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Shiv Nadar University

(SoNS) School of Natural Sciences

Physics

World University root node

APS March Meeting Abstracts

High Momentum Transfer Shallow Core-to-valence Spectroscopy in the Actinides

InAs quantum dots (QDs) were successfully formed in single-crystalline Si by sequential ion implantation and subsequent milliseconds range flash lamp annealing (FLA). Samples were characterized by μ-Raman spectroscopy, Rutherford Backscattering Spectrometry (RBS) high-resolution transmission electron microscopy (HRTEM) and low temperature photoluminescence (PL). The Raman spectrum shows two peaks at 215 and 235 cm-1 corresponding to the transverse optical (TO) and longitudinal optical (LO) InAs phonon modes, respectively. The PL band at around 1.3 μm originates from the InAs QDs with an average diameter 7.5 ± 0.5 nm and corresponds to the increased band gap energy due to the strong quantum confinement size effect. The FLA of 20 ms is sufficient for InAs QDs formation. It also prevents the out-diffusion of implanted elements. Moreover, the silicon layer amorphized during ion implantation is recrystallized by solid-phase epitaxial regrowth during FLA. © Springer-Verlag 2010.

Applied Physics B: Lasers and Optics

Formation of InAs quantum dots in silicon by sequential ion implantation and flash lamp annealing

Background: Mathematical models have formalized how free-rider effects can threaten the stability of high vaccine coverage levels under established voluntary vaccination programs. However, little research has addressed the question of when free-riding begins to develop when a new vaccine is first introduced in a population. Methodology/Principal Findings: Here, we combine a game theoretical model of vaccinating behavior with an agestructured compartmental model to analyze rational vaccinating behavior in the first years of a universal immunization program, where a new vaccine is free to all children of a specified age. The model captures how successive birth cohorts face different epidemiological landscapes that have been shaped by the vaccinating decisions of previous birth cohorts, resulting in a strategic interaction between individuals in different birth cohorts. The model predicts a Nash equilibrium coverage level of 100% for the first few birth cohorts under the new program. However, free-riding behavior emerges very quickly, with the Nash equilibrium vaccine coverage dropping significantly within 2-5 years after program initiation. Subsequently, a rich set of coupled dynamics between infection prevalence and vaccinating behaviors is possible, ranging from relatively stable (but reduced) coverage in later birth cohorts to wide fluctuations in vaccine coverage from one birth cohort to the next. Individual tolerance for vaccine risk also starts out at relatively high levels before dropping significantly within a few years. Conclusions/Significance: These results suggest that even relatively new immunization programs can be vulnerable to drops in vaccine coverage caused by vaccine scares and exacerbated by herd immunity effects, necessitating vigilance from the start.

Mathematics

Fulltext

PLoS ONE

Rapid emergence of free-riding behavior in new pediatric immunization programs

The co-chaperone Hep1 is required to prevent the aggregation of mitochondrial Hsp70 proteins. We have analyzed the interaction of Hep1 with mitochondrial Hsp70 (Ssc1) and the determinants in Ssc1 that make it prone to aggregation. The ATPase and peptide binding domain (PBD) of Hsp70 proteins are connected by a linker segment that mediates interdomain communication between the domains. We show here that the minimal Hep1 binding entity of Ssc1 consists of the ATPase domain and the interdomain linker. In the absence of Hep1, the ATPase domain with the interdomain linker had the tendency to aggregate, in contrast to the ATPase domain with the mutated linker segment or without linker, and in contrast to the PBD. The closest homolog of Ssc1, bacterial DnaK, and a Ssc1 chimera, in which a segment of the ATPase domain of Ssc1 was replaced by the corresponding segment from DnaK, did not aggregate in Δhep1 mitochondria. The propensity to aggregate appears to be a specific property of the mitochondrial Hsp70 proteins. The ATPase domain in combination with the interdomain linker is crucial for aggregation of Ssc1. In conclusion, our results suggest that interdomain communication makes Ssc1 prone to aggregation. Hep1 counteracts aggregation by binding to this aggregation-prone conformer. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.

Life Sciences

Journal of Biological Chemistry

ATPase domain and interdomain linker play a key role in aggregation of mitochondrial Hsp70 chaperone Ssc1

Observation of whistler-mode chorus emissions recorded at the Indian Antarctic station, Maitri (lat=70°46′S, long=11°50′E, L=4.5), during a quiet period on 5 February 2001 has been reported. The detailed spectral analysis of recorded chorus emissions shows that each chorus element originates from the upper edge of the underlying hiss band. To explain the observed dynamic spectra of these chorus emissions, a possible generation mechanism is presented based on the recent nonlinear theory. It is observed that the seeds of chorus emissions with rising frequency are generated near the magnetic equator as a result of a nonlinear growth mechanism that depends on the wave amplitude. On the basis of this theory, the frequency sweep rate of chorus emission is computed and compared with that of our experimentally observed values, which in general shows good agreement. © 2010 The Royal Swedish Academy of Sciences.

