Low-energy quasi-circular electron correlations with charge order wavelength in Bi2Sr2CaCu2O8+δ

Most resonant inelastic x-ray scattering (RIXS) studies of dynamic charge order correlations in the cuprates have focused on the high-symmetry directions of the copper oxide plane. However, scattering along other in-plane directions should not be ignored as it may help understand, for example, the origin of charge order correlations or the isotropic scattering resulting in strange metal behavior. Our RIXS experiments reveal dynamic charge correlations over the q_x-q_y scattering plane in underdoped Bi₂Sr₂CaCu₂O_8+δ. Tracking the softening of the RIXS-measured bond-stretching phonon, we show that these dynamic correlations exist at energies below approximately 70 meV and are centered around a quasi-circular manifold in the q_x-q_y scattering plane with radius equal to the magnitude of the charge order wave vector, q_CO. This phonon-tracking procedure also allows us to rule out fluctuations of short-range directional charge order (i.e., centered around [q_x = ±q_CO, q_y = 0] and [q_x = 0, q_y = ±q_CO]) as the origin of the observed correlations.

Highly Selective Photoelectroreduction of Carbon Dioxide to Ethanol over Graphene/Silicon Carbide Composites

Using sunlight to produce valuable chemicals and fuels from carbon dioxide (CO₂ ), i.e., artificial photosynthesis (AP) is a promising strategy to achieve solar energy storage and a negative carbon cycle. However, selective synthesis of C₂ compounds with a high CO₂ conversion rate remains challenging for current AP technologies. We performed CO₂ photoelectroreduction over a graphene/silicon carbide (SiC) catalyst under simulated solar irradiation with ethanol (C₂ H₅ OH) selectivity of>99 % and a CO₂ conversion rate of up to 17.1 mmol g_cat ^-1  h^-1 with sustained performance. Experimental and theoretical investigations indicated an optimal interfacial layer to facilitate the transfer of photogenerated electrons from the SiC substrate to the few-layer graphene overlayer, which also favored an efficient CO₂ to C₂ H₅ OH conversion pathway.

Presence of Delocalized Ti 3d Electrons in Ultrathin Single-Crystal SrTiO3

Strontium titanate (STO), with a wide spectrum of emergent properties such as ferroelectricity and superconductivity, has received significant attention in the community of strongly correlated materials. In the strain-free STO film grown on the SrRuO₃ buffer layer, the existing polar nanoregions can facilitate room-temperature ferroelectricity when the STO film thickness approaches 10 nm. Here we show that around this thickness scale, the freestanding STO films without the influence of a substrate show the tetragonal structure at room temperature, contrasting with the cubic structure seen in bulk form. The spectroscopic measurements reveal the modified Ti-O orbital hybridization that causes the Ti ion to deviate from its nominal 4+ valency (3d⁰ configuration) with excess delocalized 3d electrons. Additionally, the Ti ion in TiO₆ octahedron exhibits an off-center displacement. The inherent symmetry lowering in ultrathin freestanding films offers an alternative way to achieve tunable electronic structures that are of paramount importance for future technological applications.

Realization of Electron Antidoping by Modulating the Breathing Distortion in BaBiO3

The recent proposal of antidoping scheme breaks new ground in conceiving conversely functional materials and devices; yet, the few available examples belong to the correlated electron systems. Here, we demonstrate both theoretically and experimentally that the main group oxide BaBiO₃ is a model system for antidoping using oxygen vacancies. The first-principles calculations show that the band gap systematically increases due to the strongly enhanced Bi-O breathing distortions away from the vacancies and the annihilation of Bi 6s/O 2p hybridized conduction bands near the vacancies. Our further spectroscopic experiments confirm that the band gap increases systematically with electron doping, with a maximal gap enhancement of ∼75% when the film's stoichiometry is reduced to BaBiO_2.75. These results unambiguously demonstrate the remarkable antidoping effect in a material without strong electron correlations and underscores the importance of bond disproportionation in realizing such an effect.

Robust Surface States and Coherence Phenomena in Magnetically Alloyed SmB_{6}

Samarium hexaboride is a candidate for the topological Kondo insulator state, in which Kondo coherence is predicted to give rise to an insulating gap spanned by topological surface states. Here we investigate the surface and bulk electronic properties of magnetically alloyed Sm_{1-x}M_{x}B_{6} (M=Ce, Eu), using angle-resolved photoemission spectroscopy and complementary characterization techniques. Remarkably, topologically nontrivial bulk and surface band structures are found to persist in highly modified samples with up to 30% Sm substitution and with an antiferromagnetic ground state in the case of Eu doping. The results are interpreted in terms of a hierarchy of energy scales, in which surface state emergence is linked to the formation of a direct Kondo gap, while low-temperature transport trends depend on the indirect gap.

