Department of Physics
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Item Study of Nonlinear Interactions in Dense Plasmas(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2021) Zahid Mir; CIIT/FA14-PPHY-004/LHR; Dr. Muhammad Jamil; LHR TP 7539It is intended here in this thesis to study the nonlinear interactions evolved from the interaction of linear modes in dense plasmas like astrophysical objects, laser produced plasmas and semiconductor plasmas, etc. The dense plasma systems in focus contain electrons, ions and negatively charged dust grains. The constituent modes are electrostatic and electromagnetic in nature. In addition to the coupling mechanism, an electron beam is also considered as an energy source. A multi-fluid model consisting upon momentum balancing equation, equation of continuity, Poisson’s equation and set of Maxwell equations for quantum plasmas is used to diagnose the plasmas analytically. The thesis is composed of two-fold studies of nonlinear interactions based upon the published papers. Firstly, excitation of a beat wave generated by the parametric coupling of two modes of slightly different wavelengths is discussed. The linear analysis of the dispersive nature of the incident (pump) and scattered (sideband) is carried out. A derivation of the nonlinear dispersion expression is carried out due to the linear coupling of the pump and sideband in the assembly of a homogeneous dense dusty magneto-plasmas. This leads to the growth rate of the parametric instability of three waves. Secondly, the nonlinear growth of the parametric instability of electrostatic electron Langmuir wave, produced by the coupling of two waves with slightly different frequencies in semiconductor plasma is discussed. The linearity behaviour of an electrostatic pump (upper-hybrid wave) and electromagnetic sideband (O-mode) in semiconductor quantum plasma is studied. The second order convective term couples the fields of pair modes to derive the nonlinear beat frequency of the resultant perturbation leading to three waves parametric instability in semiconductor quantum plasma system. In both the cases the quantum mechanical effects arising due to degenerate pressure of Fermi gas, quantum tunneling effect, and exchange-correlation potential have been incorporated for the fermions. The instability is analyzed graphically for both cases on varying different parameters.Item Motion of Whiplash(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2017) ZANARA MUZAMMIL; CIIT-FA14-MSPHY-001/LHR; Dr. Shabana Nisar; LHR TP 6834We propose to study the wave motions on a whip using numerical study of associated partial differential equations with specific goals of explaining the supersonic speeds of the tip, under various boundary conditions and structural parameters, which is responsible for the crack and concentrated damaging power associated with a whip lash. In light of above, the problem for us is to study the wave motion of the whip and construct a suitable mechanical model which can explain the main distinguishing feature of the whip i.e., high speed of the tip as well as study the motion of wave on the whip and study its general characteristics. We have found that the factors which are important in the crack of a whiplash are varying thickness of the lash, stiffness of the whip, and jerk applied to the whip. The mass of the whiplash plays very vital role in the motion of the whip. Usually the whip is thicker from the end which is attached to the handle and becomes thinner as we move along the lash towards the tip. The density will decreases as the square of the length across the lash for a linear decrease in the thickness of the lash. We find that this tapering of the thickness is crucial for the speed gain of the tip and higher variation cause higher speed gain and crack. It is also confirmed that a whip having some stiffness in its lash is more effective in producing higher speeds of the tip.In modeling an actual whip, we have also found that jerk plays very important role. The speed of the tip depends on the jerk given to the lash for wave motion. For the sudden jerk, the increase in tip speed is dramatic which has that cracking sound and the painful effect which we need.For out model the tip speed dramatically increase upto about 30-40 times than the speed at the starting point. This tip speed is on the top of overall motion of the whip. This speed crosses the speed of sound easily. Speed gain is 30-40 times at the tip. It is moving with very high speed that it can cross speed of sound, this is also called sonic boom. This high speed of the tip cause that excessive pain which we need to drive an animalItem Synthesis of Al-substituted BiFeO3 powders and investigation of their properties(Library Information Services, COMSATS University Islamabad, Lahore Campus, 2018) RIZWAN ALI; CIIT/FA14-RPH-013/LHR; Dr. Mukhtar Ahmed; LHR TP 5706It is renowned that electricity and magnetism are internally combined, and their coupling was given by the four Maxwell-equations. Although the basic idea related to the crystals may well be at the same time ferromagnetism (FM) and ferroelectricity (FE) was possible offered by Pierre Curie within the 19th century however the real starting of that started in [1] during the volume of their Course of theoretical Physics. However the true modified one at an equivalent time [2] time reverse homogenous for Cr2O3 and Astrov through confirmed via an experiment [3] the same kinds of requirement within the identical substances at the mid of nineteen century. By the way Cr2O3 is to bad for the utilization as in this material antiferromagnetism its coupled with paraelectricity. Afterwards, the attempts to combine FM and FE characteristics begin in one material mainly the Soviet Union two group: the group from St.Petersburg [4, 5] and other is from Moscow [6]. After that, generally, there was no progress during this time in this area of research. The word multiferroic earlier used by H. Schmid. As he determined FM and FE in magnesium borate material [7]. The definition that he suggest to multiferroics having single phase crystal structure which at the same time having two initial ferrite characteristics. But magnesium borate materials are found not to be good practically materials due to the magneoelectric joining at low temperature and low crystal symmetry in that materials. So soon several others multiferroics that are “Cr2O3, Ti2O3, GaFeO3 , PbFe0:5Nb0:5O3” etc are exist in nature [8, 9] or can be prepared. A great interest in these field begining the study towards an main perovskite material Bismuth ferrite (BiFeO3) [5]. However, ceramic BiFeO3 was found to be conductive and the production of single phase BiFeO3 is very though task and to be used in so many device. The presence of oxygen vacancies, defects or mixed external of Fe are the main cause of conductive properties in BiFeO3. Along with the experimental studies many theoretical observations were also reported asking many questions about different real microscopic mechanisms of FM and FE in BiFeO3. Therefore in the end of 1990 there is good achievement in preparation and characterization of multiferroics. Perovskites having the structure ABO3 good impact in many groups. Between all these compounds some rare earth based compounds exihbit multiferroic characteristics because of their strange structures [10, 11]. Some Bi based compounds like “BiFeO3” [12] and “BiMnO3” [13] exihbit ME coupling at high temperature. So many articles are written on the latest progress, physics for ME characteristics and facing tough task for ongoing technology