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Physical review / A 105(5), 053524 (2022). doi:10.1103/PhysRevA.105.053524, The advent of free-electron lasers (FELs) in the soft- and hard-x-ray spectral regions has introduced the possibility to probe electronic, magnetic, and structural dynamics, in both diluted and condensed matter samples, with femtosecond time resolution. In particular, FELs have strongly enhanced the capabilities of several analytical techniques, which have taken advantage of the high degree of transverse coherence provided. Free-electron lasers based on the harmonic up-conversion of an external coherent source (seed) are characterized also by a high degree of longitudinal coherence, since electrons inherit the coherence properties of the seed. For the state of the art, the shortest wavelength delivered to user experiments by an externally seeded FEL light source is about 4 nm. In this paper we demonstrate that pulses with a high longitudinal degree of coherence (first and second order) covering the water window and with photon energy extending up to 790 eV can be generated by exploiting the so-called nonlinear harmonic regime, which allows generation of radiation at harmonics of the resonant FEL wavelength. In order to show the suitability of the nonlinear harmonics generated by a seeded FEL for research in the water window and beyond, we report the results of two proof-of-principle experiments: one measuring the oxygen $K$-edge absorption in water (∼$530$ eV) and the other analyzing the spin dynamics of Fe and Co through magnetic small-angle x-ray scattering at their $L$ edges (707 and 780 eV, respectively)., Published by Inst., Woodbury, NY

Monophonic, geodesic and 2-geodesic convexities ([Formula: see text]-convexity, [Formula: see text]-convexity and [Formula: see text]-convexity, for short) on graphs are based on the families of induced paths, shortest paths and shortest paths of length [Formula: see text], respectively. We introduce a class of graphs, the class of cross-cyclicgraphs, in which every connected [Formula: see text]-convex set is also [Formula: see text]-convex and [Formula: see text]-convex. We show that this class is properly contained in the class, say [Formula: see text], of graphs in which geodesic and monophonic convexities are equivalent and properly contains the class of distance-hereditary graphs. Moreover, we show that (1) an [Formula: see text]-hull set (i.e., a subset of vertices, with minimum cardinality, whose [Formula: see text]-convex hull equals the whole vertex set) and, hence, the m-hull number and the g-hull number of a graph in [Formula: see text] can be computed in polynomial time and that (2) both the geodesic-convex hull and the monophonic-convex hull can be computed in linear time in a cross-cyclic graph without cycles of length [Formula: see text] and, hence, in a bipartite distance-hereditary graph.

An abstract convexity space on a connected hypergraph H with vertex set V (H) is a family C of subsets of V (H) (to be called the convex sets of H) such that: (i) C contains the empty set and V (H), (ii) C is closed under intersection, and (iii) every set in C is connected in H. A convex set X of H is a minimal vertex convex separator of H if there exist two vertices of H that are separated by X and are not separated by any convex set that is a proper subset of X. A nonempty subset X of V (H) is a cluster of H if in H every two vertices in X are not separated by any convex set. The cluster hypergraph of H is the hypergraph with vertex set V (H) whose edges are the maximal clusters of H. A convexity space on H is called decomposable if it satisfies the following three properties

Given a connected hypergraph with vertex set V, a convexity space on is a subset of the powerset of V that contains ∅, V, and the singletons; furthermore, is closed under intersection and every set in is connected in . The members of are called convex sets. The convex hull of a subset X of V is the smallest convex set containing X. By a cluster of we mean any nonempty subset of V in which every two vertices are separated by no convex set. We say that a convexity space on is decomposable if it satisfies the following three axioms: (i) the maximal clusters of form an acyclic hypergraph, (ii) every maximal cluster of is a convex set, and (iii) for every nonempty vertex set X, a vertex does not belong to the convex hull of X if and only if it is separated from X by a convex cluster. We prove that a decomposable convexity space on is fully specified by the maximal clusters of in that (1) there is a closed formula which expresses the convex hull of a set in terms of certain convex clusters of and (2) is a convex geometry if and only if the subspaces of induced by maximal clusters of are all convex geometries. Finally, we prove the decomposability of some known convexities in graphs and hypergraphs taken from the literature (such as “monophonic” and “canonical” convexities in hypergraphs and “all-paths” convexity in graphs).

In a connected hypergraph a vertex set X is simple-path convex (sp-convex, for short) if either |X|≤1 or X contains every vertex on every simple path between two vertices in X (Faber and Jamison, 1986 [7]), and the sp-convex hull of a vertex set X is the minimal superset of X that is sp-convex. In this paper, we give a polynomial algorithm to compute sp-convex hulls in an arbitrary hypergraph. © 2010 Elsevier B.V.

