Single impurity Anderson model versus density functional theory for describing Ce L3 x-ray absorption spectra of CeFe2: resolution of a recent controversy

We resolved a recent controversy on the structure of the Ce L(3) x-ray absorption spectra (XAS) of CeFe(2); i.e., which of the single impurity Anderson model (SIAM) and the first-principles band calculations based on the density-functional theory (DFT) describes more appropriately the Ce 4f states and their contribution to the Ce L(3) XAS? For this purpose, we examined the core-hole effect in Ce L(3) XAS as an application of our new method taking advantage of resonant x-ray emission spectroscopy. Our result clearly shows that the Ce L(3) XAS structure is caused by the mixed valence 4f character revealed by the core-hole potential effect as indicated by SIAM, but denies the possibility that the L(3) XAS structure is caused by the 5d band structure with a very small core-hole effect as predicted by band calculations based on DFT.     

Unraveling the symmetry of the hole states near the Fermi level in the MgB2 superconductor

We use x-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) to study the fine structure at the K edge of boron in MgB(2). We observe in XAS a peak of width 0.7 eV at the edge threshold, signaling a narrow energy region with empty boron p states near the Fermi level. The changes in the near edge structure observed in EELS with direction of the momentum transfer imply that these states have p(x)p(y) symmetry. Our observations are consistent with electronic structure calculations indicating a narrow energy window of empty p(x)p(y) states that falls to zero at 0.8 eV above the Fermi level. The disappearance of the p(x)p(y) feature in EELS at grain boundaries suggests that this signature may become powerful in probing superconductivity at nanoscale.     

Diffraction of X rays at a Bragg angle of π/2 (Back reflection) with consideration of multiwave effects

The energy dependence of the back reflectivity in the dynamical diffraction of x rays at a Bragg angle of π/2 (back diffraction) in perfect crystals of cubic symmetry (silicon) is investigated theoretically. In this case strict backscattering is realized only under the conditions of multiple diffraction. The features of the influence of multiple diffraction on back reflection in the energy range near the nuclear resonance radiation energy of 14.41 keV for ⁵⁷Fe nuclei, specifically in the six-wave case, including the silicon (1,9,9) reflection (with an energy of 14.57 keV), which can be investigated experimentally with high energy resolution (1 meV) using synchrotron radiation and a monochromator developed for nuclear resonant absorption, are thoroughly studied. It is shown that the back reflectivity observed under the conditions of multiple diffraction has several maxima on the plot of its energy dependence with a value at each maximum smaller than half, in contrast to two-wave diffraction, where there is one maximum with a value close to unity. Zh. éksp. Teor. Fiz. 116, 940–952 (September 1999)     

First-principles calculations of K-shell x-ray absorption spectra for warm dense ammonia

The x-ray absorption spectroscopy is a powerful tool for the detection of thermodynamic conditions and atomic structures on warm dense matter. Here, we perform first-principles molecular dynamics and x-ray absorption spectrum calculations for warm dense ammonia, which is one of the major constituents of Uranus and Neptune. The nitrogen K-shell x-ray absorption spectrum(XAS) is determined along the Hugoniot curve, and it is found that the XAS is a good indicator of the prevailing thermodynamic conditions. The atomic structures at these conditions are ascertained. Results indicate that the ammonia could dissociate to NH_x(x = 0, 1, or 2) fragments and form nitrogen clusters, and the ratios of these products change with varying conditions. The contributions to the XAS from these products show quite different characteristics,inducing the significant change of XAS along the Hugoniot curve. Further model simulations imply that the distribution of the peak position of atomic XAS is the dominant factor affecting the total XAS.     

Unraveling orbital ordering in La0.5Sr1.5MnO4

Orbital ordering (OO) in the layered perovskite La0.5Sr1.5MnO4 has been investigated using the enhanced sensitivity of soft x-ray resonant diffraction at the Mn L edges. The energy dependence of an OO diffraction peak over the L(2,3) edges is compared to ligand-field calculations allowing a distinction between the influences of Jahn-Teller distortions and spin correlations. The energy dependence of the diffraction peak at the Mn L1 edge is remarkably different from that observed at the Mn K edge.     

