Modern Physics Letters B

Using the first-principles methods, we study the formation energetics and charge doping properties of the extrinsic substitutional defects in montmorillonite. Especially, we choose Be, Mg, Ca, Fe, Cr, Mn, Cu, Zn as extrinsic defects to substitute for Al atoms. By systematically calculating the impurity formation energies and transition energy levels, we find that all Group II defects introduce the relative shallow transition energy levels in montmorillonite. Among them, Mg_Al has the shallowest transition energy level at 0.10 eV above the valence band maximum. The transition-elemental defects Fe_Al, Cr_Al, and Mn_Al are found to have relatively low formation energies, suggesting their easy formation in montmorillonite under natural surrounding conditions. Our calculations show that the defects Cu_Al and Zn_Al have high formation energies, which exclude the possibility of their formation in montmorillonite.

A photonic quantum well (PQW) structure based on the zero-averaged refractive index gap is constructed and the transmission properties are investigated. Quantized confined photonic states resulting from the photonic confinement effect are observed. We find that the resonant transmissions are not only weakly dependent on the incident angle but also insensitive to polarization of light. More importantly, the number of the confined photonic states can be controlled by simply adjusting the cell number of the well photonic crystal. It is shown that strong field localization is found when the well photonic crystal consists of positive-index material or single negative material. Particularly for the PQW structure containing single negative material, the field in the well region can be highly enhanced by modulating the size of the well photonic crystal. These peculiar features make this PQW structure superior to the conventional PQW structure in potential applications such as omnidirectional filtering and optical bistable switching.

General formulae for coherent/incoherent nucleation fields have been obtained as functions of the thickness of the hard and soft layers within a self-contained micromagnetic model for a hard/soft trilayer. The nucleation modes are determined reliably by direct comparison of the nucleation fields for different modes. It is shown that coherent nucleation cannot take place no matter how small the layer thickness. For small soft layer thickness, the magnetic reversal mode is an incoherent nucleation. As the soft layer thickness increases, the dominant magnetic reversal process is domain-wall motion/pinning. Analyses demonstrate that negative nucleation field and very small coercivity occur when the easy axes of neighboring layers are perpendicular. The results could shade light on Brown's coercivity paradox (where the measured coercivity is much smaller than the one given by the previous theories).

The transmission properties of a periodic structure consisting of alternating metal layers and air layers are studied by numeric methods. The metal layers are the epsilon-negative (ENG) materials. The interaction of the evanescent waves in metal layers and propagation waves in air layers forms some special transmission bands. Given proper structure parameters, one transmission frequency only allows one incident direction in the low frequency range. This structure can simultaneously achieve frequency filtering and direction filtering.

A generalized Wronskian formulation is presented for a four-dimensional nonlinear evolution equation. The representative systems are explicitly solved by selecting a broad set of sufficient conditions which make the Wronskian determinant a solution to the bilinearized four-dimensional nonlinear evolution equation. The obtained solution formulas provide us with a comprehensive approach to construct explicit exact solutions to the four-dimensional nonlinear evolution equation, by which positons, negatons, solitons and complexitons are computed for the four-dimensional nonlinear evolution equation. Applying the Hirota's direct method, multi-soliton, non-singular complexiton, and their interaction solutions of the four-dimensional nonlinear evolution equation are also obtained.

A protocol for quantum dialogue is proposed to exchange directly the communicators' secret messages by using the two-qutrit entangled state. In this protocol, the security of communication is ensured by the secret transmitting order of qutrits. It is shown that two legitimate users can directly transmit the secret messages by generalized Bell-basis measurement and classical communication.

We established a simple evolutionary model based on the cancer stem cell hypothesis. By taking cellular interactions into consideration, we introduced the evolutionary games theory into the quasispecies model. The fitness values are determined by both genotypes and cellular interactions. In the evolutionary model, a cancer cell population can evolve in different patterns. For single peak intrinsic fitness landscape, the evolution pattern can transit with increasing differentiation probability from malignant cells to benign cells in four different modes. For a large enough value of differentiation probability, the evolution is always the case that the malignant cells extinct ultimately, which might give some implications on cancer therapy.

A high potential gradient of ZnO varistors were fabricated by Y₂O₃-doping and low-temperature sintering. The value of the potential gradient increased to 2460.5 V/mm with the Y₂O₃ content of 0.08 mol% and the sintering temperature of 800°C. The effects of Y₂O₃-doping and sintering temperature on the electrical properties of ZnO varistors were investigated. Under the given experimental conditions, additive Y₂O₃ exists in the form of Y₂O₃ phase after sintering at 800°C. High-energy ball-milling in the early period of the experiment induced the grain refinement and realized the sintering formation at the lower temperature of 800°C. Both Y₂O₃-doping and low-temperature sintering restrained the ZnO grain growth and increased the potential gradient.

A model of 3D RC networks was developed to describe the dielectric response of heterogeneous materials and was compared with the results of the 2D RC network model. We show that the "universal dielectric response" (UDR) of heterogeneous materials is a common feature of both 2D and 3D very large networks with randomly positioned resistors and capacitors, and that the percolation threshold of the 2D and 3D bond network are close to 0.5 and 0.25, respectively. In addition, it was found that the percolation threshold of the 3D network is in good agreement with the result of the coherent potential (CP) formula.

Single crystals of tris (Thiourea) magnesium sulphate (MTS) were grown from aqueous solution by low temperature solution growth technique. The grown crystals have been characterized by X-ray diffraction to confirm the formation of the crystalline phases. The presence of functional groups was identified by Fourier transform infrared spectroscopy. The optical transmission spectrum showed that a lower cut-off wavelength of MTS crystal is below 300 nm and it has a wide transparency window, which is suitable for second harmonic generation of laser in the blue region. The thermal stabilities were studied by differential thermal analysis (DTA) and thermo gravimetric analysis (TGA). Nonlinear optical characteristics of MTS were studied using Q-switched Nd:YAG laser (λ=1064 nm). The micro-hardness studies reveal the mechanical properties of the grown crystals.

Optical transmittance of Ba_1-xCa_xTi₂O₅ (x=0, 0.01, 0.03) bulk glasses of ferroelectric BaTi₂O₅ is measured at room temperature in the wavelength range 190–800 nm. The fundamental absorption edge located in the ultraviolet (UV) region tends to shift toward the longer wavelength region on increasing x. The optical band gap of 3.31 eV estimated for x=0 decreases on increasing x. The decrease in the optical band gap energy is related to the creation of non-bridging oxygen ions, which increase on Ca substitution. The observed increase in the Urbach energy with increasing x is associated with the possible increase of structural disorder in the ferroelectric BaTi₂O₅-based bulk glass occurred by substituting Ca for Ba.