Time-resolved RIXS experiment with pulse-by-pulse parallel readout data collection using X-ray free electron laser

Time-resolved resonant inelastic X-ray scattering (RIXS) is one of the developing techniques enabled by the advent of X-ray free electron laser (FEL). It is important to evaluate how the FEL jitter, which is inherent in the self-amplified spontaneous emission process, influences the RIXS measurement. Here, we use a microchannel plate (MCP) based Timepix soft X-ray detector to conduct a time-resolved RIXS measurement at the Ti L₃-edge on a charge-density-wave material TiSe₂. The fast parallel Timepix readout and single photon sensitivity enable pulse-by-pulse data acquisition and analysis. Due to the FEL jitter, low detection efficiency of spectrometer, and low quantum yield of RIXS process, we find that less than 2% of the X-ray FEL pulses produce signals, preventing acquiring sufficient data statistics while maintaining temporal and energy resolution in this measurement. These limitations can be mitigated by using future X-ray FELs with high repetition rates, approaching MHz such as the European XFEL in Germany and LCLS-II in the USA, as well as by utilizing advanced detectors, such as the prototype used in this study.

Disparate Exciton-Phonon Couplings for Zone-Center and Boundary Phonons in Solid-State Graphite

The exciton-phonon coupling in highly oriented pyrolytic graphite is studied using resonant inelastic x-ray scattering (RIXS) spectroscopy. With ∼70  meV energy resolution, multiple low energy excitations associated with coupling to phonons can be clearly resolved in the RIXS spectra. Using resonance dependence and the closed form for RIXS cross section without considering the intermediate state mixing of phonon modes, the dimensionless coupling constant g is determined to be 5 and 0.35, corresponding to the coupling strength of 0.42  eV+/-20  meV and 0.20  eV+/-20  meV, for zone center and boundary phonons, respectively. The reduced g value for the zone-boundary phonon may be related to its double resonance nature.

A design of resonant inelastic X-ray scattering (RIXS) spectrometer for spatial- and time-resolved spectroscopy

The optical design of a Hettrick-Underwood-style soft X-ray spectrometer with Wolter type 1 mirrors is presented. The spectrometer with a nominal length of 3.1 m can achieve a high resolving power (resolving power higher than 10000) in the soft X-ray regime when a small source beam (<3 µm in the grating dispersion direction) and small pixel detector (5 µm effective pixel size) are used. Adding Wolter mirrors to the spectrometer before its dispersive elements can realize the spatial imaging capability, which finds applications in the spectroscopic studies of spatially dependent electronic structures in tandem catalysts, heterostructures, etc. In the pump-probe experiments where the pump beam perturbs the materials followed by the time-delayed probe beam to reveal the transient evolution of electronic structures, the imaging capability of the Wolter mirrors can offer the pixel-equivalent femtosecond time delay between the pump and probe beams when their wavefronts are not collinear. In combination with some special sample handing systems, such as liquid jets and droplets, the imaging capability can also be used to study the time-dependent electronic structure of chemical transformation spanning multiple time domains from microseconds to nanoseconds. The proposed Wolter mirrors can also be adopted to the existing soft X-ray spectrometers that use the Hettrick-Underwood optical scheme, expanding their capabilities in materials research.

High temperature singlet-based magnetism from Hund's rule correlations

Uranium compounds can manifest a wide range of fascinating many-body phenomena, and are often thought to be poised at a crossover between localized and itinerant regimes for 5f electrons. The antiferromagnetic dipnictide USb₂ has been of recent interest due to the discovery of rich proximate phase diagrams and unusual quantum coherence phenomena. Here, linear-dichroic X-ray absorption and elastic neutron scattering are used to characterize electronic symmetries on uranium in USb₂ and isostructural UBi₂. Of these two materials, only USb₂ is found to enable strong Hund’s rule alignment of local magnetic degrees of freedom, and to undergo distinctive changes in local atomic multiplet symmetry across the magnetic phase transition. Theoretical analysis reveals that these and other anomalous properties of the material may be understood by attributing it as the first known high temperature realization of a singlet ground state magnet, in which magnetism occurs through a process that resembles exciton condensation.

