We report the first observation of a confined-to-propagating transition of LO phonons in GaAs/${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As superlattices (SL’s). Picosecond Raman scattering was used to investigate the generation rate of nonequilibrium GaAs LO phonons in two series of GaAs/${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As SL’s with constant GaAs layer widths (${\mathit{L}}_{\mathit{z}}$) and x, while the ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As layer widths (${\mathit{L}}_{\mathit{b}}$’s) varied. In both series we have observed a sudden increase in the generation rate of the Raman-active hot phonons as ${\mathit{L}}_{\mathit{b}}$ decreased over a narrow range. We demonstrate that this abrupt increase is due to the sharp transition from the so-called confined to propagating LO phonons. From our results we have determined the GaAs/LO phonon penetration depth into the ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As layers.

Layered transition metals dichalcogenides such as MoS₂ and WS₂ have shown a tunable bandgap, making them highly desirable for optoelectronic applications. Here, we report on one-step chemical vapor deposited MoS₂, WS₂ and Mo_xW_1-xS₂ heterostructures incorporated into photoconductive devices to be examined and compared in view of their use as potential photodetectors. Vertically aligned MoS₂ nanosheets and horizontally stacked WS₂ layers, and their heterostructure form Mo_xW_1-xS₂, exhibit direct and indirect bandgap, respectively. To analyze these structures, various characterization methods were used to elucidate their properties including Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectrometry and high-resolution transmission electron microscopy. While all the investigated samples show a photoresponse in a broad wavelength range between 400 nm and 700 nm, the vertical MoS₂ nanosheets sample exhibits the highest performances at a low bias voltage of 5 V. Our findings demonstrate a responsivity and a specific detectivity of 47.4 mA W⁻¹ and 1.4 × 10¹¹ Jones, respectively, achieved by Mo_xW_1-xS₂. This study offers insights into the use of a facile elaboration technique for tuning the performance of Mo_xW_1-xS₂ heterostructure-based photodetectors.

Using picosecond Raman scattering, hot-phonon occupation numbers (N) of GaAs and GaAs-like LO phonons have been studied over a wide range of structural parameters in more than 30 GaAs/${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As superlattices. In addition, simultaneous measurements of these LO phonon modes in bulk GaAs and ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As alloys are made for comparison. N values of both GaAs and GaAs-like modes of the superlattices are comparable to or larger than those of bulk GaAs or ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As alloys for x0.35, unless the barrier widths are very thin (<15 Å). The implications of our results on spatial properties of LO phonons are discussed and compared with relevant theoretical and experimental studies.

This paper presents the electromagnetic wave propagation in deep underground oil pipelines. The aim of this work is to analyze the attenuation of the electromagnetic waves in the pipelines caused by the oil medium under high temperature conditions. The work involves the modeling of a downhole oil pipeline. The simulations are focused on computing the scattering parameters (S-parameters) of the electromagnetic waves that propagate inside the pipeline filled with oil under varying temperature conditions. These results will be helpful to ascertain the possibilities of the wireless communication in deep underground oil pipelines.

Temperature studies of the LA[110] Brillouin shift have been conducted on the mixed ferroelectric-antiferroelectric system, ${\mathrm{Rb}}_{1\mathrm{-}\mathit{x}}$(${\mathrm{NH}}_4$ ${)}_{\mathit{x}}$ ${\mathrm{H}}_2$ ${\mathrm{PO}}_4$ for two ammonium concentrations (x=0.28 and 0.42). The shift undergoes a softening around 100 K, followed by a hardening of the mode below 50 K. This is due to both a linear (piezoelectric) and a quadratic coupling of this acoustic mode to the polarization fluctuations. The quadratic coupling results from the large electrostriction in these materials and becomes the dominant mechanism at low temperature. This is related to the frustration of the ammonium ions below 100 K. This temperature behavior is consistent with the results previously reported by Courtens et al. on other concentrations of the ${\mathrm{Rb}}_{1\mathrm{-}\mathit{x}}$(${\mathrm{NH}}_4$ ${)}_{\mathit{x}}$ ${\mathrm{H}}_2$ ${\mathrm{PO}}_4$ system.