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Prabwal Jyoti Phukon
Y8209867
From the Bulk to the Boundary
27th October 2014 (Monday)
3 pm
FB-382
Branes play an important role in our understanding of physics at different length scales: from black holes to condensed matter systems. They arise as a class of dynamical, extended objects in string theory and other closely related theories like M-theory and are specified by the number of spatial dimensions they possess (a brane in p spatial dimensions is called a p-brane). In this thesis, we are particularly interested in the D-p-branes of string theory and M-p-branes of M-theory. We primarily focus on exploring three important directions related to brane configurations: its bulk in the low energy limit,i.e supergravity theory, bulk to boundary correspondence and a class of conjectured world-volume theories on branes. The thesis addresses three illustrative issues in the context of each of these three directions in three portions.
To start with, we undertake an analysis of branes with reference to asymptotically AdS black holes and their thermodynamics. Here, we seek to understand logarithmic corrections to the Bekenstein-Hawking entropy for these black holes due to thermal fluctuations in the grand canonical ensemble.
Then, in the second part of the thesis, we carry out a bulk to boundary theory study of branes in the backdrop of gauge-gravity duality, focussing on R-charged black holes. We study the electromagnetic response functions of strongly coupled media whose dual gravitational descriptions are given by these R-charged black holes. In dimensions 4, 5 and 7, these correspond to rotating configurations of M2, D3 and M5-branes, respectively.
The third part of the thesis work attempts to understand the moduli space of a class of conjectured worldvolume theories on M2-branes. These are known as quiver gauge theories. As a specific example, we investigate the (2+1) dimensional N = 2 Chern-Simons (CS) quiver gauge theories which appear as worldvolume theories on a stack of M2-branes probing a special class of Calabi-Yau (CY) four folds, namely complex cones over smooth Fano 3-folds. In this portion, we put special emphasis on examining the Higgsing and unHiggsing of these theories via a "brane tiling" method.
We conclude by highlighting some novel results in black hole thermodynamics, holographic optic and Chern-Simons quiver gauge theories that have been obtained in course of this thesis work.
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Prabhakar Tiwari
Y9109075
Observations of Large Scale Anisotropy and Cosmological Models
8th September 2014 (Monday)
12.00 pm
FB-382
The observable Universe is simply huge! ∼10^{26} meters in every direction, ~14 billion years old and contains ∼10 ^{80} hydrogen atoms. The modern cosmology is the science of the entire Universe. We, here on a small planet, can only assume that the Universe is knowable and physics is followed everywhere in the same manner. Furthermore it is reasonable to assume that the observable Universe is statistically same for all observers, located anywhere in the Universe. Today, we demand this uniformity as “Cosmological Principle” which assumes homogeneity and isotropy at large distance scales. This thesis is a critical examination of the cosmological principle. In the thesis we review the present observations of large scale anisotropy and discuss possible theoretical models to explain these observations.
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Mr. Shyam Lal Gupta
Y6209062
Dynamics of Laser Ablated ZnO Colliding Plasma Plumes
19th August, 2014 (Tuesday)
03:00 pm
FB-382
Irradiation of the target material with a laser pulse having fluence higher than its ablation threshold fluence results in formation of its plasma. UV laser irradiation of target surface results in smaller/comparable thermal diffusion length compared to absorption length resulting in congruent ablation unlike the chunk ablation due to bulk target heating with longer wavelength irradiation. The properties of the plasma plume viz. temperature, electron number density, and ion velocities etc. affect the properties and quality of pulsed laser deposited thin films. The properties of the plasma plume can significantly be varied by using various configurations of colliding plasma plumes. The plasma plumes evolve with time due to the pressure difference with respect to the ambient and move due to the thermal kinetic energy gained by the plasma species from the ablating laser pulse. The velocity components of seed plasma plumes in the direction of the interaction contribute to processes occurring in the interaction zone, where as the forward plume front velocity components result in the movement of the interaction zone. The properties of interaction zone (interpenetration or stagnation) are tuned by varying the inter-plume distance and the ion-ion mean free path. The ion – ion mean free path depends on relative collision velocity of two seed plasmas, the ionization state, and the ion density of the plasma plumes. In the present work we report the dynamical behavior of collinearly colliding ZnO plumes and formation of interaction zone / stagnation layer. The plasma plume dynamics is studied using fast imaging and optical emission spectroscopy. Two seed plasmas initially evolve independently and then start interacting at ~15 ns along their lateral dimensions. The interaction of seed plasma plumes results in a stagnation layer having abundance of the higher ionic states, high electron number density and temperature with slower decaying rate as compared to the single plume that lasts for longer time delays as compared to single plume. At the later stages of plasma evolution the vapor phase is populated by nanoparticles /nanoclusters of ZnO. The presence of nanoparticles/clusters of ZnO in the vapor phase for longer time delays in colliding plumes is confirmed using dynamic laser light scattering (Rayleigh) and photoluminescence (PL) techniques. The configuration is used for deposition of ZnO thin films on glass substrates using 1064 nm irradiation. The deposited films are characterized using X-ray diffraction, AFM, optical transmission in the UV-visible range and photoluminescence measurements. IR ablated colliding plasma plumes results in deposition of a-plane ZnO thin films with better optical properties as compared to c-axis oriented thin films obtained using conventional UV (355 nm) single plume deposition. The observed differences in the quality and properties of thin films are attributed to the flux of mono-energetic plasma species with almost uniform kinetic energy and higher thermal velocity reaching the substrate from interaction/stagnation zone of colliding plasma plumes.
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