Efficient multiple time scale molecular dynamics: using colored noise thermostats to stabilize resonances

Multiple time scale molecular dynamics enhances computational efficiency by updating slow motions less frequently than fast motions. However, in practice the largest outer time step possible is limited not by the physical forces but by resonances between the fast and slow modes. In this paper we show that this problem can be alleviated by using a simple colored noise thermostatting scheme which selectively targets the high frequency modes in the system. For two sample problems, flexible water and solvated alanine dipeptide, we demonstrate that this allows the use of large outer time steps while still obtaining accurate sampling and minimizing the perturbation of the dynamics. Furthermore, this approach is shown to be comparable to constraining fast motions, thus providing an alternative to molecular dynamics with constraints.Comment: accepted for publication by the Journal of Chemical Physic

Magnetic domain fluctuations in an antiferromagnetic film observed with coherent resonant soft x-ray scattering

We report the direct observation of slow fluctuations of helical antiferromagnetic domains in an ultra-thin holmium film using coherent resonant magnetic x-ray scattering. We observe a gradual increase of the fluctuations in the speckle pattern with increasing temperature, while at the same time a static contribution to the speckle pattern remains. This finding indicates that domain-wall fluctuations occur over a large range of time scales. We ascribe this non-ergodic behavior to the strong dependence of the fluctuation rate on the local thickness of the film.Comment: to appear in Phys. Rev. Let

Multiangle static and dynamic light scattering in the intermediate scattering angle range

We describe a light scattering apparatus based on a novel optical scheme covering the scattering angle range 0.5\dg \le \theta \le 25\dg, an intermediate regime at the frontier between wide angle and small angle setups that is difficult to access by existing instruments. Our apparatus uses standard, readily available optomechanical components. Thanks to the use of a charge-coupled device detector, both static and dynamic light scattering can be performed simultaneously at several scattering angles. We demonstrate the capabilities of our apparatus by measuring the scattering profile of a variety of samples and the Brownian dynamics of a dilute colloidal suspension

Speckle visibility spectroscopy and variable granular fluidization

We introduce a dynamic light scattering technique capable of resolving motion that changes systematically, and rapidly, with time. It is based on the visibility of a speckle pattern for a given exposure duration. Applying this to a vibrated layer of glass beads, we measure the granular temperature and its variation with phase in the oscillation cycle. We observe several transitions involving jammed states, where the grains are at rest during some portion of the cycle. We also observe a two-step decay of the temperature on approach to jamming.Comment: 4 pages, 4 figures, experimen

Aneurisma na artéria aorta em caprinos.

Resumo: De 726 caprinos necropsiados, dois (0,28%) apresentaram aneurisma na arteria aorta, sendo um macho com idade de 48 meses e uma femea com cerca de 30 meses. Os animais eram oriundos de um rebanho mantido em regime semi-extensivo com pastejo em caatinga nativa e pernoite em aprisco. Macroscopicamente observou-se palidez das mucosas externas e das visceras. Nas cavidades toracica e abdominal estavam presentes grandes coagulos resultantes da ruptura das partes afetadas das arterias. As lesoes das arterias aorta toracica e abdominal foram carcterizadas pela dilatacao das paredes formando estruturas saculares com presenca de inumeros coagulos e tecidos necroticos de aproximadamente 10cm a 20cm de comprimento. Histologicamente, observou-se a destruicao do tecido endotelial, necrose e infiltracao de leucocitos das camadas intima, media e muscular da arteria. Acredita-se que a causa primaria dos aneurismas detectados esteja relacionado com os processos inflamatorios presentes em outros orgaos. [Aneurism in the aorta artery in goats]. Abstract: Two (0.28%) of 726 necropsyed goats presented aneurism In the aorta artery, one being a male with 48 months and the other, a female about 30 months of age. The animais carne from a herd in semi-extensive regime, grazing in caatinga vegetation and spending the nights in appropriate pan. Paleness in external mucosa and viscera was observed macroscopically. Large coagula were presented in the thoracic and abdominal cavities, dueto ruptura ol the aneurismatic parts of the blood vessels. The lesions of thoracic and abdominal aortas were characterized by wali dilations forming saccular struc-tures, with presenco of many coagula and necrotic tissues of about 10cm to 20cm of length. Histo-logically, a destruetion In the endotelial tissues, nacrosis and infiltration of leucocytes in the intimate, median, and muscular Iayers were observed. It is believed that the primary cause of the aneurisms detected in this study would be related to teh inflamatory processes of other organs

Length scale dependence of dynamical heterogeneity in a colloidal fractal gel

We use time-resolved dynamic light scattering to investigate the slow dynamics of a colloidal gel. The final decay of the average intensity autocorrelation function is well described by $g\_2(q,\tau)-1 \sim \exp[-(\tau/\tau\_\mathrm{f})^p]$, with $\tau\_\mathrm{f} \sim q^{-1}$ and $p$ decreasing from 1.5 to 1 with increasing $q$. We show that the dynamics is not due to a continuous ballistic process, as proposed in previous works, but rather to rare, intermittent rearrangements. We quantify the dynamical fluctuations resulting from intermittency by means of the variance $\chi(\tau,q)$ of the instantaneous autocorrelation function, the analogous of the dynamical susceptibility $\chi\_4$ studied in glass formers. The amplitude of $\chi$ is found to grow linearly with $q$. We propose a simple --yet general-- model of intermittent dynamics that accounts for the $q$ dependence of both the average correlation functions and $\chi$.Comment: Revised and improved, to appear in Europhys. Let

Epidemiologia do Boophilus microplus nas condições de cerrado.

O carrapato Boophilus microplus (Canestrini 1887), e um parasita temporário e obrigatório dos bovinos. Distribui-se geograficamente entre os paralelos 32 Norte e Sul, tendo ampla distribuição no México, América Central, América do Sul e Austrália.bitstream/item/137546/1/PESQ-EM-ANDAMENTO-23.pd