Vol. 68 No. 5 Sep-Oct (2022): Revista Mexicana de Física

In this review, we focus on heterogeneous, thermotropic liquid crystal (LC) mixtures our group has studied with molecular dynamics (MD) simulations. Systems considered include: (1) binary LC mixtures, (2) colloidal inclusions in a mesogenic solvent, and (3) confined mesogenic samples. An extension of the Gay-Berne model is provided to treat the mixtures investigated. Our findings are contextualized to calamitic and discotic LC systems. Structural properties of the mesogenic solvent are probed using the Maier-Saupe (nematic) order parameter. Representative snapshots from MD simulations are used to corroborate phase phenomenology. Topological defects are treated in the presence of colloidal inclusions and in confined samples. The effect of solvent flow on the behavior of topological defects is also assessed. These LC mixtures are of interest in the area of applied materials: aside from their rich mesophase behavior, these systems provide a promising platform for molecular self-assembly and organization.

Due to the increasing demand for energy, the development of new and good thermoelectric (TE) materials is very vital. In this study, with ab initio calculations, based on the density functional theory (DFT) using the self-consistent full potential linearized augmented plane wave (FPLAPW) method were performed to explore the structural, mechanical, electronic and thermoelectric properties of quaternary alloys CaKNaZ (Z = Si, Ge, Sn) with quaternary Heusler structure. optimization confirmed the most stable structure for CaKNaZ (Z = Si, Ge, Sn) compounds is Y1-type in the non-magnetic phase. All of the compounds have been shown to behave like semiconductors, with indirect band gaps of 0.82 and 0.69 for CaKNaSi, CaKNaSn respectively, and direct band gap of 0.46 for CaKNaGe. The theoretical study of thermoelectric properties for CaKNaZ (Z = Si, Ge, Sn) was carried out by Boltzmann theory as implemented in BoltzTraP code. we have obtained a high of figure of merit at moderate temperatures. This indicates that the studied alloys can be used in thermoelectric applications.

In this paper, we studied the liquid-vapor phase diagram and structural properties of discrete potential fluids using Gibbs ensemble simulations and integral equations theory. For this, we considered three discrete fluids, namely, the square well, square well-barrier, and square well-barrier-well. They represent simple models for fluids with competing interactions that exhibit a rich microscopic and macroscopic phase behavior depending on both the strength and range of the attractions and repulsions in the potential. Here, we emphasized a structural behavior near the liquid- vapor coexistence. For the square well-barrier fluid, we observed a possible scenario of a microscopic phase separation associated with a cluster-like formation near the critical region, which could be interpreted as a frustration mechanism of the liquid-vapor transition when either the strength or range of repulsion increases. This microscopic- like separation can be inhibited by suppressing the repulsion or adding an extra well to the interaction potential. However, for the square well fluid with long-range potential, we found evidence of a microscopic aggregation driven solely by attractions.

The optical properties for nano-structured semiconductor systems are of great importance nowadays, due to its possible implementation for the design of efficient optoelectronic devices. It is also well known that external fields can modify the electronic structure, and induce changes the optical properties as well. In particular the asymmetric double quantum well structures are of paramount importance because its wide range on possible configurations and also because are experimentally feasible and well controlled, particularly Al_xGa_1-xAs/GaAs heterostructures. In this work we report a systematic study on second harmonic generation (SHG) for an asymmetric Al_xGa_1-xAs/GaAs double quantum well as a function of non-resonant intense laser field and  in-plane magnetic field effect. We analyze the energy level behavior as well as the dipole matrix elements as a function of the above mentioned factors that are important for the SHG. We report a particular configuration that optimize the SHG peak, with and without intense laser field effect, as well as magnetic fields, that also tune the SHG.

Confinement effects of Kratzer potential on a Wannier-Mott Exciton(W-M) are studied in a spherical quantum dot(QD) in the presence of a static magnetic field. Time independent Schr$\ddot{o}$dinger equation is solved numerically to obtain the energy states. The excitonic transitions so realized have been used to explore the non-linear optical properties that are important for optical characterization of materials such as the optical absorption coefficients (ACs) and refractive index changes (RICs). Impact of magnetic field, strength of the laser field and transition parameters using familiar compact density matrix approach are also analyzed. It has been observed that optical properties get radically modified under confinement effects. Also, the shift of degeneracy of different excitonic energy levels with the magnetic field in confinement potential has been reported for the first time for W-M exciton in the spherical quantum dot, the study that may have crucial input to the literature and myriad of practical implications.

