The ESR study of single crystal spinel ZnCr2Se4 diluted with Sb and V

The single crystal of Sb³⁺ and V³⁺ doped zinc chromium selenide spinel ZnCr₂Se₄ were prepared by a chemical transport method and characterized by ESR spectroscopy in order to examine the effect of nonmagnetic antimony and magnetic vanadium on properties of the system. For antimony admixtures the Neel temperature is very similar to that of the parent spinel ZnCr₂Se₄ (22 K). However, upon incorporating vanadium ions, the T_N temperature decreases down to 17.5 K, determined for the maximum vanadium content (x=0.06). The temperature dependence of the ESR linewidth over paramagnetic region is interpreted by an occurrence of spin-phonon interaction. The strong broadening linewidth together with its strong temperature dependence for vanadium doped ZnCr₂Se₄ is explained by the complex paramagnetic relaxation model.     

Geometrically distributed aperiodic circular photonic crystals with broad and isotropic photonic band gaps

In this paper, a new type of circular photonic crystal (CPC) with a geometrical distribution of concentric layers is presented. A broad and isotropic photonic band gap is achieved using this geometrically distributed CPC (GCPC). Also, the influence of the number of concentric layers to the overall band gap of GCPC is studied. It is demonstrated that the band gap broadens with a red shift when the number of concentric layers is increased. The reason for the red shift of the band gap is further investigated.     

Low threshold laser cavities of two-dimensional photonic crystal line defect mode

Two-dimensional (2D) rod-type photonic crystal (PC) line defect waveguide (LDW) laser cavities based on three types of line defect modes with zero group velocity are studied by using finite-difference time-domain (FDTD) method. These laser cavities have high quality (Q) factor, better localization of light, non-uniform gain distribution and small overlap between gain medium and light field. Therefore, they have the advantages over conventional and air-bridge PC cavities with uniform gain, such as low threshold, single mode lasing and effectively avoiding thermal effect. From their comparison, one can find the mode at middle Brillouin zones (BZ) is the best one to be used as lasing mode. Its dynamic lasing process and lasing features are demonstrated by the numerical experiment where the FDTD method coupling Maxwell's equations with the rate equations of electronic population is used.     

第一性原理研究KDP晶体中Ba取代K点缺陷

 用基于第一性原理的CASTEP模拟了Ba替代K缺陷前后形成的电子结构和能态密度。发现晶体能带宽度降至6.4 eV左右,对应着380 nm的双光子吸收,这一结果可以解释掺Ba晶体在紫外波段的吸收现象。Ba替代K点缺陷仅使其周围的晶格及电子结构发生轻微畸变,对晶体整体结构影响不大。     

Study of the excitation properties of the Si3O2 cluster under an external electric field

The structure of the Si₃O_x (x=2, 3) cluster is investigated; we find that the geometry of Si₃O₂ is similar to that of Si₃O₃ except for the oxygen-deficient defect structure (Si-Si band) which exists only in the Si₃O₂ cluster. It is known that oxygen-deficient defects are used to explain visible luminescence (especially blue, purple and ultraviolet light) from silicon-based materials, which are directly bound up with the excited states of the molecules. Therefore the excitation properties of the two clusters are also studied. Our results show that the absorption spectrum of Si₃O₂ is concentrated in the visible light region. In contrast, the absorption spectrum of Si₃O₃ is mainly located in the ultraviolet light region. The calculations are perfectly consistent with experimental data and also support the theory of oxygen-deficient defects.     

Identification and elimination of inductively coupled plasma-induced defects in AlxGa1 - xN/GaN heterostructures

By using temperature-dependent Hall,variable-frequency capacitance-voltage and cathodoluminescence (CL) measurements,the identification of inductively coupled plasma (ICP)-induced defect states around the Al x Ga 1-x N/GaN heterointerface and their elimination by subsequent annealing in Al x Ga 1-x N/GaN heterostructures are systematically investigated.The energy levels of interface states with activation energies in a range from 0.211 to 0.253 eV below the conduction band of GaN are observed.The interface state density after the ICP-etching process is as high as 2.75×10 12 cm 2 ·eV 1.The ICP-induced interface states could be reduced by two orders of magnitude by subsequent annealing in N 2 ambient.The CL studies indicate that the ICP-induced defects should be Ga-vacancy related.     

Thermal and dielectric studies of nickel malonate dihydrate single crystals

Single crystals of nickel malonate dihydrate were grown by the gel technique, employing the single diffusion method. Thermal dehydration of the crystal was investigated by thermogravimetric and differential thermal analyses. The title compound exhibits a steady thermal behaviour at higher temperature range of 350-800 °C. The dielectric properties of the prepared sample were analyzed as a function of frequency in the range of 1 kHz-1 MHz and at temperatures between 40 and 140 °C.     

Structural, elastic and electronic properties of a new ternary-layered Ti2SiN

The structural, elastic and electronic properties of Ti₂SiN were studied by first-principle calculations. The calculated bond lengths of Ti-Si and Ti-C are 2.65 and 2.09 Å, respectively. The results show Ti₂SiN is mechanically stable, and its bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio μ and anisotropy factor A are determined to be 182 GPa, 118 GPa, 291 GPa, 0.233 and 1.57, respectively. The calculated electronic structure indicates that Ti₂SiN is anisotropic and conductive.     

Comparative study on BIS thiourea cadmium acetate crystals using HRXRD, etching, microhardness, UV-visible and dielectric characterizations

〈1 1 1〉 oriented bis thiourea cadmium acetate (BTCA) crystal of diameter 15 mm and length 45 mm was grown for the first time by the unidirectional Sankaranarayanan-Ramasamy (SR) method. The conventional and SR method grown BTCA crystals were characterized by using high-resolution X-ray diffraction (HRXRD), chemical etching, Vickers microhardness, UV-vis, dielectric studies and differential scanning calorimetry. The HRXRD analysis indicates that the crystalline perfection of SR method grown crystal is good without having any low angle internal structural grain boundaries. The transmittance of SR method grown BTCA is 14% higher than that of conventional grown crystal. The dielectric constant was higher and the dielectric loss was less in SR method grown crystal. The crystals grown by SR method possess less dislocation density and higher microhardness.     

Asymmetric electrical properties in Pt/Ba0.5Sr0.5Ti0.99Co0.01O3/Nb-doped SrTiO3 capacitors

Ba_0.5Sr_0.5Ti_0.99Co_0.01O₃ (BSTC) thin films have been fabricated with pulsed laser deposition on Nb-doped SrTiO₃ (STN) substrate. In Pt/BSTC/STN capacitor, we systematically investigated the capacitance, leakage current and polarization versus bias voltage characteristics, and found that curves of capacitance versus voltage and leakage current versus voltage were not symmetric, and polarization hysteresis loop exhibited large relaxation of the remnant polarization at negatively poled state. A detailed analysis of capacitance data demonstrated a difference of the built-in voltage between top Pt/BSTC interface (V_b,t=2.5 V) and bottom BSTC/STN interface (V_b,b=1.1 V). Such different built-in voltages lead to the presence of an internal electric field, which results in asymmetric electric characteristics in Pt/BSTC/STN capacitor.