Pattern formation and magnetism in pulsed laser-induced self-organized nanostructures from single and bilayer metallic films

Self-organization via nanosecond pulsed laser melting of thin metal films is attractive as a reliable and low cost method to create surface metallic nanostructures. The potential applications of these resulting structures are related to their magnetic, optical, plasmonic, and magneto-optical properties, such as in high density magnetic storage, energy harvesting, and sensing. In this dissertation experimental and theoretical mechanisms of nanoscale patterning by dewetting self-organization was explored in nanoscopic thickness single and bilayer metal films. Magnetism in the resulting nanostructures was also explored. Experimental techniques included thin film deposition, laser irradiation, scanning and transmission electron microscopy, atomic force and magnetic force microscopy. A thermodynamic model of dewetting based on comparing the rate of change of free energy to frictional loss via viscous dissipation was developed. From this model the length scale characteristics of the self-organization in single and bilayer metal systems was determined. The model was successfully applied to experimental behavior of pulsed-laser induced dewetting of Ag, as well as for Co, where intrinsic thermocapillary effects influenced dewetting. Another finding was that the metal films show a thickness dependent dewetting morphology. This was investigated in detail for Ag on SiO2 substrates and it was found that for films less then 9.5 nm, the early stages of dewetting consisted of bicontinuous structures, while above, it consisted of regular sized holes. This observation was consistent with a previously published theoretical argument that morphology was based on the film-thickness dependent location of the minimum in the free energy curvature. In this model, bicontinuous patterns occur for thickness below the curvature minimum, while holes occur above it. For bilayer self-organization, the immiscible metals Ag and Co were investigated on SiO2 substrates. The thermodynamic theory predicted substantially different length scales for the two bilayer configurations, Ag/Co/SiO2 and Co/Ag/SiO2. This behavior was corroborated by experimental measurements based on pulsed laser dewetting. The difference arises due to change in the sign and magnitude of intermolecular forces for the two configurations. The nanoparticles resulting from bilayer self-organization consisted of granular nanostructure with each nanoparticle containing phase separatedl grains of Ag and Co, thus representing nanoscale nanocomposites. Extensive magnetic investigations were performed on hemispherical shaped nanoparticles of elements: Co, Fe, Ni), alloys: FeCo) and composites: CuCo), created by the laser dewetting process. By utilizing magnetic force microscopy technique, the magnetic anisotropy of individual nanoparticles was determined. The most important discovery was a size-dependent single-domain magnetic anisotropy due to which, smaller particles had an in-plane magnetic orientation while larger particles were out-of-plane. The reason for the unusual out-of-plane orientation for the hemispherical particles was a residual strain in the nanoparticles following the large cooling rates under laser melting. The resulting magnetoelastic energy was sufficient to overcome shape and magnetocrystalline anisotropy energies. These investigations present a fundamental picture of nanostructure synthesis via pulsed laser-induced dewetting and self-organization in single and bilayer films and the magnetic behavior of the processed materials

Celestial gluon and graviton OPE at loop level

In this paper, we analyze the loop corrections to celestial OPE for gluons and gravitons. Even at the loop level, the soft gluons and gravitons have $\Delta=1-\mathbb Z_{\geq 0}$. The only novelty is the presence of higher poles. At one loop level, there are two types of conformal soft gluons with a single pole and a double pole in the $\Delta$ plane. The celestial OPEs are obtained using the collinear splitting functions. In the case of gluons, the splitting functions receive loop corrections. After taking the holomorphic soft limit, we find the OPE of conformal soft gluons. In the case of gravitons, where splitting functions are known to be all loop exact, we still find a $w_{\infty}$ algebra for soft gravitons which is in addition to the $w_{1+\infty}$ algebra already found by Strominger.Comment: 15 pages+appendice