The turbulent kinetic energy dissipation is an essential quantity in the study of turbulence in fluids. In particular, on the transference of turbulent energy from large to small scales and determining its state of equilibrium and stationarity. This work has the purpose of understanding turbulence generation by non-breaking waves from turbulent kinetic energy dissipation analysis. Different groups of nonbreaking monochromatic waves with different slopes were mechanically constructed in a laboratory, whereby an acoustic device, the wave orbitals velocities were measure in various depths. Considering the inertial subrange in the power spectrum of components of turbulence velocity, a turbulent kinetic energy dissipation rate was quantified. It was detected that the magnitude of the turbulent kinetic energy dissipation rate increases with the wave slope and that deeper is invariant to axis rotations. It was distinguished that most of the profiles of the turbulent kinetic energy dissipation agree with an atypical logarithmic layer. Finally, a term of turbulence production was introduced, relative to nonbreaking wave orbital velocities. Unlike other turbulence production terms or approximations, it adequately reproduces the values of the turbulent kinetic energy dissipation rate regardless of wave slope, which established that the wave orbitals velocities shear is the generator mechanism of turbulence in a fluid under nonbreaking waves.

Recently, fractional calculus has got considerable attention from researchers since many problems in natural sciences and engineering are modelled with differential equations having fractional order. The nonlinear coupled time-fractional Boussinesq-Burger (B-B) equation, the nonlinear time-fractional long water wave (ALW) equation, and the nonlinear (2+1)-dimensional space-time fractional generalized Nizhnik-Novikov-Veselov (GNNV) equation are used to express the structure of shallow water waves (SWWs) with different distributions. The analytical solutions of these equations play a substantial role in explaining the properties of complex phenomena in applied sciences. In the current work, we utilize the exponential rational function (ERF) method with the definition of fractional derivative in the conformable sense to achieve new exact traveling wave solutions of these fractional systems. The correctness, validity, and graphics of the new traveling wave solutions are achieved with the aid of Mathematica. Results demonstrate the effectiveness and strength of this technique to solve the system of fractional differential equations (FDEs).

In this work, the improved Hulthen plus Hellmann potentials model in the presence of temperature-dependent confined Coulomb potential ´ IHHPTd model is adopted as the quark-antiquark interaction potential for studying the mass spectra of heavy mesons in the three-dimensional nonrelativistic quantum mechanics noncommutative phase space (3DNRQm-NCSP) symmetries. In addition, we found another application for this potential through its description for hydrogen atoms He⁺, Li⁺² and Be⁺. We solved the deformed Schrödinger equation analytically using the generalized Bopp’s shift method and standard perturbation theory. The new energy eigenvalues E (u,d)hy nc−n and (E hlm n−g, E hlm n−mand E hlm n−l ) for hydrogen atoms and heavy mesons such as charmonium cc and bottomonium cb and corresponding deformed Hamiltonian operators H nc hhp(r, Θ, θ, λ, λ, σ, σ, ²) and H nc hhp(r, Θ, θ, λ, λ, σ, σ, gs) were obtained, respectively. The present results are applied for calculating the new mass of heavy mesons. Four special cases were considered when some of the improved potential parameters were set to zero, resulting into improved Hellmann potential, improved Yukawa potential, improved Coulomb potential, and improved Hulthen potential, in ´ (3DNRQm-NCSP) symmetries. The limiting cases are analyzed for Θ, σ and χ −→ 0 are compared with those of literature.

In the present study, we determined quantum entanglement in a full trapped ion (TRI)-coherent system and its dependence on the LambDicke parameter (LDP). We investigated the entanglement in view of two elaborated measurements of the family: entropy and fidelity. We selected three values of the deep LDP to demonstrate the benefits of these two critical measures. The findings obtained in this study showed that the maximum value of fidelity for entangled states is quantified approximately to be 0.35, and the long lifetime is also observed with entropy measurement. The findings suggest that three coupling parameters play a significant role in developing quantum entanglement.