Oxidation of Pt(111) under near-ambient conditions

The oxidation of Pt(111) at near-ambient O2 pressures has been followed in situ using x-ray photoelectron spectroscopy (XPS) and ex situ using x-ray absorption spectroscopy (XAS). Polarization-dependent XAS signatures at the O K edge reveal significant temperature- and pressure-dependent changes of the Pt-O interaction. Oxide growth commences via a PtO-like surface oxide that coexists with chemisorbed oxygen, while an ultrathin α-PtO2 trilayer is identified as the precursor to bulk oxidation. These results have important implications for understanding the chemical state of Pt in catalysis.     

Soft X-ray resonant magnetic diffraction

We have conducted the first soft x-ray diffraction experiments from a bulk single crystal, studying the bilayer manganite La2-2xSr1+2xMn2O7 with x=0.475 in which we were able to access the (002) Bragg reflection using soft x rays. The Bragg reflection displays a strong resonant enhancement at the L(III) and L(II) manganese absorption edges. We demonstrate that the resonant enhancement of the magnetic diffraction of the (001) is extremely large, indeed so large that it exceeds that of the nonresonant Bragg diffraction. Resonant soft x-ray scattering of 3d transition metal oxides is the only technique for the atomic selective measurement of spin, charge, and orbital correlations in materials, such as high temperature superconductors, colossal magnetoresistance manganites, and charge stripe nickelates.     

Structure and morphology of Cu/Ni film grown by electrodeposition method: A study of neutron reflectivity and AFM

We present detailed study of structure and interface morphology of an electrodeposited Cu/Ni film using X-ray diffraction, X-ray reflectivity, neutron reflectivity and atomic force microscopy (AFM) techniques. The crystalline structure of the film has been determined by X-ray diffraction, which suggest polycrystalline growth of the film. The depth profile of density in the sample has been obtained from specular X-ray and neutron reflectivity measurements. AFM image of the air-film interface shows that the surface is covered by globular islands of different sizes. The AFM height distribution of the surface clearly shows two peaks and the relief structure (islands) on the surface in the film, which can be treated as a quasi-two-level random rough surface structure. We have demonstrated that the detailed morphology of air-film interfaces, the quasi-two-level surface structure as well as morphology of the buried interfaces can be obtained from off-specular neutron reflectivity data. AFM and off-specular neutron reflectivity measurements also show that the morphologies of electrodeposited surface is distinctively different as compared to that of sputter-deposited surfaces in the sample.     

Thermal unbinding of highly oriented phospholipid membranes

We present a temperature dependent x-ray reflectivity study of highly oriented, fully hydrated multilamellar phospholipid membranes. Both the specular and diffuse (nonspecular) x-ray reflectivity were measured for dimyristoyl-sn-glycero-phosphocholine (DMPC) and oleoyl-palmitoyl-sn-glycero-phosphocholine (POPC) on silicon substrates in excess water. In this configuration the repeat distance as well as the fluctuation spectra can be determined as a function of temperature. Both model systems studied exhibit a discontinuous unbinding transition from a substrate bound, multilamellar state to a state of freely dispersed bilayers in water. In the unbound phase a single membrane remains on the substrate.     

Probing the local order of single phospholipid membranes using grazing incidence x-ray diffraction

We report the first grazing incidence x-ray diffraction measurements of a single phospholipid bilayer at the solid-liquid interface. Our grazing incidence x-ray diffraction and reflectivity measurements reveal that the lateral ordering in a supported DPPE (1, 2-Dipalmitoyl-sn-Glycero-3-Phosphoethanolamine) bilayer is significantly less than that of an equivalent monolayer at the air-liquid interface. Our findings also indicate that the leaflets of the bilayer are uncoupled in contrast to the scattering from free standing phosphatidylcholine bilayers. The methodology presented can be readily implemented to study more complicated biomembranes and their interaction with proteins.     

Anomalous x-ray reflectivity characterization of ion distribution at biomimetic membranes

Anomalous x-ray reflectivity measurements provides detailed information on ion binding to biomembrane surfaces. Using a monochromatic beam tuned to various x-ray energies at the Argonne National Laboratory Advanced Photon Source and utilizing a newly commissioned x-ray liquid surfaces reflectometer, measurements at and away from ion absorption edges allow determination of the distribution of these ions as they accumulate near lipid membranes. As a model, the interaction of Ba2+ ions with DMPA- (1,2-dimyristoyl-sn-glycero-3-phosphatidic acid) monolayers at the aqueous surface is studied. We find an unexpectedly large concentration of barium at the interface, approximately 1.5 per DMPA-, forming a Stern layer of bound ions and a cloud of less densely bound ions near the lipid headgroups. This result can be understood only if one assumes that bound cations are partially speciated, e.g., as BaOH+.     

