9,028 research outputs found
Magneto-Conductance Anisotropy and Interference Effects in Variable Range Hopping
We investigate the magneto-conductance (MC) anisotropy in the variable range
hopping regime, caused by quantum interference effects in three dimensions.
When no spin-orbit scattering is included, there is an increase in the
localization length (as in two dimensions), producing a large positive MC. By
contrast, with spin-orbit scattering present, there is no change in the
localization length, and only a small increase in the overall tunneling
amplitude. The numerical data for small magnetic fields , and hopping
lengths , can be collapsed by using scaling variables , and
in the perpendicular and parallel field orientations
respectively. This is in agreement with the flux through a `cigar'--shaped
region with a diffusive transverse dimension proportional to . If a
single hop dominates the conductivity of the sample, this leads to a
characteristic orientational `finger print' for the MC anisotropy. However, we
estimate that many hops contribute to conductivity of typical samples, and thus
averaging over critical hop orientations renders the bulk sample isotropic, as
seen experimentally. Anisotropy appears for thin films, when the length of the
hop is comparable to the thickness. The hops are then restricted to align with
the sample plane, leading to different MC behaviors parallel and perpendicular
to it, even after averaging over many hops. We predict the variations of such
anisotropy with both the hop size and the magnetic field strength. An
orientational bias produced by strong electric fields will also lead to MC
anisotropy.Comment: 24 pages, RevTex, 9 postscript figures uuencoded Submitted to PR
Reação de cultivares e clones de batata a Meloidogyne javanica. reaction of potato cultivars and clones to Meloidogyne javanica.
Analytical results on quantum interference and magnetoconductance for strongly localized electrons in a magnetic field: Exact summation of forward-scattering paths
We study quantum interference effects on the transition strength for strongly
localized electrons hopping on 2D square and 3D cubic lattices in the presence
of a magnetic field B. These effects arise from the interference between phase
factors associated with different electron paths connecting two distinct sites.
For electrons confined on a square lattice, with and without disorder, we
obtain closed-form expressions for the tunneling probability, which determines
the conductivity, between two arbitrary sites by exactly summing the
corresponding phase factors of all forward-scattering paths connecting them. An
analytic field-dependent expression, valid in any dimension, for the
magnetoconductance (MC) is derived. A positive MC is clearly observed when
turning on the magnetic field. In 2D, when the strength of B reaches a certain
value, which is inversely proportional to twice the hopping length, the MC is
increased by a factor of two compared to that at zero field. We also
investigate transport on the much less-studied and experimentally important 3D
cubic lattice case, where it is shown how the interference patterns and the
small-field behavior of the MC vary according to the orientation of B. The
effect on the low-flux MC due to the randomness of the angles between the
hopping direction and the orientation of B is also examined analytically.Comment: 24 pages, RevTeX, 8 figures include
Interface Fluctuations under Shear
Coarsening systems under uniform shear display a long time regime
characterized by the presence of highly stretched and thin domains. The
question then arises whether thermal fluctuations may actually destroy this
layered structure. To address this problem in the case of non-conserved
dynamics we study an anisotropic version of the Burgers equation, constructed
to describe thermal fluctuations of an interface in the presence of a uniform
shear flow. As a result, we find that stretched domains are only marginally
stable against thermal fluctuations in , whereas they are stable in .Comment: 3 pages, shorter version, additional reference
Qualidade das silagens de leucena (Leucaena leucocephala) e gliricídia (Gliricídia sepium) sob diferentes épocas de abertura dos silos.
