Electronic Properties of a New Structure of BC2N and Its Intercalation with Magnesium: Possibility for a New High-Tc Superconductor

We propose a new structure for the graphite-like BC₂N, other than the one proposed in Liu et al. (Phys. Rev. B 39, 1760, 1989). We compare it with the older structure and show that it has a lower energy. The band structure calculations of a single sheet of this new structure show that it is a semiconductor with a very small band gap of 0.25 eV, whereas an AA stacking of BC₂N layers of this structure behaves like a metal. Because of the similarity of this structure with the boron layers in MgB₂, we propose to intercalate the layers of the new structure of BC₂N with the magnesium atoms to obtain Mg₂BC₂N. The band structure calculations of this new structure show an unusually large metallic density of states at the Fermi level, much higher than that of MgB₂. This leads us to expect Mg₂BC₂N to be a superconductor with a higher T _c.     

Synthesizing and Physical Properties of GdPr-123 High-Tc Superconductor

Ceramic Gd_1-xPr_xBa₂Cu₃O_7-y (GdPr-123) high-T_c superconductors have been synthesized by the standard solid state reaction method and characterized by XRD, SEM, TGA, and DT techniques in the range of x, 0.0 ≤ x ≤ 1.0 Samples have orthorhombic structure with Pmmm symmetry and there is a small percentage (less than 1%) of the Ba sites occupied by Pr ions in some Pr-rich samples. In these samples a small trace of BaCuO₂ and Pr BaO₃ secondary phases persist with the main peaks at 2θ = 29.3° and 28.9° respectively. Microstructure analysis indicates a uniform grain size distribution with a mean size of 5 μm. No significant change of grain size is shown throughout the range of x studied. The valence of Pr and Cu were determined by magnetization measurements in the temperature range 100 to 250°K. These data reveal a valence of 3.86+ for Pr in all samples independant of x. The similarity of the superconducting and insulating properties in this system to those of the oxygen deficient RBa₂Cu₃O₇ (R-123) (R: Y or rare earth) system implies that the mechanism of superconductivity in high-T_c. superconductors cannot be two-dimensional, even though the superconductive current occurs in two dimensions.     

Ta2N3电子结构与光学性质的第一性原理研究

基于密度泛函第一性原理的GGA方法计算研究了Ta2N3的能带结构、态密度、分态密度和光学性质.计算结果表明,Ta2N3具有明显的金属能带结构特征,且在费米能级附近,Ta的5d态与N的2p态杂化,Ta-N以共价键相互作用.Ta2N3的静态介电常数为77.428,静态的折射率n0为8.88,而介电函数的虚部随能量的增加而减小.Ta2N3多晶体的反射系数在0～1.65eV区域随能量的增加而逐渐减小,在1.65eV附近达极小值,此后随能量的增加而增大,但在15eV时发生陡降.Ta2N3多晶体的吸收系数数量级达105 cm-1,且在高能区对光子的吸收较少,其电子能量损失谱(EELS)的共振峰在15eV处,与此能量时反射系数的陡降相对应.     

Electronic Structure and Magnetic Properties of Zinc-Blende Co-Doped GaN with N Vacancies

The electronic structure and magnetic properties of zinc-blende structure of (Ga,Co)N phase with N vacancy defects are investigated using the Korringa–Kohn–Rostoker (KKR) method combined with coherent potential approximation (CPA). The results show that (Ga,Co)N phase is ferromagnetically polarized with an enhancement of the polarization and that the electronic structure can be modified simply by changing the concentration of N vacancies. Moreover, the (Ga,Co)N with high density of N vacancies shows a drastic increase of the magnetic moment of cobalt in the parent GaN compound, to reach a maximum value of 1.7 μ_B/Co at 8 at.%, which is in good agreement with the experimental values reported in the literature.     

High Pressure Effects on Electronic and Magnetic Properties of LaOFeAs Superconductor

The effect of pressure has been studied on structural and electronic properties of LaOFeAs high-T _c superconductor by ab initio density functional theory by using pseudopotential Quantum Espresso code. The lattice parameters and ionic positions in the ambient pressure and some high pressure up to 20 GPa have been calculated. The obtained data versus the simple scaling relation for the ionic positions and distances for mechanical pressures have been discussed. The results of band structure and magnetic moment calculations of this compound versus the applied pressure are presented in this paper. The results are compared with the other experimental and computational data in the literature.     

