1,774 research outputs found
Real-time extraction of growth rates from rotating substrates during molecular-beam epitaxy
We present a method for measuring molecular‐beam epitaxy growth rates in near real‐time on rotating substrates. This is done by digitizing a video image of the reflection high‐energy electron diffraction screen, automatically tracking and measuring the specular spot width, and using numerical techniques to filter the resulting signal. The digitization and image and signal processing take approximately 0.4 s to accomplish, so this technique offers the molecular‐beam epitaxy grower the ability to actively adjust growth times in order to deposit a desired layer thickness. The measurement has a demonstrated precision of approximately 2%, which is sufficient to allow active control of epilayer thickness by counting monolayers as they are deposited. When postgrowth techniques, such as frequency domain analysis, are also used, the reflection high‐energy electron diffraction measurement of layer thickness on rotating substrates improves to a precision of better than 1%. Since all of the components in the system described are commercially available, duplication is straightforward
Critical behavior of 3D Z(N) lattice gauge theories at zero temperature
Three-dimensional lattice gauge theories at zero temperature are
studied for various values of . Using a modified phenomenological
renormalization group, we explore the critical behavior of the generalized
model for . Numerical computations are used to simulate
vector models for for lattices with linear extension up
to . We locate the critical points of phase transitions and establish
their scaling with . The values of the critical indices indicate that the
models with belong to the universality class of the three-dimensional
model. However, the exponent derived from the heat capacity is
consistent with the Ising universality class. We discuss a possible resolution
of this puzzle. We also demonstrate the existence of a rotationally symmetric
region within the ordered phase for all at least in the finite
volume.Comment: 25 pages, 4 figures, 8 table
TMD PDF's: gauge invariance, RG properties and Wilson lines
The UV divergences associated with transverse-momentum dependent (TMD) parton
distribution functions (PDF) are calculated together with the ensuing one-loop
anomalous dimensions in the light-cone gauge. Time-reversal-odd effects in the
anomalous dimensions are observed and the role of Glauber gluons is discussed.
A generalized renormalization procedure of TMD PDFs is proposed, relying upon
the renormalization of contour-dependent operators with obstructions.Comment: 4 pages, 1 figure. Talk presented at the International Workshop on
Diffraction in High Energy and Nuclear Physics, La Londe-les-Maures, France,
9-14 Sept 2008. v2: 5 pages, preprint number and e-mail addresses adde
The phase transitions in 2D Z(N) vector models for N>4
We investigate both analytically and numerically the renormalization group
equations in 2D Z(N) vector models. The position of the critical points of the
two phase transitions for N>4 is established and the critical index \nu\ is
computed. For N=7, 17 the critical points are located by Monte Carlo
simulations and some of the corresponding critical indices are determined. The
behavior of the helicity modulus is studied for N=5, 7, 17. Using these and
other available Monte Carlo data we discuss the scaling of the critical points
with N and some other open theoretical problems.Comment: 19 pages, 8 figures, 10 tables; version to appear on Phys. Rev.
Edge State Tunneling in a Split Hall Bar Model
In this paper we introduce and study the correlation functions of a chiral
one-dimensional electron model intended to qualitatively represent narrow Hall
bars separated into left and right sections by a penetrable barrier. The model
has two parameters representing respectively interactions between top and
bottom edges of the Hall bar and interactions between the edges on opposite
sides of the barrier. We show that the scaling dimensions of tunneling
processes depend on the relative strengths of the interactions, with repulsive
interactions across the Hall bar tending to make breaks in the barrier
irrelevant. The model can be solved analytically and is characterized by a
difference between the dynamics of even and odd Fourier components. We address
its experimental relevance by comparing its predictions with those of a more
geometrically realistic model that must be solved numerically.Comment: 13 pages, including 4 figures,final version as publishe
Scaling Exponents in the Incommensurate Phase of the Sine-Gordon and U(1) Thirring Models
In this paper we study the critical exponents of the quantum sine-Gordon and
U(1) Thirring models in the incommensurate phase. This phase appears when the
chemical potential exceeds a critical value and is characterized by a
finite density of solitons. The low-energy sector of this phase is critical and
is described by the Gaussian model (Tomonaga-Luttinger liquid) with the
compactification radius dependent on the soliton density and the sine-Gordon
model coupling constant .
For a fixed value of , we find that the Luttinger parameter is
equal to 1/2 at the commensurate-incommensurate transition point and approaches
the asymptotic value away from it. We describe a possible phase
diagram of the model consisting of an array of weakly coupled chains. The
possible phases are Fermi liquid, Spin Density Wave, Spin-Peierls and Wigner
crystal.Comment: 10pages; Improved version; Submitted to Physical Review
Phase transitions in strongly coupled 3d Z(N) lattice gauge theories at finite temperature
We perform an analytical and numerical study of the phase transitions in
three-dimensional Z(N) lattice gauge theories at finite temperature for N>4
exploiting equivalence of these models with a generalized version of the
two-dimensional vector Potts models in the limit of vanishing spatial coupling.
In this limit the Polyakov loops play the role of Z(N) spins. The effective
couplings of these two-dimensional spin models are calculated explicitly. It is
argued that the effective spin models have two phase transitions of BKT type.
This is confirmed by large-scale Monte Carlo simulations. Using a cluster
algorithm we locate the position of the critical points and study the critical
behavior across both phase transitions in details. In particular, we determine
various critical indices, compute the helicity modulus, the average action and
the specific heat. A scaling formula for the critical points with N is
proposed.Comment: 28 pages, 12 figures, 12 tables; version to appear on Phys. Rev.
Procedimentos metodológicos utilizados na compilação de dados do Modeflora para construção de mapas dinâmicos no cartão microSD para uso no GPS.
O planejamento da exploração florestal em florestas tropicais, realizada com técnicas de precisão recomendadas pelo Modelo Digital de Exploração Florestal (Modeflora), permite significativas vantagens. Porém, para que o planejamento da exploração florestal seja executado com precisão e agilidade pelas equipes de campo, é necessário que todas as informações estejam consolidadas num mapa dinâmico. Para que todas as informações fiquem consolidadas num único conjunto de mapas dinâmicos, há necessidade de compilar os dados em um formato de arquivo digital (IMG), o qual é reconhecido pelos receptores GPS e navegadores automotivos. As informações permanecem armazenadas num cartão de memória (SD ou microSD) e o aparelho de GPS fica com memória livre para a coleta de pontos e trilhas. Caso não se faça a compilação dos dados para o cartão de memória, a quantidade de waypoints e trilhas a serem inseridas no GPS será limitada, além de impossibilitar a gestão em campo para adicionar ou retirar um mapa de exploração de determinado pátio.bitstream/item/27594/1/Circular-tec-52.pd
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