404 research outputs found
The continuum limit of the non-commutative lambda phi^4 model
We present a numerical study of the \lambda \phi^{4} model in three Euclidean
dimensions, where the two spatial coordinates are non-commutative (NC). We
first show the explicit phase diagram of this model on a lattice. The ordered
regime splits into a phase of uniform order and a ``striped phase''. Then we
discuss the dispersion relation, which allows us to introduce a dimensionful
lattice spacing. Thus we can study a double scaling limit to zero lattice
spacing and infinite volume, which keeps the non-commutativity parameter
constant. The dispersion relation in the disordered phase stabilizes in this
limit, which represents a non-perturbative renormalization. From its shape we
infer that the striped phase persists in the continuum, and we observe UV/IR
mixing as a non-perturbative effect.Comment: 3 pages, 3 figures, talk presented by W.B. at the 11th Regional
Conference on Mathematical Physics, Tehran, May 3-6, 200
Non--Commutative Field Theories beyond Perturbation Theory
We investigate two models in non-commutative (NC) field theory by means of
Monte Carlo simulations. Even if we start from the Euclidean lattice
formulation, such simulations are only feasible after mapping the systems onto
dimensionally reduced matrix models. Using this technique, we measure Wilson
loops in 2d NC gauge theory of rank 1. It turns out that they are
non-perturbatively renormalizable, and the phase follows an Aharonov-Bohm
effect if we identify \theta = 1/B. Next we study the 3d \lambda \phi^{4} model
with two NC coordinates, where we present new results for the correlators and
the dispersion relation. We further reveal the explicit phase diagram. The
ordered regime splits into a uniform and a striped phase, as it was
qualitatively conjectured before. We also confirm the recent observation by
Ambjorn and Catterall that such stripes occur even in d=2, although they imply
the spontaneous breaking of translation symmetry. However, in d=3 and d=2 we
observe only patterns of two stripes to be stable in the range of parameters
investigated.Comment: 8 pages, 8 figures, talk presented at 35th Ahrenshoop Symposiu
Numerical results on the Non-commutative \lambda \phi^4 Model
The UV/IR mixing in the \lambda \phi^4 model on a non-commutative (NC) space
leads to new predictions in perturbation theory, including Hartree-Fock type
approximations. Among them there is a changed phase diagram and an unusual
behavior of the correlation functions. In particular this mixing leads to a
deformation of the dispersion relation. We present numerical results for these
effects in d=3 with two NC coordinates.Comment: 3 pages, 6 figures, Talk presented at Lattice2003(theory
Optimally Stabilized PET Image Denoising Using Trilateral Filtering
Low-resolution and signal-dependent noise distribution in positron emission
tomography (PET) images makes denoising process an inevitable step prior to
qualitative and quantitative image analysis tasks. Conventional PET denoising
methods either over-smooth small-sized structures due to resolution limitation
or make incorrect assumptions about the noise characteristics. Therefore,
clinically important quantitative information may be corrupted. To address
these challenges, we introduced a novel approach to remove signal-dependent
noise in the PET images where the noise distribution was considered as
Poisson-Gaussian mixed. Meanwhile, the generalized Anscombe's transformation
(GAT) was used to stabilize varying nature of the PET noise. Other than noise
stabilization, it is also desirable for the noise removal filter to preserve
the boundaries of the structures while smoothing the noisy regions. Indeed, it
is important to avoid significant loss of quantitative information such as
standard uptake value (SUV)-based metrics as well as metabolic lesion volume.
To satisfy all these properties, we extended bilateral filtering method into
trilateral filtering through multiscaling and optimal Gaussianization process.
The proposed method was tested on more than 50 PET-CT images from various
patients having different cancers and achieved the superior performance
compared to the widely used denoising techniques in the literature.Comment: 8 pages, 3 figures; to appear in the Lecture Notes in Computer
Science (MICCAI 2014
Superconducting properties of very high quality NbN thin films grown by high temperature chemical vapor deposition
Niobium nitride (NbN) is widely used in high-frequency superconducting
electronics circuits because it has one of the highest superconducting
transition temperatures ( 16.5 K) and largest gap among
conventional superconductors. In its thin-film form, the of NbN is very
sensitive to growth conditions and it still remains a challenge to grow NbN
thin film (below 50 nm) with high . Here, we report on the superconducting
properties of NbN thin films grown by high-temperature chemical vapor
deposition (HTCVD). Transport measurements reveal significantly lower disorder
than previously reported, characterized by a Ioffe-Regel ()
parameter of 14. Accordingly we observe 17.06 K (point of
50% of normal state resistance), the highest value reported so far for films of
thickness below 50 nm, indicating that HTCVD could be particularly useful for
growing high quality NbN thin films
The Bright Side of Coulomb Blockade
We explore the photonic (bright) side of dynamical Coulomb blockade (DCB) by
measuring the radiation emitted by a dc voltage-biased Josephson junction
embedded in a microwave resonator. In this regime Cooper pair tunneling is
inelastic and associated to the transfer of an energy 2eV into the resonator
modes. We have measured simultaneously the Cooper pair current and the photon
emission rate at the resonance frequency of the resonator. Our results show two
regimes, in which each tunneling Cooper pair emits either one or two photons
into the resonator. The spectral properties of the emitted radiation are
accounted for by an extension to DCB theory.Comment: 4 pages, 4 figures + 3 pages, 1 figure supplementary materia
On the Quantum Geometry of String Theory
The IKKT or IIB matrix model has been proposed as a non-perturbative
definition of type IIB superstring theories. It has the attractive feature that
space--time appears dynamically. It is possible that lower dimensional
universes dominate the theory, therefore providing a dynamical solution to the
reduction of space--time dimensionality. We summarize recent works that show
the central role of the phase of the fermion determinant in the possible
realization of such a scenario.Comment: 3 pages, 2 figures, Lattice2001(surfaces
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