Understanding and optimizing the sensitization of anatase titanium dioxide surface with hematite clusters

The presence of hematite (Fe2O3) clusters at low coverage on titanium dioxide (TiO2) surface has been observed to enhance photocatalytic activity, while excess loading of hematite is detrimental. We conduct a comprehensive density functional theory study of Fe2O3clusters adsorbed on the anatase TiO2(101) surface to investigate the effect of Fe2O3on TiO2. Our study shows that TiO2exhibits improved photocatalytic properties with hematite clusters at low coverage, as evidenced by a systematic study conducted by increasing the number of cluster adsorbates. The adsorption of the clusters generates impurity states in the band gap improving light absorption and consequently affecting the charge transfer dynamics. Furthermore, the presence of hematite clusters enhances the activity of TiO2in the hydrogen evolution reaction. The Fe valence mixing present in some clusters leads to a significant increase in H2evolution rate compared with the fixed +3 valence of Fe in hematite. We also investigate the effect of oxygen defects and find extensive modifications in the electronic properties and local magnetism of the TiO2-Fe2O3system, demonstrating the wide-ranging effect of oxygen defects in the combined system.

Comparing first-principles density functionals plus corrections for the lattice dynamics of YBa2Cu3O6

The enigmatic mechanism underlying unconventional high-temperature superconductivity, especially the role of lattice dynamics, has remained a subject of debate. Theoretical insights have long been hindered due to the lack of an accurate first-principles description of the lattice dynamics of cuprates. Recently, using the r2SCAN meta-generalized gradient approximation (meta-GGA) functional, we have been able to achieve accurate phonon spectra of an insulating cuprate YBa2Cu3O6 and discover significant magnetoelastic coupling in experimentally interesting Cu-O bond stretching optical modes [Ning et al., Phys. Rev. B 107, 045126 (2023)]. We extend this work by comparing Perdew-Burke-Ernzerhof and r2SCAN performances with corrections from the on-site Hubbard U and the D4 van der Waals (vdW) methods, aiming at further understanding on both the materials science side and the density functional side. We demonstrate the importance of vdW and self-interaction corrections for accurate first-principles YBa2Cu3O6 lattice dynamics. Since r2SCAN by itself partially accounts for these effects, the good performance of r2SCAN is now more fully explained. In addition, the performances of the Tao-Mo series of meta-GGAs, which are constructed in a different way from the strongly constrained and appropriately normed (SCAN) meta-GGA and its revised version r2SCAN, are also compared and discussed.

First-Principles Perspective on Gas Adsorption by [Fe4S4]-Based Metal-Organic Frameworks

[Fe₄S₄] or [4S-4Fe] clusters are responsible for storing and transferring electrons in key cellular processes and interact with their microenvironment to modulate their oxidation and magnetic states. Therefore, these clusters are ideal for the metal node of chemically and electromagnetically tunable metal-organic frameworks (MOFs). To examine the adsorption-based applications of [Fe₄S₄]-based MOFs, we used density functional theory calculations and studied the adsorption of CO₂, CH₄, H₂O, H₂, N₂, NO₂, O₂, and SO₂ onto [Fe₄S₄]⁰, [Fe₄S₄]²⁺, and two 1D MOF models with the carboxylate and 1,4-benzenedithiolate organic linkers. Our reaction kinetics and thermodynamics results indicated that MOF formation promotes the oxidative and hydrolytic stability of the [Fe₄S₄] clusters but decreases their adsorption efficiency. Our study suggests the potential industrial applications of these [Fe₄S₄]-based MOFs because of their limited capacity to adsorb CO₂, CH₄, H₂O, H₂, N₂, O₂, and SO₂ and high selectivity for NO₂ adsorption.

Quantum view of Li-ion high mobility at carbon-coated cathode interfaces

Lithium-ion batteries (LIBs) are among the most promising power sources for electric vehicles, portable electronics and smart grids. In LIBs, the cathode is a major bottleneck, with a particular reference to its low electrical conductivity and Li-ion diffusivity. The coating with carbon layers is generally employed to enhance the electrical conductivity and to protect the active material from degradation during operation. Here, we demonstrate that this layer has a primary role in the lithium diffusivity into the cathode nanoparticles. Positron is a useful quantum probe at the electroactive materials/carbon interface to sense the mobility of Li-ion. Broadband electrical spectroscopy demonstrates that only a small number of Li-ions are moving, and that their diffusion strongly depends on the type of carbon additive. Positron annihilation and broadband electrical spectroscopies are crucial complementary tools to investigate the electronic effect of the carbon phase on the cathode performance and Li-ion dynamics in electroactive materials.