Electronic Properties and Lattice Dynamics Studies of the Nickel-Based Superconductor ThNiAsN

We perform theoretical studies of the newly discovered nickel-based pnictide ThNiAsN. The obtained large density of states may explain the normal-state Sommerfeld coefficient observed by former experiments. The obtained band structure and Fermi surfaces are rather two-dimensional with a small hole-like Fermi surface around X point and three electron-like ones around M point, which share great similarities with LaNiAsO. Meanwhile, the lattice dynamics of ThNiAsN are also studied within density functional perturbation theory (DFPT). The electron-phonon coupling constant is 0.67, suggesting that ThNiAsN is a phonon-mediated superconductor. In the end, as a prototype of nickel-based superconductor, we obtain an effective tight-binding model by means of the maximally localized Wannier function (MLWF). Based on this model, the nesting properties have been studied using Lindhard function for nominal doping and hole doping. We find the hole doping will introduce better nesting properties, making this system a potential candidate to develop magnetic and unconventional superconducting instabilities.     

Magnetic Properties of a Superconductor with no Inversion Symmetry

We study the magnetic properties of a superconductor in a crystal without symmetry, in particular how the lack of this symmetry exhibits itself. We show that, though the penetration depth itself shows no such effect, for suitable orientation of magnetic field, there is a magnetic field discontinuity at the interface which shows this absence of symmetry. The magnetic field profile of a vortex in the x − y plane is shown to be identical to that of an ordinary anisotropic superconductor to second order in a small parameter . For a vortex along z, there is an induced magnetization along the radial direction.     

Fullerene Single Crystals: Structure and Electronic Properties

This work covers an advanced investigation of the single crystal growth process for fullerenic materials and the contribution of Raman and IR spectroscopy to the understanding of the fcc high temperature rotor phase and the low temperature sc ratchet phase of pristine C₆₀. In addition substantial research contributions to the understanding of alkali metal doped fullerenes are presented.

Alkali metal doped crystals are studied with particular attention to the line broadening in the superconducting A₃C₆₀ phase and to the phases with stoichiometry A₁C₆₀. For the former electron-phonon coupling constants could be determined for all fivefold degenerated gerade modes and a total coupling strenght of λ = 0.90 was found. For the latter the different experimental conditions for obtaining the phase separated intermediate state or alternatively the orthorhombic polymeric state are analysed. Vibrational spectra for the polymeric state obtained from phototransformation and from mono alkali metal doping are found to be characteristically different, mainly with respect to the intensity of the Raman or IR response. Finally, research results obtained from the single crystals by various other techniques like thermal expansion or low frequency elastic deformation are summarized.     

Fullerene Single Crystals: Structure and Electronic Properties

Abstract

This work covers an advanced investigation of the single crystal growth process for fullerenic materials and the contribution of Raman and IR spectroscopy to the understanding of the fcc high temperature rotor phase and the low temperature sc ratchet phase of pristine C₆₀. In addition substantial research contributions to the understanding of alkali metal doped fullerenes are presented.

Alkali metal doped crystals are studied with particular attention to the line broadening in the superconducting A ₃C₆₀ phase and to the phases with stoichiometry A₁C₆₀. For the former electron-phonon coupling constants could be determined for all fivefold degenerated gerade modes and a total coupling strenght of λ = 0.90 was found. For the latter the different experimental conditions for obtaining the phase separated intermediate state or alternatively the orthorhombic polymeric state are analysed. Vibrational spectra for the polymeric state obtained from phototransformation and from mono alkali metal doping are found to be characteristically different, mainly with respect to the intensity of the Raman or IR response. Finally, research results obtained from the single crystals by various other techniques like thermal expansion or low frequency elastic deformation are summarized.     

Transport Properties of a Single Crystal of Pressure-Induced Superconductor CePtSi2

We synthesized single crystals of pressure-induced superconductor CePtSi₂ by an In-flux method, and measured its electrical resistivity ρ under hydrostatic pressure. At ambient pressure, ρ shows two maxima at T ₁=3.7 K and T ₂=33 K, and a downward bend at the Néel temperature T _N. With increasing pressure T _N decreases and disappears above 1.11 GPa, as reported in a polycrystalline sample. On the other hand, the maximum at T ₁ becomes a shoulder above P _v=1.21 GPa, and begins to increase with increasing pressure, indicating the beginning of a valence crossover.     

