In this work, we present an end-station with two perpendicular high-efficiency soft x-ray spectrometers at the plane grating monochromator beamline in the Physikalisch-Technische Bundesanstalt laboratory at the electron storage ring BESSY II. It is designed for polarization-dependent x-ray emission spectroscopy and resonant inelastic x-ray scattering spectroscopy and covers the photon energy range from 80 to 1100 eV. Depending on the energy range to be monitored, either one of two spherical mirrors and either one of three variable line spacing gratings in each spectrometer is selected to provide optimal performance in terms of resolving power and detection yield. A calibration of the optical components and detection devices enables quantitative measurements and the possibility of the determination of x-ray fluorescence fundamental parameters in the soft x-ray range.

While highly successful, density functional theory is known to have limitations owing to its neglect of many-body electron-electron interactions. This neglect leads to errors in the single-particle energies, leading to underestimated bandgaps and bandwidths as well as errors in band alignment at interfaces. Many-body perturbation theory, in the form of the GW self-energy correction, has been widely used to improve upon these shortcomings. Although less well studied, the same GW method is also able to predict the finite quasiparticle lifetime that is seen to cause anomalous broadening in the lowest-lying lines of valence emission spectra. Using near-edge x-ray absorption and emission, we probe the electronic structure of Li2CO3. Our measurements are compared to first-principles calculations, including GW self-energy corrections to the single-particle energies and excitonic effects from the Bethe-Salpeter equation.

In Opt. Express 24, 22528 (2016)10.1364/OE.24.022528, D. Sarenac et al. reported neutron holography using a Laue crystal interferometer and a spiral phase plate, interpreting the resulting interference pattern as a hologram reconstructing neutron beams carrying orbital angular momentum (OAM). However, this interpretation is highly questionable: the interferometer's markedly unequal lateral coherence lengths are far too small and, moreover, should have produced an asymmetric pattern, which was not observed. Our simulations demonstrate that the reported images can be fully explained as conventional interference patterns without invoking OAM. Furthermore, we highlight the inherent challenges of using crystal interferometry for holography and question whether neutron OAM states are necessary - or even sufficient - to account for the observed results.

Extreme broadening previously observed in certain N x-ray fluorescence lines excited near the N K edge in nitrates has been observed in the S L x-ray fluorescence in sodium sulfate, Na2SO4. It is explained as a large imaginary self-energy corresponding to an anomalously short lifetime of quasiparticles in one of the valence bands of these compounds. Our latest measurement and the corresponding many-body theory indicate that this is a general effect not specific to nitrates.

A neutron Laue crystal interferometer has been reported by Saranac et al . to demonstrate neutron holography of a spiral phase plate. Using its two coherent beams as the object and reference beams, the resulting interference pattern was interpreted as a hologram. This interference pattern was then reported to reconstruct neutron beams with various intrinsic orbital angular momenta. There are serious doubts about the method for generating neutron orbital angular momentum with a crystal interferometer. Due to the extremely different lateral coherence lengths in the interferometer, one should expect the pattern described as a hologram to be asymmetric. In addition, a neutron crystal interferometer always produces a Moire image where the beams are combined in the final crystal, that appears as a one-dimensional enlargement of the interference pattern. We present computer simulations showing that the images presented as holograms can be computed as conventional interference patterns assuming the phase shifts of ordinary neutrons passing through objects located in one or the other beam path of the interferometer. Additionally, we discuss the complications of using a crystal interferometer for holography, while raising the question of whether the intrinsic orbital angular momentum states of neutrons are necessary or sufficient to explain the recorded images.

Microcalorimeter x-ray detectors offer the specific advantage of being high -resolution energy -dispersive detectors. Furthermore, they can be designed to cover almost any energy range, from soft x-rays to gamma rays. Many of the current energy values of L, M, and N x-ray lines in the soft x-ray range (below 1.2 keV) have not been established through a chain of calibration. Based on our experience, we propose here a method of measuring the energies of these lines that should establish peak positions to a few tenths of an electron volt. It would involve the calibration of a microcalorimeter detector with diagram line energy values determined by a grating x-ray spectrometer calibrated by a plane grating monochromator using synchrotron radiation. We present L -line spectra from Cu, Co, and Ni obtained with a microcalorimeter detector to demonstrate the feasibility of obtaining high -resolution spectra in the energy range below 1 keV.

