Effective field theory analysis of 3D random field Ising model on isometric lattices

Ising model with quenched random magnetic fields is examined for single Gaussian, bimodal and double Gaussian random field distributions by introducing an effective field approximation that takes into account the correlations between different spins that emerge when expanding the identities. Random field distribution shape dependencies of the phase diagrams and magnetization curves are investigated for simple cubic, body centered and face centered cubic lattices. The conditions for the occurrence of reentrant behavior and tricritical points on the system are also discussed in detail.Comment: 13 pages, 8 figure

Nonequilibrium phase transitions and stationary state solutions of a three-dimensional random-field Ising model under a time dependent periodic external field

Nonequilibrium behavior and dynamic phase transition properties of a kinetic Ising model under the influence of periodically oscillating random-fields have been analyzed within the framework of effective field theory (EFT) based on a decoupling approximation (DA). Dynamic equation of motion has been solved for a simple cubic lattice ($q=6$) by utilizing a Glauber type stochastic process. Amplitude of the sinusoidally oscillating magnetic field is randomly distributed on the lattice sites according to bimodal and trimodal distribution functions. For a bimodal type of amplitude distribution, it is found in the high frequency regime that the dynamic phase diagrams of the system in temperature versus field amplitude plane resemble the corresponding phase diagrams of pure kinetic Ising model. Our numerical results indicate that for a bimodal distribution, both in the low and high frequency regimes, the dynamic phase diagrams always exhibit a coexistence region in which the stationary state (ferro or para) of the system is completely dependent on the initial conditions whereas for a trimodal distribution, coexistence region disappears depending on the values of system parameters.Comment: 11 pages, 11 figure

TeV Gamma Rays from Geminga and the Origin of the GeV Positron Excess

The Geminga pulsar has long been one of the most intriguing MeV-GeV gamma-ray point sources. We examine the implications of the recent Milagro detection of extended, multi-TeV gamma-ray emission from Geminga, finding that this reveals the existence of an ancient, powerful cosmic-ray accelerator that can plausibly account for the multi-GeV positron excess that has evaded explanation. We explore a number of testable predictions for gamma-ray and electron/positron experiments (up to ~100 TeV) that can confirm the first "direct" detection of a cosmic-ray source.Comment: 4 pages and 3 figures; Minor revisions, accepted for publication in Physical Review Letter

Best-Bet Astrophysical Neutrino Sources

Likely astrophysical sources of detectable high-energy (>> TeV) neutrinos are considered. Based on gamma-ray emission properties, the most probable sources of neutrinos are argued to be GRBs, blazars, microquasars, and supernova remnants. Diffuse neutrino sources are also briefly considered.Comment: 6 pages, 2 figures, in Proc. of TeV-Particle Astrophysics II, Madison, WI, 28-31 Aug, 200

OLPT CONDUCTIVITY IN WOLLASTONITE INLAID NR/SBR TYPE ELASTOMER BASED MATERIAL

The electrical properties of wollastonite inlaid NR/SBR type elastomer based material have been evaluated. Electrical properties of the samples were measured in the temperature range of 303 to 453 K and the frequency range of 100 Hz – 40 MHz. All electrically measured parameters were given anomalies at 385 K. Only one type of dielectric relaxation process have been observed for all measurements. Physical parameters characterizing the dielectric behavior have been obtained by fitting the experimental results in the modified Debye equation. The activation energy which is thermally activated by dielectric relaxation process have been calculated to be 0.58 eV. DC conductivity increasing by temperature has been explained with the help of VFT model whereas the AC one has been clarified by the OLPT model

Etkili yönetici görüşlerine göre zaman yönetiminde belirgin olan kişisel faktörler

Bireylerin kişilikleri ve işlerindeki çalışma alışkanlıkları, kendilerinin zamanı kullanma biçimlerini önemli derecede etkilemektedir. Bir başka ifade ile, kişilik yapısının ve bazı kişisel özelliklerin zaman ihtiyaçları ve çalışma alışkanlıkları üzerinde belirleyici etkilerinin de olduğu söylenebilir. Geri dönüşü mümkün olmayan ve etkin bir şekilde kullanılması gereken en değerli kaynaklardan biri olan zaman örgütlerin başarıya ulaşmasında önemli bir rol oynamaktadır. Zamanın kullanımı ile ilgili süreci yönlendirenler ise örgütün temel unsurlarından olan yöneticilerdir. Günümüzde yöneticiler çok yoğun ve karmaşık ortamda çalışmaktadırlar. Yöneticilerin zamanı etkin kullanmaları ve yönetmeleri; örgütün bütün kaynaklarının da aynı şekilde kullanılması yönünde önemli ölçüde fayda sağlayacak ve örgütün başarı düzeyini yükseltecektir. Bu çalışmada analitik metot kullanılarak, zaman yönetimini etkileyen önemli bazı kişisel faktörlerin yöneticiler açısından ele alınıp değerlendirilmesi amaçlanmıştır. Konu ile ilgili literatür taraması yapılarak elde edilen veriler sonucunda zamanın yöneticiler tarafından nasıl en etkin biçimde kullanılabileceği hususunda değerlendirme yapılmıştır

