Study on the electronic structure and Fermi surface of 3d-transition-metal disilisides CoSi2

We have investigated the electronic structure, the momentum density distribution ρ(p), and the Fermi surface FS of single crystals of the Pyrite-type 3d-transition-metal disilisides CoSi₂. The band structure calculations, the density of states DOS, and the FS, in vicinity of Fermi level, have been carried out using the full-potential linearized augmented plane wave FP-LAPW method within generalized gradient approximation GGA for exchange and correlation potential. The measurements have been performed via the 2D angular correlation of annihilation radiation ACAR experiments. ρ(p) has been reconstructed by using the Fourier transformation technique. The FS has been reconstructed within the first Brillion zone BZ through the Locks, Crisp, and West LCW folding procedures. The analysis confirmed that Si 3sp states hybrid with both Co 3d–t _2g and Co 3d–e _g states around Γ and X points, respectively. The dimensions of the FS of CoSi₂ have been compared to the present calculations as well as to the earlier results.     

Ultrasound Imaging of Treatment-Needed Cardiac Rejection with CD4-Targeted Nanobubbles

Acute cardiac allograft rejection (ACAR) is one of the main reasons for mortality after heart transplantation. The timely diagnosis of treatment-needed ACAR is of great importance in the clinic. Molecular imaging may make accurate diagnoses at the early stage to visualize the activity of specific pathological processes. The amount of CD4⁺-lymphocyte infiltration is a crucial index in ACAR grading. CD4-targeted nanobubbles (NB_CD4) are fabricated to assess the infiltrating CD4⁺ lymphocytes, while isotype control nanobubbles (NB_Iso) are also fabricated. Animal models with ACAR grades from 0R to 3R are established. Ultrasound molecular imaging (USMI) is performed to distinguish different rejection grades with NB_CD4. In the grade 2R and 3R groups, the USMI signals of NB_CD4 are significantly higher than those of NB_Iso. The signals of NB_CD4 in the 2R group are significantly lower than those in the 3R group and significantly higher than those in the 1R and 0R groups. Moreover, the signals of NB_CD4 are strongly correlated with the rejection grades and the number of infiltrating CD4⁺ lymphocytes. As asymptomatic rejection with 2R or higher grades should be seriously treated, USMI with NB_CD4 may be a new approach to timely diagnosis of treatment-needed ACAR.     

Characterization of the Fermi surface of the organic superconductor - by measurements of Shubnikov-de Haas and angle-dependent magnetoresistance oscillations and by electronic band-structure calculations

The electronic structure of the quasi two-dimensional (2D) organic superconductor -(ET)₂SF₅CH₂CF₂SO₃ was examined by measuring Shubnikov-de Haas (SdH) and angle-dependent magnetoresistance (AMRO) oscillations and by comparing with electronic band-structure calculations. The SdH oscillation frequencies follow the angular dependence expected for a 2D Fermi surface (FS), and the observed fundamental frequency shows that the 2D FS is 5% of the first Brillouin zone in size. The AMRO data indicate that the shape of the 2D FS is significantly non-circular. The calculated electronic structure has a 2D FS in general agreement with experiment. From the temperature and angular dependence of the SdH amplitude, the cyclotron and band effective masses were estimated to be and ,where g is the conduction electron g factor and the free electron mass. The band effective mass is estimated to be from the calculated electronic band structure. Received: 3 March 1997 / Revised: 5 May 1997 / Received in final form: 5 November 1997 / Accepted: 10 November 1997     

Positron annihilation study of the influence of doping on the 3<Emphasis Type="Italic">d</Emphasis> electron states in the Ni<Subscript>3</Subscript>Al intermetallic compound

The 3d electron states in Ni₃Al single crystals doped with Fe, Co, and Nb have been investigated using angular correlation of annihilation radiation (ACAR). The ACAR spectra contain information on the momentum distribution of valence electrons and strongly bound 3d electrons of the intermetallic compound. It has been established that the positrons in the Ni₃Al crystals predominantly annihilate in the nickel sublattice from delocalized states. The doping of the compound by the third element leads to a variation in the momentum distribution of Ni 3d electrons due to the change in the character of interatomic bonds. An analysis of the momentum distribution has demonstrated that the niobium atoms increase the covalent component of the chemical bond as compared to the binary compound due to the d _Nb-d _Ni hybridization. The doping with cobalt atoms also enhances the tendency toward the formation of the covalent bond. At the same time, iron atoms have a weak effect on the electronic structure of the intermetallic compound.     