In this work, we study the quasi-normal modes for scalar and electromagnetic perturbations for an Einstein–Gauss–Bonnet black hole surrounded by quintessence using the third-order WKB method and the Eikonal limit. First, the critical values of the Gauss–Bonnet coupling constant and the normalization factor related to the quintessence are established, to describe the event horizons of the solution. We have studied the condition of extreme horizons and the quasi–normal modes are performed by setting the quintessence state parameter in the particular case of ωq = −2/3

In this paper, the deformation of special relativity within the frame of conformable derivative is formulated. Within this context, the two postulates of the theory are re-stated. Then, the addition of velocity laws are derived and used to verify the constancy of the speed of light. The invariance principle of the laws of physics is demonstrated for some typical illustrative examples, namely, the conformable wave equation, the conformable Schrodinger equation, the conformable Klein-Gordon equation, and conformable Dirac equation. The current formalism may be applicable when using special relativity in a nonlinear or dispersive medium.

This study is focused on the investigation of three-dimensional (3D) printed SnS thin film and the optimisation of SnS thin film thickness by additive layer deposition of the film using three-dimensional printing system based on liquid deposition modelling (LDM). Voids in separate island-like state and traps associated with certain film thickness affect charge carriers due to the presence of large grain boundaries associated with small grains which acts as electron trap thus affecting SnS thin film's optical band gap energy and electrical conductivity among others. SnS thin films were printed on glass substrate using LDM-3D printing. Surface Profilometer, Energy dispersive X-ray spectroscopy, X-ray diffractometer, Scanning electron microscope, Uv-vis spectrophotometer and four point probe were used to characterise the SnS thin films. The conductivity of 0.002987 (Ωm)^-1 and optical energy band gap of 1.37 eV of 0.6 μm 3D printed SnS thin film was optimum and favours the attainment of the threshold voltage for optoelectronic and electronic application. The results demonstrate the potential of the LDM-3D printing of thin film for materials deposition and application which provides a new way of layer thickness variation and levelling of semiconductor thin film.

Metal artifacts cause errors in the exact delineation of implants and dose changes in radiotherapy. In this study, the dose distribution differences in the region of interest (ROI) were calculated by deliberately making contouring errors from the real size of model implants by using both Monaco treatment planning system (TPS) and Geant4 toolkit. In Sec. 2, the computed tomography images were acquired by placing known uniform cylindrical geometry titanium (Ti6Al4V) and cobalt (CoCrMo) alloys into water phantoms separately. The metal alloys were artificially contoured as 2 mm contracted and expanded from their real dimensions in Monaco TPS. The plans were generated with 6 MV photon beams for contouring of three different sizes, real, contracted and expanded, for each metal alloy. In addition, all configurations were simulated in Geant4 by using the photon energy spectrum data of the Elekta Synergy linear accelerator. Then, the 3D dose data obtained from ROIs near the implant in Monaco TPS and Geant4 were analyzed with in-house programs. In Sec. 3, the depth dose values of Geant4 were compatible with TPS calculations and ion chamber measurements. When the alloys were contoured to real dimensions, it was observed that the local isodose values have changed up to 15% in ROI. The mean dose values were found to be higher in contracted and lower in expanded contours. It was observed that ±2 mm error in contouring the implants changed the mean dose up to ±8%. In Sec. 4, this study emphasized that a few millimeters of error in contouring different implant materials can have a significant effect on dose distribution in a region close to the implant.

This study looks at the confinement effects of Aharonov-Bohm (AB) flux and magnetic fields, as well as topological defects in a quantum plasma, on the hydrogen atom. The joint effects show that the system is extremely attractive. Furthermore, as we've shown, the joint effect of the fields is greater than the sum of the individual effects, resulting in a significant change in the system's bound state energy. The magnetic field can be used as a control parameter or booster, whereas the topological defect and AB field are needed to hold the hydrogen atom in quantum plasmas at a low energy. The findings of our research may be extended to atomic structure and plasma collisions.

Analytical solutions for a continuum model of a DC gas discharge were obtained by asymptotic approximations. The solutions are one-dimensional and stationary. Three different cavity configurations of electrode pairs were considered, namely, parallel flat plates, concentric cylinders and concentric spheres. The asymptotic approximations consider nonlinear effects that are present in the dynamics of the plasma which are neglected in most of analytical solutions found in the literature. The obtained solutions determine the distribution of positively and negatively charged particles, as well as the electric potential along the space domain. Analytical results agree quantitatively with results from a spectral numerical solution developed for validation purposes. Such agreement was possible in a regime where the rate of charged particle production was assumed to be very small. Finally the limits of this regime are reported.

