High-resolution Soft X-ray Research Articles

Evolution of electronic structure across the rare-earth RNiO3 series

The perovksite rare-earth nickelates, RNiO3 (R = La… Lu), are a class of materials displaying a rich phase-diagram of metallic and insulating phases associated with charge and magnetic order. Being in the charge transfer regime, Ni3+ in octahedral coordination displays a strong hybridization with oxygen to form 3d-2p mixed states, which results in a strong admixture of 3d8L_ into 3d7, where L_ denotes a hole on the oxygen. To understand the nature of this strongly hybridized ground state, we present a detailed study of the Ni and O electronic structure using high-resolution soft X-ray absorption spectroscopy (XAS). Through a comparison of the evolution of the XAS line-shape at Ni L- and O K-edges across the phase diagram, we explore the changes in the electronic signatures in connection with the insulating and metallic phases that support the idea of hybridization playing a fundamental role.

High-order multilayer coated blazed gratings for high resolution soft x-ray spectroscopy.

A grand challenge in soft x-ray spectroscopy is to drive the resolving power of monochromators and spectrometers from the 10(4) achieved routinely today to well above 10(5). This need is driven mainly by the requirements of a new technique that is set to have enormous impact in condensed matter physics, Resonant Inelastic X-ray Scattering (RIXS). Unlike x-ray absorption spectroscopy, RIXS is not limited by an energy resolution dictated by the core-hole lifetime in the excitation process. Using much higher resolving power than used for normal x-ray absorption spectroscopy enables access to the energy scale of soft excitations in matter. These excitations such as magnons and phonons drive the collective phenomena seen in correlated electronic materials such as high temperature superconductors. RIXS opens a new path to study these excitations at a level of detail not formerly possible. However, as the process involves resonant excitation at an energy of around 1 keV, and the energy scale of the excitations one would like to see are at the meV level, to fully utilize the technique requires the development of monochromators and spectrometers with one to two orders of magnitude higher energy resolution than has been conventionally possible. Here we investigate the detailed diffraction characteristics of multilayer blazed gratings. These elements offer potentially revolutionary performance as the dispersive element in ultra-high resolution x-ray spectroscopy. In doing so, we have established a roadmap for the complete optimization of the grating design. Traditionally 1st order gratings are used in the soft x-ray region, but we show that as in the optical domain, one can work in very high spectral orders and thus dramatically improve resolution without significant loss in efficiency.

Probing two- and three-dimensional electrons inMgB2with soft x-ray angle-resolved photoemission

The electronic band structure of ${\mathrm{MgB}}_{2}$ involves a unique combination of two- and three-dimensional (3D) electrons derived from the boron $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ states, respectively. We have mapped out the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ bands over the complete Brillouin zone, including the full disconnected Fermi surface, using high-resolution soft x-ray angle-resolved photoelectron spectroscopy. The measured band structure, which is closely related to that of graphene, is in overall good agreement with the density functional theory--general gradient approximation (DFT-GGA), though differences in Fermi surface volume are seen. Surprisingly, the measured bands are wider than calculated, by $\ensuremath{\sim}8$% for the $\ensuremath{\sigma}$ and $\ensuremath{\sim}10$--15% for the $\ensuremath{\pi}$ bands. This solves the long-standing challenge of establishing the full 3D electronic structure of the model compound ${\mathrm{MgB}}_{2}$, and it demonstrates the tendency of DFT-GGA to overestimate the band narrowing due to exchange correlations effects.

Distinguishing between High- and Low-Spin States for Divalent Mn in Mn-Based Prussian Blue Analogue by High-Resolution Soft X-ray Emission Spectroscopy.