Photoemission Spectroscopy Using Virtual Photons Emitted by Positron Sticking: A Complementary Probe for Top-Layer Surface Electronic Structures

We present a spectroscopic method which utilizes virtual photons to selectively measure the electronic structure of the topmost atomic layer. These virtual photons are created when incident positrons transition from vacuum states to bound surface states on the sample surface and can transfer sufficient energy to excite electrons into the vacuum. The short interaction range of the virtual photons restricts the penetration depth to approximately the Thomas-Fermi screening length. Measurements and analysis of the kinetic energies of the emitted electrons made on a single layer of graphene deposited on Cu and on the clean Cu substrate show that the ejected electrons originate exclusively from the topmost atomic layer. Moreover, we find that the kinetic energies of the emitted electrons reflect the density of states at the surface. These results demonstrate that this technique will be a complementary tool to existing spectroscopic techniques in determining the electronic structure of 2D materials and fragile systems due to the absence of subsurface contributions and probe-induced surface damage.

Re-examining the giant magnetization density in α''-Fe16N2 with the SCAN+U method

We present an in-depth discussion of the magnetic ground state of α''-Fe₁₆N₂ within the framework of the density functional theory (DFT). The exchange-correlation effects are treated using a variety of schemes, including the local-spin-density approximation, the generalized-gradient approximation, and the Strongly-Constrained-and-Appropriately-Normed (SCAN) scheme. We also delineate effects of adding an on-site interaction parameter U on the Fe sites. Among all the schemes considered, only SCAN+U is found to capture the surprisingly large magnetization density in α''-Fe₁₆N₂ that has been observed experimentally. Our study shows how the combination of SCAN and self-interaction corrections applied on different Fe sites through the parameter U can reproduce both the correct equilibrium volume and the giant magnetization density of α''-Fe₁₆N₂.

Atomic structure of an FeCrMoCBY metallic glass revealed by high energy x-ray diffraction

Amorphous bulk metallic glasses with the composition Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Y₂have been of interest due to their special mechanical and electronic properties, including corrosion resistance, high yield-strength, large elasticity, catalytic performance, and soft ferromagnetism. Here, we apply a reverse Monte Carlo technique to unravel the atomic structure of these glasses. The pair-distribution functions for various atomic pairs are computed based on the high-energy x-ray diffraction data we have taken from an amorphous sample. Monte Carlo cycles are used to move the atomic positions until the model reproduces the experimental pair-distribution function. The resulting fitted model is consistent with ourab initiosimulations of the metallic glass. Our study contributes to the understanding of functional properties of Fe-based bulk metallic glasses driven by disorder effects.

Atomic arrangements in an amorphous CoFeB ribbon extracted via an analysis of radial distribution functions

We discuss the atomic structure of amorphous ferromagnetic FeCoB alloys, which are used widely in spintronics applications. Specifically, we obtain the pair-distribution functions for various atomic pairs based on high-energy x-ray diffraction data taken from an amorphous Co₂₀Fe₆₁B₁₉specimen. We start our reverse Monte Carlo cycles to determine the disordered structure with a two-phase model in which a small amount of cobalt is mixed with Fe₂₃B₆as a second phase. The structure of the alloy is found to be heterogeneous, where the boron atoms drive disorder through the random occupation of the atomic network. Our analysis also indicates the presence of small cobalt clusters that are embedded in the iron matrix and percolating the latter throughout the structure. This morphology can explain the enhanced spin polarization observed in amorphous magnetic materials.

Structural properties of PbTe quantum dots revealed by high-energy x-ray diffraction

High-energy x-ray diffraction (HE-XRD) experiments combined with an analysis based on atomic-pair-distribution functions can be an effective tool for probing low-dimensional materials. Here, we show how such an analysis can be used to gain insight into structural properties of PbTe nanoparticles (NPs). We interpret our HE-XRD data using an orthorhombic Pnma phase of PbTe, which is an orthorhombic distortion of the rocksalt phase. Although local crystal geometry can vary substantially with particle size at scales below 10 nm, and for very small NPs the particle size itself influences x-ray diffraction patterns, our study shows that HE-XRD can provide a unique nano-characterization tool for unraveling structural properties of nanoscale systems.