A standard method to detect thermal neutrons is the nuclear interaction 3He(n,p)3H. The spin dependence of this interaction is also the basis of a neutron spin-polarization filter using nuclear polarized 3He. We consider the corresponding interaction for neutrons placed in an intrinsic orbital angular momentum (OAM) state. We derive the relative polarization-dependent absorption cross sections for neutrons in an L=1 OAM state. The absorption of those neutrons results in compound states Jπ=0-, 1-, and 2-. Varying the three available polarizations tests that an OAM neutron has been absorbed and probes which decay states are physically possible. We describe the energetically likely excited states of 4He after absorption, taking account of the odd parity of the compound state. This provides a definitive method for detecting neutron OAM states and suggests that intrinsic OAM states offer the possibility to observe new physics, including anomalous cross sections and new channels of radioactive decay.

In the analysis of neutron scattering measurements of condensed matter structure, it normally suffices to treat the incident and scattered neutron beams as if composed of incoherent distributions of plane waves with wavevectors of different magnitudes and directions that are taken to define an instrumental resolution. However, despite the wide-ranging applicability of this conventional treatment, there are cases, such as specular neutron reflectometry, in which the structural length scales of the scattering object require that the wavefunction of an individual neutron in the beam be described by a spatially localized packet - in particular with respect to the transverse extent of its wavefronts (i.e. normal to the packet's mean direction of propagation). It is shown in the present work that neutron diffraction patterns observed for periodic transmission phase gratings, as well as specular reflection measurements from patterned thin films with repeat units of the order of micrometres, can be accurately described by associating an individual neutron with a wave packet and treating a beam as a collection of independent packets. In these cases, accurate analysis requires that the transverse spatial extent of a neutron packet wavefront be accounted for in addition to the angular divergence of the beam that is characterized by a distribution of packet mean wavevector directions. It is shown how a measure of the effective transverse spatial extent of the neutron packet - over which its wavefronts are of sufficient uniformity to produce coherent scattering - can be determined by employing reference diffraction gratings and patterned thin films of known structure and composition.

The comparability and reliability of the analysis of the electronic structure of selected cyanates and thiocyanates at the nitrogen $K$ edge based on BSE calculations have been investigated in this work. Using high-resolution x-ray spectroscopy with calibrated instrumentation reliable and reproducible experimental results for x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) were achieved. These results were used to validate theoretical modeling with first-principles calculations based on the Bethe-Salpeter equation approach for the excited-state interactions. Furthermore, radiation-induced damages occurring in the samples are correlated with absolute doses.

Microcalorimeter X-ray detectors employing a transition-edge sensor are capable of very high energy resolution, but achieving this in practice depends on an understanding of the readout process. The combination of thermal cooling and thermoelectric feedback present in most circumstances produces a pulse whose area and amplitude both vary nonlinearly with photon energy. The resolution is further affected by the presence of pileup pulses in the pulse or baseline that need to be detected and rejected down to an arbitrarily small level. We describe here a method that includes both the rejection of pileup pulses and the correction of the nonlinearities. We have implemented the process in a system that accepts pulses from multiple detectors and analyzes them in real time. An initial brief X-ray spectrum from a standard sample supplies sufficient data for pulse characterization and energy calibration. The system is then also enabled to combine pulses from multiple detectors into an energy-calibrated histogram in real time.

X-ray absorption and resonant inelastic x-ray scattering measurements are carried out on lithium nitrate LiNO3. The nitrogen σ orbitals exhibit a large lifetime effect. Experimentally, this is manifest as an apparent weakening of the x-ray emission signal from these states, but a closer examination shows that instead it is due to extreme broadening. This echos previous studies on ammonium nitrate, which, despite large differences in the cation and space group, showed a similar effect associated with the nitrate. Using first-principles GW self-energy and Bethe-Salpeter equation calculations we show that this effect is due in part to short quasi-hole lifetimes for the orbitals constituting the NO σ bonds.

