Interplay of force constants in the lattice dynamics of disordered alloys : An ab-initio study

A reliable prediction of interatomic force constants in disordered alloys is an outstanding problem. This is due to the need for a proper treatment of multisite (atleast pair) correlation within a random environment. The situation becomes even more challenging for systems with large difference in atomic size and mass. We propose a systematic density functional theory (DFT) based study to predict the ab-initio force constants in random alloys. The method is based on a marriage between special quasirandom structures (SQS) and the augmented space recursion (ASR) to calculate phonon spectra, density of states (DOS) etc. bcc TaW and fcc NiPt alloys are considered as the two distinct test cases. Ta-Ta (W-W) bond distance in the alloy is predicted to be smaller (larger) than those in pure Ta (W), which, in turn, yields stiffer (softer) force constants for Ta (W). Pt-Pt force constants in the alloy, however, are predicted to be softer compared to Ni-Ni, due to a large bond distance of the former. Our calculated force constants, phonon spectra and DOS are compared with experiments and other theoretical results, wherever available. Correct trend of present results for the two alloys pave a path for further future studies in more complex alloy systems

Thermal conductivity and diffusion-mediated localization in Fe_{1-x}Cr_{x} Alloys

We apply a new Kubo-Greenwood type formula combined with a generalized Feynman diagram- matic technique to report a first principles calculation of the thermal transport properties of disordered Fe_{1-x}Cr_{x} alloys. The diagrammatic approach simplifies the inclusion of disorder-induced scattering effects on the two particle correlation functions and hence renormalizes the heat current operator to calculate configuration averaged lattice thermal conductivity and diffusivity. The thermal conductivity K(T) in the present case shows an approximate quadratic T-dependence in the low temperature regime (T < 20 K), which subsequently rises smoothly to a T-independent saturated value at high T . A numerical estimate of mobility edge from the thermal diffusivity data yields the fraction of localized states. It is concluded that the complex disorder scattering processes, in force-constant dominated disorder alloys such as Fe-Cr, tend to localize the vibrational modes quite significantly.Comment: 5 pages, 5 figure

Anomalous random correlations of force constants on the lattice dynamical properties of disordered Au-Fe alloys

Au-Fe alloys are of immense interest due to their biocompatibility, anomalous hall conductivity, and applications in various medical treatment. However, irrespective of the method of preparation, they often exhibit a high-level of disorder, with properties sensitive to the thermal or magnetic annealing temperatures. We calculate lattice dynamical properties of Au$_{1-x}$Fe$_x$ alloys using density functional theory methods, where, being a multisite property, reliable interatomic force constant (IFC) calculations in disordered alloys remain a challenge. We follow a two fold approach: (1) an accurate IFC calculation in an environment with nominally zero chemical pair correlations to mimic the homogeneously disordered alloy; and (2) a configurational averaging for the desired phonon properties (e.g., dispersion, density of states, and entropy). We find an anomalous change in the IFC's and phonon dispersion (split bands) near $x$=0.19, which is attributed to the local stiffening of the Au-Au bonds when Au is in the vicinity of Fe. Other results based on mechanical and thermo-physical properties reflect a similar anomaly: Phonon entropy, e.g., becomes negative below $x$=0.19, suggesting a tendency for chemical unmixing, reflecting the onset of miscibility gap in the phase diagram. Our results match fairly well with reported data, wherever available

Implementation of High Speed Railway Mobile Communication System

High speed railways (HSR) provide highly efficient transport mode which improves the quality of railway services, saves time of the passengers which leads to greater customer satisfaction as well as improves the economics of the society. This has introduced significant challenges like developing new technologies, improving the existing architecture and controlling costs etc. Due to the improvements in the speed, ability to access internet and stream live media there is a requirement of an advanced high speed communication and signaling system. This system demands higher bandwidth, higher reliability and shorter response time for efficient operation and safety. This paper introduces the existing system deployed by the railway based on Global System for Mobile communication (GSM) , analyzes it and presents a much more advanced communication and signaling system based on 4G Long Term Evolution (LTE) technology

Enhanced Li Capacity in Functionalized Graphene: A First Principle Study with van der Waals Correction

We have investigated the adsorption of Li on graphene oxide using density functional theory. We show a novel and simple approach to achieve a positive lithiation potential on epoxy and hydroxyl functionalized graphene, compared to the negative lithiation potential that has been found on prestine graphene. We included the van der Waals correction into the calculation so as to get a better picture of weak interactions. A positive lithiation potential suggests a favorable adsorption of Li on graphene oxide sheets that can lead to an increase in the specific capacity, which in turn can be used as an anode material in Li-batteries. We find a high specific capacity of ~860 mAhg-1 by functionalizing the graphene sheet. This capacity is higher than the previously reported capacities that were achieved on graphene with high concentration of Stone-Wales (75%) and divacancy (16%) defects. Creating such high density of defects can make the entire system energetically unstable, whereas graphene oxide is a naturally occurring substance