3D Gravity, Chern-Simons and Higher Spins: A Mini Introduction

These are notes of introductory lectures on (a) elements of 2+1 dimensional gravity, (b) some aspects of its relation to Chern-Simons theory, (c) its generalization to couple higher spins, and (d) cosmic singularity resolution as an application in the context of flat space higher spin theory. A knowledge of the Einstein-Hilbert action, classical non-Abelian gauge theory and some (negotiable amount of) maturity are the only pre-requisites.Comment: 23 pages, Based on talks/lectures by CK at Goteborg, Tehran and Bangkok. v2:acknowledgments and references added, v3:published versio

Design and control of a smart fin using piezoelectric actuators

The objective of this research work is to design and implement control algorithms for smart fin of a projectile, which is currently under development in the Army Research Laboratory (ARL). The smart fin is used to maneuver small aerial vehicles by controlling the rotation angle of the fin. The fin is activated by a composite laminated plate that has two active piezoelectric layers. The prototype of the smart fin is assembled using Macro Fiber Composite (MFC actuator model M8557, Smart Material Co); Control algorithms for rotating the fin when subject to external aerodynamic forces are proposed. These controllers use a finite element model of the system. The three controllers are designed using Integral, Adaptive and Fuzzy Logic techniques respectively. Effects of Aerodynamic forces and uncertainties are included in these controllers; An experimental setup of the fin and actuator has been made for verifying and implementing the controllers with a real-time controller (dSPACE DS1102 controller board), which can be interfaced with the code developed in MATLAB and Simulink. Final tuning of the model is done using experimental data

Strings vs Spins on the Null Orbifold

We study the null orbifold singularity in 2+1 d flat space higher spin theory as well as string theory. Using the Chern-Simons formulation of 2+1 d Einstein gravity, we first observe that despite the singular nature of this geometry, the eigenvalues of its Chern-Simons holonomy are trivial. Next, we construct a resolution of the singularity in higher spin theory: a Kundt spacetime with vanishing scalar curvature invariants. We also point out that the UV divergences previously observed in the 2-to-2 tachyon tree level string amplitude on the null orbifold do not arise in the $\alpha^\prime\to \infty$ limit. We find all the divergences of the amplitude and demonstrate that the ones remaining in the tensionless limit are physical IR-type divergences. We conclude with a discussion on the meaning and limitations of higher spin (cosmological) singularity resolution and its potential connection to string theory.Comment: 27 pages, 5 figures, v2: comments about boundary conditions added, strengthening the conclusio

Resummation at finite conformal spin

We generalize the computation of anomalous dimension and correction to OPE coefficients at finite conformal spin considered recently in \cite{arXiv:1806.10919, arXiv:1808.00612} to arbitrary space-time dimensions. By using the inversion formula of Caron-Huot and the integral (Mellin) representation of conformal blocks, we show that the contribution from individual exchanges to anomalous dimensions and corrections to the OPE coefficients for "double-twist" operators $[\mathcal{O}_1\mathcal{O}_2]_{\Delta,J}$ in $s-$channel can be written at finite conformal spin in terms of generalized Wilson polynomials. This approach is democratic {\it wrt} space-time dimensions, thus generalizing the earlier findings to cases where closed form expressions of the conformal blocks are not available.Comment: Typos corrected, references added. JHEP versio

