• Volume 91, Issue 3

September 2018

• Optical properties of $\rm{TlGa(S_{x}Se_{1−x})_{2}}$ layered mixed crystals ($0 \leq x \leq 1$): Absorption edge and photoluminescence study at $T = 10 K$

Transmittance measurements of $\rm{TlGa(S_{x}Se_{1−x})_{2}}$ layered mixed crystals ($0 \leq x \leq 1$) were performed in the 1.80–2.80 eV photon energy range at $T = 10 K$. Band-gap energies of the studied crystals were estimated by means of the derivative spectra of transmittance and photon energy dependence of absorption coefficient. The compositional dependence of direct band-gap energy at $T = 10 K$ revealed that as sulphur (selenium) composition is increased (decreased) in the mixed crystals, the direct band-gap energy increases from 2.19 eV (x = 0) to 2.67 eV (x = 1). Photoluminescence spectra of $\rm{TlGa(S_{x}Se_{1−x})_{2}}$ mixed crystals were investigated in the photon energy region of 2.00–3.10 eV at $T = 10 K$. The observed bands were assigned to the transitions of electrons from conduction band to the shallow acceptor levels in the band gap. The shift of the PL bands to higher energies with increasing sulphur content was revealed. In addition, the composition ratio of the mixed crystals was obtained from the energy-dispersive spectroscopy experiments.

• Asymptotics of activity series at the divergence point

For statistical models of imperfect gases, a new method is proposed to evaluate the reducible cluster integrals of very high (actually unlimited) orders on the basis of information on the irreducible integrals (virial coefficients) or information on the corresponding radius of convergence for the virial series in powers of activity. This method is used to transform conventional expansions for pressure and density in powers of activity to a functional form that allows the analytical study of those series at the vicinity of their divergence point. In particular, the results of this study confirm the adequacy of the cluster-based approach at the condensation region and agreewith the results of the previous studies of partition function in terms of irreducible integrals.

• Sensing of ammonia gas by undoped and aluminum-doped tin oxide nanoparticles by Raman spectroscopy

The study of gas sensing properties of $\rm{SnO_{2}}$ has been widely carried out mainly using electrical methods where a change in resistance or/and conductance of $\rm{SnO_{2}}$ is measured when it is exposed to the sample. In this work, a spectroscopic approach was employed using Raman intensity as a tool to study the sensing of ammonia by undoped and aluminum-doped $\rm{SnO_{2}}$ nanoparticles. The study showed a variation of intensity of the classical Raman modes of $\rm{SnO_{2}}$, when $\rm{SnO_{2}}$ was exposed to ammonia. The response to ammonia by nanoparticles of different sizes and doping concentrations was calculated. This study also revealed the optimum crystallite size and doping concentration suitable for sensing ammonia. However, the most important conclusion that could be drawn from this study was that the response of $\rm{SnO_{2}}$ to ammonia could be detected at room temperature through Raman spectroscopy unlike in the case of electrical studies, where a high temperature is required for sensing.

• A new 5D hyperchaotic system with stable equilibrium point, transient chaotic behaviour and its fractional-order form

Hidden attractors with the family of stable equilibrium points in higher-dimensional systems are more interesting and difficult discover compared to other families of hidden attractors. In this paper, a new 5D hyperchaotic system is reported. The proposed system has only one stable equilibrium point. Hence, the new system belongs to the category of hidden attractors. Although some lower-dimensional chaotic systems with stable equilibrium points are available in the literature, but very few hyperchaotic systems with stable equilibrium points are reported. The new system is simple considering the number of terms, compared with the existing similar type of systems. The proposed system exhibits multistability and transient chaotic behaviour. The fractional-order counterpart ofthe proposed system is analysed using Adams–Bashforth–Moulton algorithm and the chaotic nature is validated bybifurcation diagram. The simulation results confirm the claims made in the paper.