Electrons in uranium-based materials are often on the border between localised and itinerant behaviour, which can lead to unusual magnetic behaviour. Here the authors combine experiment and theory to show that USb₂ may be an unusually high temperature example of a singlet-ground-state magnet.

A setup for extreme-ultraviolet ultrafast angle-resolved photoelectron spectroscopy at 50-kHz repetition rate

Time- and angle-resolved photoelectron spectroscopy (trARPES) is a powerful method to track the ultrafast dynamics of quasiparticles and electronic bands in energy and momentum space. We present a setup for trARPES with 22.3 eV extreme-ultraviolet (XUV) femtosecond pulses at 50-kHz repetition rate, which enables fast data acquisition and access to dynamics across momentum space with high sensitivity. The design and operation of the XUV beamline, pump-probe setup, and ultra-high vacuum endstation are described in detail. By characterizing the effect of space-charge broadening, we determine an ultimate source-limited energy resolution of 60 meV, with typically 80-100 meV obtained at 1-2 × 10¹⁰ photons/s probe flux on the sample. The instrument capabilities are demonstrated via both equilibrium and time-resolved ARPES studies of transition-metal dichalcogenides. The 50-kHz repetition rate enables sensitive measurements of quasiparticles at low excitation fluences in semiconducting MoSe₂, with an instrumental time resolution of 65 fs. Moreover, photo-induced phase transitions can be driven with the available pump fluence, as shown by charge density wave melting in 1T-TiSe₂. The high repetition-rate setup thus provides a versatile platform for sensitive XUV trARPES, from quenching of electronic phases down to the perturbative limit.

High-efficiency in situ resonant inelastic x-ray scattering (iRIXS) endstation at the Advanced Light Source

An endstation with two high-efficiency soft x-ray spectrographs was developed at Beamline 8.0.1 of the Advanced Light Source, Lawrence Berkeley National Laboratory. The endstation is capable of performing soft x-ray absorption spectroscopy, emission spectroscopy, and, in particular, resonant inelastic soft x-ray scattering (RIXS). Two slit-less variable line-spacing grating spectrographs are installed at different detection geometries. The endstation covers the photon energy range from 80 to 1500 eV. For studying transition-metal oxides, the large detection energy window allows a simultaneous collection of x-ray emission spectra with energies ranging from the O K-edge to the Ni L-edge without moving any mechanical components. The record-high efficiency enables the recording of comprehensive two-dimensional RIXS maps with good statistics within a short acquisition time. By virtue of the large energy window and high throughput of the spectrographs, partial fluorescence yield and inverse partial fluorescence yield signals could be obtained for all transition metal L-edges including Mn. Moreover, the different geometries of these two spectrographs (parallel and perpendicular to the horizontal polarization of the beamline) provide contrasts in RIXS features with two different momentum transfers.

Revealing the Size-Dependent d-d Excitations of Cobalt Nanoparticles Using Soft X-ray Spectroscopy

Cobalt-based catalysts are widely used to produce liquid fuels through the Fischer-Tropsch (FT) reaction. However, the cobalt nanocatalysts can exhibit intriguing size-dependent activity whose origin remains heavily debated. To shed light on this issue, the electronic structures of cobalt nanoparticles with size ranging from 4 to 10 nm are studied using soft X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) spectroscopies. The RIXS measurements reveal the significant size-dependent d-d excitations, from which we determine that the crystal-field splitting energy 10Dq changes from 0.6 to 0.9 eV when the particle size is reduced from 10 to 4 nm. The finding that larger Co nanoparticles have smaller 10Dq value is further confirmed by the Co L-edge RIXS simulations with atomic multiplet code. Our RIXS results demonstrate a stronger Co-O bond in smaller Co nanoparticles, which brings in further insight into their size-dependent catalytic performance.

