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M. Aslam

Publications and source records attributed to M. Aslam.

12 recordsLinked to original sources

Study of Dynamical Instability of Collapsing Charged Spherically Symmetric Anisotropic Matter Configurations within Non-Minimally Coupled Gravity

We investigate the dynamical instability and gravitational collapse of charged, spherically symmetric anisotropic matter configurations within $f(R,\mathcal{L}_{m})$ gravity. The analysis focuses on the combined effects of anisotropic pressure, perturbations, electric charge, and modified-gravity source terms on the stability of compact objects. A specific equation of state is adopted to relate the static and perturbed variables through the adiabatic index. Using a perturbation scheme, we derive the modified hydrostatic equilibrium and collapse equations and obtain instability constraints in both Newtonian and post-Newtonian regimes. The results show that the stability of the system is governed by the competition between inward gravitational attraction and outward pressure support. In addition, the dark source terms generated by the non-minimal matter-geometry coupling modify the collapse conditions and can enhance the stability of the charged fluid. These findings provide a useful framework for understanding the evolution and collapse of dense self-gravitating compact objects in non-minimally coupled gravity.

gr-qc

Optoelectronic and Transport Properties of Vacancy Ordered Double Perovskite Halides: A First-principles Study

In the search for stable lead (Pb) free perovskites, Vacancy ordered double perovskite (VODP), A$_2$BX$_6$ has emerged as a promising class of materials for solar harvesting owing to their nontoxicity, better stability, and unique optoelectronic properties. Here, we present the stability and the key physical attributes of few selected compounds in a systematic manner using state-of-the-art first-principle calculations. A careful structural and stability analysis via simulating convex hull and compositional phase diagrams for different structural prototypes discloses 14 stable and 1 metastable compounds in this class. The electronic structure calculations using hybrid functional reveals six compounds to acquire band gap in the ideal visible region. These six compounds, namely Cs$_2$SnI$_6$, Cs$_2$PdI$_6$, Cs$_2$TeI$_6$, Cs$_2$TiI$_6$, Cs$_2$PtI$_6$, and Cs$_2$PdBr$_6$, show high optical absorption ($\approx$ 10$^{5}$ cm $^{-1}$) giving rise to high spectroscopic limited maximum efficiency, SLME (15-23\%) in the thin-film thickness range. Close inspection of transport properties reveals polar optical phonon scattering to be the dominant mechanism limiting the overall mobility. Further analysis of the polaron excitations discloses the possibility of large polaron formation at low to moderate defect concentrations. At high defect concentrations, ionized impurity scattering takes over. This suggests that, a simulation based guided control of defect concentrations during synthesis can yield a desired candidate for promissing device application. Additionally, few selected compounds show moderate to high electron mobility values ($\sim$13-63 cm$^2$V$^{-1}$ s$^{-1}$) at room temperature. Overall, the present study paves an important path to help design VODP as Pb-free potential candidates for future optoelectronic applications.

cond-mat.mtrl-sci

Optoelectronic Properties and Defect Physics of Lead-free Photovoltaic Absorbers Cs$_2$Au$^{I}$Au$^{III}$X$_6$ (X=I, Br)

Stability and toxicity issues with the hybrid lead iodide perovskite MAPbI$_3$ necessitate a hunt for potential alternatives. Here, we shed light on promising photovoltaic properties of gold mixed-valence halide perovskites Cs$_2$Au$_2$X$_6$ (X = I, Br, Cl). They satisfy fundamental requirements such as nontoxicity, better stability, a band gap in the visible range, and a low excitonic binding energy. Our study shows a favorable electronic structure, resulting in a high optical-transition strength, and thus a sharp rise in the absorption spectrum near the band gap. This, in turn, yields a very high short-circuit current density and hence higher simulated efficiency compared with MAPbI$_3$. However, careful investigation of defect physics reveals the possibility of deep-level defects (such as V$_X$ , V$_{Cs}$, X$_{Au}$, X$_{Cs}$, Au$_i$, and Au$_X$ , X = I, Br), depending on the growth conditions. These can act as carrier traps and become detrimental to photovoltaic performance. The present study should help in taking necessary precautions in synthesizing these compounds in a controlled chemical environment, which should minimize performance limiting defects and pave the way for future studies on this class of materials.

cond-mat.mtrl-sci

Superconductivity of Cobalt in Thin Films

Due to competing long range ferromagnetic order, the transition metals Fe, Co and Ni are not superconductors at ambient pressure. While superconductivity was observed in a non-magnetic phase of Fe, stabilized under pressure, it is yet to be discovered in Co and Ni under any experimental conditions. Here, we report emergence of superconductivity in the recently discovered high-density nonmagnetic face centered cubic phase in Co thin films below a transition temperature (Tc) of ~5.4 K, as revealed in experiments based on point-contact spectroscopy and resistance, and four-probe measurements of resistance at ambient pressure. We confirm the non-magnetic nature of the dense fcc phase of Co within first-principles density functional theory, and show that its superconductivity below 5 K originates from anomalous softening of zone-boundary phonons and their enhanced coupling with electrons upon biaxial strain.