Let G be a connected graph on V. A subset X of V is all-paths convex (or ap -convex) if X contains each vertex on every path joining two vertices in X and is monophonically convex (or m-convex) if X contains each vertex on every chordless path joining two vertices in X. First of all, we prove that ap -convexity and m-convexity coincide in G if and only if G is a tree. Next, in order to generalize this result to a connected hypergraph H, in addition to the hypergraph versions of ap -convexity and m-convexity, we consider canonical convexity (or c-convexity) and simple-path convexity (or sp -convexity) for which it is well known that m-convexity is finer than both c-convexity and sp -convexity and sp -convexity is finer than ap -convexity. After proving sp -convexity is coarser than c-convexity, we characterize the hypergraphs in which each pair of the four convexities above is equivalent. As a result, we obtain a convexity-theoretic characterization of Berge-acyclic hypergraphs and of γ-acyclic hypergraphs.

The electronic structure in the normal state of CeFeAsO0.89F0.11 oxypnictide superconductors has been investigated with x-ray absorption and photoemission spectroscopy. All of the data exhibit signatures of Fe d-electron itinerancy. Exchange multiplets appearing in the Fe 3s core level indicate the presence of itinerant spin fluctuations. These findings suggest that the underlying physics and the origin of superconductivity in these materials are likely to be quite different from those of the cuprate high-temperature superconductors. These materials provide opportunities for elucidating the role of magnetic fluctuations in high-temperature superconductivity.

In an on-line statistical database, the query-answering system should prevent answers to statistical queries from leading to disclosure of confidential data. On the other hand, a statistical user is inclined to data mining, that is, to disclose pieces of information that are implicit in the (explicit) answers to his queries. A key task for both is to find data that is derivable from given summary statistics. We show that this task is easy if data is additive and the set of given summary statistics can be modelled by a graph.

Given an edge-weighted graph where all weights are nonnegative reals, an edge reweighting is an assignment of nonnegative reals to edges such that, for each vertex, the sums of given and new weights assigned to the edges incident on the vertex do coincide. An edge is then said to be invariant if its weight is the same for any edge reweighting. We show that the set of invariant edges of an arbitrary edge-weighted graph can be determined in time linear in the size of the underlying graph. Moreover, an application to the security of statistical data is discussed.

The notion of vertex separability by partial edges for a simple hypergraph is introduced and the related structural properties of the hypergraph are analyzed in terms of maximal (with respect to set-theoretic inclusion) compacts and of dividers, where a compact is a vertex set in which every two vertices are separated by no partial edge, and a divider is a partial edge X for which there exists a pair of vertices that are separated by X and by no proper subset of X. It is proven that, given a hypergraph H, the hypergraph (called the compaction of H) made up of maximal compacts of H is acyclic and coincides with H if and only if H is acyclic; furthermore, it has the same dividers as H, and can be characterized as being the unique acyclic hypergraph that has the same compacts as H. Polynomial algorithms for finding maximal compacts and dividers of a given hypergraph are provided. Finally, an application to the problem of computing the maximum-entropy extension of a system of marginals over a hypergraph is discussed.

This communication presents and discusses an experimental proof of the correlation among local structure and second harmonic generation (SHG) in bulk nanostructured potassium niobiosilicate (KNS) glasses. In particular, SHG shows a maximum in correspondence of the early stages of nanostructuring, that are characterized by the segregation within the amorphous matrix of nanosized inhomogeneities. EXAFS experiments indicate that these inhomogeneities are determined by the maximum size reached by the niobium second coordination shell combined with the sudden inclusion of potassium atoms in it. Such rearrangement at the local level determines the maximum fluctuation of the bulk glass refractive index and in turn its maximum SHG activity.

We report an extensive study on the intrinsic bulk electronic structure of the high-temperature superconductor CeFeAsO0.89F0.11 and its parent compound CeFeAsO by soft and hard x-ray photoemissions, x-ray absorption, and soft x-ray emission spectroscopies. The complementary surface/bulk probing depth, and the elemental and chemical sensitivity of these techniques allow resolving the intrinsic electronic structure of each element and correlating it with the local structure, which has been probed by extended x-ray absorption fine-structure spectroscopy. The measurements indicate a predominant 4f1 i.e., Ce3+ initial-state configuration for cerium and an effective valence-band-to-4f charge-transfer screening of the core hole. The spectra also reveal the presence of a small Ce f0 initial-state configuration, which we assign to the occurrence of an intermediate-valence state. The data reveal a reasonably good agreement with the partial density of states as obtained in standard density-functional calculations over a large energy range. Implications for the electronic structure of these materials are discussed