Modular soft x-ray spectrometer for applications in energy sciences and quantum materials

Over the past decade, the advances in grating-based soft X-ray spectrometers have revolutionized the soft X-ray spectroscopies in materials research. However, these novel spectrometers are mostly dedicated designs, which cannot be easily adopted for applications with diverging demands. Here we present a versatile spectrometer design concept based on the Hettrick-Underwood optical scheme that uses modular mechanical components. The spectrometer's optics chamber can be used with gratings operated in either inside or outside orders, and the detector assembly can be reconfigured accordingly. The spectrometer can be designed to have high spectral resolution, exceeding 10 000 resolving power when using small source (∼1μm) and detector pixels (∼5μm) with high line density gratings (∼3000 lines/mm), or high throughput at moderate resolution. We report two such spectrometers with slightly different design goals and optical parameters in this paper. We show that the spectrometer with high throughput and large energy window is particularly useful for studying the sustainable energy materials. We demonstrate that the extensive resonant inelastic X-ray scattering (RIXS) map of battery cathode material LiNi_1/3Co_1/3Mn_1/3O₂ can be produced in few hours using such a spectrometer. Unlike analyzing only a handful of RIXS spectra taken at selected excitation photon energies across the elemental absorption edges to determine various spectral features like the localized dd excitations and non-resonant fluorescence emissions, these features can be easily identified in the RIXS maps. Studying such RIXS maps could reveal novel transition metal redox in battery compounds that are sometimes hard to be unambiguously identified in X-ray absorption and emission spectra. We propose that this modular spectrometer design can serve as the platform for further customization to meet specific scientific demands.

Ultrafast dynamics of localized magnetic moments in the unconventional Mott insulator Sr2IrO4

We report a time-resolved study of the ultrafast dynamics of the magnetic moments formed by the [Formula: see text] states in Sr2IrO4 by directly probing the localized iridium 5d magnetic state through resonant x-ray diffraction. Using optical pump-hard x-ray probe measurements, two relaxation time scales were determined: a fast fluence-independent relaxation is found to take place on a time scale of 1.5 ps, followed by a slower relaxation on a time scale of 500 ps-1.5 ns.

Reproducibly creating hierarchical 3D carbon to study the effect of Si surface functionalization on the oxygen reduction reaction

We report a new method to reproducibly fabricate functional 3D carbon structures directly on a current collector, e.g. stainless steel. The 3D carbon platform is formed by direct growth of upright arrays of carbon nanofiber bundles on a roughened surface of stainless steel via the seed-assisted approach. Each bundle consists of about 30 individual carbon nanofibers with a diameter of 18 nm on average. We have found that this new platform offers adequate structural integrity. As a result, no reduction of the surface area during downstream chemical functionalization was observed. With a fixed and reproducible 3D structure, the effect of the chemistry of the grafted species on the oxygen reduction reaction has been systematically investigated. This investigation reveals for the first time that non-conductive Si with an appropriate electronic structure distorts the carbon electronic structure and consequently enhances ORR electrocatalysis. The strong interface provides excellent electron connectivity according to electrochemical analysis. This highly reproducible and stable 3D platform can serve as a stepping-stone for the investigation of the effect of carbon surface functionalization on electrochemical reactions in general.

Why LiFePO4is a safe battery electrode: Coulomb repulsion induced electron-state reshuffling upon lithiation

A combined spectroscopic and theoretical study clarifies the electron states associated with the intrinsic safety of LiFePO₄electrodes.

Restricting lignin and enhancing sugar deposition in secondary cell walls enhances monomeric sugar release after low temperature ionic liquid pretreatment

BACKGROUND

Lignocellulosic biomass has the potential to be a major source of renewable sugar for biofuel production. Before enzymatic hydrolysis, biomass must first undergo a pretreatment step in order to be more susceptible to saccharification and generate high yields of fermentable sugars. Lignin, a complex, interlinked, phenolic polymer, associates with secondary cell wall polysaccharides, rendering them less accessible to enzymatic hydrolysis. Herein, we describe the analysis of engineered Arabidopsis lines where lignin biosynthesis was repressed in fiber tissues but retained in the vessels, and polysaccharide deposition was enhanced in fiber cells with little to no apparent negative impact on growth phenotype.

RESULTS

Engineered Arabidopsis plants were treated with the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate 1-ethyl-3-methylimidazolium acetate ([C2C1im][OAc]) at 10 % wt biomass loading at either 70 °C for 5 h or 140 °C for 3 h. After pretreatment at 140 °C and subsequent saccharification, the relative peak sugar recovery of ~26.7 g sugar per 100 g biomass was not statistically different for the wild type than the peak recovery of ~25.8 g sugar per 100 g biomass for the engineered plants (84 versus 86 % glucose from the starting biomass). Reducing the pretreatment temperature to 70 °C for 5 h resulted in a significant reduction in the peak sugar recovery obtained from the wild type to 16.2 g sugar per 100 g biomass, whereas the engineered lines with reduced lignin content exhibit a higher peak sugar recovery of 27.3 g sugar per 100 g biomass and 79 % glucose recoveries.