Different ways of dealing with Compton scattering and positron annihilation experimental data

Different ways of dealing with one-dimensional (1D) spectra, measured e.g., in the Compton scattering or angular correlation of positron annihilation radiation (ACAR) experiments, are presented. Using the example of divalent hexagonal close packed metals, we show what kind of information on the electronic structure one can get from 1D profiles interpreted in terms of either 2D or 3D momentum densities.2D and 3D densities are reconstructed from merely two and seven 1D profiles, respectively. Applied reconstruction techniques are particular solutions of the Radon transform in terms of orthogonal Gegenabauer polynomials. We propose their modification connected with so-called two-step reconstruction.The analysis is performed both in the extended p and reduced k zone schemes. It is demonstrated that if the positron wave function or many-body effects are strongly momentum dependent, analysis of 2D densities folded into k space may lead to wrong conclusions concerning the Fermi surface. In the case of 2D ACAR data in Mg, we found very strong many-body effects. PACS 71.18.+y; 13.60.Fz; 87.59.Fm     

Positron annihilation study of the electronic structure of CeCu6 and LaCu6

Summary The results of a prelimianry study of the electronic structure of the heavy-fermion system CeCu₆ via the two-dimensional angular correlation of electron-positron annihilation radiation (2D ACAR) technique were compared with those obtained for the isostructural reference non-f-electron material LaCu₆. The spectra relating to the 2 compounds displayed extremely similar anisotropic structures, with features related to the symmetry of the Brillouin zone. At present, these structures cannot however be attributed to Fermi-surface or positron wave function effects or to anisotropies of some full valence band. In the case of LaCu₆, data processing prior to the LCW procedure via new filtering techniques revealed a reasonably good agreement between the LCW data and LAPW Fermi-surface calculations while in the case of CeCu₆ it appeared difficult to relate the results, still of controversial interpretation, to any theoretical model.     

LCAO model for 3D Fermi surface of high-Tc cuprate Tl2Ba2CuO6+δ

A simple analytical formula for three-dimensional Fermi surface (3D FS) of ${\rm Tl}_2{\rm Ba}_2{\rm CuO}_{6+\delta}$ is derived in the framework of LCAO approximation spanned over Cu 4s, Cu 3d $3d_{x^2 - y^2 }$ , O 2p_x and O 2p_y states. This analytical result can be used for fitting of experimental data for 3D FS such as polar angle magnetoresistance oscillation. The model takes into account effective copper-copper hopping amplitude t_ss between Cu 4s orbitals from neighbouring ${\textrm{CuO}_2}$ layers. The acceptable correspondence with the experimental data gives a hint that the t_ss amplitude dominates in formation of coherent 3D FS, and other oxygen-oxygen and copper-oxygen amplitudes are rather negligible. For absolute determination of the hopping parameters a simple electronic experiment with a field effect transistor type microstructure is suggested. The thin superconductor layer is the source-drain channel of the layered structure where an AC current is applied.     

Electron–positron momentum density distribution of Gd from 2D ACAR data via Maximum Entropy and Cormack’s methods

A successful application of the Maximum Entropy Method (MEM) to the reconstruction of electron–positron momentum density distribution in gadolinium out of the experimental of 2D ACAR data is presented. Formally, the algorithm used was prepared for two-dimensional reconstructions from line integrals. For the first time the results of MEM, applied to such data, are compared in detail with the ones obtained by means of Cormack’s method. It is also shown how the experimental uncertainties may influence the results of the latter analysis. Preliminary calculations, using WIEN2k code, of band structure and Fermi surface have been done as well.     