Physica Scripta

An explanation of the observation of whistler-mode chorus emissions at the Indian Antarctic station, Maitri (L = 4.5)

Magnetic atoms at surfaces are a rich model system for solid-state magnetic bits exhibiting either classical or quantum behaviour. Individual atoms, however, are difficult to arrange in regular patterns. Moreover, their magnetic properties are dominated by interaction with the substrate, which, as in the case of Kondo systems, often leads to a decrease or quench of their local magnetic moment. Here, we show that the supramolecular assembly of Fe and 1,4-benzenedicarboxylic acid molecules on a Cu surface results in ordered arrays of high-spin mononuclear Fe centres on a 1.5 nm square grid. Lateral coordination with the molecular ligands yields unsaturated yet stable coordination bonds, which enable chemical modification of the electronic and magnetic properties of the Fe atoms independently from the substrate. The easy magnetization direction of the Fe centres can be switched by oxygen adsorption, thus opening a way to control the magnetic anisotropy in supramolecular layers akin to that used in metallic thin films.

Nature Materials

Supramolecular control of the magnetic anisotropy in two-dimensional high-spin Fe arrays at a metal interface

Refractive indices for molecular crystals are obtained from Hartree-Fock wavefunctions constrained to reproduce a set of experimental X-ray structure factors. Coupled-perturbed Hartree-Fock theory is used to calculate the in-crystal effective polarizabilities from which the refractive indices are obtained, thus eliminating the need for the calibration procedure used in earlier work by Whitten et al. [J. Chem. Phys. 2006, 125, 174505]. The results clearly demonstrate that these X-ray constrained Hartree-Fock (XCHF) wavefunctions reflect genuine effects of intermolecular interactions in crystals. Molecular dipole moments are consistently in excellent agreement with ab initio MP2 estimates that incorporate the effects of the crystal field. Consistent agreement of the XCHF refractive indices with experimental measurements at optical frequencies confirms that this approach can provide both meaningful results and considerable insight into the relative importance of molecular properties and crystal field effects in determining the detailed nature of the refractivity tensor. © 2009 the Owner Societies.

Chemistry

Physical Chemistry Chemical Physics

Refractive indices for molecular crystals from the response of X-ray constrained Hartree-Fock wavefunctions

We report (11)B and (195)Pt NMR Knight shift K and spin-lattice relaxation rate 1/T(1) in CePt(2)B(2)C in the range 4-315 K. The quadrupolar coupling constant, nu(Q) for boron nuclei is 790 +/- 10 kHz. The change of hyperfine field, H(hf), is observed below 30 K in the K versus susceptibility, chi, plot. The calculated value of H(hf) at the (11)B ((195)Pt) is 0.156 (6.86) kOe/mu(B) in the range 30-300 K and approximately 0 (0.22) kOe/mu(B) below 30 K. The 1/T(1) versus T curve shows some exotic behavior. The Ce 4f spin contribution to the nuclear relaxation rate (1/T(1f)) in each case is obtained by subtracting the T(1K)(-1) estimated from its La analog, i.e. LaPt(2)B(2)C. In the case of (11)B resonance, in the temperature range of 300-100 K, (1/T(1f)) is independent of T, suggesting a Curie-Weiss behavior of the imaginary part of the dynamic susceptibility. It then shows a slow but continuous increment in the range 100-70 K, indicating a signature of the development of short-range magnetic correlation among the Ce 4f spins. Below 70 K, this enhancement of 1/T(1f) is completely suppressed and it decreases sharply, indicating a suppression of the effect of magnetic correlation, due to the dominance of the Kondo effect over the RKKY interaction. 1/T(1f), follows approximately T(alpha), with an exponent alpha approximately 0.7 in the range 4-30 K for (195)Pt and in the range 8-30 K for (11)B resonance. This is a characteristics of a non-Fermi-liquid like behavior. However, in the case of (11)B, there is again a clear change in the slope of the 1/T(1f) versus T curve below 8 K, with the value of alpha = 1.0, as if the behavior of the conduction electrons approaches towards a Fermi liquid, when probed near the (11)B site.

Journal of Physics: Condensed Matter

11 B and 195 Pt NMR study of heavy-fermion compound CePt 2 B 2 C

The anomalous wear-out phenomena of Eu-implanted metal-oxide-semiconductor devices were investigated. It will be shown that in contrast to other rare earth elements the electroluminescence (EL) intensity of Eu-implanted SiO2 layers can rise under constant current injection before the known EL quenching will start. Under certain circumstances, this rise may amount up to two orders of magnitude. The EL behavior will be correlated with the microstructural and electrical properties of the devices. Transmission electron microscopy and Rutherford backscattering spectroscopy were applied to trace the development of Eu/Eu oxide clusters and the diffusion of Eu to the interfaces of the gate oxide layer. The hydrogen profile within the SiO2-SiON interface region was determined by nuclear reaction analysis. Current-voltage characteristics, EL decay times, and the progression of the voltage and the EL spectrum with increasing charge injection were measured to study charge and trapping phenomena in the oxide layer to reveal details of the EL excitation mechanism. A first qualitative model for the anomalous life time behavior is proposed. © 2009 American Institute of Physics.

Journal of Applied Physics

Anomalous wear-out phenomena of europium-implanted light emitters based on a metal-oxide-semiconductor structure

We demonstrate that the recent paper by Jana and Roy entitled "Non-Hermitian quantum mechanics with minimal length uncertainty" [SIGMA 5 (2009), 083, 7 pages, arXiv:0908.1755] contains various misconceptions. We compare with an analysis on the same topic carried out previously in our manuscript [arXiv:0907.5354]. In particular, we show that the metric operators computed for the deformed non-Hermitian Swanson models differs in both cases and is inconsistent in the former.