Visualization of the surface roughness by x-ray scanning

In the x-ray region the reflectivity of a superpolished surface strongly depends on its roughness. This effect may be used to obtain a two-dimensional map of the roughness spatial distribution for flat surfaces with an average roughness height of the order of one nanometer or less. The method described in the paper lies in the illumination of the sample by a highly collimated x-ray beam, and a linear one-dimensional scanning of the sample with simultaneous registration of the specular component of the reflected beam by multielement linear detector. This method may be used to monitor the surface quality of silicon semiconductor wafers, computer hard disks, x-ray and laser mirror substrates etc.     

采用紫外光照法在磷酸盐玻璃中合成CdS/Ag复合微粒

掺杂有Ⅱ-Ⅵ族半导体纳米颗粒(如CdS)或者过渡金属(如Ag)的玻璃由于其较大的非线性光学效应而引起人们的极大兴趣,而同时掺杂有半导体/金属的复合微粒则可以进一步增强玻璃的三阶非线性效应,因此成为目前的研究热点。我们利用玻璃沉淀技术及随后的热处理和紫外光还原技术制备了含高浓度(1%)Ag微粒的玻璃,并采用X射线衍射分析了其物相,用高分辨扫描电镜分析了其形貌,以及测试了其吸收和发光性能。从CdS/Ag复合微粒的扫描照片可以发现晶粒均匀分布在玻璃中,尺寸约为1μm。X射线衍射发现经过热处理和紫外光照的样品衍射峰中含有CdS和Ag,而只进行热处理的样品则只含有CdS,未处理的样品则显非晶态。CdS/Ag复合微粒的吸收峰呈现典型的表面等离子共振峰(420nm)以及CdS的峰(600nm),只含有CdS微粒的样品的吸收峰则在480nm附近,未处理的样品在320nm附近有一个吸收峰,这可能是由于样品在快速冷却过程中的微小晶化造成的。只含有CdS微粒的样品有三个明显的发光峰,然而CdS/Ag复合微粒的发过峰则消失。我们提出了共振能量转移机制来解释该现象。讨论了紫外光照还原Ag微粒的机制。可以认为通过紫外光照,CdS表面的电子被激发出来还原Ag+,从而形成银颗粒,伴随着空穴则被表面缺陷所捕获。     

Ferrimagnetism in EuFe4Sb12 due to the interplay of f-electron moments and a nearly ferromagnetic host

We combine x-ray magnetic circular dichroism spectroscopy at Fe L2,3 edges, at Eu M4,5 edges, x-ray absorption spectroscopy (XAS) investigation of Eu valence, and local spin density calculations, to show that the filled skutterudite Eu0.95Fe4Sb12 is a ferrimagnet in which the Fe 3d moment and the Eu2+ 4f moment are magnetically ordered with dominant antiferromagnetic coupling. From Eu L3 edge XAS, we find that about 13% of the Eu have a formal valence of 3+. We ascribe the origin of ferrimagnetism at a relatively high transition temperature TC of 85 K in Eu0.95Fe4Sb12 to f-electron interaction with the nearly ferromagnetic [Fe4Sb12]2.2- host lattice.     

Theoretical analysis of the spectra of X-ray resonant magnetic reflectivity

We have developed the general computer code for the calculations of reflectivity with polarization analysis from an arbitrary anisotropic multilayer, which allows us to test different approaches for the Bragg reflectivity spectrum treatment. We have proved the validity of the usage of the Bragg peak position for the determination of the energy dependence of the diagonal component of resonant susceptibility tensor, but revealed the essential discrepancy of this procedure for the off-diagonal term determination. The explanation lies in the polarization mixture by multiple reflections at large glancing angles. By the model calculations we have shown that in L-MOKE geometry the observed difference of the integrated Bragg peak reflectivity for the (+) and (−) field direction is predominantly caused by the magnetization of the central part of resonant layer at the first-order Bragg peak, but it is very sensitive to the interface magnetization at the second-order Bragg peak.     

