Investigating the thermoelectric properties of Na0.74Co1−xNbxO2 (x = 0.05,0.10) at high temperature region

Here, the thermoelectric (TE) properties of Na_0.74Co${}_{1-x}$Nb_xO₂ ($x=0.05,0.10$) compounds are investigated experimentally and computationally. The experimental measurements are conducted in $300-620$ K. Positive sign of Seebeck coefficient for both the compounds indicates the dominating p-type character. The maximum experimental values of ZT are observed as ∼ 0.12 and ∼ 0.19 at 620 K for $x=0.05$ and $x=0.10$, respectively. The experimental transport properties of these compounds are understood by employing spin-polarized GGA+U (= 4 eV) electronic structure calculations on $x=0.0625$ compound. On the basis of best experimental and computational matching of transport properties, we have estimated ZT till 1200 K computationally. The highest calculated values of ZT are ∼ 1.36 and ∼ 1.22 at 1200 K for $x=0.05$ and $x=0.10$, respectively. The optimum value of efficiency for $x=0.05$ is calculated as ∼ 6.4%, whereas it reaches ∼ 7.5% for $x=0.10$.     

PDM creation and annihilation operators of the harmonic oscillators and the emergence of an alternative PDM-Hamiltonian

The exact solvability and impressive pedagogical implementation of the harmonic oscillator's creation and annihilation operators make it a problem of great physical relevance and the most fundamental one in quantum mechanics. So would be the position-dependent mass (PDM) oscillator for the PDM quantum mechanics. We, hereby, construct the PDM creation and annihilation operators for the PDM oscillator via two different approaches. First, via von Roos PDM Hamiltonian and show that the commutation relation between the PDM creation ${\hat{A}}^{+}$ and annihilation $\hat{A}$ operators, $[\hat{A},{\hat{A}}^{+}]=1\iff \hat{A}{\hat{A}}^{+}-1/2={\hat{A}}^{+}\hat{A}+1/2$, not only offers a unique PDM-Hamiltonian ${\hat{H}}_1$ but also suggests a PDM-deformation in the coordinate system. Next, we use a PDM point canonical transformation of the textbook constant mass harmonic oscillator analog and obtain yet another set of PDM creation ${\hat{B}}^{+}$ and annihilation $\hat{B}$ operators, hence an “apparently new” PDM-Hamiltonian ${\hat{H}}_2$ is obtained. The “new” PDM-Hamiltonian ${\hat{H}}_2$ turned out to be not only correlated with ${\hat{H}}_1$ but also represents an alternative and most simplistic user-friendly PDM-Hamiltonian, $\hat{H}={(\hat{p}/\sqrt{2m\left(x\right)})}^2+V\left(x\right)$; $\hat{p}=-iħ{\partial }_x$, that has never been reported before.     

Terahertz generation in quantum cascade structures by two-color deriving fields: An approach to reach inversion population via optical pumping

In this paper, a quantum cascade laser (QCL) design is proposed based on GaAs/AlGaAs material system, which simultaneously operates at three widely separated wavelengths (${\lambda }_1=11.1\mu m,{\lambda }_2=14.1\mu m$ and ${\lambda }_{THz}=60\mu m$). In the design, all the wavelength radiations are achieved by the engineering of the electronic spectrum via the quantum-well widths and the applied electric field in a single active region within a same waveguide. The mid-infrared (mid-IR) wavelengths are obtained by adoption a dual-upper-state active region, and the proposed design aims to use both the mid-IR radiations as the coherent deriving fields to populate the upper THz lasing state to aid the THz-laser population inversion via optical pumping instead of direct electrical injection. A detailed analysis of electronic transport in the structure is carried out using a multi-level rate-equation model. The results show that the proposed structure offers an alternative approach to room temperature THz generation in QCLs.     

CNN-aided multiple PU spectrum sensing in uncalibrated massive antennas SU system

This paper deploys the Convolutional Neural Network (CNN) to learn and set the statistical test in Spectrum Sensing (SS) task of multiple primary user (PU) sources in massive uncalibrated antennas of secondary users (SU) sharing the same spectrum resources. The proposed deep learning-based SS method (DL-SS) is based on the CNN architecture that has the capability of extracting features of the sample covariance matrices (SCMs) that are given as the network input, improving the overall performance and robustness. The proposed CNN-SS method is compared with nine recent multiple-antennas SS methods, namely the arithmetic–geometric detector (AGM), John’s detector (JD), sphericity detector (SD), generalized likelihood test (GLRT), locally most powerful invariant test (LMPIT), maximum–minimum eigenvalue detector (MME), covariance detector (CAV), Hadamard detector (HD) and volume detector (VD) methods; besides, the proposed method is also compared with five recent state-of-art CNN-based SS methodologies. Performance-complexity trade-off of the proposed and reference SS methods are corroborated via Monte Carlo Simulations (MCS). The proposed CNN-SS method under uncalibrated massive antennas reveals substantial benefits w.r.t. the reference methods and is competitive with others CNN-SS methodologies, both in terms of complexity and performance, achieving detection probability of $P_d=0.9$ $(@SNR=-20\mathrm{dB})$ under very low false alarm probability $P_f=0.1$. Under different figures of merit, the performance of the CNN-based SS detector has revealed to be indubitably superior regarding the state-of-art SS detectors. However, the proposed CNN-based SS detector presents relative computational complexity increases. Hence, to be effective, such a superior operational performance requires a very efficient processing structure in the SU base stations.     