CONCLUSIONS

The engineered Arabidopsis lines generate high sugar yields after pretreatment at 70 °C for 5 h and subsequent saccharification, while the wild type exhibits a reduced sugar yield relative to those obtained after pretreatment at 140 °C. Our results demonstrate that employing cell wall engineering efforts to decrease the recalcitrance of lignocellulosic biomass has the potential to drastically reduce the energy required for effective pretreatment.

Speed limit of the insulator-metal transition in magnetite

As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator-metal, or Verwey, transition has long remained inaccessible. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase. Here we investigate the Verwey transition with pump-probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator-metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics.

Impact of high biomass loading on ionic liquid pretreatment

BACKGROUND

Ionic liquid (IL) pretreatment has shown great potential as a novel pretreatment technology with high sugar yields. To improve process economics of pretreatment, higher biomass loading is desirable. The goal of this work is to establish, the impact of high biomass loading of switchgrass on IL pretreatment in terms of viscosity, cellulose crystallinity, chemical composition, saccharification kinetics, and sugar yield.

RESULTS

The pretreated switchgrass/IL slurries show frequency dependent shear thinning behavior. The switchgrass/IL slurries show a crossover from viscous behavior at 3 wt% to elastic behavior at 10 wt%. The relative glucan content of the recovered solid samples is observed to decrease with increasing levels of lignin and hemicelluloses with increased biomass loading. The IL pretreatment led to a transformation of cellulose crystalline structure from I to II for 3, 10, 20 and 30 wt% samples, while a mostly amorphous structure was found for 40 and 50 wt% samples.

CONCLUSIONS

IL pretreatment effectively reduced the biomass recalcitrance at loadings as high as 50 wt%. Increased shear viscosity and a transition from 'fluid' like to 'solid' like behavior was observed with increased biomass loading. At high biomass loadings shear stress produced shear thinning behavior and a reduction in viscosity by two orders of magnitude, thereby reducing the complex viscosity to values similar to lower loadings. The rheological properties and sugar yields indicate that 10 to 50 wt% may be a reasonable and desirable target for IL pretreatment under certain operating conditions.

Real-time manifestation of strongly coupled spin and charge order parameters in stripe-ordered La(1.75)Sr(0.25)NiO(4) nickelate crystals using time-resolved resonant x-ray diffraction

We investigate the order parameter dynamics of the stripe-ordered nickelate, La(1.75)Sr(0.25)NiO(4), using time-resolved resonant x-ray diffraction. In spite of distinct spin and charge energy scales, the two order parameters' amplitude dynamics are found to be linked together due to strong coupling. Additionally, the vector nature of the spin sector introduces a longer reorientation time scale which is absent in the charge sector. These findings demonstrate that the correlation linking the symmetry-broken states does not unbind during the nonequilibrium process, and the time scales are not necessarily associated with the characteristic energy scales of individual degrees of freedom.

Doubling of the bands in overdoped Bi2Sr2CaCu2O(8+delta): evidence for c-axis bilayer coupling

We present high resolution angle resolved photoemission data of the bilayer superconductor Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) showing a clear doubling of the near E(F) bands. This splitting approaches zero along the (0,0)-->(pi,pi) nodal line and is not observed in single layer Bi(2)Sr(2)CuO(6+delta) (Bi2201), indicating that the splitting is due to the long sought after bilayer splitting effect. The splitting has a magnitude of approximately 75 meV near the middle of the zone, extrapolating to about 110 meV near the (pi,0) point. The existence of these two bands also helps to clear up the recent controversy concerning the topology of the Fermi surface.

Fermi surface nesting and nanoscale fluctuating charge/orbital ordering in colossal magnetoresistive oxides

We used high-resolution angle-resolved photoemission spectroscopy to reveal the Fermi surface and key transport parameters of the metallic state of the layered colossal magnetoresistive oxide La1.2Sr1.8Mn2O7. With these parameters, the calculated in-plane conductivity is nearly one order of magnitude larger than the measured direct current conductivity. This discrepancy can be accounted for by including the pseudogap, which removes at least 90% of the spectral weight at the Fermi energy. Key to the pseudogap and to many other properties are the parallel straight Fermi surface sections, which are highly susceptible to nesting instabilities. These nesting instabilities produce nanoscale fluctuating charge/orbital modulations, which cooperate with Jahn-Teller distortions and compete with the electron itinerancy favored by double exchange.