Stationary State Solutions of a Bond Diluted Kinetic Ising Model: An Effective-Field Theory Analysis

We have examined the stationary state solutions of a bond diluted kinetic Ising model under a time dependent oscillating magnetic field within the effective-field theory (EFT) for a honeycomb lattice $(q=3)$. Time evolution of the system has been modeled with a formalism of master equation. The effects of the bond dilution, as well as the frequency $(\omega)$ and amplitude $(h/J)$ of the external field on the dynamic phase diagrams have been discussed in detail. We have found that the system exhibits the first order phase transition with a dynamic tricritical point (DTCP) at low temperature and high amplitude regions, in contrast to the previously published results for the pure case \cite{Ling}. Bond dilution process on the kinetic Ising model gives rise to a number of interesting and unusual phenomena such as reentrant phenomena and has a tendency to destruct the first-order transitions and the DTCP. Moreover, we have investigated the variation of the bond percolation threshold as functions of the amplitude and frequency of the oscillating field.Comment: 8 pages, 4 figure

Direct X-ray Constraints on Sterile Neutrino Warm Dark Matter

Warm dark matter (WDM) might more easily account for small scale clustering measurements than the heavier particles typically invoked in Lambda cold dark matter (LCDM) cosmologies. In this paper, we consider a Lambda WDM cosmology in which sterile neutrinos nu_s, with a mass m_s of roughly 1-100 keV, are the dark matter. We use the diffuse X-ray spectrum (total minus resolved point source emission) of the Andromeda galaxy to constrain the rate of sterile neutrino radiative decay: nu_s -> nu_{e,mu,tau} + gamma. Our findings demand that m_s < 3.5 keV (95% C.L.) which is a significant improvement over the previous (95% C.L.) limits inferred from the X-ray emission of nearby clusters, m_s < 8.2 keV (Virgo A) and m_s < 6.3 keV (Virgo A + Coma).Comment: 8 pages, 2 figures, minor revisions, accepted for publication in Physical Review

SDR-based demonstration system and applicability of SNR aggregation for multistage distributed cooperative communication in MANETs

Multistage or hierarchical distributed cooperative communication on the physical layer is a promising approach to overcome the scalability limitation of Mobile Ad-hoc Networks (MANETs). Standalone nodes that could successfully decode messages join transmission and support each other. They become a virtual transmit cluster and send simultaneously. While information theoretic research has demonstrated that an approximately linear scaling behaviour can be achieved, imperfections and constraints of practical systems have not been taken into account. Within this paper, we present a scalable and modular low-cost demonstration system based on software-defined radios (SDRs) to study distributed cooperative communication in practical MANETs. Furthermore, we apply SNR aggregation in combination with distributed cooperative transmission. To this end, we show a practical implementation approach and investigate the performance by measurements and simulations. Our results clearly highlight the advantages of combining distributed cooperative communication and SNR aggregation, e. g. to overcome larger distances in a distributed long haul multiple-input single-output (MISO) scenario or to enable a more efficient broadcast

Multi-carrier (OFDM) cooperative transmission in MANETs with multiple carrier frequency offsets

Cooperative transmission, realized by aggregating several nodes to a virtual multiple input system, is an auspicious approach to establish a more robust and effective communication in MANETs. In such a setup, impairments, i. e. multiple timing and carrier frequency offsets (TO, CFO) will occur. While multi-carrier schemes, e. g. Orthogonal Frequency Division Multiplexing (OFDM), are well-known to mitigate the impact of multipath propagation and TO, multiple CFO causes inter-carrier-interference (ICI) which typically degrades the performance significantly. Within this paper, we propose an effective code and equalizer structure that allows to overcome this limitation. It mitigates the impact of multiple CFO that can be significantly larger than the subcarrier spacing with a reasonable computational effort. For that, we utilize inherent code properties of Linear-Scalable Dispersion Codes (LSDCs) and propose a communication system composed of an equalizer structure in combination with LSDCs that enables multi-carrier distributed cooperative transmission for practical MANETs with high node mobility. We demonstrate the benefits of cooperative transmission in comparison to classical non-cooperative multi-hop or concurrent transmission by outage simulations, which clearly indicate that our proposal can be of crucial importance for the overall MANET scalability. Lastly, we compare our OFDM system with a recently proposed time-domain equalization single-carrier system and point out use cases, where the OFDM system can be more advantageous