We combine Mn L2,3-edge X-ray absorption, high resolution Mn 2p-3d-2p resonant X-ray emission, and configuration-interaction full-multiplet (CIFM) calculation to analyze the electronic structure of Mn-based Prussian blue analogue. We clarified the Mn 3d energy diagram for the Mn(2+) low-spin state separately from that of the Mn(2+) high-spin state by tuning the excitation energy for the X-ray emission measurement. The obtained X-ray emission spectra are generally reproduced by the CIFM calculation for the Mn(2+) low spin state having a stronger ligand-to-metal charge-transfer effect between Mn t2g and CN π orbitals than the Mn(2+) high spin state. The d-d-excitation peak nearest to the elastic scattering was ascribed to the Mn(2+) LS state by the CIFM calculation, indicating that the Mn(2+) LS state with a hole on the t2g orbital locates near the Fermi level.

Suppression of stochastic pinning in magnetic nanowire devices using “virtual” domain walls

We have investigated the pinning and depinning of “virtual” domain walls in planar magnetic nanowires. Such virtual walls are created when a conventional domain wall becomes annihilated at a narrow gap between two segments of a discontinuous nanowire. By using focused magneto-optical Kerr effect magnetometry to study the repeatability of their depinning, we show that virtual walls exhibit single-mode depinning distributions, characterized by remarkably low, sub-Oersted standard deviations. This is in stark contrast to the depinning of domain walls from conventional notch-shaped defects, which typically exhibit multi-mode depinning field distributions spanning tens to hundreds of Oersteds. High-resolution magnetic soft x-ray microscopy measurements are used to reveal that this high level of repeatability is the result of a simple mediated-nucleation process, which decouples the depinning mechanism from structure of the initially injected DWs. Our work serves as an example of how the complex and dynamical stochastic behaviors exhibited by domain walls in nanowires can be controlled.

Integrated reflectivity measurements of hydrogen phthalate crystals for high-resolution soft x-ray spectroscopy

The integrated x-ray reflectivity of Potassium Hydrogen Phthalate (KAP) and Rubidium Hydrogen Phthalate (RAP) crystals is studied at a photon energy of (1740±14) eV using a double-crystal setup. The absolute measured reflectivities are in < 5% agreement with the values predicted by the dynamic diffraction theory for perfect crystals when absorption is included. Within 4% experimental error margins, specimen that were exposed to ambient conditions over many years show identical reflectivity as specimen that were cleaved just before the measurement. No differences are observed between cleaving off a 10 μm surface layer and splitting the entire crystal bulk of 2 mm thickness. We conclude that at 1.7 keV photon energy the penetration depth of ∼ 1 μm is large compared to a potentially deteriorated surface layer of a few 10 nm.

Design of the high-resolution soft X-ray imaging system on the Joint Texas Experimental Tokamak.

A new soft X-ray diagnostic system has been designed on the Joint Texas Experimental Tokamak (J-TEXT) aiming to observe and survey the magnetohydrodynamic (MHD) activities. The system consists of five cameras located at the same toroidal position. Each camera has 16 photodiode elements. Three imaging cameras view the internal plasma region (r/a < 0.7) with a spatial resolution about 2 cm. By tomographic method, heat transport outside from the 1/1 mode X-point during the sawtooth collapse is found. The other two cameras with a higher spatial resolution 1 cm are designed for monitoring local MHD activities respectively in plasma core and boundary.

A portable high-resolution soft x-ray and extreme ultraviolet spectrometer designed for the Shanghai EBIT and the Shanghai low energy EBITs.

A portable high resolution soft x-ray and extreme ultraviolet (EUV) spectrometer has been developed for spectroscopic research at the Shanghai Electron Beam Ion Trap (EBIT) laboratory. A unique way of aligning the grazing incidence spectrometer using the zero order of the grating is introduced. This method is realized by extending the range of the movement of the CCD detector to cover the zero order. The alignment can be done in a few minutes, thus leading to a portable spectrometer. The high vacuum needed to be compatible with the EBITs is reached by mounting most of the translation and rotation stages outside the chamber. Only one high vacuum compatible linear guide is mounted inside the chamber. This is to ensure the convenient interchange of the gratings needed to enable wavelength coverage of the whole range of 10 to 500 Å. Spectra recorded with one of our low energy EBITs shows that a resolving power of above 800 can be achieved. In the slitless configuration used in this work, we found the resolving power to be limited by the width of the EBIT plasma. When mounted on the Shanghai EBIT which is a high energy EBIT and has a narrower EBIT plasma width, the estimated resolving power will be around 1400 at 221.15 Å.

