Vol. 64 No. 3 May-Jun (2018): Revista Mexicana de Física

In this work are presented the results obtained from the deposition of Cd1-xZnxTe nanolayers using as precursor the vapours of the elements Zn, Te, and a mixture of Cd and Zn on GaAs and GaSb (001) substrates by Atomic Layer Deposition technique (ALD), which allows the deposition of layers of nanometric dimensions. At each exposure of the growth surface to the of cation or anion precursors vapours, this surface is saturated. Therefore, it is considered that the process is self-regulated. The ZnTe layers were grown in a wide range of temperatures; however, ZnTe nanolayers with a shiny mirror-like surface could be grown at temperatures between 370 and 410oC. Temperatures higher than 400oC were necessary for the CdTe growth. The layers of the Cd1-xZnxTe ternary alloy were deposited at temperature range of 400 and 425oC. The grown nanofilms were characterized by Raman spectroscopy and high-resolution X-ray diffraction. The Raman spectrum shows the peak corresponding to LO-ZnTe at 208 cm-1, which is weak and is slightly redshifted in comparison with the reported for the bulk ZnTe. For the case of the CdTe nanolayers, Raman spectrum presents the LO-CdTe peak, which is indicative of the successfully growth of the nanolayers, its weakness and its slight redshifted in comparison with the reported for the bulk CdTe can be related with the nanometric characteristic of this layer. The performed high resolution X-ray diffraction measurements allowed to study some important characteristics, as the crystallinity of the grown layers. Additionally, the performed HR-XRD measurements suggest that the crystalline quality have dependence with the growth temperature.

The quaternary chalcogenide compound Ag2MnSnS4 belonging to the system I2-II-IV-VI4 and synthesized by the melt and anneal technique, was characterized by Rietveld refinement of the powder X-ray diffraction data and differential thermal analysis (DTA). It was found that nS4 crystallizes in the orthorhombic space group Pmn21, with unit cell parameters a = 8:1705(5) Å, b = 6:9413(5) Å, c = 6:6532(5) Å, and V = 377:33(5) Å3, in a wurtzite-stannite structure. The DTA indicates that this compound melts at 790°C and that the phase relations which occurs in the material would be: α ➝ α + α1 ➝ α1 ➝ α1 + β ➝ β ➝ 1 + L ➝ L, were α is the orthorhombic wurtzite-stannite Pmn21 structure; α1 is a high temperature modification; and β and β1 are the zinc-blende structure and its high-temperature modification, respectively.

 

In this research, we report a detailed study of the structural, electrial and magnetic properties of the ruthenium pyrochlore with the composition (Er_2-x Sr_x) Ru_2O_7 0<=x<=0.10 prepared by solid-state reaction in air at ambient pressure.  The synthesized products were characterized using powder X-ray diffraction. The structure of the samples was refined with the Rietveld method, showing that the lattice parameters are more sensitive to the Strontium and Erbium sites. Scanning electron microscopy shows that the crystal size varies between 0.27 and 0.62 mu m. In all polycrystalline samples, the electrical resistance decreases with increasing temperature, indicating that the samples are nonmetallic. The slope of the temperature-dependent resistance profiles systematically decreases with increasing x, proving that the carrier concentration increases with increasing the Sr content. Zero-field-cooled and field cooled magnetization measurements show an irreversible behavior where the split is systematically enhanced by increasing x.

We report the Ca0.90Sr0.10RuO3 compound synthesized by solid-state reaction method at ambient pressure using temperatures between 700 and 800◦C in air. By X-ray powder diffraction (XRD), we determine a solid solution until Ca0.85Sr0.15RuO3 compound. Scanning electron microscopy (SEM)
indicates than the particle size varies between 0.422 and 1.598 µm. The resistance measurement, as a function of temperature measurement from 10 to 300 K for Ca0.90Sr0.10RuO3 compound shows a metallic behavior. Finally, photoluminescence (PL) and its temperature dependence of  Ca0.90Sr0.10RuO3  compound in the temperature range 6.7-296 K were measured. It was observed than the main broad band centered at ∼ 1.73 eV with the shoulders at ∼ 1.38 eV and ∼ 2.05 eV exists in the entire temperature range. It can be well fitted by three Gaussian curves B1, B2 and B3, centered at ∼ 1.38, ∼ 1.73 and ∼ 2.05 eV, respectively. The transitions identified as B1 and B2  are studied with temperature. The photoluminescence mechanics for Ca0.90Sr0.10RuO3  compound are
presented based on the electronic structures formed by the interactions among spin, charge and lattice, in which B1 was identified with the charge transfer excitation of an electron from the lower Jahn-Teller split eg level of a Ru3+ ion to the eg level of an adjacent Ru4+ ion, B2 was assigned to the transition between the spin up and spin down eg bands separated by Hund’s coupling energy Ej , whereas B3 is attributed to the transition, determined by the crystal field energy Ec between a t2g core electron of Ru3+ to the spin up eg bands of Ru4+ by a dipole allowed charge transfer rocess.

