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Pawan Kumar,Rakesh Kumar Dwivedi,Pawan Kumar,Ravikant Prasad,Praveen Kumar Siwach,Hari Krishna Singh 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.58 No.1
Thin films of La_(0.80)Sr_(0.20)Mn_(1−x)Cu_xO_3 with x = 0.00, 0.04, 0.08, 0.10, 0.15, and 0.20 were deposited on single-crystal LaAlO_3 (100) (LAO) substrates at 250 ± 10 ℃ by using a nebulized spray pyrolysis technique. Structural, surface, and transport characterizations were done by X-ray diffraction (XRD), atomic force microscopy (AFM) and the standard four probe technique, respectively. XRD patterns revealed the formation of single-phase polycrystalline films with a rhombohedral crystal structure. Cu substitution resulted in a uniform decrease in the metal insulator transition (T_(IM)) temperature up to x = 0.10, followed by a drastic decrease in T_(IM) for x = 0.15. Finally, the metal-insulator transition vanished for higher Cu concentrations (x = 0.20) in the observed temperature range. The variation in T_(IM) with the amount of added Cu is explained on the basis of the double exchange mechanism. Cu substitution has been found to perturb the Mn^(3+)-O-Mn^(4+) network and has a strong influence on T_(IM). The variation in resistivity maxima with temperature for different Cu concentrations showed a systematic trend up to x ∼ 0.10. We also explored the electrical conduction at T > T_(IM) in the framework of small polaron hopping in the adiabatic limit. The activation energy showed a strong dependence on the Cu concentration. The maximum magneto-resistance (MR) up to 10%, was observed for x = 0.15 at low temperatures.
Hybrid porous thin films: Opportunities and challenges for sensing applications
Kumar, Pawan,Kim, Ki-Hyun,Vellingiri, Kowsalya,Samaddar, Pallabi,Kumar, Parveen,Deep, Akash,Kumar, Naresh Elsevier 2018 Biosensors & bioelectronics Vol.104 No.-
<P><B>Abstract</B></P> <P>In this paper, the scientific progress in the field of thin film materials and their associated sensing technologies are described comprehensively to address the directions for future research and developments as per the need of modern-day technologies. To begin with, we briefly discussed the fundamental synthesis approaches for advanced thin films with an emphasis on the properties necessary for controlled fabrication (e.g., the elemental ratio and spatial arrangement). Subsequently, we explored the control, characterization, and optimization of hybrid porous thin films with respect to diverse sensing applications. The application of hybrid porous thin film materials has also been discussed in relation to the mechanisms used for biological, optical, electrical, acoustic, and other advanced sensing techniques (e.g., surface-enhanced Raman scattering (SERS)). Finally, conclusions are drawn to highlight the current status of thin film-based sensing technology along with its opportunities and challenges.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Fundamental synthesis approaches of advanced thin films. </LI> <LI> Here optimization of hybrid porous thin films for advanced sensing techniques is reviewed. </LI> <LI> Current status, opportunities, and challenges are discussed. </LI> </UL> </P>
Anomalous Weak Ferromagnetism in Electron-doped Nd_(1−x)Sr_xMnO_3 (0.50 ≤ x ≤ 0.62) Thin Films
Pawan Kumar,Ravikant Prasad,Hari Krishna Singh,Rakesh Kumar Dwivedi,Avanish Kumar Srivastava,Mangala Prasad Singh 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.4
In the present work we demonstrate that an anomalous weak ferromagnetism occurs in the composition range 0.50 ≤ x ≤ 0.62 in nanostructured thin films of Nd_(1−x)Sr_xMnO_3 (NSMO). Nanocrystalline thin films of overdoped manganite Nd_(1−x)Sr_xMnO_3 (<I>x</I> ∼ 0.50, 0.55, 0.60 and 0.62) on single crystal LaAlO_3 (001) substrates are grown by nebulized chemical spray pyrolysis technique. These single phase films possess average crystallite size ∼15 nm and the width of the grain boundaries is ∼1 - 2 nm. In the composition range 0.50 ≤ x ≤ 0.62, the ground state of NSMO is A-type antiferromagnetic (AFM) metallic. However, in the present films a broad paramagnetic (PM) to ferromagnetic (FM) transition is observed in all the compositions at T_C ∼ 226 K for x = 0.50 and 235 K for rest of the samples. All the films show a well-defined M-H hysteresis loop at 5 K. The coercivity (H_C) of these films is much larger than those having composition range 0.35 < x < 0.45. For x = 0.50 (0.62) film H_C = -1210 (-1484) Oe and +1162 (+1476) Oe. The asymmetry in coercivity suggests the presence of weak exchange bias effect in these films. It is observed that the FM ground state in these films have smaller magnetic moment per Mn atom than the expected full moment using the rigid model, we term this as a weak ferromagnetic (WFM) state arising due to the destabilization of the AFM ordering. We propose a possible scenario based on the combined effect of spin reorganization and enhanced orbital disordering in nanosized manganites, to explain the observed anomalous weak ferromagnetism in the A-type AFM spin ordered regime.
Jain Pawan Kumar,Kaushik Shiv Kumar,Vashisht Lalit Kumar Korean Mathematical Society 2007 대한수학회보 Vol.44 No.1
Some stability theorems (Paley-Wiener type) for Banach frames in Banach spaces have been derived.
Temporal multifractal analysis of extreme events in the crude oil market
Devi Pooja,Kumar Sunil,Kumar Pawan,Kumar Sushil,Khan Imran,Kumar Pawan 한국물리학회 2022 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.81 No.4
The ‘crude oil price’ has been one of the integral components of rising globalization. This paper delves into the static and dynamic multifractal analysis of daily prices of Brent crude oil for the period of 33 years (1987–2020) using the multifractal detrended fuctuation analysis. We discuss the results and limitations of the static multifractal analysis of historical data of crude oil. The crude oil market contains much more complexity in its dynamics that needs to be examined to draw inferences. These are important for socio-economic and political policy formulation at the global level. Thus the temporal multifractal analysis of historical data of crude oil is performed to understand such complexity. Further, the efect of extreme events on the multifractal properties of the crude oil market is also investigated. It is shown that multifractal spectra evolve through diferent shapes with time and the multifractal asymmetry coefcient can be used as an indicator in the detection of extreme events.
Controlling Band Gap and Refractive Index in Dopant-Free α-Fe2O3 Films
Pawan Kumar,Nitin Rawat,Da-Ren Hang,이흥노,Rajesh Kumar 대한금속·재료학회 2015 ELECTRONIC MATERIALS LETTERS Vol.11 No.1
Dopant-free hematite (α-Fe2O3) films are formed at a liquid-vaporinterface by means of an easy method in order to control the bandgap and refractive index of the films. The α-Fe2O3 films after beingtransferred to a glass substrate are studied for their structural andoptical properties. Control over the thickness of the films in therange from 75 to 400 nm and the constituent nanocrystallite sizefrom 3 to 46 nm is achieved by controlling the synthesisparameters. By controlling the film thickness, crystallite size, andcrystallinity of dopant-free α-Fe2O3 films, the optical band gap isincreased significantly (by ≈ 0.64 eV) from 2.30 to 2.94 eV, alongwith increase in the refractive index from 1.35 to 2.8. Theobserved increase in the optical band gap is explained on thebasis of change in lattice symmetry (via change in the c/a ratio) ofα-Fe2O3 crystallites.