International Journal of Modern Physics B

In this paper, the review of recent results of calculations of surface relaxations, energetics, and bonding properties for ABO₃ perovskite (001), (011) and (111) surfaces using mostly a hybrid description of exchange and correlation is presented. Both AO and BO₂-terminations of the nonpolar (001) surface and A, BO, and O terminations of the polar (011) surface, as well as B and AO₃-terminations of the polar (111) surface were considered. On the AO-terminated (001) surface, all upper-layer A atoms relax inwards, while all second layer atoms relax outwards. For the BO₂-terminated (001) surface, in most cases, the largest relaxations are on the second-layer metal atoms. For almost all ABO₃ perovskites, the surface rumpling is much larger for the AO-terminated than for the BO₂-terminated (001) surface, but their surface energies are always quite similar. In contrast, different terminations of the (011) ABO₃ surface lead to very different surface energies for the O-terminated, A-terminated, and BO-terminated (011) surface, respectively. A considerable increase in the Ti–O or Zr–O, respectively, chemical bond covalency near the (011) surface as compared both to the bulk and to the (001) surface in ABO₃ perovskites were predicted. According to the results of ab initio calculations for Nb doped SrTiO₃, Nb is a shallow donor; six nearest O ions are slightly displaced outwards from the Nb ion. The F center in ABO₃ perovskites resembles electron defects in the partially-covalent SiO₂ crystal rather than usual F centers in ionic crystals like MgO and alkali halides. The results of calculations for several perovskite KNb_xTa_1-xO₃ (KTN) solid solutions, as well as hole and electron polarons in ABO₃ perovskites are analyzed.

The absorption edge shifted to long wavelength direction and short wavelength direction of two opposite experimental conclusions have been reported, when the band-gap and absorption spectra of Nb-doped anatase TiO₂ were studied. In order to solve this contradiction, the electronic structure and the optical property of Nb heavy doped anatase TiO₂ have been studied by the first-principles plane-wave ultrasoft pseudopotential method based on the density functional theory with +U method modification. The calculated results indicate that the higher the Nb-doping is, the higher the total energy is, the worse the stability is, the higher the formation energy is, the more difficult the doping is, the wider the optical band-gap is, the more obvious the absorption edge shifting to short wavelength direction is, the lower the absorptivity and the reflectivity is, which is in agreement with the experimental results. The reasonable interpretation of the contradiction has been reported in this paper, too.

Au/n-type 4H-SiC diodes were fabricated and their electrical characteristics have been investigated by using the capacitance/conductance-voltage-frequency (C–V–f and G/w–V–f) measurements method at room temperature. The main parameters such as the doping atoms (N_D), diffusion potential (V_D) and barrier height (Φ_B(C–V)) values were obtained from the reverse bias C^-2–V plots for each frequency. C and G/ω values decrease with increasing frequency as almost exponential for each voltage and these changes in C and G/ω are considerably high at low frequencies due to the contribution of surface states (N_ss) to the measured C and G/ω. The resistivity (Ri) versus V plots were also obtained by using the C and G data and they exhibit an anomalous peak which is corresponding to the depletion region at each frequencies and its magnitude decreases with increasing frequency. The energy density distribution of N_ss and their relaxation time (τ) were obtained from the conductance method and they range from 1.53 × 10¹⁴ eV^-1 cm^-2 to 1.03 × 10¹⁴ eV^-1 cm^-2 and 1.29 × 10^-4 s to 3.35 × 10^-5 s, respectively, in the energy range of (0.585-E_v) – (0.899-E_v) eV. The voltage dependent of N_ss was also obtained from C_HF – C_LF method. The obtained value of N_ss is about 10¹⁴ eV^-1 cm^-2 order and these values are suitable for an electronic device.

The structural, stability and electronic properties of C15-AB₂ (A = Ti, Zr; B = Cr) isomeric intermetallic compounds were systematically investigated by using density functional theory (DFT) and plane-wave pseudo-potential (PW-PP) method. The macroscopic properties including the lattice constant, bulk modulus and stability for these compounds were studied before and after hydrogenation. For parent compounds, the enthalpy of formation was evaluated with regard to their bulk modules and electronic structures. After hydrogenation of compounds at different interstitial tetrahedral sites (A₂B₂, A₁B₃, B₄), a volume expansion was found for hydrides. The stability properties of hydrides characterized the A₂B₂ sites as the site preference of hydrogen atoms for both compounds. The Miedema's "reverse stability" rule is also satisfied in these compounds as lower the enthalpy of formation for the host compound, the more stable the hydride. Analysis of microscopic properties (electronic structures) after hydrogenation at more stable interstitial site (A₂B₂) shows that the H atoms interact stronger with the weaker (or non) hydride forming element B (Cr) than the hydride forming element A (Ti/Zr). A correlation was also found between the stability of the hydrides and their electronic structure: the deeper the hydrogen band, the less stable the hydride.