• Tailoring multilayer quantum wells for spin devices

Time-resolved Kerr rotation and resonant spin amplification techniques were used to study the spin dynamics in multilayer GaAs/AlGaAs quantum wells. The spin dynamics was regulated through the wave function engineering and quantum confinement in multilayer quantum wells. We observed the spin coherence with remarkably long dephasing time $T^{\ast}_{2}$ >13 ns for the structure doped beyond metal–insulator transition. Dyakonov–Perel spin relaxation mechanism, as well as the inhomogeneity of electron g-factor, was suggested as the major limiting factor for the spin coherence time. In the metallic regime, we found that the electron–electron collisions become dominant over microscopic scattering on the electron spin relaxation with the Dyakonov–Perel mechanism. Furthermore, the data analysis indicated that in our structure, due to the spin relaxation anisotropy, the Dyakonov–Perel spin relaxation mechanism is efficient for the spins oriented in-plane and suppressed along the quantum well growth direction resulting in the enhancement of $T^{\ast}_{2}$ . Our findings, namely, long-lived spin coherence persisting up to high temperature, spin polarisation decay time with and without magnetic field, the spin–orbit field, single electron relaxation time, transport scattering time and the electron–electron Coulomb scattering time highlight the attractiveness of $n$-doped multilayer systems for spin devices.

• Optimisation of a hybrid photoneutron source in a linear accelerator using GEANT4 and MCNPX Monte Carlo codes

Radiotherapy is important for treating oral cancer in advanced stages. Boron neutron capture therapy represents a unique modality in which neutron beams penetrate into the tissue within an epithermal range. Different methods are available for neutron production, and in this study, we used an electron accelerator. A photoneutron source based on three different energy values (10, 25, and 50 MeV) of a linear accelerator electron beam wasdesigned using GEANT4 (GEometry ANd Tracking) and MCNPX (Monte Carlo N-Particle eXtended) simulation codes. The results indicate that a hybrid photoneutron source introduced tungsten and uranium with BeO. The statistical uncertainties in all simulations were less than 0.3 and 0.07% for MCNPXand the standard electromagnetic physics packages of GEANT4, respectively. Various cross-sectional and stopping-power data and different physics simulations produced different distributions.

• The envelope travelling wave solutions to the Gerdjikov–Ivanov model

In this paper, by using the complete discrimination system of the polynomial method, the classification of the envelope travelling wave solutions to the Gerdjikov–Ivanov model is obtained. The complete result shows thatthere exist rich patterns of travelling wave solutions to the Gerdjikov–Ivanov model, including solitary solutions, periodic solutions, rational singular solutions and double periodic continuous and non-continuous solutions. Among those, some new solutions are given.

• Design and development of low-cost room temperature electric field poling system for the fabrication of quasi-phase matching devices

We discuss the construction of a low-cost room-temperature electric field poling system and the fabrication of high-quality periodically poled lithium niobate (PPLN) devices. The various poling parametersof the indigenously developed quasi-phase matching (QPM) fabrication facility are studied and optimised for the wastage-/breakage-free device fabrication. Furthermore, we present a simple way to monitor the domain reversal process in the poling system using a crossed polariser arrangement. It has been observed that the pulse-by-pulse poling process is more favourable for monitoring the domain reversal and for estimating the exact poling pulse required for the fabrication of (1 : 1) duty cycle PPLN devices. In addition to the fabrication of PPLN, we further discuss the quality checking of the fabricated devices using a simple far-field diffraction experiment.

• Improving amplitude-modulated signals by re-scaled and twice sampling vibrational resonance methods

We present the re-scaled vibrational resonance (VR) method and the twice sampling VR method to improve the amplitude-modulated signal. Two different kinds of signals are considered. One is the amplitude modulated harmonic signal. The other is the amplitude-modulated aperiodic binary signal. Both the VR methods have an excellent effect on the signal improvement. For the re-scaled VR method, the scale parameter is the key factor to determine the resonance output. For the twice sampling VR method, the frequency reduced ratio or the minimal random pulse width stretched ratio is the crucial factor. By choosing appropriate key factors, the outputcan achieve the strongest resonance and lead to the optimal signal improvement.