A single-band generalized Hubbard model that describes two-dimensional superconductivity with d-wave symmetry on a square lattice within the BCS formalism is considered. For a set of Hamiltonian parameters and varying the ratio between nearest-neighbor and next-nearest neighbor hoppings (t'/t), an optimal doping (nop) can be found for each t'/t value, where the critical temperature is maximum (Tc-max). After calculating the superconducting gap at T=0K and the corresponding ground state (Eg ) for all the carrier concentrations, a ground state energy minimum (Eg-min) is found close to half filling. Since Tc-max is the highest critical temperature for a given ratio t'/t, the minimum of all the Tc-max values defines a supreme of this set of temperatures, named as Tc-max-sup. The corresponding optimal doping for Tc-max-sup will be called nop-sup, and the the results show that  Eg-min is located at nop-sup. The Fermi surface (FS) is analyzed for carrier concentrations close to nop-sup and it is suggested that the location for over (OD) and under (UD) doping regimes (nOD>nop-sup>nUD) could define a pseudogap zone for high critical temperature superconductors.

It presents the characterization of rare earths (Eu,Ce)-doped CdS nanofilms that were synthesised by the growth technique chemical bath deposition (CBD) at the reservoir temperature of 70±2°C. The doping of CdS with rare earths is performed by varying the synthesis time from 60 to 135 min. The rare earths molar concentration was range from 0.0≤x≤3.5, which was determined by energy dispersive X-ray spectroscopy. X-ray diffraction (XRD) analysis and Raman scattering reveal that CdS nanofilms showed the zinc blende (ZB) crystalline phase. The CdS average nanocrystal size was ranged from 1.84 to 2.67 nm that was determined by the Debye–Scherrer equation from ZB (111) direction, which was confirmed by transmission electron microscopy. Raman scattering shows that the lattice dynamics is characteristic of bimodal behaviour and the multipeaks adjust of the first optical longitudinal mode for the (Eu,Ce)-doped CdS, which denotes the Raman shift of the characteristic peak about 305 cm⁻¹ of the CdS nanocrystals. The CdS nanofilms exhibit a direct bandgap that slightly decreases with increasing doping, from 2.50 to 2.42 eV, which was obtained by room temperature transmittance. The room-temperature photoluminescence of CdS shows the band-to-band transition at 2.88 eV, which is associated to quantum confinement and a dominant radiative band at 2.37 eV that is called the optical signature of interstitial oxygen. The Eu³⁺-doped CdS photoluminescence shows the dominant radiative band at 2.15 eV, which is associated to the intra-4f radiative transition of Eu³⁺ ions that corresponds to the magnetic dipole transition, (⁵D0→⁷F¹). For the Ce³⁺-doped CdS the dominant radiative transition, at 2.06 eV, is clearly redshifted, although the passivation of the CdS nanofilms by Ce was approximately by a factor about 21 for the best results.

Planar waveguides were generated in samples of rutile crystal (TiO2) by bombarding with two types
of ion: silicon and carbon. Rutile is used because of its anisotropic properties, particularly its birefrin-
gence. The guide is generated due to damage caused by the ions in the crystal which change its index of
refraction. Three parameters were used: the implantation ion energy, the implantation
uence, and the
orientation of the crystallographic planes. The refractive index prole of the irradiated sample was cal-
culated and together with the value of the optical barrier the comparison was made between the dierent
waveguides generated.

In this work the influence of GaN substrate thickness on the near and far-field patterns of InGaN lasers structures is analyzed. In simulation a conventional separate confinement heterostructure of InGaN-MQW / GaN / AlGaN is considered. A fluctuating behavior is found, showing that for some values of the substrate thickness the near and far- field patterns can be optimized and there are critical values of the substrate thickness that produce the lowest values of the confinement factor and the higher values of the full wide half-maximum of the far field. It is also shown that by substituting the GaN contact layer by an AlxGa1-xN layer with a parabolic variation of the Al content it is possible to reduce the optical field leakage to substrate. Results indicated that properly choosing the thickness of the substrate and replacing the n-GaN contact layer by a graded-index (GRIN) AlxGa1-xN layer it is possible to improve both the confinement factor and Far field pattern in nitride lasers.

In the recent experiments [1] the unusual oscillatory magnetoresistance in superconductors was discovered with a periodicity essentially independent on magnetic field direction and even material parameters. The nearly universal period points to a subatomic mechanism of the phenomenon. This mechanism is related to formation inside samples of subatomically thin (10−11 cm) threads in the form of rings of approximately Bohr radius. Electron states of rings go over into conduction electrons which carry the same spin imbalance in energy as rings. The imbalance occurs due to spin interaction with the orbital momentum of the
ring. The conductivity near Tc is determined by fluctuating Cooper pairs consisting of electrons with shifted energies. Due to different angular momenta of rings these energies periodically depend on magnetic field resulting in the observed oscillatory magnetoresistance. Calculated universal positions of peaks (n + 1/2)∆H (∆H \simeq 0.18T and n = 0, 1, 2...) on the R(H) curve are in a good agreement with measurements.