Thermospin effects in a T-shaped spin valves with spin-flip scattering

With the help of the nonequilibrium Green's function technique, we theoretically analyze the thermospin property through a typical T-shaped spin valve with spin-flip scattering in the linear regime. The influences of spin-flip coefficient of interdot λ, spin-flip coefficient of intradot η and interdot hopping coefficient $t+{\delta }_{\sigma }\mathrm{\Delta }t$ on thermospin property are discussed. As interdot hopping coefficient t is equal to energy level ε, the spectrum of $G_s$ shows Fano-like effect with ε variation. Antiresonance position of $G_s$ is almost unchanged and its width becomes narrower with ε increasing. Spin thermopower $S_s$ is close to the maximum of the peak and charge thermopower $S_c$ is equal to zero for $t=\epsilon$. As a result, the pure spin thermopower $S_s$ can be obtained, which means that a pure spin current may be produced by a temperature gradient in our system. It is found that spin figure of merit $Z{T}_s$ can reach a considerable value by adjusting key parameters of the system, such as Δt, β, α, ?. The typical T-shaped spin valve can be treated as a stable thermospin battery which allows to convert the heat energy to spin voltage, thus produces the pure spin current in the device.     

Dynamics of FREI with/without cool flame interaction

The interactions between cool flames and flames with repetitive extinction and ignition (FREI) of stoichiometric n-heptane/air mixture were studied using a micro flow reactor with a controlled temperature profile from 373 to 1300 K. Two different flame dynamics with and without cool flames were observed in reactors with inner diameters $d_{\text{inner}}$ of 1 and 2 mm. Cool flames and FREI are spatially separated at $d_{\text{inner}}=$ 1 mm, whereas interactions between cool flames and FREI are observed at $d_{\text{inner}}=$ 2 mm. At $d_{\text{inner}}=$ 1 mm, the brightness intensity from cool flames depends on the inlet velocity ($u_{\text{inlet}}$). Approximately above $u_{\text{inlet}}=$ 10 cm/s, the brightness intensity from cool flames decreases with increasing inlet velocity, despite a large amount of mixture input. This is because before low temperature ignition occurs under higher inlet velocity conditions, the mixture archives temperature where negative temperature coefficient is dominant. Reaction front propagation speed of FREI decreases monotonically due to heat loss because the extinction points of FREI are located in higher temperatures than the cool flame region. At $d_{\text{inner}}=$ 2 mm, the acceleration of the reaction front in the cool flame region is confirmed experimentally, as predicted in our previous two-dimensional numerical simulations. Additionally, the instantaneous reaction front speed after autoignition is analyzed at $d_{\text{inner}}=$ 1 mm. The instantaneous reaction front speed decreases as the time from extinction to ignition $t_{\text{ex}\_\text{ig}}$ becomes longer because a moderate mixing zone of reactants and products is formed.     

Experimental determination of interfacial energy for solid Al in the Al-Sn-Mg eutectic system with Bridgman-type apparatus

The equilibrated grain boundary groove shape of solid Al in equilibrium with Al-Sn-Mg eutectic liquid was observed by using a Bridgman type directional solidification apparatus. The ratio of the thermal conductivity of the equilibrated liquid to the thermal conductivity of solid Al has been obtained as 0.91. In addition, the average Gibbs-Thomson coefficient, $\bar{\mathrm{\Gamma }}=(4.20\pm 0.35)\times {10}^{-8}\mathrm{Km}$, the solid-liquid interfacial energy, ${\sigma }_{SL}=180.68\pm 23.48\text{mJ}/{\text{m}}^2$ and the grain boundary energy, ${\sigma }_{GB}=309.30\pm 29.47\text{mJ}/{\text{m}}^2$, in the Al/Al-Sn-Mg system have been calculated from the measured grain boundary shapes.     

Synthesis and microwave dielectric properties of an electronic ceramic Y2WO6 for wireless communications

Y₂WO₆ ceramics were fabricated via a solid-state reaction method and investigated structure stability, densification, microstructure, and dielectric properties at microwave frequency range. Y₂WO₆ crystallized in a monoclinic structure and stabilized to 1500 ^∘C, beyond which the decomposition of Y₆WO₁₂ occurred. Y₂WO₆ ceramic could be sintered into a compact bulk at 1450 ^∘C, which was characterized by a high relative density ∼ 97.6% and a dense microstructure. The favorable dielectric performances were achieved at 1450 ^∘C with a relative permittivity ${\epsilon }_r\sim 11.4$, a quality factor $Q\times f\sim 42,380$ GHz ($f=8.6$ GHz), and a temperature coefficient of resonant frequency ${\tau }_f\sim -49.0$ ppm/^∘C. The MW properties of Y₂WO₆ suggest that it could be useful candidate material for low-loss dielectric resonators.