened to
PRISM\u27s PPC, we will obtain VB for general probabilistic
models, far wider than BNs and PCFGs.
The last observation is that once VB becomes available in
PRISM, it saves us a lot of time and energy. First we do
not have to derive a new VB algorithm from scratch for
each model and implement it. All we have to do is just to
write a probabilistic model at predicate level. The rest
of work will be carried out automatically in a unified
manner by the PRISM system as it happens in the case of EM
learning. Deriving and implementing a VB algorithm is a
tedious error-prone process, and ensuring its correctness
would be difficult beyond PCFGs without formal semantics.
PRISM augmented with VB will completely eliminate such
needs and make it easy to explore and test new Bayesian
models by helping the user cope with data sparseness and
avoid over-fitting
Suzaku X-Ray Observations of the Accreting NGC 4839 Group of Galaxies and the Radio Relic in the Coma Cluster
Based on Suzaku X-ray observations, we study the hot gas around the NGC4839
group of galaxies and the radio relic in the outskirts of the Coma cluster. We
find a gradual decline in the gas temperature from 5 keV around NGC4839 to 3.6
keV at the radio relic, across which there is a further, steeper drop down to
1.5 keV. This drop as well as the observed surface brightness profile are
consistent with a shock with Mach number M = 2.2 pm 0.5 and velocity vs = (1410
pm 110) km s^-1. A lower limit of B > 0.33 mu G is derived on the magnetic
field strength around the relic from upper limits to inverse Compton X-ray
emission. Although this suggests that the non-thermal electrons responsible for
the relic are generated by diffusive shock acceleration (DSA), the relation
between the measured Mach number and the electron spectrum inferred from radio
observations are inconsistent with that expected from the simplest,
test-particle theory of DSA. Nevertheless, DSA is still viable if it is
initiated by the injection of a pre-existing population of non-thermal
electrons. Combined with previous measurements, the temperature profile of Coma
in the southwest direction is shallower outside NGC4839 and also slightly
shallower in the outermost region. The metal abundance around NGC4839 is
confirmed to be higher than in its vicinity, implying a significant peak in the
abundance profile that decreases to 0.2 solar toward the outskirts. We
interpret these facts as due to ram pressure stripping of metal-enriched gas
from NGC4839 as it falls into Coma. The relic shock may result from the
combined interaction of pre-existing intracluster gas, gas associated with NGC
4839, and cooler gas flowing in from the large-scale structure filament in the
southwest.Comment: 13 page, accepted for publication in Publications of the Astronomical
Society of Japa
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