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Monolayer germanium telluride (GeTe) exhibits tunable electronic and optical properties, making it promising for device applications. Using Density Functional Theory (DFT), we investigate the impact of uniaxial strain, ϵ, on its band structure and dielectric function (DF), ε ̃ . Tensile strain along the —zigzag direction induces a direct-to-indirect bandgap transition at ϵ = 2%, reducing the gap from 1.80 to 1.62 eV. Compressive strain narrows the gap while maintaining its indirect nature. Strain also induces in-plane anisotropy in the DF and shifts the optical absorption spectrum. We determine a deformation potential (DP) of −6.45 eV for the bandgap under tensile strain and DPs for other above bandgap high-symmetry points. These findings highlight the potential of strain engineering for tuning GeTe's properties in devices. [ABSTRACT FROM AUTHOR]

Hyperloop systems, where a pod travels at high speed within a tube under rarefied conditions, have a maximum speed limit due to the Kantrowitz effect. One solution to overcome this limit is to include a circuit with a fan that can also assist the pod's propulsion through a nozzle at the vehicle's rear. This paper focuses on analyzing the propulsive efficiency of these coaxial jets within a tube at low-pressure conditions. The paper's objective is to use a computational fluid dynamics tool to design an experiment in a wind tunnel with a steady tube and vehicle that could reproduce the actual operation of a stationary tube and a moving vehicle. Several issues are dealt with. First, the effect of the vehicle's front design on the coaxial jets, which resulted be negligible. Additionally, the increase in temperature in the compressor circuit can be neglected, simplifying the experimental arrangement. Third, scaling the wind tunnel prototype shows that the difference in size can be compensated by setting the test pressure at ambient conditions. Finally, considering steadiness in the vehicle in the test leads to a different velocity pattern in the coaxial jets. Several changes in the tube's geometry are proposed and analyzed to address this problem. The results demonstrate that it is possible to replicate the actual coaxial jets in steady conditions with a small tapered section in the tube. Furthermore, this modification can be used over a relatively large range of operating conditions and for different rear pod designs. [ABSTRACT FROM AUTHOR]

The SARS-CoV-2 (COVID-19) pandemic has highlighted the crucial role of preventive measures in avoiding the spread of disease and understanding the transmission of airborne viruses in indoor spaces. This study focuses on a novel personal protective equipment consisting of a fan-peaked cap that creates a jet flow of air in front of the individual's face to reduce the concentration of airborne viruses and decrease the risk of infection transmission. Direct numerical simulation is used to analyze the effectiveness of the device under certain conditions, such as the velocity of the airflow, flow orientation, ambient conditions, and geometrical factors. [ABSTRACT FROM AUTHOR]

We show that in spatially resolved reflectance anisotropy (RA) spectrometers, off-axis optical rays introduce a spurious signal component that cannot be addressed by optical alignment. Such a component is associated with the difference between the reflectivities s and p of the sample and depends, in a complex manner, on the incidence position of the incident light on the surface of the sample. We report a data-reduction procedure to easily identify and remove spurious RA signals associated with the off-axis optical rays, based on the singular value decomposition analysis of spatially resolved RA spectra. We validated this approach by developing a spatially resolved RA spectrometer based on an 8 × 8 multi-anode photomultiplier (PMT). The PMT allowed the use of phase-sensitive detection techniques to enhance the signal-to-noise ratio, which is essential for the evaluation of the proposed data reduction procedure. [ABSTRACT FROM AUTHOR]

Graphene nanoribbons (GNRs) are nanostructures considered to be promising building blocks for the realization of graphene-based devices. The optical properties of GNRs are hard to determine due to their nanoscopic dimensions. Reflectance Anisotropy Spectroscopy/Reflectance Difference Spectroscopy (RAS/RDS) is a powerful optical tool to characterize highly anisotropic structures. RAS/RDS has shown to be very useful to measure the optical response of materials including semiconductor heterostructures. The technique is non-destructive and can be used in air or in vacuum conditions. Considering the highly anisotropic geometry of the GNRs, the RAS/RDS becomes a quite convenient technique to characterize the optical properties of GNRs and in general to study the dependence on the thickness of the optical properties of graphene. The GNRs used in the present work were synthesized on 6H-SiC stepped substrates and annealed in air to obtain quasifree-standing bilayer graphene (widths: 240 nm, 210, and 120 nm). For this system, the isolation of the optical signal coming from the GNRs in the RAS spectra is not an easy task due to the fact that both GNRs and the 6H-SiC stepped substrate are highly anisotropic. To study and characterize the GNRs, we present and discuss an experimental approach to isolate the RAS signal coming from the GNRs. We also have performed nano-RAS measurements by using a near-field scanning optical microscopy technique (nanometric resolution) that supports our method. We show that RAS and nano-RAS are powerful complementary optical probes that can be used to characterize GNRs and also properties such as the visual transparency of one-, two-, or few-layer thick graphene. [ABSTRACT FROM AUTHOR]

