Exact Gravitational Quasinormal Frequencies of Topological Black Holes

We compute the exact gravitational quasinormal frequencies for massless topological black holes in d-dimensional anti-de Sitter space. Using the gauge invariant formalism for gravitational perturbations derived by Kodama and Ishibashi, we show that in all cases the scalar, vector, and tensor modes can be reduced to a simple scalar field equation. This equation is exactly solvable in terms of hypergeometric functions, thus allowing an exact analytic determination of the gravitational quasinormal frequencies.Comment: 14 pages, Latex; v2 additional reference

Existence of a tribo-modified surface layer of BR/S-SBR elastomers reinforced with silica or carbon black

The existence of a modified surface layer on top of a rubber disk, in contact with a rigid counter-surface, is still a point of discussion. In this study, we show that a modified surface layer with different mechanical properties exists. Modification of the reinforced elastomers is discussed and the subsequent effect of existence of such a layer on the friction is emphasized. The elasticity modulus both inside and outside the wear track is determined by atomic force microscope nano-indentations and compared with the macroscale mechanical properties measured using dynamical mechanical analysis. The elastic modulus inside the wear track is lower than the bulk. Intensifying the tribological conditions by means of increasing normal pressure and sliding velocity will result in a greater loss of mechanical propertie

Volume-preserving normal forms of Hopf-zero singularity

A practical method is described for computing the unique generator of the algebra of first integrals associated with a large class of Hopf-zero singularity. The set of all volume-preserving classical normal forms of this singularity is introduced via a Lie algebra description. This is a maximal vector space of classical normal forms with first integral; this is whence our approach works. Systems with a non-zero condition on their quadratic parts are considered. The algebra of all first integrals for any such system has a unique (modulo scalar multiplication) generator. The infinite level volume-preserving parametric normal forms of any non-degenerate perturbation within the Lie algebra of any such system is computed, where it can have rich dynamics. The associated unique generator of the algebra of first integrals are derived. The symmetry group of the infinite level normal forms are also discussed. Some necessary formulas are derived and applied to appropriately modified R\"{o}ssler and generalized Kuramoto--Sivashinsky equations to demonstrate the applicability of our theoretical results. An approach (introduced by Iooss and Lombardi) is applied to find an optimal truncation for the first level normal forms of these examples with exponentially small remainders. The numerically suggested radius of convergence (for the first integral) associated with a hypernormalization step is discussed for the truncated first level normal forms of the examples. This is achieved by an efficient implementation of the results using Maple

Stability of Topological Black Holes

We explore the classical stability of topological black holes in d-dimensional anti-de Sitter spacetime, where the horizon is an Einstein manifold of negative curvature. According to the gauge invariant formalism of Ishibashi and Kodama, gravitational perturbations are classified as being of scalar, vector, or tensor type, depending on their transformation properties with respect to the horizon manifold. For the massless black hole, we show that the perturbation equations for all modes can be reduced to a simple scalar field equation. This equation is exactly solvable in terms of hypergeometric functions, thus allowing an exact analytic determination of potential gravitational instabilities. We establish a necessary and sufficient condition for stability, in terms of the eigenvalues $\lambda$ of the Lichnerowicz operator on the horizon manifold, namely $\lambda \geq -4(d-2)$. For the case of negative mass black holes, we show that a sufficient condition for stability is given by $\lambda \geq -2(d-3)$.Comment: 20 pages, Latex, v2 refined analysis of boundary conditions in dimensions 4,5,6, additional reference

The effect of endurance and circuit resistance training on serum brain-derived neurotrophic factor and cortisol in inactive male students: A randomized clinical trial