A compound figure of merit for photonic applications of metal nanocomposites

Selecting nanocomposites for photonic switching applications requires optimizing their thermal, nonlinear and two-photon absorption characteristics. We simplify this step by defining a compound figure of merit (FOM_{C}) for nanocomposites of noble metals in dielectric based on criteria that limit these structures in photonic applications, i.e. thermal heating and two-photon absorption. The device independent results predict extremely large values of FOM_{C} for a specific combination of the metal and insulator dielectric constant given by \epsilon_{h}=\frac{\epsilon_{1}-\epsilon_{2}}{2}, where \epsilon_{h} is the dielectric constant of the host and \epsilon_{1} and \epsilon_{2} are the real and imaginary parts for the metal.Comment: Appearing in Appl. Phys. Lett. (2006

Universality of Loop Corrected Soft Theorems in 4d

In \cite{1808.03288}, logarithmic correction to subleading soft photon and soft graviton theorems have been derived in four spacetime dimensions from the ratio of IR-finite S-matrices. This has been achieved after factoring out IR-divergent components from the traditional electromagnetic and gravitational S-matrices using Grammer-Yennie prescription. Although the loop corrected subleading soft theorems are derived from one-loop scattering amplitudes involving scalar particles in a minimally coupled theory with scalar contact interaction, it has been conjectured that the soft factors are universal (theory independent) and one-loop exact (don't receive corrections from higher loops). This paper extends the analysis conducted in \cite{1808.03288} to encompass general spinning particle scattering with non-minimal couplings permitted by gauge invariance and general coordinate invariance. By re-deriving the $\ln\omega$ soft factors in this generic setup, we establish their universal nature. Furthermore, we summarize the results of loop corrected soft photon and graviton theorems up to sub-subleading order, which follows from the analysis of one and two loop QED and quantum gravity S-matrices. While the classical versions of these soft factors have already been derived in the literature, we put forth conjectures regarding the quantum soft factors and outline potential strategies for their derivation.Comment: 56 pages + appendice

Light-Cone Reduction vs. TsT transformations : A Fluid Dynamics Perspective

We compute constitutive relations for a charged $(2+1)$ dimensional Schr\"odinger fluid up to first order in derivative expansion, using holographic techniques. Starting with a locally boosted, asymptotically $AdS$, $4+1$ dimensional charged black brane geometry, we uplift that to ten dimensions and perform $TsT$ transformations to obtain an effective five dimensional local black brane solution with asymptotically Schr\"odinger isometries. By suitably implementing the holographic techniques, we compute the constitutive relations for the effective fluid living on the boundary of this space-time and extract first order transport coefficients from these relations. Schr\"odinger fluid can also be obtained by reducing a charged relativistic conformal fluid over light-cone. It turns out that both the approaches result the same system at the end. Fluid obtained by light-cone reduction satisfies a restricted class of thermodynamics. Here, we see that the charged fluid obtained holographically also belongs to the same restricted class.Comment: 0+22 pages, 1 figur

Modified supersymmetric indices in AdS3_3/CFT2_2

We consider the $\mathcal{N}=(2,2)$ AdS$_3$/CFT$_2$ dualities proposed by Eberhardt, where the bulk geometry is AdS$_3\times(S^3\times T^4)/\mathbb{Z}_k$, and the CFT is a deformation of the symmetric orbifold of the supersymmetric sigma model $T^4/\mathbb{Z}_k$ (with $k=2,\ 3,\ 4,\ 6$). The elliptic genera of the two sides vanish due to fermionic zero modes, so for microstate counting applications one must consider modified supersymmetric indices. In an analysis similar to that of Maldacena, Moore, and Strominger for the standard $\mathcal{N}=(4,4)$ case of $T^4$, we study the appropriate helicity-trace index of the boundary CFTs. We encounter a strange phenomenon where a saddle-point analysis of our indices reproduces only a fraction (respectively $\frac{1}{2},\ \frac{2}{3},\ \frac{3}{4},\ \frac{5}{6}$) of the Bekenstein-Hawking entropy of the associated macroscopic black branes.Comment: v2: an outline added in the introductio