For zinc blende semiconductors, such as InP, the Raman selection rules for a backscattering configuration from the (001) surface forbid the transversal optical (TO) phonon mode, whereas the longitudinal optical mode is allowed. However, when InP is highly doped with Si atoms, InP-Si clusters with the reduced C 3 v symmetry allow TO modes in the Raman spectrum with the backscattering configuration. Here, we demonstrate that the amplitude of the TO modes can be modulated spatially by using a highly doped InP grating. By exciting the sample with a laser linearly polarized parallel and perpendicular to the grating grooves, we observe a change in amplitude of the phonon optical response for the TO mode. [ABSTRACT FROM AUTHOR]

We report reflectance (R) and reflectance difference spectroscopy (RDS) spectra of wurtzite heterostructures grown on Si(111) and Si(110) substrates in the energy range from 2.0 to 3.5 eV. Due to the threefold symmetry of the Si(111) surface, the heterostructures grown on this surface will relax isotropically through the formation of misfit dislocations, preferably at the AlN/Si interface, and no in-plane anisotropies are expected. In fact, only a small in-plane anisotropy of reflectance is observed, due to the initial residual off-cut of the silicon substrate that leads to wurtzite layers with a c-axis slightly tilted with respect to the surface. In contrast, for the Si(110) substrate, strong differences in lattice parameters appear between silicon and GaN/AlN depending on the considered in-plane direction, leading to a large in-plane anisotropy. By using a multiple reflection model for R and the in-plane anisotropies described, we developed a model to describe the RDS spectra in the vicinity and below the fundamental gap of GaN. [ABSTRACT FROM AUTHOR]

Reports on the measurement of the linear electro-optic reflectance spectrum of CdTe(001) in an energy range around the E1 and E1+...1 interband transitions. Experiment; Theoretical model; Experimental results; Discussion; Conclusions.

We report on differential reflectance contrast (DRC) sub-microscopic images measured of graphene layers exfoliated on a SiO2/Si substrate by using a near field scanning optical microscope (NSOM) with a spatial resolution of 40 nm. In general, high-quality mechanically exfoliated graphene flakes have sizes of some micrometers and exhibit a distribution of different thicknesses; thus an approach to characterize the topography of the flakes in the sub-micrometric regime is fundamental. DRC in the near field limit is a very useful technique to characterize the flakes in the sub-microscopic scale. The DRC signal is obtained by taking the numerical difference between the reflectivity coming from a region with no graphene (substrate) and a region containing a graphene layer. It is shown by a multiple reflection model (graphene/SiO2/Si) and spectroscopic ellipsometry measurements that the optical contrast in such system can be modulated by changing the thickness of the SiO2 layer or/and the wavelength of the incident light. The results open the possibility to use this optical technique for the thicknesses characterization in the sub-micrometer scale of 2D materials. [ABSTRACT FROM AUTHOR]

We have used transmission anisotropy (TA) and photoreflectance-difference (PRD) spectroscopies as tools for the characterization of heterostructures consisting of 100 Å GaxIn1-xAsySb1-y quantum wells with 200 Å AlxGa1-xAsySb1-y barriers. Exctions associated to two allowed and two forbidden transitions of the confined levels in the quantum wells (QWs) are detected with PRD at 15 K. Using TA spectroscopy they all can be clearly resolved at room temperature. The signal of the anisotropy is proved experimentally to stem exclusively from the QWs and the surrounding interfaces. We show TA spectroscopy to be a powerful tool for interface characterization in heterostructures with energy gap below that of the substrate. [ABSTRACT FROM AUTHOR]

Discusses a study which determined the crystal structure and magnetic properties of the layered compound Co(HPO[sub3]) H[sub2]O. Formation and refinement of the structure; Description of the structure; Characterization of the magnetic properties of the sample.