Characterization of advanced composites- a nondestructive approach

Adhesively bonded sandwich structures comprising of particulate composites as core and graphite epoxy skins as stiffeners are widely used for various applications in the marine and aerospace industry. The core material and the stiffener are held together by an adhesive bond. Particulate composites are made from a mixture of a polymer resin and hollow or solid particles. Hollow particulate composites are known as syntactic foams. Particulate composites possess attractive mechanical and physical properties such as high compressive strength etc, making them attractive materials for use in structural applications. Characterization of the adhesive bondline and core material in sandwich structures is important for ensuring structural stability and reliability. Nondestructive evaluation [NDE] techniques such as ultrasound are used for better evaluation of these sandwich structured materials. The present study addresses the problems of detection of disbonds, bond surface characteristics and porosity in the adhesive panels along with characterization of particulate composites separately using NDE. The importance of the attenuation coefficient in computing the longitudinal velocities of the ultrasonic wave in particulate composite samples is also discussed. Five sets of adhesively bonded carbon epoxy composite specimens with varying bond surface preparation, twenty four different types of hollow syntactic foams and six different types of solid particulate composites, are fabricated. The adhesively bonded panels are made by including known defects in the bond layer of the samples. The particulate composites (syntactic foams and solid particulates) are fabricated by varying the volume fraction of each of the four types of microballoons and solid particle from 10% to 60%. Pulse echo UI method is selected for use in the present work. The results of this research provides a better understanding of adhesive joints and particulate composites and thus help in characterizing structures composed of these constituents. One of the major findings in this research is the discovery of a nondestructive method to determine the dynamic modulus of particulate composites. In addition, a constitutive model explaining the effect of particle size, porosity, radius ratio on the ultrasonic attenuation coefficient in particulate composites is developed

Structural, optical and nanomechanical properties of (1 1 1) oriented nanocrystalline ZnTe thin films

Structural, optical and nanomechanical properties of nanocrystalline Zinc Telluride (ZnTe) films of thickness upto 10 microns deposited at room temperature on borosilicate glass substrates are reported. X-ray diffraction patterns reveal that the films were preferentially oriented along the (1 1 1) direction. The maximum refractive index of the films was 2.74 at a wavelength of 2000 nm. The optical band gap showed strong thickness dependence. The average film hardness and Young’s modulus obtained from loaddisplacement curves and analyzed by Oliver-Pharr method were 4 and 70 GPa respectively. Hardness of (1 1 1) oriented ZnTe thin films exhibited almost 5 times higher value than bulk. The studies show clearly that the hardness increases with decreasing indentation size, for indents between 30 and 300 nm in depth indicating the existence of indentation size effect. The coefficient of friction for these films as obtained from the nanoscratch test was ∼0.4.Financial support in the form of fellowships to MSRNK and SK from the ACRHEM project of DRDO is acknowledged

Comparative evaluation of micro-tensile bond strength between zirconia core and all-ceramic layering with different surface treatments: an in vitro study

Background: The quest for newer and stronger materials for replacing teeth has resulted in Zirconium oxide\u27s introduction, which possesses excellent mechanical strength and toughness. However, uncertainty exists in the relationship between its bond strength and surface treatment method adopted and the mode of failure at the interface. Aim - The study aimed to evaluate the comparison of micro-tensile bond strength between the zirconia core and all-ceramic layering with different surface treatments and analyse their failure mode by Scanning Electron Microscope (SEM). Materials and methods: Zirconia cores (Ceramill ZI 71 XS) were fabricated by CAD-CAM into discs with 5mm diameter and 3.5mm height. Then the cores were divided into four groups. Among which, Group-I was the control group, and the remaining are surface treated. Group-II specimens were treated with sandblasting, followed by acid etching; Group-III and Group-IV were treated with zirliner and glass beads, respectively. After that, the veneering material (IPS Empress, E.max Ceram Dentin) of 2×2 mm was adhered to the zirconia core and then kept in the ceramic furnace. The specimens were mounted on a Universal Testing Machine, and tensile stress is applied. The obtained data were subjected to One-way ANOVA and Tukey-HSD tests for statistical analyses. Results - The samples treated with sandblasting followed by acid etching showed more micro-tensile bond strength between core and veneer interface. Furthermore, the SEM study revealed a cohesive failure in Group-II, whereas, in Group-I and -III, there was an adhesive failure. Group-IV specimens exhibited a mixed failure. One-way ANOVA showed significant differences (p=0.001) within the groups. In posthoc analysis, Group-III showed significant differences with Groups -I, II, and IV. Conclusion: Increased surface roughness of zirconia obtained by sandblasting with aluminium oxide particles, when coupled along with chemical etching with hydrofluoric acid, enhanced the micro-tensile bond strength between the Y-TZP zirconia core and veneering ceramic