This work proposes a photonic method to generate optical pulses with a triangular wave shape using a low coherence source. We modulate this source using a sinusoidal radio frequency signal as a local oscillator. We eliminate the second harmonic of the spectral components generated by the modulation process tuning a reject filter to get a triangular wave. We implement the filter using an interferometer with a delay line in one of their branches. To tune the filter, the only adjustment required is over the optical delay line. We developed our scheme in a simulation environment but based on the use of optical and optoelectronic elements with parameters of commercial devices, some of them available in our laboratory. We generate triangular waveform signals with pulse frequencies of 100, 150, 250, and 300 MHz as proof of concept. The proposed scheme has the advantage of being low cost and easy to use because it does not require bias control nor adjustment to  Vpi voltage. Usually, other authors must vary these parameters to get the desired electrical signal. Still, we generate the triangular waveform by adjusting only the local oscillator frequency and the optical delay line. Therefore, because we use a broad-spectrum optical source and do not require a strictly controlled variable voltage, our system will be cheaper.

Using phenomenological and microscopic potentials, the experimental angular distributions for the ^4,6,8He nuclei elastically scattered from a ²⁰⁸Pb target at  are investigated. Both the modified version of CDM3Y6 interaction based on the inclusion of the rearrangement term (RT) and those obtained from the Sao Paulo Potentials are used for the microscopic potentials. The cluster folding potential for ⁶He+²⁰⁸Pb is calculated using the triton + triton cluster structure for ⁶He. This analysis revealed that the real cluster folding potential strength must be reduced by about 90%. Using the extracted potentials, the total reaction cross sections were successfully reproduced.

In the present study, the approximate fractal morphometry of spherical-type essential oil microemulsions was performed. The geometric fractal characterization was carried out by a recently published continuous half-fractal model which allowed to model microemulsions as systems in their stable thermodynamic equilibrium phase with high degree of homogeneity. Regarding the characteristic of high homogeneity an equation was obtained to roughly describe the volume fractal dimension and the fractal volume of two special cases elaborated from Rosmarinus officinalis and Melaleuca alternifolia previously investigated. In addition, referring to the characteristic of high homogeneity, it was possible to approximate the fractal dimension of area and the fractal area for each microemulsion. Our numerical estimates showed coherence with the principles of Hausdorff-Besicovitch geometry and with the experimental evidence about the physical dimension as a non-integer dimension.

The effect of sound vibration (SV) towards plants had been studied for more than 30 years. Many results confirmed that SV influenced various parts of plants, e.g.: the stomata. However, the scientific community was still in doubt of these results. Hence, it is now a matter of giving further proofs and/or insights using new methods. This study aimed to determine the stomata pore movement influenced by SV with new observation and analysis technique based on edge detection. We had directly observed the abaxial stomata of Rhoeo discolor plant exposed by single SV frequencies of 0 Hz to 7000 Hz with an interval of 1000 Hz. The main device used in the observation was a microscope. The observation was conducted before, during, and after the SV exposure. The measurement of the stomata pore area utilized an image capture, which was then analyzed with edge detection technique. These edges were used as an indicator in the calculation of the stomata pore area in the pixel unit. The result showed that SV with frequency of 6000 Hz produced the largest stomata pore area, whereas the frequencies of 2000 Hz and 3000 Hz gave smaller stomata pore areas. Therefore, a single frequency of SV influenced the stomata pore area based on the edge detection analysis.

In this study we investigate Boiti-Leon-Manna-Pempinelli equation in three dimensions, which describes the evolution of the horizontal velocity component of water waves propagating in the xy-plane in an infinite narrow channel of constant depth and that can be considered as a model for incompressible fluid. The new (F/G)-expansion approach and the unified approach are employed to construct some new traveling wave solutions to the nonlinear model. A large numbers of traveling wave solutions for the nonlinear model are demonstrated respectively in the form of hyperbolic and trigonometric function solutions. The proposed methods are also proved to be effective in solving nonlinear evolution problems in mathematical physics and engineering