14,097 research outputs found
Metropolis-Hastings within Partially Collapsed Gibbs Samplers
The Partially Collapsed Gibbs (PCG) sampler offers a new strategy for
improving the convergence of a Gibbs sampler. PCG achieves faster convergence
by reducing the conditioning in some of the draws of its parent Gibbs sampler.
Although this can significantly improve convergence, care must be taken to
ensure that the stationary distribution is preserved. The conditional
distributions sampled in a PCG sampler may be incompatible and permuting their
order may upset the stationary distribution of the chain. Extra care must be
taken when Metropolis-Hastings (MH) updates are used in some or all of the
updates. Reducing the conditioning in an MH within Gibbs sampler can change the
stationary distribution, even when the PCG sampler would work perfectly if MH
were not used. In fact, a number of samplers of this sort that have been
advocated in the literature do not actually have the target stationary
distributions. In this article, we illustrate the challenges that may arise
when using MH within a PCG sampler and develop a general strategy for using
such updates while maintaining the desired stationary distribution. Theoretical
arguments provide guidance when choosing between different MH within PCG
sampling schemes. Finally we illustrate the MH within PCG sampler and its
computational advantage using several examples from our applied work
Deep levels and radiation effects in p-InP
A survey was conducted on past studies of hole traps in InP. An experiment was designed to evaluate hole traps in Zn-doped InP after fabrication, after electron irradiation and after annealing using deep level transient spectroscopy. Data similar to that of Yamaguchi was seen with observation of both radiation-induced hole and electron traps at E sub A=0.45 eV and 0.03 eV, respectively. Both traps are altered by annealing. It is also shown that trap parameters for surface-barrier devices are influenced by many factors such as bias voltage, which probes traps at different depths below the surface. These devices require great care in data evaluation
Quasi-Rip: A New Type of Rip Model without Cosmic Doomsday
The fate of our universe is an unceasing topic of cosmology and the human
being. The discovery of the current accelerated expansion of the universe
significantly changed our view of the fate of the universe. Recently, some
interesting scenarios concerning the fate of the universe attracted much
attention in the community, namely the so-called "Little Rip" and "Pseudo-Rip".
It is worth noting that all the Big Rip, Little Rip and Pseudo-Rip arise from
the assumption that the dark energy density is monotonically
increasing. In the present work, we are interested to investigate what will
happen if this assumption is broken, and then propose a so-called "Quasi-Rip"
scenario, which is driven by a type of quintom dark energy. In this work, we
consider an explicit model of Quasi-Rip in detail. We show that Quasi-Rip has
an unique feature different from Big Rip, Little Rip and Pseudo-Rip. Our
universe has a chance to be rebuilt from the ashes after the terrible rip. This
might be the last hope in the "hopeless" rip.Comment: 9 pages, 2 figures, 1 table, revtex4; v2: discussions added, Phys.
Rev. D in press; v3: published versio
Is it possible to observe experimentally a metal-insulator transition in ultra cold atoms?
Kicked rotors with certain non-analytic potentials avoid dynamical
localization and undergo a metal-insulator transition. We show that typical
properties of this transition are still present as the non-analyticity is
progressively smoothed out provided that the smoothing is less than a certain
limiting value. We have identified a smoothing dependent time scale such that
full dynamical localization is absent and the quantum momentum distribution
develops power-law tails with anomalous decay exponents as in the case of a
conductor at the metal-insulator transition. We discuss under what conditions
these findings may be verified experimentally by using ultra cold atoms
techniques. It is found that ultra-cold atoms can indeed be utilized for the
experimental investigation of the metal-insulator transition.Comment: 7 pages, 3 figure
A totally laparoscopic associating liver partition and portal vein ligation for staged hepatectomy assisted with radiofrequency (radiofrequency assisted liver partition with portal vein ligation) for staged liver resection
In order to induce liver hypertrophy to enable liver resection in patients with a small future liver remnant, various methods have been proposed in addition to portal vein embolisation. Most recently, the ALPPS technique has gained significant international interest. This technique is limited by the high morbidity associated with an in-situ liver splitting and the patient undergoing two open operations. We present the case of a variant ALPPS technique performed entirely laparoscopically with no major morbidity or mortality. An increased liver volume of 57.9% was seen after 14 days. This technique is feasible to perform and compares favourably to other ALPPS methods whilst gaining the advantages of laparoscopic surgery
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