cond-mat.supr-con

Topology On BCK-Modules

In this paper, we introduce the notion of a BCK-topological module in a natural way and establish that every decreasing sequence of submodules on a BCK-module M over bounded commutative BCK-algebra X is indeed a BCK- topological module. We have defined the notion of compatible and strict BCK- module homomorphisms, and establish that a strict BCK-module homomorphism is an open as well as a continuous mapping. Also, we establish the necessary and sufficient condition for a compatible mapping to be strict.

math.GM

Energy optimization and Performance Analysis of Cluster Based Routing Protocols Extended from LEACH for WSNs

An energy efficient routing protocol is the major attentiveness for researcher in field of Wireless Sensor Networks (WSNs). In this paper, we present some energy efficient hierarchal routing protocols, prosper from conventional Low Energy Adaptive Clustering Hierarchy (LEACH) routing protocol. Fundamental objective of our consideration is to analyze, how these ex- tended routing protocols work in order to optimize lifetime of network nodes and how quality of routing protocols is improved for WSNs. Furthermore, this paper also emphasizes on some issues experienced by LEACH and also explains how these issues are tackled by other enhanced routing protocols from classi- cal LEACH. We analytically compare the features and performance issues of each hierarchal routing protocol. We also simulate selected clustering routing protocols for our study in order to elaborate the enhancement achieved by ameliorate routing protocols.

cs.NI

A new approach to soft set through applications of cubic set

The notions of (internal, external) cubic soft sets, P-(R-)order,P-(R-)union, P-(R-)intersection and P-OR, R-OR, P-AND and R-AND are introduced, and related properties are investigated. We show that the P-union and the P-intersection of internal cubic soft sets are also internal cubic soft sets. We provide conditions for the P-union (resp. P-intersection) of two external cubic soft sets to be an internal cubic soft set. We give conditions for the P-union (resp. R-union and R-intersection) of two external cubic soft sets to be an external cubic soft set. We consider conditions for the R-intersection (resp.P-intersection) of two cubic sof sets to be both an external cubic soft set and an internal cubic soft set.

math.LO

A Comprehensive Survey of MAC Protocols for Wireless Body Area Networks

In this paper, we present a comprehensive study of Medium Access Control (MAC) protocols developed for Wireless Body Area Networks (WBANs). In WBANs, small batteryoperated on-body or implanted biomedical sensor nodes are used to monitor physiological signs such as temperature, blood pressure, ElectroCardioGram (ECG), ElectroEncephaloGraphy (EEG) etc. We discuss design requirements for WBANs with major sources of energy dissipation. Then, we further investigate the existing designed protocols for WBANs with focus on their strengths and weaknesses. Paper ends up with concluding remarks and open research issues for future work.

cs.NI

CEEC: Centralized Energy Efficient Clustering A New Routing Protocol for WSNs

Energy efficient routing protocol for Wireless Sensor Networks (WSNs) is one of the most challenging task for researcher. Hierarchical routing protocols have been proved more energy efficient routing protocols, as compare to flat and location based routing protocols. Heterogeneity of nodes with respect to their energy level, has also added extra lifespan for sensor network. In this paper, we propose a Centralized Energy Efficient Clustering (CEEC) routing protocol. We design the CEEC for three level heterogeneous network. CEEC can also be implemented in multi-level heterogeneity of networks. For initial practical, we design and analyze CEEC for three level advance heterogeneous network. In CEEC, whole network area is divided into three equal regions, in which nodes with same energy are spread in same region.

cs.NI

Adaptive-Reliable Medium Access Control Protocol for Wireless Body Area Networks

Extensive energy is consumed by Transceiver communication operation [1]. Existing research on MAC layer focuses to maximize battery-powered sensor node's life. Bottleneck of MAC layer protocol design for WBAN is to achieve high reliability and energy minimization. Majority of MAC protocols designed for WBANs are based upon TDMA approach. However, a new protocol needs to be defined to achieve high energy efficiency, fairness and avoid extra energy consumption due to synchronization.

cs.NI

Survey of Extended LEACH-Based Clustering Routing Protocols for Wireless Sensor Networks

An energy efficient routing protocol is the major concern in Wireless Sensor Networks (WSNs). In this survey paper, we present energy efficient hierarchical routing protocols, developed from conventional LEACH routing protocol. Main focus of our study is how these extended protocols work in order to increase the life time and how quality routing protocol are improved for WSNs. Furthermore, this paper also highlights some of the issues faced by LEACH and also explains how these issues are tackled by extended versions of LEACH. We compare the features and performance issues of the selected hierarchal routing protocols.

cs.NI