Celestial holography from Chiral strings

In this paper, we studied the relationship between celestial holography and chiral strings. Chiral strings differ from the usual string theory by a change of boundary conditions on the string propagators. It is shown that chiral strings would reproduce graviton amplitudes and could serve as an alternative description of Einstein's gravity. Celestial holography is a proposed duality between gravity in asymptotically flat space-time and a CFT living on its conformal boundary. Since both are CFT descriptions of gravity, we investigate the potential connection between these two formalisms. In this paper, we find that both the energetic as well as conformal soft theorems could be derived from the OPEs of vertex operators of chiral strings. All operators in the CCFT can be described by Mellin transforming the vertex operators in the chiral string theories, and the OPE coefficients of CCFT can also be obtained from the world-sheet description.Comment: 28 pages, 2 figures. Include additional reference

Chemical characterization, adverse effects and potential toxicological effects of medical devices applied in gastrointestinal disease: a review

Medical devices have gained popularity as a therapeutic or diagnostic purpose in gastrointestinal obstructions caused by malignant tumour or benign stricture or any other pre-existing conditions. The problems of stents are like sense of foreign body, migration or incomplete expansion or re obstruction. In this scenarios, self-expandable metallic stents (SEMS), or biodegradable self-expanding stents, wall flex stents, drug eluted stents, etc prevents the obstruction related difficulties. SEMS associated with re-obstruction and migration along with difficulty in removal and chances of leaks due to corrosion.  In case of biodegradable self-expanding stents, which are either made of magnesium-based materials or synthetic polymers, such as polylactide or polyglycolide, or co-polymers, such as polydioxanone. Magnesium-based materials are very biocompatible but due to the property of dissolving in the body by rapid corrosion, degradation can occur before the therapeutic objective is reached. Synthetic polymers associated severe mucosal hyperplastic reaction with overgrowth and/or ingrowth. Wall flex stents, drug eluted stents can be used to prevent complications associated with above variants but their clinical significance and toxicological effects were not evaluated completely. Under this given scenario toxicological evolution of various medical devices used in gastrointestinal disease and their potential toxicological effects are required to understand their tolerability and acceptability

On Type-II Progressively Hybrid Censoring

The progressive Type-II censoring scheme has become quite popular. A drawback of a progressive censoring scheme is that the length of the experiment can be very large if the items are highly reliable. Recently, Kundu and Joarder (2006) introduced the Type-II progressively hybrid censored scheme and analyzed the data assuming that the lifetimes of the items are exponentially distributed. This article presents the analysis of Type-II progressively hybrid censored data when the lifetime distributions of the items follow Weibull distributions. Maximum likelihood estimators and approximate maximum likelihood estimators are developed for estimating the unknown parameters. Asymptotic confidence intervals based on maximum likelihood estimators and approximate maximum likelihood estimators are proposed. Different methods are compared using Monte Carlo simulations and one real data set is analyzed

A Software Defined Radio based UHF Digital Ground Receiver System for Flying Object using LabVIEW

This study demonstrates the design and implementation of a software defined radio based digital ground receiver system using LabVIEW. In flight testing centre, command transmission system is used to transmit specific commands to execute some operation inside the flight vehicle. One ground receiver system is needed to monitor the transmitted command and monitor the presence of the command in air. The newly implemented ground receiver system consists of FPGA, RTOS and general processing unit. The analog to digital conversion and RF down conversions are carried out in high speed PCI extension for instrumentation express cards. The communication algorithms, digital down conversion are implemented in FPGAs. The communication system uses digital demodulation and decoding scheme and realised by NI PXI-7966R with Xilinx Virtex 5, SXT, FPGA. The performance of the receiver system has been analysed by linearity measurement of pre-amplifier Gain, Noise figure, frequency, power and also measurement of sensitivity. The results show successful implementation of the ground receiver system