Stokes amplification regimes in quasi-cw pumped hydrogen-filled hollow-core photonic crystal fiber

Pure rotational stimulated Raman scattering spectra containing nine strong spectral components were generated from a approximately 11 m long hollow-core photonic crystal fiber filled with hydrogen and pumped with nanosecond pulses having energies around 100-300 nJ. Observation of both transient and steady-state scattering threshold behavior is reported. Passage from the transient to the steady state is observed with a pulse as long as 14 ns. Convenient analytical expressions for energy and power threshold are deduced for the present configuration.     

Detecting chaos,determining the dimensions of tori and predicting slow diffusion in Fermi–Pasta–Ulam lattices by the Generalized Alignment Index method

The recently introduced GALI method is used for rapidly detecting chaos, determining the dimensionality of regular motion and predicting slow diffusion in multi-dimensional Hamiltonian systems. We propose an efficient computation of the GALI_k indices, which represent volume elements of k randomly chosen deviation vectors from a given orbit, based on the Singular Value Decomposition (SVD) algorithm. We obtain theoretically and verify numerically asymptotic estimates of GALIs long-time behavior in the case of regular orbits lying on low-dimensional tori. The GALI_k indices are applied to rapidly detect chaotic oscillations, identify low-dimensional tori of Fermi–Pasta–Ulam (FPU) lattices at low energies and predict weak diffusion away from quasiperiodic motion, long before it is actually observed in the oscillations.     

The role of an interface in stabilizing reaction intermediates for hydrogen evolution in aprotic electrolytes

By combining idealized experiments with realistic quantum mechanical simulations of an interface, we investigate electro-reduction reactions of HF, water and methanesulfonic acid (MSA) on the single crystal (111) facets of Au, Pt, Ir and Cu in organic aprotic electrolytes, 1 M LiPF₆ in EC/EMC 3:7W (LP57), the aprotic electrolyte commonly used in Li-ion batteries, 1 M LiClO₄ in EC/EMC 3:7W and 0.2 M TBAPF₆ in 3 : 7 EC/EMC. In our previous work, we have established that LiF formation, accompanied by H₂ evolution, is caused by a reduction of HF impurities and requires the presence of Li at the interface, which catalyzes the HF dissociation. In the present paper, we find that the measured potential of the electrochemical response for these reduction reactions correlates with the work function of the electrode surfaces and that the work function determines the potential for Li⁺ adsorption. The reaction path is investigated further by electrochemical simulations suggesting that the overpotential of the reaction is related to stabilizing the active structure of the interface having adsorbed Li⁺. Li⁺ is needed to facilitate the dissociation of HF which is the source of protons. Further experiments on other proton sources, water and methanesulfonic acid, show that if the hydrogen evolution involves negatively charged intermediates, F⁻ or HO⁻, a cation at the interface can stabilize them and facilitate the reaction kinetics. When the proton source is already significantly dissociated (in the case of a strong acid), there is no negatively charged intermediate and thus the hydrogen evolution can proceed at much lower overpotentials. This reveals a situation where the overpotential for electrocatalysis is related to stabilizing the active structure of the interface, facilitating the reaction rather than providing the reaction energy.

By combining idealized experiments with realistic quantum mechanical simulations of an interface, we investigate electroreduction reactions of HF, water and methanesulfonic acid on the single crystal (111) facets of Au, Pt, Ir and Cu in a variety of aprotic electrolytes.     

Dynamical analysis of the infection status in diverse communities due to COVID-19 using a modified SIR model

In this article, we model and study the spread of COVID-19 in Germany, Japan, India and highly impacted states in India, i.e., in Delhi, Maharashtra, West Bengal, Kerala and Karnataka. We consider recorded data published in Worldometers and COVID-19 India websites from April 2020 to July 2021, including periods of interest where these countries and states were hit severely by the pandemic. Our methodology is based on the classic susceptible–infected–removed (SIR) model and can track the evolution of infections in communities, i.e., in countries, states or groups of individuals, where we (a) allow for the susceptible and infected populations to be reset at times where surges, outbreaks or secondary waves appear in the recorded data sets, (b) consider the parameters in the SIR model that represent the effective transmission and recovery rates to be functions of time and (c) estimate the number of deaths by combining the model solutions with the recorded data sets to approximate them between consecutive surges, outbreaks or secondary waves, providing a more accurate estimate. We report on the status of the current infections in these countries and states, and the infections and deaths in India and Japan. Our model can adapt to the recorded data and can be used to explain them and importantly, to forecast the number of infected, recovered, removed and dead individuals, as well as it can estimate the effective infection and recovery rates as functions of time, assuming an outbreak occurs at a given time. The latter information can be used to forecast the future basic reproduction number and together with the forecast on the number of infected and dead individuals, our approach can further be used to suggest the implementation of intervention strategies and mitigation policies to keep at bay the number of infected and dead individuals. This, in conjunction with the implementation of vaccination programs worldwide, can help reduce significantly the impact of the spread around the world and improve the wellbeing of people.