RESOLVING THE ORIGIN OF THE DIFFUSE SOFT X-RAY BACKGROUND

The ubiquitous diffuse soft (1/4 keV) X-ray background was one of the earliest discoveries of X-ray astronomy. At least some of the emission may arise from charge exchange between solar wind ions and neutral atoms in the heliosphere, but no detailed models have been fit to the available data. Here we report on a new model for charge exchange in the solar wind, which when combined with a diffuse hot plasma component filling the Local Cavity provides a good fit to the only available high-resolution soft X-ray and extreme ultraviolet (EUV) spectra using plausible parameters for the solar wind. The implied hot plasma component is in pressure equilibrium with the local cloud that surrounds the solar system, creating for the first time a self-consistent picture of the local interstellar medium.

Design and performance of AERHA, a high acceptance high resolution soft x-ray spectrometer.

A soft x-ray spectrometer based on the use of an elliptical focusing mirror and a plane varied line spacing grating is described. It achieves both high resolution and high overall efficiency while remaining relatively compact. The instrument is dedicated to resonant inelastic x-ray scattering studies. We set out how this optical arrangement was judged best able to guarantee performance for the 50 - 1000 eV range within achievable fabrication targets. The AERHA (adjustable energy resolution high acceptance) spectrometer operates with an effective angular acceptance between 100 and 250 μsr (energy dependent) and a resolving power well in excess of 5000 according to the Rayleigh criterion. The high angular acceptance is obtained by means of a collecting pre-mirror. Three scattering geometries are available to enable momentum dependent measurements with 135°, 90°, and 50° scattering angles. The instrument operates on the Synchrotron SOLEIL SEXTANTS beamline which serves as a high photon flux 2 × 200 μm(2) focal spot source with full polarization control.

Ultra High Resolution Soft X-Ray Microscopy Studies of Intercalation Mechanism in LiFePO4 Cathodes

With the rising demand in renewable energy storage devices for portable electronic applications and vehicle electrification, the development of novel techniques to study and characterize materials’ functions and their impact on performance of energy storage systems has become crucial. Olivine structured LiFePO4, currently cathode of choice in state-of-the art commercial Li ion batteries, has been specifically and extensively studied to elucidate its charge/discharge mechanism, kinetics and sources of capacity fade through various characterization methods in recent years. While these studies have resulted in significant improvements in the rate capabilities of this compound, the exact lithium intercalation mechanism remains controversial. Various studies on single-particle, many-particle and bulk LiFePO4 through a multitude of characterization techniques, have mainly pointed to two different possible intercalation pathways: non-equilibrium solid solution or a two-phase separation. 1 To follow phase, structural and chemical transformation in these cathodes, a technique with an increased resolution conducive to the study of nano-particles is required.In this work, the intercalation pathway and phase transition mechanism in single particle nano-LiFePO4 is studied utilizing an ultra high-resolution soft X-ray ptychographic microscope. The high penetration depth and short exposure times of soft X-rays enables scanning transmission X-ray microscopes (STXM) to probe the structural and chemical states of materials in nano scale with high resolution. X-ray ptychography is a coherent diffraction imaging technique that extracts structural information from inversion of diffraction data, hence, offering a spatial resolution that is not limited by the X-ray spot size.2 Measurements on chemically delithiated single nanoplates were conducted at Fe L3 edge in both normal scanning X-ray microscopy mode and in ptychography mode on beamlines 5.3.2.1. and 11.0.2 of the Advanced Light Source. In this study, a spatial resolution better than 10 nm was achieved on Beamline 11.0.2, utilizing a 60 nm outer zone-width zone plate and a CCD detector for diffraction measurements. Particles were chemically mapped through linear combination fitting of partially delithiated particles with LiFePO4 (LFP) and FePO4 (FP) as end member standards. While normal STXM measurements on these particles appeared to point to the existence of solid solution reaction, increased resolution in ptychography mode clearly showed the existence of a two-phase equilibrium within a single particle.Increased spatial resolution offered by ptychography also allows for studying intercalation and phase transformation inhomogeneities in bulk electrodes in operando conditions. In-situ experiments will be conducted on bulk LiFePO4 electrodes to examine the intercalation mechanism, lithium/electron mobility and phase boundary growth in the bulk as charge/discharge cycles progress. With such high spatial resolution ptychography can be the ideal technique for characterizing mesoscale chemical and structural transformations in electrochemical systems. Figure 1. STXM (right) and Ptychography STXM (left) images of partially delithiathed particle outlined in yellow; ptychography mode allows for chemical mapping of the particle at higher spatial resolution than STXM. Linear combination fitting of the ptychgraphic image points to the clear existence of a two phase equilibrium of lithium-rich and lithium poor regions, with a 70.9:29.1 LFP:FP ratio in the red colored region and 2.5:97.5 LFP:FP in the blue colored region.