Comparison of PD BLADE with fat saturation (FS), PD FS and T2 3D DESS with water excitation (WE) in detecting articular knee cartilage defects

The purpose of this study, is to compare the sequences: 1) proton density (PD) BLADE (BLADE is a PROPELLER-equivalent implementation of the Siemens Medical System) with fat saturation (FS) coronal (COR), 2) PD FS COR, 3) multi-planar reconstruction (MPR) with 3 mm slice thickness and 4) multi-planar reconstruction (MPR) with 1.5 mm slice thickness, both from the T2 3D-double-echo steady state (DESS) with water excitation (WE) sagittal (SAG), regarding their abilities to identify changes in the femorotibial condyle cartilage in knee MRI examinations. Thirty three consecutive patients with osteoarthritis (18 females, 15 males; mean age 56 years, range 37–71 years), who had been routinely scanned for knee examination using the previously mentioned image acquisition techniques, participated in the study. A quantitative analysis was performed based on the relative contrast (ReCON) measurements, which were taken both on normal tissues as well as on pathologies. Additionally, a qualitative analysis was performed by two radiologists. Motion and pulsatile flow artifacts were evaluated. The PD BLADE FS COR sequence produced images of higher contrast between Menisci and Cartilage, Fluid and Cartilage, Pathologies and Cartilage as well as of the Conspicuousness Superficial Cartilage and it was found to be superior to the other sequences (p < 0.001). The sequences T2 3D DESS 1.5 mm and T2 3D DESS 3 mm were significantly superior to the PD BLADE FS COR and the PD FS COR sequences in the visualization of Bone and Cartilage and the Conspicuousness Deep Surface Cartilage. This pattern of results is also confirmed by the quantitative analysis. PD FS BLADE sequences are ideal for the depiction of the cartilage pathologies compared to the conventional PD FS and T2 3D DESS sequences.     

Role of ion energy flux on the structural and morphological properties of silicon oxy-nitride composite films deposited by plasma focus device

ABSTRACT

Crystalline silicon oxy-nitride (SiON) composite films are deposited on Si substrate for multiple (5, 15, 25 and 50) focus shots (FS) by plasma focus device. The X-rays diffraction patterns reveal the development of various diffraction peaks related to Si, Si₃N₄, and SiO₂ phases which confirms the formation of SiON composite film. The intensity of Si₃N₄ (1 0 2) plane is linearly increased with the increase of FS. The Si₃N₄ (1 0 2) phase does not nucleate for 5 FS. Raman analysis confirms the formation of β–Si–N phase. Raman and Fourier transform infrared spectroscopy analysis reveals that the strength of chemical bonds like Si–N, Si–O formed during the deposition process of SiON composite films is associated with the bonds intensity which in turn depends on the number of FS. The field emission scanning electron microscopic analysis reveals that the surface morphology like size, shape and distribution of micro/nano-dimensional particles, film compactness and the formation of micro-rods, micro-teethes and micro-tubes of SiON composite films is entirely associated with the rise in substrate surface transient temperature which in turn depends on the increasing number of FS. The EDX spectrum confirms the presence of Si (22.5?±?4.7 at. %), N (13.4?±?4.5 at. %) and O (54.7?±?11.3 at. %) in the SiON composite film. The thickness of SiON composite film deposited for 50 FS is found to ～15.47?µm.     

Direct observation of the high-momentum components of the conduction band of copper by two-dimensional angular correlation of annihilation radiation

A high precision study of the two-dimensional angular correlation (2D ACAR) in copper using a multicounter apparatus reveals directly the high-momentum components of the conduction band. The general features of the anisotropy of these “umklapp annihilations” are discussed.     