等离子体中X射线透过率分析及潜在通信应用研究

X射线具有波长短、光子能量高等特点,有望在等离子体环境中实现信息的有效传输.本文首先采用基于连续介质中的WKB分层法,研究了黑障条件下, X射线在非均匀等离子体鞘套中的透过率特性,仿真了不同等离子体电子密度和碰撞频率下X射线信号的透过率,理论上证明了X射线可用于黑障区信息传输的可行性.其次通过搭建环形扩散辉光放电等离子体发生器及实验验证系统,进行了国内外首次X射线穿过等离子体鞘套的验证实验.实验结果表明,等离子体对X射线信号的透过率存在一定程度的衰减,透过等离子体前后的X射线信号能谱轮廓相似度优于95.5%,能谱峰值点的偏移量小于1.3%.此外,在原有理论模型的基础上,考虑等离子中的粒子与X射线的碰撞、吸收效应,优化了X射线在等离子体中的透过率模型,与传统的理论方法相比,该模型可对实验现象进行更好的解释.同时计算了X射线在临近空间的透过率,并分析了X射线通信所能达到的潜在指标.这些结果有望为解决黑障区信号传输提供一定的理论与实验依据.     

Proposal for an x-ray free electron laser oscillator with intermediate energy electron beam

Harmonic lasing of low-gain free electron laser oscillators has been experimentally demonstrated in the terahertz and infrared regions. Recently, the low-gain oscillator has been reconsidered as a promising candidate for hard x-ray free electron lasers, through the use of high reflectivity, high-resolution x-ray crystals. In this Letter, it is proposed to utilize a crystal-based cavity resonant at a higher harmonic of the undulator radiation, together with phase shifting, to enable harmonic lasing of the x-ray free electron laser oscillator, and hence allow the generation of hard x-ray radiation at a reduced electron beam energy. Results show that fully coherent free electron laser radiation with megawatt peak power, in the spectral region of 10-25 keV, can be generated with a 3.5 GeV electron beam.     

Spatial chemical inhomogeneity and local electronic structure of Mn-doped Ge ferromagnetic semiconductors

We have investigated the chemical distributions and the local electronic structure of potential diluted magnetic semiconductor Ge0.94Mn0.06 single crystals using scanning photoelectron microscopy (SPEM), x-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES). The SPEM image shows the stripe-shaped microstructures, which arise from the chemical phase separation between the Mn-rich and Mn-depleted phases. The Mn 2p XAS shows that the Mn ions in the Mn-rich region are in the divalent high-spin Mn2+ states but that they do not form metallic Mn clusters. The Mn 3d PES spectrum exhibits a peak centered at approximately 4 eV below E(F) and the negligible spectral weight near E(F). This study suggests that the observed ferromagnetism in Ge1-xMnx arises from the phase-separated Mn-rich phase.     

Interference between resonance and potential scattering of high energy electrons from crystal surfaces

A solution of the tight-binding model of resonance diffraction of high-energy electrons from a crystal surface is found which describes the behaviour of the reflectivity in the vicinity of the intersection of a resonace parabola and a horizontal Kikuchi line. A simple analytical formula is obtained which makes it possible to evaluate the wave function for the case where both resonance and potential contributions to the reflectivity are of the same order of magnitude. The intensity of the specular reflection is shown to be directly related to the effective displacement Δ of the beam in the direction parallel to the surface, and a general formula suitable for evaluation of Δ from calculated RHEED rocking curves is derived.     

Solid and Liquid AgI at High Pressure and High Temperature: A X-ray Absorption Spectroscopy Study

We report about recent X-ray absorption spectroscopy (XAS) measurements on solid and liquid AgI under high pressure. The structural behaviour of AgI has been investigated to pressures P～4.3 GPa at room temperature and to P～1.8 GPa at 1100 K. The high temperature/high pressure conditions have been obtained by means of a large-volume press of the Paris-Edinburgh type, coupled with a 10 mm boron/epoxy biconical gasket. The absorption spectra have been collected in transmission mode, both at the K-edge of Ag and I, and the samples have been characterized in situ by energy scanning X-ray diffraction at fixed angles. Our XAS results for solid AgI are compatible with previous X-ray diffraction measurements. For liquid AgI, we observe a slight change in the intensity and a shift in the frequency of the XAS oscillations with respect to what obtained in the case of the ambient pressure liquid.