High-temperature thermal stability study of 1 nm Al2O3 deposited on InAs surfaces investigated by synchrotron radiation based photoemission spectroscopy

High-resolution soft x-ray photoemission spectroscopy (SXPS) has been used to study the high-temperature thermal stability of ultra-thin atomic layer deposited (ALD) Al2O3 layers (∼1 nm) on sulfur passivated and native oxide covered InAs surfaces. While the arsenic oxides were removed from both interfaces following a 600 °C anneal, a residual indium oxide signal remained. No significant differences were observed between the sulfur passivated and native oxide surfaces other than the thickness of the interfacial oxide layer while the Al2O3 stoichiometry remained unaffected by the anneals. The energy band offsets were determined for the Al2O3 on the sulfur passivated InAs surface using both valence band edge and shallow core-level photoemission measurements.

高分辨宽能段软X射线光束线热负载影响与修正

高分辨宽能段软X射线光束线热负载影响与修正

High temperature thermal stability of the HfO2/Ge (100) interface as a function of surface preparation studied by synchrotron radiation core level photoemission

High resolution soft x-ray photoemission spectroscopy (SXPS) have been used to study the high temperature thermal stability of ultra-thin atomic layer deposited (ALD) HfO2 layers (∼1nm) on sulphur passivated and hydrofluoric acid (HF) treated germanium surfaces. The interfacial oxides which are detected for both surface preparations following HfO2 deposition can be effectively removed by annealing upto 700°C without any evidence of chemical interaction at the HfO2/Ge interface. The estimated valence and conduction band offsets for the HfO2/Ge abrupt interface indicated that effective barriers exist to inhibit carrier injection.

Magnetic Microstructure of Rolled‐Up Single‐Layer Ferromagnetic Nanomembranes

The magnetic microstructure of rolled-up magnetic nanomembranes is revealed both theoretically and experimentally. Two types of nanomembranes are considered, one with a non-magnetic spacer layer and the other without. Experimentally, by using different materials and tuning the dimensions of the rolled-up nanomembranes, domain patterns consisting of spiral-like and azimuthally magnetized domains are observed, which are in qualitative agreement with the theoretical predictions.

Development of Ta-Based Superconducting Tunnel Junction X-Ray Detector Arrays

We are developing new Ta-based superconducting tunnel junction (STJ) X-ray detectors for high-resolution soft X-ray spectroscopy at synchrotrons. STJ detectors combine the high-energy resolution of cryogenic detectors with the high count rate capabilities of athermal devices and the high efficiencies of solid state detectors, which increases the sensitivity for material analysis by fluorescence-detected X-ray absorption spectroscopy. Our STJ detectors are fabricated using thick, high-Z Ta absorber films that enhance quantum efficiency and spectral purity, and extend operational range to several keV compared with earlier Nb-based STJs. They offer an energy resolution of ~ 5 to 10 eV FWHM for soft X-rays up to ~1 keV, and count rates of several 1000 counts/s per detector pixel. For increased solid angle coverage, we have fabricated 36- and 112-pixel Ta-based STJ detector arrays with total areas of 1.4 and 4.5 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively. The 208 × 208 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> pixels have an energy resolution between 6.8 and 7.6 eV FWHM at 525 eV with a low-energy shoulder, and their responsivity is uniform to within 2% across the array. Here we discuss the performance of the array in the context of synchrotron science.