Avaliou-se silagens de Leucena (Leucaena leucocephala) e Gliricídia (Gliricídia sepium), produzidas em tubos de PVC, com 7, 14, 28 e 56 dias de fermentação, determinando-se os valores de matéria seca (MS), proteína bruta (PB), fibra em de tergente neutro (FDN), fibra em detergente ácido (FDA), extrato etéreo (EE), carboidratos não estruturais (CNE), digestibilidade in vitro da matéria seca (DIVMS) e pH. Os teores médios de MS, PB, FDN, FDA, DIVMS, pH e CNE apresentaram diferenças significativas entre as espécies forrageiras, não diferindo em relação ao teor de EE. Houve diferenças significativas para todos os parâmetros avaliados, exceto a MS, quando avaliados em função do tempo de incubação. As silagens de leucena e gliricídia apresentaram teores médios de MS (28,24 e 28,20%, respectivamente) inferiores a faixa recomendada como requisitos para a confecção de uma boa silagem. As silagens em estudo apresentaram altos níveis de PB, no entanto apresentaram baixa digestibilidade. Os valores de pH para ambas silagens foram elevados em todos os tempos de incubação, o que pode prejudicar o processo fermentativo
Continuous feedback on a quantum gas coupled to an optical cavity
We present an active feedback scheme acting continuously on the state of a
quantum gas dispersively coupled to a high-finesse optical cavity. The quantum
gas is subject to a transverse pump laser field inducing a self-organization
phase transition, where the gas acquires a density modulation and photons are
scattered into the resonator. Photons leaking from the cavity allow for a
real-time and non-destructive readout of the system. We stabilize the mean
intra-cavity photon number through a micro-processor controlled feedback
architecture acting on the intensity of the transverse pump field. The feedback
scheme can keep the mean intra-cavity photon number constant, in
a range between and , and
for up to 4 s. Thus we can engage the stabilization in a regime where the
system is very close to criticality as well as deep in the self-organized
phase. The presented scheme allows us to approach the self-organization phase
transition in a highly controlled manner and is a first step on the path
towards the realization of many-body phases driven by tailored feedback
mechanisms
A BRIEF REVIEW ON THE ADVANTAGES, HINDRANCES AND ECONOMIC FEASIBILITY OF STIRLING ENGINES AS A DISTRIBUTED GENERATION SOURCE AND COGENERATION TECHNOLOGY
The present paper aims to provide a brief review of the potentiality and economic feasibility of the Stirling engine as a distributed generation source and cogeneration technology. Another objective was the determination of hindrances which may be preventing the feasibility of the Stirling technology. With these intentions, a research based on a combination of preselected keywords was performed at the Metasearch of CAPES (Brazil's Higher Education Coordination of Personnel Perfecting). No filters in relation to the research period or to particular geographical regions were employed, thus publications until 2017’s middle were included and the research was conducted on a global level. Next, the selection of papers which contained some of the keywords was made, consisting initially of the read of the publications’ abstracts. The remaining ones were then further explored and had their relevant information incorporated, according to the scope of this work. It is worth mentioning that other accredited sources which dealt with important aspects of the topic were also included. Furthermore, a table containing some examples of products concerning the application of the Stirling engine as a distributed generation and cogeneration technology is presented. Ultimately, it is concluded that the Stirling technology, despite its advantages and suitability regarding the proposed applications, is not yet commercially feasible, having currently only a minor presence in the market. This scenario can be attributed to the need for further research and technical development as well as cost reduction
THE HIGH CADENCE TRANSIENT SURVEY (HITS). I. SURVEY DESIGN AND SUPERNOVA SHOCK BREAKOUT CONSTRAINTS
Indexación: Web of Science; Scopus.We present the first results of the High Cadence Transient Survey (HiTS), a survey for which the objective is to detect and follow-up optical transients with characteristic timescales from hours to days, especially the earliest hours of supernova (SN) explosions. HiTS uses the Dark Energy Camera and a custom pipeline for image subtraction, candidate filtering and candidate visualization, which runs in real-time to be able to react rapidly to the new transients. We discuss the survey design, the technical challenges associated with the real-time analysis of these large volumes of data and our first results. In our 2013, 2014, and 2015 campaigns, we detected more than 120 young SN candidates, but we did not find a clear signature from the short-lived SN shock breakouts (SBOs) originating after the core collapse of red supergiant stars, which was the initial science aim of this survey. Using the empirical distribution of limiting magnitudes from our observational campaigns, we measured the expected recovery fraction of randomly injected SN light curves, which included SBO optical peaks produced with models from Tominaga et al. (2011) and Nakar & Sari (2010). From this analysis, we cannot rule out the models from Tominaga et al. (2011) under any reasonable distributions of progenitor masses, but we can marginally rule out the brighter and longer-lived SBO models from Nakar & Sari (2010) under our best-guess distribution of progenitor masses. Finally, we highlight the implications of this work for future massive data sets produced by astronomical observatories, such as LSST.http://iopscience.iop.org/article/10.3847/0004-637X/832/2/155/meta;jsessionid=76BDFFFE378003616F6DBA56A9225673.c4.iopscience.cld.iop.or
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