Physics of the Solid State最新文献

We investigated the phonon and thermal properties of the silicon- and germanium-nanowires, covered by Si_xGe_1–x, plastic, diamond and SiO₂ shells as well as Si-based segmented nanowires, consisting of segments of different sizes and/or materials. Acoustic phonon energies were calculated in the framework of the face-centered cubic cell model of the lattice vibrations, while thermal conductivity was investigated in the framework of Boltzmann transport equation approach within the relaxation time approximation. It was shown, that claddings with higher (lower) sound velocity strongly affect the phonon energy spectra and increase (decrease) the average phonon group velocity in core nanowire. It was demonstrated, that redistribution of the phonon modes in Si/Ge and Si/SiO₂ segmented nanowires leads to a localization of the great amount of the phonon modes in nanowire segments, resulting in exclusion of such modes from the heat flow and suppression of the phonon thermal conduction (by a factor of 2–8) in comparison with generic silicon nanowires. Low values of the thermal conductivity of segmented nanowires make them prospective for thermoelectric and thermoinsulating applications.

This study investigated the structural, mechanical, piezoelectric, and electromechanical properties of AlScN thin films using density functional theory (DFT) under varying levels of applied pressure, ranging from 0 to 20 GPa. The primary focus of this research is to explore the feasibility of optimizing AlScN thin films for surface acoustic wave (SAW) applications through pressure-induced modifications. Our findings reveal two significant outcomes. First, we observe a notable increase in the elastic constant C₃₃ as a function of pressure. This increase signifies a substantial enhancement in material stiffness, directly influencing wave propagation and velocity within the thin films. Second, a remarkable 68% improvement in the piezoelectric constant, d₃₃, is identified for Al_0.75Sc_0.25N at an applied pressure of 20 GPa compared to Al_0.75Sc_0.25N at 0 GPa. This enhancement has a profound impact on the electromechanical coupling characteristics of the material. These results underscore the potential for tuning the piezoelectric response of AlScN thin films using applied pressure, offering a promising avenue for enhancing the performance of SAW-based AlScN devices.

The electronic structure and elastic and thermodynamic properties of the AgMO₃ (M = Nb, Ta) were investigated using first-principles calculations. The lattice parameters and volumes are in reasonable agreement with the experimental results. The calculated Cauchy’s pressure, Poisson’s ratio, and B/G ratio confirm the ductile nature of both the compounds. The variation in entropy (S), thermal expansion coefficient (α), constant volume heat capacity (C_v), and the constant pressure heat capacity C_p with temperature have been studied.

The purpose of this study was to investigate the influence of dual doping with Fe and Co on the microstructural, morphological, and optical properties of ZnO nanoparticles (NPs). Zn_0.97–xFe_0.03Co_xO (x = 0, 0.01, 0.02, and 0.03) NPs were prepared via a solid-state reaction method using high-purity ZnO, Fe, and Co NPs. This study was performed using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), UV-visible spectrophotometry, photoluminescence (PL), and energy-dispersive X‑ray spectroscopy (EDS). XRD analysis revealed that all samples presented a pure hexagonal wurtzite str-ucture without any trace of Fe, Co, or their oxides, indicating that the dopant ions were well-substituted Zn ions. However, some peaks appear in the spectrum of the Zn_0.94Fe_0.03Co_0.03O sample, corresponding to the secondary spinel phases ZnCo₂O₄ and CoFe₂O₄. FE-SEM micrographs showed that all samples exhibited sphere-like particles, and their sizes, aggregation degree, and morphology were slightly influenced by the dopant content. The estimated bandgap values decreased from 3.24 eV for undoped ZnO to 3.17 eV for Zn_0.95Fe_0.03Co_0.02O NPs and then slightly increased. Moreover, the refractive index was evaluated from the bandgap energy using Moss, Ravindra Hervé-Vandamme, and Reddy models, and then compared. The PL spectra of all samples revealed strong and sharp emission peaks in the UV region, which increased in intensity as the Co content increased.

Cu–Fe–Pd alloys have great versatility and the promise of extensive applications across numerous fields of industry and have become a focal point of scientific inquiry and exploration. The effect of order-disorder phase transformation on structure and thermal properties has been investigated in ternary Cu₇₅Fe₀₅Pd₂₀ alloy by using high-temperature X-ray diffraction and differential scanning calorimetry. High-temperature X-ray diffraction experiments for the sample pre-annealed at 723 K have revealed the formation of an L1₂‑type ordered structure up to 805 K and disordered face centered cubic (f.c.c.) structure above 805 K. The lattice parameter is observed to be larger than that predicted by Vegard’s rule. This is due to the fact that the addition of Fe weakens the interatomic forces in the alloy. The integrated intensity data was used to determine thermal parameters. The phase transition occurring at T_c is of the first order because a sudden change in lattice parameter and linear thermal expansion coefficient is observed. These parameters collectively suggest that the investigated alloys could be advantageous for the automobile and space industries.