Development of near atomically perfect diffraction gratings for EUV and soft x-rays with very high efficiency and resolving power

Multilayer-coated Blazed Gratings (MBG) can offer high diffraction efficiency in a very high diffraction order and are therefore of great interest for high-resolution EUV and soft x-ray spectroscopy techniques such as Resonance Inelastic X-ray Scattering. However, realization of the MBG concept requires nano-scale precision in fabrication of a saw-tooth substrate with atomically smooth facets, and reproduction of the blazed groove profile in the course of conformal growth of a multilayer coating. We report on recent progress achieved in the development, fabrication, and characterization of ultra-dense MBGs for EUV and soft x-rays. As a result of thorough optimization of all steps of the fabrication process, an absolute diffraction efficiency as high as 44% and 12.7% was achieved for a 5250 l/mm grating in the EUV and soft x-ray regions respectively. This work now shows a direct route to achieving high diffraction efficiency in high order at wavelengths throughout the soft x-ray energy range with revolutionary applications in synchrotron science.

2D XAFS-XEOL Spectroscopy – Some recent developments

The use of optical photons to measure the modulation of the absorption coefficient upon X-ray excitation, or optical XAFS, is of particular interest for application to the study of light emitting semiconducting nanomaterials due to the additional information that may be gained. The potential for site-selectivity, elemental and excitation energy specific luminescence decay channels, and surface vs. bulk effects all make the use of X-ray excited optical luminescence (XEOL) desirable as a detection method. Previous experiments have made use of a monochromator to select the optical emission wavelength used to monitor optical XAFS. This method of detection suffers from the primary limitation of only being able to monitor the optical response at one emission wavelength. By combining the high resolution soft X-ray Spherical Grating Monochromator beam-line at the Canadian Light Source with an Ocean Optics QE 65000 fast CCD spectrophotometer and custom integration software we have developed a technique for collecting 2D XAFS-XEOL spectra, in which the excitation energy is scanned and a XEOL spectra is collected for every energy value. Herein we report the development of this technique and its capabilities using the study of the luminescence emitted from single crystal zinc oxide as an example.

A high resolution soft X-ray scintillation detector based on the Young-Weierstrass points of a lens shaped YAG crystal

Shaping a YAG scintillator crystal into a truncated-ball lens enables to image with a high numerical aperture its front surface, where the image converted from x-rays to visible is localized. Hence, both resolution and luminosity gains can be expected. Moreover if the plane surface is set at the Young-Weierstrass point of the spherical refractive surface, stigmatic imaging is achieved. On this principle, we have constructed an imaging detector from a 10 mm diameter YAG:Ce sphere and a long working distance plane-apochromatic microscope objective which does not limit the numerical aperture. The effective numerical aperture of the built system is 1.08, giving a Rayleigh resolution limit of 0.3 μm. Images of test objects (diatoms) have been recorded, in contact mode, with 103 eV. Periodic features of 0.4 μm pitch are visible on these images. The field of view is close to 200μm. The device is intended as an aid for X-ray optics fine-tuning and characterization.

Growth of Multilayer Optics for Synchrotron Radiation Sources

W/B4C multilayers have been deposited using magnetron sputtering with period ranging from 3.5 nm to 1.6 nm and layer pairs up to 300 to be used as soft x-ray /hard x-ray monochromators and soft x-ray polarizers. Normal incidence mirrors have been developed to be used in 12.5−13.5 nm using Mo/Si with reflectivity of ~ 67 % at 13 nm. In addition, high thermal stability NbC/Si multilayers in this wavelength range have been developed. The reflectivity of these mirrors is comparable with Mo/Si but have much higher thermal stability compared to Mo/Si. We have also deposited thermally stable C/B4C, low Z/low Z multilayer. These mirrors can be used as polarizer and high resolution soft x-ray monochromators.