The hot injection technique is used to produce CdSe nanoparticles with quantum dot properties. Several reports have considered an inert atmosphere (nitrogen) as necessary for a successful synthesis, which complicates the experimental set-up. In this work, CdSe nanoparticles were synthesized by hot injection in air instead of in nitrogen, simplifying the experimental set-up. To avoid undesirable interactions with oxygen, well-defined concentrations of the organic species were used during the synthesis, but air still influenced the growth rate of the particles. To establish a comparison, the same experimental methodology was applied in nitrogen and in air. The nanoparticles synthesized in air showed a higher growth rate than those synthesized in nitrogen at the same reaction times. Additionally, similar optical behaviors and band gaps were observed in both cases, showing that an inert atmosphere is not necessary for the synthesis of quantum dots made of CdSe nanoparticles.

A wavelet scaling numerical characterization of time series based on the variance of the wavelet coefficients is used for three well-known four-dimensional and one five-dimensional hyper-chaotic systems. We report several scaling behaviors for the states of these hyper-chaotic systems.

Fractal geometry eects in capillary imbibition process are studied. Capillary rise analysis in Koch's curve-like tubes were be carried out with iterations i = 0; 1; 2; 3; 4; 5. The behaviour was characterized in function of time, fractal geometry and height of capillary rise. An geometrical relationship for fractal dimension of  ow tortuosity (dr) in porous media is obtained. The analytical model of Lucas-Washburn-Cai to describe the capillary rise by spontaneus imbibition in tubes with deterministic fractal geometry is adjusted. The equilibrium height time as function of fractal dimension of  ow tortuosity in capillary tubes with tortuous path is also derived.

In this work, we present a study to determine the transit times and frequencies of pulses by using
the Short-Time Fourier Transform (STFT). We consider the case of an acoustic signal composed of
five Gaussian pulses which have a high overlapping in time but oscillate at different frequencies. We
proceeded in three steps. First, we illustrate how the STFT calculated through a sliding window
produces a spectrogram where transit time is on one axis and frequency on the other. Second, we
derive an exact analytical solution of the STFT to develop an intuitive vision of the mathematical
technique. Finally, in a third step, we present an experiment to demonstrate that the STFT is a
useful technique to characterize a complex acoustical signal.

We use the linear sigma model with quarks to study the QCD phase diagram from the point of view of chiral symmetry restoration. We compute the leading order effective potential for high and low temperatures and finite quark chemical potential, up to the contribution of the ring diagrams to account for the plasma screening effects. We fix the values of the model couplings using physical values for the input parameters such as  the vacuum pion and sigma masses, the critical temperature at vanishing quark chemical potential and the conjectured end point value of the baryon chemical potential of the transition line at vanishing temperature. We find that the critical end point (CEP) is located at low temperatures and high quark chemical potentials $(\mu^{\text{CEP}}>320\ {\mbox{MeV}},T^{\text{CEP}}<40\ {\mbox{MeV}})$.

The data for elastically scattered charged pions from few nuclei, namely ^{12}C, ^{16}O, ^{28}Si, ^{32}S, ^{40}Ca, ^{56}Fe, ^{58}Ni, and ^{90}Zr have been analyzed by obtained potentials using a suggested scaling procedure. Originally the \pi ^{\pm}-^{12}C elastic scattering data at 50 MeV was nicely fitted by a parameterized simple local optical potential extracted from available phase shifts using inverse scattering theory. The potential parameters of the  \pi ^{\pm}-^{12}C systems were scaled to  \pi ^{\pm}-^{16}O systems and then successively to other few systems covering the scattering of charged pions from target nuclei, namely \pi ^{\pm}-^{28}Si, \pi ^{\pm}-^{32}S, \pi ^{\pm}-^{40}Ca, \pi ^{\pm}-^{56}Fe, \pi ^{\pm}-^{58}Ni and \pi ^{\pm}-^{90}Zr . The obtained scaled potentials showed a remarkable success in explaining the available measured elastic differential cross sections, and in predicting other ones for the systems under consideration. The reaction cross sections have been calculated for all these systems at the three incident pion's kinetic energies, T\pi = 40, 30, 20 MeV. Unfortunately, experimental reaction cross sections are totally absent or cloudy and unconfident. As such, and at this stage, we consider our calculated values useful and pending for future investigations.  For the systems and energies considered herein, simple scaling relations are well established. This will be beneficial in analyzing similar nuclear scattering data, as low-energy pion-nucleus and kaon-nucleus elastic scattering data; and, hopefully, in explaining pionic atom data.