• Harmonic oscillatorWigner function extension to exceptional polynomials

In this paper, we construct isospectral Hamiltonians without shape invariant potentials for a harmonic oscillator Wigner function on a real line. In this case, we actually remove the ground state of the second Hamiltonian, which forms a special case, $m = 0$, of an exceptional Laguerre differential equation with solutions $\{L^{−2}_{n}\}^{\infty}_{n=2}$ as eigenfunctions form a complete orthogonal set in the Hilbert space.

• Influence of quadrupole–quadrupole-type interaction on the chaotic dynamics of $\alpha$-helical proteins

By proposing a model Hamiltonian in the first quantised form we investigate the chaotic dynamics of $\alpha$-helical proteins by taking into account the quadrupole–quadrupole-type interaction. The dynamics is studied byderiving Hamilton’s equations of motion and by plotting the time-series evolution and phase-space trajectories. Chaotic trajectories are observed in the phase-space plots. The effect of the interaction parameters on the stabilityof proteins is also discussed.

• Fabrication and photoresponse of $\rm{n-WS_{2}/p-V_{0.25}W_{0.75}Se_{2}}$ van der Waals heterojunction

Transition metal dichalcogenides (TMDCs) have shown tremendous potential for application in the field of optoelectronics owing to their extraordinary characteristics. The $\rm{WS_{2}/V_{0.25}W_{0.75}Se_{2}}$ van der Waals heterostructure was fabricated by layer transfer technique and its $I – V$ characteristic was measured at room temperature. The fabricated pn-junction heterostructure shows obvious current rectification with a rectification ratio of $\thicksim{39}$ at $\pm{1}$ V. The heterostructure was analysed in the dark and under polychromatic illumination. The noticeable rise in reverse current is observed at higher intensity of illumination. The photocurrent and photoresponsivity are found to be enhanced as intensity and bias voltage are increased. The higher value of the ideality factor of $\thicksim{2}$ is attributed to the inhomogeneity of the heterojunction.

• Genetic programming modelling for the electrical resistivity of Cu–Zn thin films

Electrical resistivity measurement is an exact way to find defects in metals and alloys. Defects contribute to the residual resistivity, and determining their number is very important.Defining the inner electrical structure of an alloy is difficult, and especially it is unpredictable in alloys. This article offers a genetic programming formulation to learn how deposition conditions and alloy constituents affect the electrical resistivity of Cu–Zn alloy. Inputparameters selected were: measurement temperature (K), Cu and Zn% content in the deposition bath and thin films, bath temperature, deposition potential, and the grain size of the samples. Electrical resistivity values were the output parameters. A total of 130 training and testing sets were selected. The comparative results prove the superior performance in predicting electrical resistivity of the films. The produced model proposes a close relationship for all the input parameters with the electrical resistivity property.

• Lumps and rouge waves for a (3 + 1)-dimensional variable-coefficient Kadomtsev–Petviashvili equation in fluid mechanics

In this paper, a (3 + 1)-dimensional variable-coefficient Kadomtsev–Petviashvili equation, which describes the long water waves and small-amplitude surface waves with the weak nonlinearity, weak dispersionand weak perturbation in fluid mechanics, is investigated. Lump, lump–soliton and rouge–soliton solutions are obtained with the aid of symbolic computation. For the lump and soliton, amplitudes are related to the nonlinearity coefficient and dispersion coefficient, while velocities are related to the perturbation coefficients. Fusion and fission phenomena between the lump and soliton are observed, respectively. Graphic analysis shows that: (i) soliton’s amplitude becomes larger after the fusion interaction, and becomes smaller after the fission interaction; (ii) afterthe interaction, the soliton propagates along the opposite direction to before when any one of the perturbation coefficients is a time-dependent function. For the interactions between the rogue wave and two solitons, the roguewave splits from one soliton and merges into the other one, and the two solitons exchange the amplitudes through the energy transfer by the rogue wave.

• Suppressed transmission of light through single slit in gold film

A new phenomenon of an obvious dip of the transmission from subwavelength slits can be obtained if we change the wavelength of the incident light in the visible regime is proposed. Theory shows that there exist maximumreflectivities if we change wavelengths for different slit widths. The theoretical results are consistent with both finite differencetime-domain (FDTD) simulations and experiments.

• # Pramana – Journal of Physics

Volume 94, 2020
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019