زمینه و هدف: عامل رشد عصبی مشتق از مغز نقش مهمی در رشد و تکامل دستگاه عصبی دارد. تحقیقات حیوانی نشان داده اند که سطوح سرمی این فاکتور تحت تأثیر فعالیت ورزشی قرار می گیرد. هدف از انجام تحقیق حاضر تعیین تأثیر تمرین استقامتی و مقاومتی دایره ای بر عامل رشد عصبی مشتق شده از مغز و کورتیزول سرمی در مردان غیر فعال بود. روش بررسی: در این مطالعه کارآزمایی بالینی، 30 دانشجوی پسر غیر فعال به طور تصادفی به سه گروه تمرین استقامتی، تمرین مقاومتی و کنترل تقسیم شدند. آزمودنی های گروه استقامتی برنامه تمرینی استقامتی شامل 45-30 دقیقه دوی تناوبی با شدت 75-60 درصد ضربان قلب بیشینه را به مدت چهار هفته اجرا کردند. آزمودنی های گروه های تمرین مقاومتی نیز سه جلسه در هفته، به مدت چهار هفته تمرین مقاومتی دایره ای با شدت 75-60 درصد یک تکرار بیشینه را انجام دادند. قبل و 48 ساعت بعد از دوره‌ی تحقیق، نمونه گیری خونی برای سنجش مقادیر سرمی عامل رشد عصبی مشتق شده از مغز و کورتیزول از آزمودنی ها به عمل آمد. یافته ها: تمرین استقامتی و مقاومتی دایره ای غلظت سرمی عامل رشد عصبی مشتق شده از مغز را به طور معنی داری افزایش داد. در بررسی نتایج پس آزمون تفاوتی بین گروه های تمرینی مشاهده نشد؛ ولی بین دو گروه تمرین استقامتی و گروه کنترل تفاوت معنی دار بود. تمرین استقامتی و مقاومتی تأثیر معنی داری بر سطوح کورتیزول سرمی نداشت. نتیجه گیری: بر اساس یافته های این مطالعه، تمرین استقامتی و مقاومتی دایره ای باعث افزایش فاکتورهای نروتروفیک می شود که ممکن است بدین طریق باعث ایجاد سازگاری های ساختاری و عملکردی در دستگاه عصبی شود

Nanofibers-based piezoelectric energy harvester for self-powered wearable technologies

The demands for wearable technologies continue to grow and novel approaches for powering these devices are being enabled by the advent of new energy materials and novel manufacturing strategies. In addition, decreasing the energy consumption of portable electronic devices has created a huge demand for the development of cost-effective and environment friendly alternate energy sources. Energy harvesting materials including piezoelectric polymer with its special properties make this demand possible. Herein, we develop a flexible and lightweight nanogenerator package based on polyvinyledene fluoride (PVDF)/LiCl electrospun nanofibers. The piezoelectric performance of the developed nanogenator is investigated to evaluate effect of the thickness of the as-spun mat on the output voltage using a vibration and impact test. It is found that the output voltage increases from 1.3 V to 5 V by adding LiCl as additive into the spinning solution compared with pure PVDF. The prepared PVDF/LiCl nanogenerator is able to generate voltage and current output of 3 V and 0.5 µA with a power density output of 0.3 µW cm−2 at the frequency of 200 Hz. It is found also that the developed nanogenerator can be utilized as a sensor to measure temperature changes from 30◦C to 90◦C under static pressure. The developed electrospun temperature sensor showed sensitivity of 0.16%/◦C under 100 Pa pressure and 0.06%/◦C under 220 Pa pressure. The obtained results suggested the developed energy harvesting textiles have promising applications for various wearable self-powered electrical devices and systems

Carrier-induced ferromagnetism in n-type ZnMnAlO and ZnCoAlO thin films at room temperature

The realization of semiconductors that are ferromagnetic above room temperature will potentially lead to a new generation of spintronic devices with revolutionary electrical and optical properties. Transition temperatures in doped ZnO are high but, particularly for Mn doping, the reported moments have been small. We show that by careful control of both oxygen deficiency and aluminium doping the ferromagnetic moments measured at room temperature in n-type ZnMnO and ZnCoO are close to the ideal values of 5mB and 3mB respectively. Furthermore a clear correlation between the magnetisation per transition metal ion and the ratio of the number of carriers to the number of transition metal donors was established as is expected for carrier induced ferromagnetism for both the Mn and Co doped films. The dependence of the magnetisation on carrier density is similar to that predicted for the transition temperature for a dilute magnetic semiconductor in which the exchange between the transition metal ions is through the free carriers.Comment: 14 pages pd

On Using Magnetic and optical methods to determine the size and characteristics of nanoparticles embedded in oxide semiconductors

Films of oxides doped with transition metals are frequently believed to have magnetic inclusions. Magnetic methods to determine the amount of nanophases and their magnetic characteristics are described. The amount of the sample that is paramagnetic may also be measured. Optical methods are described and shown to be very powerful to determine which defects are also magnetic.Comment: Manuscript of poster to be presented at MMM-Intermag 2010. Accepted for publication in Magnetic Trans of IEE