Presents a study which measured the complex alternating current magnetic initial susceptibility of Ho[sub2]Fe[sub14]B single crystal. Materials and methods used; Overview of a study on the domain structure of Ho[sub2]Fe[sub14]B on polycrystalline samples; Calculation of the coherent rotational susceptibility for a system with free energy.

Photoreflectance-difference (PR/PRD) and reflectance-difference (RD) spectroscopies employ synchronic detection usually with lock-in amplifiers operating at moderate (200-1000 Hz) and high (50-100 KHz) modulation frequencies, respectively. Here, we report a measurement system for these spectroscopies based on a multichannel CCD spectrometer without a lock-in amplifier. In the proposed scheme, a typical PRD or RD spectrum consists of numerical subtractions between a thousand CCD captures recorded, while a photoelastic modulator is either operating or inhibited. This is advantageous and fits the slow response of CCD detectors to high modulation frequencies. The resulting spectra are processed with Savitzky-Golay filtering and compared well with those measured with conventional scanning systems based on lock-in amplifiers. [ABSTRACT FROM AUTHOR]

In this work, we report on photoreflectance (PR) and photoreflectance-difference (PR-D) measurements of GaAs(001) upon deposition of Gd2O3 thin films. The study is focused on two different substrates: a semi-insulating (SI) with Cr impurities and a Si-doped n-type. PR-D results show that Gd2O3 induces a tensile strain on the GaAs surface and a direct piezo-electric dipole is created. Such strain changes the crystal symmetry from cubic to orthorhombic and renders the quadratic electro-optic (QEO) component anisotropic. For the SI substrate, both linear electro-optic (LEO) and QEO components contribute to the PR-D spectrum, whereas the n-type PR-D spectrum is dominated by the LEO component. In both cases, a tensile strain induces a rigid redshift of ~20 meV to low energies of the E1 and E1 + Δ1 optical transitions. [ABSTRACT FROM AUTHOR]

The aim of this work is to generalize a very important type of Lie algebras and superalgebras, i.e., filiform Lie (super)algebras, into the theory of Lie algebras of order F. Thus, the concept of filiform Lie algebras of order F is obtained. In particular, for F=3 it has been proved that by using infinitesimal deformations of the associated model elementary Lie algebra it can be obtained families of filiform elementary lie algebras of order 3, analogously as that occurs into the theory of Lie algebras [M. Vergne, "Cohomologie des alg`ebres de Lie nilpotentes. Application à l'étude de la variété des algèbres de Lie nilpotentes," Bull. Soc. Math. France 98, 81-116 (1970)]. Also we give the dimension, using an adaptation of the sl(2,C)-module Method, and a basis of such infinitesimal deformations in some generic cases. [ABSTRACT FROM AUTHOR]

We report on real time-resolved Reflectance-difference (RD) spectroscopy of GaAs(001) grown by molecular beam epitaxy, with a time-resolution of 500 ms per spectrum within the 2.3-4.0 eV photon energy range. Through the analysis of transient RD spectra we demonstrated that RD line shapes are comprised of two components with different physical origins and determined their evolution during growth. Such components were ascribed to the subsurface strain induced by surface reconstruction and to surface stoichiometry. Results reported in this paper render RD spectroscopy as a powerful tool for the study of fundamental processes during the epitaxial growth of zincblende semiconductors. [ABSTRACT FROM AUTHOR]

We describe a spectrometer (visible-ultraviolet photon energy range) for the measurement of reflectance-difference spectra of cubic semiconductors. The spectrometer employs a photoelastic modulator to modulate the polarization of the light incident on the sample and allows for a simple procedure to correct the measured spectra for parasitic components associated with such modulation. The instrument reported has a higher throughput than more conventional setups. To illustrate the spectrometer performance we report on reflectance-difference spectra of (001) oriented GaAs single crystals doped with silicon donors at a level of 10[sup 18]/cm³. [ABSTRACT FROM AUTHOR]