This article shows the characteristics on a-SiGe:H based solar cells with various structures. Based on these results, we proposed the a-SiC:H/a-SiGe:H hetero-junction thin film solar cells with a-Si:H buffer layer at the p/i interface to improve the performance of pin-type a-SiGe:H based solar cell. All films of amorphous materials were deposited by 13.56 MHz PECVD method. The effects of deposited parameters on the characteristics of a-SiC:H and a-SiGe:H films have been investigated by ultraviolet–visible–near infrared (UV–VIS–NIR) spectrophotometer. The various values of V_oc, J_sc, FF and η were measured under 100 mW/cm² (AM 1.5) solar simulator irradiation. In the proposed structure, we achieved a higher conversion efficiency than general a-SiGe:H solar cell and a-SiC:H/a-SiGe:H hetero-junction thin film solar cells without a-Si:H buffer layer at the p/i interface. Based on results obtained from this study, we discuss the roles of a-Si:H buffer layer in a-SiC:H/a-SiGe:H hetero-junction thin film solar cells.

As part of a series of experiments based on the application of ultrasound to structural materials and metal alloys, a sample of alpha iron (Ferrite) was subjected to ultrasounds in order to investigate the possible emission of nuclear particles and the possible formation of new isotopes. The sample was subsequently subjected to Instrumental Neutron Activation Analysis (INAA) to highlight the possible transformations that occurred, or the formation of new natural isotopes or the variation of the natural isotopic composition of the material.

Transition metal doped topological insulators (TI) Co_xBi_2-xSe₃ were prepared and the phase structure, electrical and magnetic transport properties, Hall mobility and Hall resistivity were studied. The lattice constant c decreased with the increasing Co concentration. All samples exhibited weak metallic resistivity. With increasing Co concentration, the value of ρ_xx increased, reaching a maximum at x = 0.02, and then decreased. The resistivity data could be fitted by different formulas below and above 30 K in all samples result from different mechanism corresponding to different temperature regions. The magnetoresistance (MR) of Bi₂Se₃ TI showed a change dependence of concentration. The MR was enhanced by small quantity of Co doping, attaining a maximum value in Bi_1.98Co_0.02Se₃ crystal; but MR was suppressed in high-doped sample (x > 0.02). Those results were ascribed to the rivalry between the phonon scattering effect and the magnetic ordering in single crystals caused by Co ions. For low-doped samples doped, the contribution of phonon scattering was much higher than that of magnetic ordering, leading to larger MR value. However, in samples doped with higher Co concentrations, the magnetic ordering effect became dominant and caused the MR decreases. The MR increased with decreasing temperature and increasing magnetic field. The Hall mobility data indicated that incorporating Co in Bi₂Se₃ single crystal can reduce native defect of Bi₂Se₃.

The dynamics of Davydov's model of α-helical proteins is considered by including the influence of inhomogeneities in the monomer units. Using the D2 ansatz for the exciton–phonon quantum state, the model Hamiltonian is transformed into a pair of classical lattice equations, which is further reduced in the continuum limit to a sole perturbed nonlinear Schrödinger (NLS) equation. The results of the perturbation theory of this equation show that the inhomogeneities in the localized form do not affect the velocity and amplitude of the solitary waves during propagation. We employ also the sine–cosine functions method to construct the exact solitary wave solutions in the presence of a variety of nonlinear inhomogeneities such as biquadratic, exponential and periodic inhomogeneities and it reveals that the coherent energy transport in the α-helical proteins is very much influenced by these nonlinear inhomogeneities.

Deterministic network models have been attractive media for discussing dynamical processes' dependence on network structural features. On the other hand, the heterogeneity of weights affect dynamical processes taking place on networks. In this paper, we present a family of weighted expanded Koch networks based on Koch networks. They originate from a r-polygon, and each node of current generation produces m r-polygons including the node and whose weighted edges are scaled by factor w in subsequent evolutionary step. We derive closed-form expressions for average weighted shortest path length (AWSP). In large network, AWSP stays bounded with network order growing (0 < w < 1). Then, we focus on a special random walks and trapping issue on the networks. In more detail, we calculate exactly the average receiving time (ART). ART exhibits a sub-linear dependence on network order (0 < w < 1), which implies that nontrivial weighted expanded Koch networks are more efficient than un-weighted expanded Koch networks in receiving information. Besides, efficiency of receiving information at hub nodes is also dependent on parameters m and r. These findings may pave the way for controlling information transportation on general weighted networks.