     

A correlation of the isentropic exponents of real gases

In previous publications, three isentropic exponents, k_pv, k_Tv, k_pT, have been introduced, which when used in place of the classical isentropic exponent k = c_p/c_v in the ideal gas isentropic change equations, the latter can describe very accurately the isentropic change of real gases. The present work provides a general method for determining the values of k_pv, k_Tv, k_pT within the ranges of reduced pressure p_r = 0 to 10 and of reduced temperature T_r = 1 to 3.5, thus allowing the calculation of the isentropic flow of those real gases for which no detailed thermodynamic data are available. The accuracy obtained is limited only by the accuracy of the generalized Lee-Kesler equation of state, which is employed in the method developed.     

Geometrical properties of local dynamics in Hamiltonian systems: The Generalized Alignment Index (GALI) method

We investigate the detailed dynamics of multi-dimensional Hamiltonian systems by studying the evolution of volume elements formed by unit deviation vectors about their orbits. The behavior of these volumes is strongly influenced by the regular or chaotic nature of the motion, the number of deviation vectors, their linear (in)dependence and the spectrum of Lyapunov exponents. The different time evolution of these volumes can be used to identify rapidly and efficiently the nature of the dynamics, leading to the introduction of quantities that clearly distinguish between chaotic behavior and quasiperiodic motion on N-dimensional tori. More specifically we introduce the Generalized Alignment Index of order k () as the volume of a generalized parallelepiped, whose edges are k initially linearly independent unit deviation vectors with respect to the orbit studied whose magnitude is normalized to unity at every time step. We show analytically and verify numerically on particular examples of N-degree-of-freedom Hamiltonian systems that, for chaotic orbits, tends exponentially to zero with exponents that involve the values of several Lyapunov exponents. In the case of regular orbits, fluctuates around non-zero values for 2≤k≤N and goes to zero for N<k≤2N following power laws that depend on the dimension of the torus and the number m of deviation vectors initially tangent to the torus: ∝t^{−2(k−N)+m} if 0≤m<k−N, and ∝t^−(k−N) if m≥k−N. The is a generalization of the Smaller Alignment Index (SALI) (). However, provides significantly more detailed information on the local dynamics, allows for a faster and clearer distinction between order and chaos than SALI and works even in cases where the SALI method is inconclusive.     

Generalized and exact solutions for oblique shock waves of real gases with application to real air

Calculation of the oblique shock wave of real gases is a difficult and time consuming problem because it involves numerical solution of a set of 10 equations, two of which (i.e., the equation of state and enthalpy function)—if available—are of a very complicated algebraic form. The present work presents a generalized method for calculating oblique shock waves of real gases, based on the Redlich-Kwong equation of state. Also described is an exact method applicable when the exact equation of state and enthalpy function of a real gas are available. Application of the generalized and the exact methods in the case of real air showed that the former is very accurate and at least twenty times faster than the latter. An additional contribution of the study is the derivation of real gas oblique shock wave equations, which are of the same algebraic form as the well known ideal gas normal shock wave relations.     

Multi-band,highly absorbing,microwave metamaterial structures

A further example of multi-band absorption using ultra-thin, polarization-insensitive, wide-angled metamaterial absorbers that operate in multi-frequency bands within the microwave regime is presented in this work. The basic structure geometry is utilised to create multi-band highly absorbing structures by incorporating the scalability property of the metamaterials. Simulation results verify the structure’s ability for high absorption. The multi-band absorbers are promising candidates as absorbing elements in scientific and technical applications because of its multi-band absorption, polarization insensitivity, and wide-angle response. Finally, the current distributions for those structures are presented to gain a better insight into the physics behind the multiple absorption mechanism.     

Study of group-III binary and ternary nitrides using X-ray absorption fine structure measurements

It is demonstrated that the NEXAFS spectra are a “fingerprint” of the symmetry and the composition of the binary nitrides GaN, AlN and InN, as well as of their ternary alloys In_0.16Ga_0.84N and Al_yGa_1−yN. From the angular dependence of the N-K-edge NEXAFS spectra, the hexagonal symmetry of the under study compounds is deduced and the (p_x, p_y) or p_z character of the final state is identified. The energy position of the absorption edge (E_abs) of the binary compounds GaN, AlN and InN is found to red-shift linearly with the atomic number of the cation. The E_abs of the Al_yGa_1−yN alloys takes values in between those corresponding to the parent compounds AlN and GaN. Contrary to that, the E_abs of In_0.16Ga_0.84N is red-shifted relative to that of GaN and InN, probably due to ordering and/or phase separation phenomena. The EXAFS analysis results reveal that the first nearest-neighbour shell around the N atom, which consists of Ga atoms, is distorted in both GaN and Al_xGa_1−xN for x<0.5.     

Pyramidal slip in electron beam heated deformed Titanium

The incidence of perfect glide dislocations, moving on parallel pyramidal slip bands on a particular grain boundary of deformed Titanium is studied by means of Transmission Electron Microscopy. Static experiments, performed by using the electron beam as a heating source, proved that slip propagation across the interface is possible when the angle of intersections between the activated slip planes of the incoming and the outgoing dislocations with the boundary plane is minimised. Additionally, the Burgers vector of the residual dislocations left in the boundary after slip transmission occurred should also be minimised. Due to their very small Burgers vector, residual dislocations are visualised with satisfactory results by an image simulation method.