Optical Properties and Electronic Structure of PdO

The complete set of optical spectra is calculated for the first time for crystalline PdO in the range 2–5.5 eV for the E c and E || c polarizations using experimental dielectric permittivity data. The main features of the spectra are discussed. The origin of the strongest peaks in the spectra is inferred from comparison with band-structure calculations and polarized dielectric permittivity spectra.     

Properties of Photonic Band Gaps in Superconductor Multilayer Structure with Periodically Varying Ambient Temperature

The properties of the reflectance for a superconductor multilayer structure with periodically varying ambient temperature have been theoretically investigated by the transfer matrix method (TMM). From the numerical results, it has been shown that such system has the photonic band gap (PBG) properties of photonic crystals (PCs), so it can also be called superconductor photonic crystals (SPCs). It is found that the locations and bandwidths of PBGs can be modulated by the incident angle. The frequency ranges and central frequencies of PBGs can be tuned by the temperature and thickness of superconductor layer A (higher temperature superconductor layer), respectively. The bandwidths of PBGs can be notably enlarged with increasing the temperature of superconductor layer A. The frequency ranges of PBGs can be controlled by increasing the thickness of superconductor layer A, and the more PBGs appear. The damping coefficient of superconductor and the number of periods have little effect on the bandwidths of PBGs under low-temperature conditions. It is shown that this kind of SPCs has potential applications in filters, microcavities, and fibers, etc.     

Preparation of a Fine Powder of a Bi-Based High-Tc Ceramic Superconductor by Wet Ball-Milling: Effect of Fluids on Grinding Performance

A Bi-based high-Tc ceramic superconductor (Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O _x ) was milled in various fluids and the effect of fluids on the crystal structure and morphology of the milled powder was studied. It was found that methyl acetate is an excellent fluid for wet grinding without decomposition of the 110 K phase. Dehydrated pure ethanol acts as a reagent to make cracks on the specific surface of Bi-based high-Tc superconducting oxide crystals during the operation, and this yields plate-like particles with many 002 faces.     

Ab Initio Study of Hydrogen Interaction with Defective BC2N Nanotubes

The spin polarized density functional theory is used to investigate the incorporation of hydrogen adatoms and the interaction between molecular H₂ with antisites and vacancies in both zigzag (4,0) and armchair (3,3) BC₂N nanotubes. We find that the presence of antisites and vacancies increases the binding energy of hydrogen adatoms on the tube surface. In the most stable antisites (C_B, C_N, N_CI and B_CII), the hydrogen adatoms bind preferentially on carbon atoms of the defective site (C_B, and C_N) or closer to it (N_CI and B_CII). For a single adsorbed H, the calculated binding energies show that the H adsorption on a carbon vacancy (V_CII) is the most stable site with a binding energy of ?4.23?eV. The adsorption of a second H atom near the previous one is an exothermic process compared to of a single H₂ molecule physisorbed on the nanotube surface.     

金属卤化物石墨层间化合物结构与性能研究新进展

金属卤化物石墨层间化合物 (MXGIC)是一种具有广泛应用前景的新型功能材料 ,因其突出的稳定性和优良的物理化学性能而引起了材料界的关注。特别是近年来成为GIC研究的一个热点领域。本文评述了MXGIC的结构、性能的最新研究成果 ,展望了GIC的未来发展方向。     

金属Pt的电子结构和物理性质

依据纯金属单原子理论(OA)确定了面心立方结构(fcc)金属Pt的电子结构为[Xe](5dn)6.48(5dc)2.02-(6sc)1.48(6sf)0.02,由这种电子结构计算的晶格常数、结合能等与实验值很符合,并与第一原理方法计算的结果较符合,在此基础上初步解释了金属Pt的电子结构与其催化性能的关系,通过计算得到了fcc金属Pt的势能曲线、热膨胀系数、晶格常数、比热、焓和Gibbs能随温度变化的曲线.     

Full Relativistic Electronic Structure and Fermi Surface Sheets of the First Honeycomb-Lattice Pnictide Superconductor SrPtAs

We report full-potential density functional theory (DFT)-based ab initio band structure calculations to investigate electronic structure properties of the first pnictide superconductor with a honeycomb-lattice structure: SrPtAs. As a result, electronic bands, density of states, Fermi velocities and the topology of the Fermi surface for SrPtAs are obtained. These quantities are discussed in comparison to the first available experimental data. Predictions for future measurements are provided.