Modern Physics Letters B

Under investigation in this paper is a variable-coefficient higher-order nonlinear Schrödinger equation, which has certain applications in the inhomogeneous optical fiber communication. Through the Hirota method, bilinear forms, dark one- and two-soliton solutions for such an equation are obtained. We graphically study the solitons with $d_1(z)$, $d_2(z)$ and $d_3(z)$, which represent the variable coefficients of the group-velocity dispersion, third-order dispersion and fourth-order dispersion, respectively. With the different choices of the variable coefficients, we obtain the parabolic, periodic and V-shaped dark solitons. Head-on and overtaking collisions are depicted via the dark two soliton solutions. Velocities of the dark solitons are linearly related to $d_1(z)$, $d_2(z)$ and $d_3(z)$, respectively, while the amplitudes of the dark solitons are not related to such variable coefficients.

For describing the propagation of ultrashort pulses in the birefringent or two-mode fiber, we consider the coupled nonlinear Schrödinger equations with higher-order effects. According to the Ablowitz–Kaup–Newell–Segur system, 3$\times$3 Lax pair for such equations is derived. Based on the Lax pair, we construct the conservation laws and Darboux transformation (DT). One- and two-soliton solutions are obtained via the DT, and we graphically present the one soliton and bound-state two solitons.

Nanocrystalline tin dioxide (SnO₂) thin films have been successfully prepared by sol–gel spin-coating technique on p-type Si (100) substrates. A stable solution was prepared by mixing tin(II) chloride dihydrate, pure ethanol, and glycerin. Temperature affects the properties of SnO₂ thin films, particularly the crystallite size where the crystallization of SnO₂ with tetragonal rutile structure is achieved when thin films that prepared under different aging heat times are annealed at 400${}^{\circ }$C. By increasing aging heat time in the presence of annealing temperatures the FESEM images indicated that the thickness of the fabricated film was directly proportional to solution viscosity, increasing from approximately 380 nm to 744 nm, as well as the crystallization of the thin films improved and reduced defects.

The electronic structure and the lattice dynamics of N-doped 3C–SiC have been studied using the first-principles calculations. Electrons are introduced to the conduction band bottom at the X-point of the Brillouin zone. It is revealed that the acoustical modes and optical modes soften upon N doping. The softened acoustical modes contribute over twice of what the optical modes do to the electron–phonon coupling strength. We predict that the superconductivity will occur with N doping concentration beyond about 3 at.% and the transition temperature can reach 11.36 K upon doping of N at about 5 at.% for 3C–SiC.

This paper presents the low-frequency acoustic properties of a new single and double slotting radial acoustic metamaterial plate which is arranged in the axial coordinate. The band structures, transmission spectra, and eigenmode displacement fields of this kind of acoustic metamaterial are different from previous studies. Numerical calculation results show that the first-order band gap (BG) of the radial flexible elastic metamaterial plate is below 200 Hz. A multiple-vibration coupling mechanism is proposed to explain the low-frequency band gaps. By changing the geometric parameters a, t, and g, the location and width of the low-frequency band gaps can be varied neatly. In summary, rational design of geometries and materials is the crucial pathway to open and lower BGs, and could restrain low-frequency vibration similarly. This can be used to protect infrasound, generate filters, and design acoustic devices.

In this paper, a vector Ramani equation is proposed by using the bilinear approach. With the help of the bilinear exchange formulae, bilinear Bäcklund transformation and the corresponding Lax pair for the vector Ramani equation are derived. Besides, multi-soliton solution expressed by pfaffian is given and proved by pfaffian techniques.

The dielectric and impedance spectra of TlGaSe₂ crystals have been studied at temperatures in the 100–500 K range in the alternating current (AC $\sim$1 V). It has been shown that the conductivity of TlGaSe₂ crystals is mainly an ionic characteristic at temperatures above 400 K. The well-defined peak at the frequency dependence of the imaginary part of impedance $(Z^{″})$ is observed in the 215–500 K temperature range. In a constant field, there occurs a significant decrease in electrical conductivity $\sigma$ in due course. The ionic contribution to conductivity (76% at $T=470\mathrm{\text{K}}$) has been estimated from a kinetic change in electrical conductivity $(\sigma )$ under the influence of a constant electric field. The diagram analysis in a complex plane $(Z^{″}-Z^{\prime })$ has been conducted by applying the method of an equivalent circuit of the substation. It has been determined that the average relaxation time of the electric module of the sample is $\tau =1.54\cdot 10^{-6}\mathrm{\text{s}}$.