Fermi surface of the noble metals

We report calculations of extremal areas of four Fermi surface (FS) orbits of the noble metals using the linear muffin tin orbital method in the atomic sphere approximation. Our calculations indicate that thel = 3 potential parameters and increase in the number of k-points in the Brillouin zone (BZ) summation from 240 to 916 have no significant effect on the FS. All calculations were performed self-consistently including up tol = 2 potential parameters and with 240 k points in the BZ summation. Calculations were performed with the exchange, correlation potentials (XCP) of Barth-Hedin, Barth-Hedin modified by Janak, SlaterX _α, and the Vosko-Wilk-Nussair. Results compared with other theoretical calculations indicate that none of the above XC potentials give an accurate representation of the FS for all the noble metals. We feel that the inclusion of the non-locality of XCP may give a better account of the FS geometry.     

吸收制冷循环极限制冷温度研究

提出了吸收制冷循环的极限制冷温度概念,以自行复叠吸收制冷(ACAR)循环为例分析了多种因素对极限制冷温度和COP等性能参数的影响。分析结果表明制冷剂的配比是影响ACAR循环极限制冷温度等性能的关键因索,为此计算得到了理沦最佳制冷剂配比。同时,分析了传统吸收制冷循环的特性,并在相同条件下和ACAR循环进行了比较,结果表明ACAR循环可以获得低得多的制冷温度,具有高得多的COP,用于深度冷冻具有独特的优势。     

Influence of Fermi surface topology on kinetic properties of Fe-Co alloys in the domain close to an equiatom composition

The electron energy spectrum for the superstructure FeCo is computed by a symmetrized Green's function method. A curve of the density of the electron states N() and the Fermi surface (FS) section is constructed for the ferromagnetic state obtained by a rigid shift in conformity with the value of the atomic magnetic moment of the alloy. A FS scheme is proposed, consisting of definite sheets belonging to different energy bands emerging at the Fermi level. A semiquantitative estimate is made of the magnitude of the FS area, and the contribution of separate FS sheets to the magnitude of the anomalous Hall R_s, Nernst-Ettingshausen Q_s and thermal emf S coefficients are examined. The nature of the change in the separate FS sections as the alloy composition changes is examined within the framework of the rigid band model, and a qualitative explanation is obtained for the concentration dependence of R_s and S in Fe-Co alloys.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 3–9, December, 1981.     

分子团簇表面吸附敏化ZnO纳米线的第一性原理研究

ZnO纳米线作为新型太阳能电池结构的重要组成部件之一, 其导电能力直接影响到太阳能电池的性能. 采用密度泛函理论平面波超软赝势方法, 计算并分析了C₂H₆O(乙醇)、 C₆H₅FS(4-氟苯硫酚)、 C₇HF₇S(4-(三氟甲基L)-2, 3, 5, 6-四氟硫代苯酚) 等小分子吸附的六边形结构\langle0001angle ZNWs (ZnO 纳米线) 的几何结构、 吸附能和电子结构. 首先, 通过几何优化得到了不同基团吸附的ZNWs的稳定结构, 同时吸附能计算结果表明C₇HF₇S吸附的体系结构最为稳定, 且吸附呈现放热反应; 其次, 为研究表面敏化对导电性能的影响, 计算了不同小分子基团吸附下的能带结构和态密度, 并利用能带理论分析了表面吸附敏化对禁带宽度的调控机理, 结果分析表明小分子表面吸附敏化对ZNWs的电学性能有一定的影响, 其中C₇H₇FS和C₆H₅FS分子均发生了不同程度的电荷转移.     

Effect of interstitial impurities on positronium formed in voids of vanadium

Angular correlation of two-photon annihilation radiation (ACAR) measurements have been performed to study the effect of interstitial impurities (O, C and D) on positronium (Ps) formation in irradiation-induced voids of vanadium. It has been observed that Ps formation is sensitively affected by doping with the interstitial impurities, irradiation dose, irradiation temperature, and also by post-irradiation annealing. The Ps component intensity is found to be related to segregation of the interstitial impurities and provides a new experimental method to study void surfaces.     