Rare-earth oxides are among the materials which are presently studied as possible replacements of amorphous silicon dioxide as gate insulators in nanometric Si devices; in fact, they generally exhibit high values of the dielectric constant ('high kappa'), a necessary requirement to obtain a high capacitance with layer thickness greater than the value below which tunneling currents become unacceptably high. Lu2O3 is one of the rare-earth oxides which may have the required properties in view of its quite high values of kappa and forbidden band gap. Since the envisaged dielectric layers are only a few nanometers thick, a description and a physical understanding of the atomic and electronic structure of the interface are of great importance. In this paper, we report a study by synchrotron radiation photoemission and transmission electron microscopy of the growth of Lu2O3 on Si(001). Thanks to the high spectral and spatial resolution, we provide clear evidence of a rather complex structure in which all silicon suboxides and SiO2 are present at the same time, along with a silicatelike phase and Lu2O3 itself; moreover, some grains and both crystalline and amorphous portions are present. In the photoemission experiment, the contribution of the Si surface to the core level line shapes has been taken into account; in the electron microscopy measurements, we present line scans on the nanometer scale of O, Si, and Lu concentrations and a Fourier transform discussion of the structure of the crystalline portions of the overlayer. The valence band discontinuity, which is measured in situ and is relative to the structurally well characterized interface, is found to be 3.16 +/- 0.16 eV. These findings are discussed in relation to the suitability of Lu2O3 as a high-kappa dielectric and in the context of available theoretical predictions of thermodynamic stability versus the formation of silicon oxide, silicates, and silicides and of the band discontinuity problem. (c) 2007 American Institute of Physics.

We investigated the local structure around N and Sb atoms in GaAsSbN/GaAs epilayers as a function of growth conditions and annealing time via soft and hard x-ray absorption spectroscopies in order to find out if short range ordering (SRO) in the group-V sublattice is present. SRO is one of the potential origins of the huge blueshift of the band gap observed upon annealing in these materials. By combining a Sb K- and L- and N K-edge x-ray absorption fine structure spectroscopy analysis, we demonstrate that neither strong Sb clustering nor preferential Sb-N association is possible, and that Sb atoms see a random number of N next nearest neighbors except for growth temperatures smaller than 400 degrees C, for which Sb-N neighbors in the type-V sublattice are in excess with respect to statistical disorder. On the other hand, the evolution of SRO around N anions (breaking of nitrogen pairs and randomization) can play a role in the annealing-induced band gap blueshift. Varying growth conditions and concentration modifies the band gap but, surprisingly, it does not affect the position of the conduction band minimum when Sb is incorporated.

Yttria sY2O3d is one of the candidate materials actively studied as a high dielectric constant replacement of amorphous silicon dioxide in downscaled microelectronic devices. An understanding of the physical properties of yttria epilayers on silicon is both a required prerequisite for applications and of fundamental interest. Using x-ray absorption spectroscopy at both the yttrium and oxygen K edges we study the local atomic and electronic structure in Y2O3 epilayers deposited on Sis001d. We find that above a thin near - interface layer the epilayers have a local atomic and electronic structure bearing a close similarity to that of bulk yttria; however, there is an increase in static disorder with decreasing thickness which can be correlated to an increase of the defectivity. The unoccupied electronic structure, as probed by near edge spectra at the cation and oxygen edges, is very similar to that of bulk yttria; we present and discuss simulations within the real - space full multiple scattering formalism which are quite successful in reproducing the experimental line shape. In the near - interface layer we detect the presence of silicon oxide, as reported for this and other systems previously; a new finding is that yttrium -silicon bonds are present in the interface layer, forming the precursors to the formation of yttrium silicide which develops into a long range ordered phase at higher deposition temperatures. We compare our observations with available theoretical predictions on the thermodynamic stability of yttria in comparison to silicon dioxide, yttrium silicide, and yttrium silicate; the predictions are not confirmed by experiment as far as the 2 nm thick interface layer is concerned.

The search for high dielectric constant (high- k ) oxides to replace silicon dioxide for future CMOS devices is a formidable task. Key issues are definitely good structural and electrical properties as well as high quality interfaces between the high- k and the semiconductor substrate. In this work we study the local structure of ultra thin Y 2 O 3 films and Y 2 O 3 /Si interface, by using Y k -edge X-ray absorption spectroscopy (XAS) in grazing incidence geometry. Y 2 O 3 epilayers (2–20 nm thick) are grown on Si (0 0 1) by MBE at 450 °C and some of them are in-situ annealed at 500 °C for 30 min. In the as grown epilayer, XAS reveals the presence of Y–Si and Y–O correlations at the Y 2 O 3 /Si interface. The in-situ annealing produces a rearrangement of the local atomic environment at the interface and only Y–O correlations are detected. The local structural quality of the epilayers improves with thickness and annealing.