The integration of zincblende semiconductors on silicon demands for a real-time control of the crucial steps of epitaxial growth process at a microscopic level. Optical probes, being non-invasive, are very useful in monitoring such processes at a microscopic level. By using the reflectance anisotropy technique with microscopic resolution (μ-RD/RA), which detects the difference in reflectance for two orthogonal crystal directions, we measured the optical anisotropies below and above band gap of orientation-patterned GaP structures deposited on both Si(100) and GaP(100) vicinal substrates. We have developed a physical model to describe the line shape of the spectra below and above the fundamental gap of GaP. By using this model, we have successfully analyzed μ-RD/RA spectra, and we were able to do anisotropy topographic maps of the surface and buried interface, which are consistent to those measured with scanning electron microscopy. [ABSTRACT FROM AUTHOR]

Macroreflectance and microreflectance difference spectroscopies have been used to measure the strain induced optical anisotropies of semiconductor structures comprised of strained Si(110) thin films deposited on top of step-graded SiGe virtual substrates. The stress relaxation mechanism mainly occurs by the introduction of microtwin formation, whose fluctuation depends strongly on growth conditions. Correlations of such optical diagnostics with x-ray diffraction measurements and atomic force microscopy images, allow for the in situ study of the strain within both the top Si layer and the SiGe underneath with an spatial resolution of at least 5 μm. [ABSTRACT FROM AUTHOR]

The distribution of the activation energies, m(U0), of undoped and SiC-doped MgB2 wires has been estimated from magnetic relaxation measurements using the Hagen-Griessen inversion scheme. Ni-sheathed wires fabricated by the powder-in-tube (PIT) technique, using the ex situ reaction, have been analyzed. The enhancement of the superconducting properties under high magnetic fields (B>2 T) on doped wires is evidenced by the increase of the upper critical, Bc2, and irreversibility, Birr, fields, and by the shift of the pinning energy distribution to higher energy values. The obtained m(U0) distributions might be also explained by a nonlinear current dependence of the pinning energies: U(J,B,T)=[U*(B,T)/μ]*[(Jc/J)μ-1] with μ=0.2–0.4. [ABSTRACT FROM AUTHOR]

The first stages of epitaxial growth of CdTe on ZnTe and ZnTe on CdTe are monitored with reflectance difference spectroscopy. Spectroscopic reflectance difference data show strong optical anisotropy responses at the critical points of the bulk dielectric function at the E[sub 0], E[sub 1], and E[sub 1]+Δ[sub 1] interband transitions of ZnTe, respectively, CdTe, which indicate that anisotropic in-plane strain occurs during epitaxial growth. Applying a model it is possible to determine the in-plane strain due to the disbalance of 60° dislocations along [11¯0] and [110]. Kinetic reflectance difference data taken at the E[sub 1] transition of the respective material exhibit with an accuracy of one monolayer the onset of the formation of misfit dislocations for these material systems. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents an abstract of a study on the structure and magnetic properties of Bi[sub2]CuO[sub4]. Reason for the great interest in the magnetic properties of copper oxides; Description of the nuclear structure.

We report on the use of reflectance difference (RD) spectroscopy aimed to detect and quantify the degree of strain in thin Si films grown on top of misoriented Si1-xCx(001), where it was found that the Si layers are compressively strained. It is demonstrated that RD spectroscopy is suitable for in situ monitoring of strained thin Si films growing on vicinal substrates. [ABSTRACT FROM AUTHOR]

We report on a rapid, 32-channel reflectance-difference (RD) spectrometer with sub-second spectra acquisition times and ΔR/R sensitivity in the upper 10-4 range. The spectrometer is based on a 50 kHz photo-elastic modulator for light polarization modulation and on a lock-in amplifier for signal harmonic analysis. Multichannel operation is allowed by multiplexing the 32 outputs of the spectrometer into the input of the lock-in amplifier. The spectrometer spans a wavelength range of 230 nm that can be tuned to cover E1 and E1 + Δ1 transitions for a number of III-V semiconductors at epitaxial growth temperatures, including GaAs, InAs, AlAs, and their alloys. We present two examples of real-time measurements to demonstrate the performance of the RD spectrometer, namely, the evolution of the RD spectrum of GaAs (001) annealed at 500 °C and the time-dependent RD spectrum during the first stages of the epitaxial growth of In0.3Ga0.7As on GaAs (001) substrates. [ABSTRACT FROM AUTHOR]