Magnetic field dependence ofT c and temperature dependence ofH c2 in layered superconductors with open normal state Fermi surface

A theoretical framework for treating the effects of magnetic fieldH on the pairing theory of superconductivity is considered, where the field is taken in an arbitrary direction with respect to crystal axes. This is applicable to closed, as well as open normal state Fermi surface (FS), including simple layered metals. The orbital effects of the magnetic field are treated semiclassically while retaining the full anisotropic paramagnetic contribution. Explicit calculations are presented in the limits |H| → |H _c2(T)|,T ∼ 0 andT →T _c(|H|), |H| ∼ 0. Effects of weak nonmagnetic impurity scattering, without vertex corrections, have also been taken into account in a phenomenological way. The final results for the case of open FS and layered materials are found to differ considerably from those of the closed FS. For example, an important parameter,h(T=0)=|H_c2(0)|/[-Tδ|H _c2 T|δT]_T{s0} for the case of a FS open ink _z-direction with thek _z-bandwidth, 4t ₃, very small compared to the Fermi energy,E _F, is close to 0.5906, compared to 0.7273 for the closed FS, in the clean limit. Analytical results are given for the magnetic field dependence ofT _c and the temperature dependence of H _c2 for a model of layered superconductors with widely open FS. For a set of band structure parameters for YBa₂Cu₃O₇ used elsewhere, we find reasonable values for the upper critical fieldH _c2(0), the slope (dH _c2/dT)_{T c0}, anisotropic coherence lengths ζ_i(T=0),i=x, y, z, and (dT _c/d|H|)_{|H| → 0}.     

Positronium interactions with Cu(II) Ions in water

Summary Positronium reactions with Cu(II) ions in aqueous solution were investigated by measuring the concentration dependence of lifetime spectra and of 1D-ACAR curves for the following Cu (II) complexes: [Cu(H₂O)₆[²⁺, [Cu(NH₃)₄ (H₂O)₂]²⁺ and [Cu(EDTA)(H₂O)]²⁻. The combined analysis of lifetime and ACAR data shows that Cu(II) ions:a) reduce the formation of positronium (inhibition effect),b) promote both ortho ⇌ para conversion and redox reactions. It was also found that inhibition and reaction rate constants are affected by the ligand type.     

Optimization and validation of a rapid high-resolution T1-w 3D FLASH water excitation MRI sequence for the quantitative assessment of articular cartilage volume and thickness

In view of follow up, survey and development of therapeutic strategies for osteoarthritis where cartilage deterioration plays an important role, a non invasive, reliable and quantitative assessment of the articular cartilage is desirable. The currently available high resolution T(1)-weighted (T1-w) 3D FLASH pulse sequences with frequency selective fat suppression are very time consuming. We have 1) optimized a high resolution T1-w 3D FLASH water excitation (WE) sequence for short acquisition time and cartilage visualization, and 2) validated this sequence for cartilage volume and thickness quantification. The spectral fat presaturation was replaced by selective water excitation. The flip angle of the WE sequence was optimized for the contrast to noise (C/N(cart)) ratio of cartilage. Sagittal datasets (voxel size: 0.31 x 0.31 x 2 mm(3)) of the knees of nine healthy volunteers were acquired both, with the 3D FLASH WE (17.2/6.6/30 degrees ) sequence (WE) and a previously validated 3D FLASH fat saturated (42/11/30 degrees ) sequence (FS). For validation of the WE sequence, cartilage volume, mean and maximal cartilage thickness of the two sequences were compared. Reproducibility was assessed by calculating the coefficient of variation (COV %) of 4 consecutive WE data sets in the volunteers. The acquisition time was reduced from 16'30" (FS) down to 7'14" for the WE sequence. Image contrast and visualization of the cartilage was very similar, but delineation of the basal layer of the cartilage was slightly improved with the WE sequence. A flip angle of 30 degrees provided the best C/N(cart) ratios (WE). Reproducibility (COV) was between 1.9 and 5.9%. Cartilage volume and thickness agreed within 4% between FS and WE sequence. The WE sequence allows for rapid, valid and reproducible quantification of articular cartilage volume and thickness, prerequisites for follow-up examinations. The reduced acquisition time (50% of FS) enables routine clinical application and thus may contribute to a broader assessment of osteoarthritis.