In the analysis of neutron scattering measurements of condensed matter structure, it normally suffices to treat the incident and scattered neutron beams as if composed of incoherent distributions of plane waves with wavevectors of different magnitudes and directions which are taken to define an instrumental resolution. However, despite the wide-ranging applicability of this conventional treatment, there are cases in which the wave function of an individual neutron in the beam must be described more accurately by a spatially localized packet, in particular with respect to its transverse extent normal to its mean direction of propagation. One such case involves the creation of orbital angular momentum (OAM) states in a neutron via interaction with a material device of a given size. It is shown in the work reported here that there exist two distinct measures of coherence of special significance and utility for describing neutron beams in scattering studies of materials in general. One measure corresponds to the coherent superposition of basis functions and their wavevectors which constitute each individual neutron packet state function whereas the other measure can be associated with an incoherent distribution of mean wavevectors of the individual neutron packets in a beam. Both the distribution of the mean wavevectors of individual packets in the beam as well as the wavevector components of the superposition of basis functions within an individual packet can contribute to the conventional notion of instrumental resolution. However, it is the transverse spatial extent of packet wavefronts alone that determines the area within which a coherent scattering process can occur in the first place. This picture is shown to be consistent with standard quantum theory. It is also demonstrated that these two measures of coherence can be distinguished from one another experimentally.

It has been shown that single-particle wave functions, of both photons and electrons, can be created with a phase vortex, i.e., an intrinsic orbital angular momentum (OAM). A recent experiment has claimed similar success using neutrons [C. W. Clark et al., Nature, 525, 504 (2015)NATUAS0028-083610.1038/nature15265]. We show that their results are insufficient to unambiguously demonstrate OAM, and they can be fully explained as phase contrast interference patterns. Furthermore, given the small transverse coherence length of the neutrons in the original experiment, the probability that any neutron was placed in an OAM state is vanishingly small. We highlight the importance of the relative size of the coherence length, which presents a unique challenge for neutron experiments compared to electron or photon work, and we suggest improvements for the creation of neutron OAM states.

Near-edge x-ray spectroscopies, including emission and resonant inelastic x-ray scattering (RIXS), are widely used to probe the local electronic and molecular structure of materials.The spectra, however, provide only an indirect measure of the interesting parameters of a system: a transition metal L edge might reveal charge state in a battery cathode whereas the oxygen K edge can reflect the hydrogen bond network in water.In both cases it is not the spectra themselves that are of interest, but the electronic and structural configurations that give rise to them.Accurate modeling provides an invaluable tool for not only the interpretation of measured results, but also the design of experiments.The OCEAN code simulates near-edge spectra by solving the Bethe-Salpeter equation on top of a density-functional theory foundation, without system-dependent fitting parameters [1].By contrasting OCEAN results with high-resolution measurements we are able to identify discrepancies that arise from specific effects or physical processes originally neglected in the calculations.We present examples showcasing the importance of correctly accounting for valence-band quasiparticle lifetimes [2], intrinsic disorder including zero-point motion, and phonon scattering when modeling x-ray emission and RIXS.

The electronic structure of hexagonal boron nitride (h-BN) is explored using measurements of x-ray absorption and resonant inelastic x-ray scattering (RIXS) at the nitrogen K edge (1s) in tandem with calculations using many-body perturbation theory within the GW and Bethe-Salpeter equation (BSE) approximations. Our calculations include the effects of lattice disorder from phonons activated thermally and from zero point energy. They highlight the influence of disorder on near-edge x-ray spectra.

It has been previously shown that two effects cause dramatic changes in the x-ray absorption and emission spectra from the N K edge of the insulating crystal ammonium nitrate. First, vibrational disorder causes major changes in the absorption spectrum, originating not only from the thermal population of phonons, but, significantly, from zero-point motion as well. Second, the anomalously large broadening (~ 4 eV) of the emission originating from nitrate σ states is due to unusually short lifetimes of quasiparticles in an otherwise extremely narrow band. In this work we investigate the coupling of these effects to core and valence excitons that are created as the initial x-ray excitation energy is progressively reduced toward the N edge. Using a GW/Bethe-Salpeter approach, we show the extent to which this anomalous broadening is captured by the GW approximation. The data and calculations demonstrate the importance that the complex self-energies (finite lifetimes) of valence bands have on the interpretation of emission spectra. We produce a scheme to explain why extreme lifetimes should appear in σ states of other similar compounds.