We design and numerically investigate a wide angle and broadband perfect absorber with compact multilayer structures. The proposed structure is composed of three stacks of semiconductor films and one metal layer. The absorber presents a broadband absorption between 450 nm and 700 nm with an average absorption above 99.2%. For oblique incidence, the absorber shows an average absorption about 90% for a wide range of incident angles from 0${}^{\circ }$ to 60${}^{\circ }$ with p polarization and s polarization. The electrical field intensity distributions are also studied to disclose the broadband absorption mechanism. This designed broadband absorber appears to have very promising applications in solar thermal energy harvesting, thermal emitters and detection.

In this paper, we employed first-principle calculations to investigate the elastic properties, electronic properties and hardness of diamond and hexagonal W₂B₅ compounds under high pressure. The elastic properties were carried out to discuss the structural stability and the bond components of diamond and hexagonal W₂B₅. The electronic properties were presented to analysis the change of the bond components for W₂B₅. In addition, the hardness of these two crystals under high pressure was calculated by a semi-empirical method considering the role of metallic components and the effect of pressure on the hardness of diamond and hexagonal W₂B₅ was discussed.

In this work, we have applied the first-principles calculations to investigate the structural stability, elastic properties and electronic structure of Mo₂FeB₂ with alloying elements Cr, Ni and Mn. The calculated cohesive energy shows that Cr, Ni and Mn prefer to occupy the Fe atom of Mo₂FeB₂. However, only when Mn is doped at the Mo atom of Mo₂FeB₂, it is converted from dynamic unstable state to stable state. The calculated elastic modulus shows that Mo₂FeB₂ will have better mechanical properties when alloying elements are at Fe site instead of Mo site. Moreover, Cr addition can improve the volume deformation resistance of Mo₂FeB₂, Mn addition can improve the shear deformation resistance for Mo₂FeB₂. The calculated B/G ratio shows that Ni addition can improve the brittleness of borides. Furthermore, the hardness of Mo₂FeB₂ can be enhanced by adding Cr and Mn element. The calculated electronic structure indicates that the increasing of elastic modulus is attributed to the formation of Cr–B and Mn–B covalent bonds.

The inorganic–organic perovskite CH₃NH₃PbI₃ is a hot research material owing to its outstanding performances as one light absorbing layer of solid-state dye-sensitized solar cells. In this study, we focused on the atomic size effect on CH₃NH₃BI₃ (B = Sn, Pb), provided the best atomic size with which CH₃NH₃BI₃ absorbs widest range of different wavelengths of light, by first-principles calculation. We found that the halogen I–p states are mainly composed of the valence band maximum (VBM) of CH₃NH₃BI₃, and the cation B–p states are primarily composed of the conduction band minimum (CBM). Besides, the bandgap of CH₃NH₃BI₃ decreases and absorptive capacities of different wavelengths of light expand when we reduced the size of the atom and changed B atom from Pb to Sn during the change of suitable range. From all of the above, it is discovered that when the atomic size is 20% less than the normal size, CH₃NH₃PbI₃ has the best optical properties, and its light-absorption range is the widest among all sizes of CH₃NH₃BI₃ compounds. All these results reveal that the stress and strain on CH₃NH₃BI₃ change the atomic size which leads to alteration of bandgap and optical properties in high-efficiency solar cells among all CH₃NH₃BI₃ compounds, namely we can enhance the efficiency of the inorganic–organic perovskite solar cells by setting up suitable pressure on the material in future.

Using the first-principle methods, the pressure dependence of the electronic structure and band structure of kaolinite were studied within the local-density approximation. The calculated results show that pressure would chiefly alter the band structure of kaolinite, while pressure can have its main effect on the band gap of kaolinite. At p = 0.6 GPa, band structure of kaolinite first converts an indirect gap into a direct gap, and then recovers an indirect gap structure at p = 66.2 GPa because CBM shift in the band structure is under high-pressure. The bond Si–O is more stable than bond Al–O under pressure, in addition, pressure has a significant effect on the inner hydroxyl bond of kaolinite and leads to a large variation of H–O(inner) bond lengths. The calculated results will not only help to understand the electronic structure of kaolinite under pressure, but also provide theoretical guidance for deal with the safe problems of soft-rock tunnel engineering.