It is well established, both theoretically and experimentally, that unstrained monolayer graphene shows linear dispersion as defined by Dirac equation of massless Fermions. But, when it is subjected to anisotropic strain, the two Dirac points get shifted from their equilibrium positions and they merge when the applied strain attains a threshold value. Near the merging point, dispersion energy is found to deviate from linearity and band gap opens up turning graphene to behave as semiconductor. A detailed calculation shows that unlike normal semiconductors with direct band gap its dispersion energy is non-parabolic around the merging point and the curvature of non-parabolicity changes with the variation of the direction of the applied anisotropic strain. Not only that, the threshold value of strain for band gap opening varies periodically between specified maximum and minimum as the strain is applied in the directions further away from the zigzag edge. To study these atypical features, a generalized expression for strain induced non-linear dispersion relation of monolayer intrinsic graphene has been formulated under tight-binding approximation (TBA). Also, the band gap energy, density of states (DOS) and electron effective mass (EEM) have been determined as a function of the magnitude of strain as well as its direction of application.

Here, a new method of preparing a polycrystalline mixture with a large volume of stable composition from binary solid solutions is presented. For this purpose, several holes of small diameter (0.5–1 mm) are opened at the bottom of the quartz pot prepared for melting the alloy. The upper end of the mold made of quartz, according to the geometrical structure of the mixture, is connected to the bottom of the puta in such a way that the holes in the puta remain inside it and both volumes are hermetically connected. Such a “puta-mould” system is fixed in the device in such a way that the template (mould) of the mixture is in a vertical position. Appropriate masses of the components of the solid solution are placed in a crucible and melted under high vacuum conditions. After the molten liquid becomes homogeneous, inert gas with a pressure of 0.5–0.8 atm is injected into the working volume. Due to the pressure force exerted by the gas on the surface of the liquid, it rushes through the holes and fills the mold, where it crystallizes at a high speed. The mold is placed inside a thick-walled, heat-conducting cylinder connected to a running water-cooled body of the lower end unit. This ensures that crystallization occurs at a high speed. Thus, the composition has the same value throughout the prepared mixture. Using the method, an alloy containing 10 at % Si was prepared from the Ge–Si solid solution system. Calculation of the density of samples taken from different parts of the mixture confirmed that the composition was the same throughout the mixture.

The theoretical investigation of acoustic phonons in Si/Ge three-dimensional quantum dots superlattices (supracrystals) is presented. The acoustic phonon energy spectra are calculated in the framework of the face-centered cubic cell molecular-dynamic model. The dependencies of phonon density of states and phonon group velocity on the phonon energy are studied; the average phonon velocity is found to be close to zero in the wide range of phonon energies (hbar omega > 10) meV. The results allow us to predict the extremely low value of the lattice thermal conductivity in supracrystals and correspondingly high value of the thermoelectric figure of merit ZT.

In recent decades, much interest has been shown in nonlinear lattice vibrations because crystalline materials are subjected to high-amplitude impacts in many fields of human activity. One of the effects of nonlinearity in discrete periodic structures is the possibility of existence of spatially localized high-amplitude vibrations, referred to as discrete breathers (DBs), or intrinsic localized modes. The problem of searching for DBs in nonlinear chains (i.e., one-dimensional crystals) can be solved in a fairly simple way, because the variety of possible DBs is small in this case. However, no general approaches to the search for DBs have been developed for high-dimension crystal lattices. Such an approach was derived based on the works by Chechin, Sakhnenko et al., who developed the theory of bushes of nonlinear normal modes, which (as applied to crystals) were later referred to as delocalized nonlinear vibrational modes (DNVMs). It has recently been noted that all known DBs can be obtained by superimposing localizing functions on DNVMs with a frequency beyond the phonon spectrum of the lattice. Since the Chechin and Sakhnenko theory makes it possible to find all possible DNVMs by considering the lattice symmetry, it has become possible to formulate the problem of determining all possible DBs in a given lattice. This approach has recently been applied with success to the search for DBs in a two-dimensional triangular lattice. The purpose of this study is to analyze and describe DBs in a two-dimensional square lattice obtained using a localizing function. As a result, new types of DBs of a square lattice are obtained, including one-dimensional DBs (i.e., those localized only in one of two orthogonal directions) and zero-dimensional DBs (i.e., those localized in two directions).