Competing stripe and magnetic phases in the cuprates from first principles

Realistic description of competing phases in complex quantum materials has proven extremely challenging. For example, much of the existing density-functional-theory-based first-principles framework fails in the cuprate superconductors. Various many-body approaches involve generic model Hamiltonians and do not account for the interplay between the spin, charge, and lattice degrees of freedom. Here, by deploying the recently constructed strongly constrained and appropriately normed (SCAN) density functional, we show how the landscape of competing stripe and magnetic phases can be addressed on a first-principles basis both in the parent insulator YBa2Cu3O6 and the near-optimally doped YBa2Cu3O7 as archetype cuprate compounds. In YBa2Cu3O7, we find many stripe phases that are nearly degenerate with the ground state and may give rise to the pseudogap state from which the high-temperature superconducting state emerges. We invoke no free parameters such as the Hubbard U, which has been the basis of much of the existing cuprate literature. Lattice degrees of freedom are found to be crucially important in stabilizing the various phases.

Bacteriorhodopsin Enhances Efficiency of Perovskite Solar Cells

Recently, halide perovskites have upstaged decades of solar cell development by reaching power conversion efficiencies that surpass the performance of polycrystalline silicon. The efficiency improvement in the perovskite cells is related to repeated recycling between photons and electron-hole pairs, reduced recombination losses, and increased carrier lifetimes. Here, we demonstrate a novel approach toward augmenting the perovskite solar cell efficiency by invoking the Förster Resonance Energy Transfer (FRET) mechanism. FRET occurs in the near-field region as the bacteriorhodopsin (bR) protein, and perovskite has similar optical gaps. Titanium dioxide functionalized with the bR protein is shown to accelerate the electron injection from excitons produced in the perovskite layer. FRET predicts the strength of long-range excitonic transport between the perovskite and bR layers. Solar cells incorporating TiO₂/bR layers are found to exhibit much higher photovoltaic performance as compared to baseline cells without bR. These results open the opportunity to develop a new class of bioperovskite solar cells with improved performance and stability.

Nature of the Positron State in CdSe Quantum Dots

Previous studies have shown that positron-annihilation spectroscopy is a highly sensitive probe of the electronic structure and surface composition of ligand-capped semiconductor quantum dots (QDs) embedded in thin films. The nature of the associated positron state, however, whether the positron is confined inside the QDs or localized at their surfaces, has so far remained unresolved. Our positron-annihilation lifetime spectroscopy studies of CdSe QDs reveal the presence of a strong lifetime component in the narrow range of 358-371 ps, indicating abundant trapping and annihilation of positrons at the surfaces of the QDs. Furthermore, our ab initio calculations of the positron wave function and lifetime employing a recent formulation of the weighted density approximation demonstrate the presence of a positron surface state and predict positron lifetimes close to experimental values. Our study thus resolves the long-standing question regarding the nature of the positron state in semiconductor QDs and opens the way to extract quantitative information on surface composition and ligand-surface interactions of colloidal semiconductor QDs through highly sensitive positron-annihilation techniques.

Visualizing redox orbitals and their potentials in advanced lithium-ion battery materials using high-resolution x-ray Compton scattering

Reduction-oxidation (redox) reactions are the key processes that underlie the batteries powering smartphones, laptops, and electric cars. A redox process involves transfer of electrons between two species. For example, in a lithium-ion battery, current is generated when conduction electrons from the lithium anode are transferred to the redox orbitals of the cathode material. The ability to visualize or image the redox orbitals and how these orbitals evolve under lithiation and delithiation processes is thus of great fundamental and practical interest for understanding the workings of battery materials. We show that inelastic scattering spectroscopy using high-energy x-ray photons (Compton scattering) can yield faithful momentum space images of the redox orbitals by considering lithium iron phosphate (LiFePO₄ or LFP) as an exemplar cathode battery material. Our analysis reveals a new link between voltage and the localization of transition metal 3d orbitals and provides insight into the puzzling mechanism of potential shift and how it is connected to the modification of the bond between the transition metal and oxygen atoms. Our study thus opens a novel spectroscopic pathway for improving the performance of battery materials.

Characterization of Thin Film Materials using SCAN meta-GGA, an Accurate Nonempirical Density Functional

We discuss self-consistently obtained ground-state electronic properties of monolayers of graphene and a number of ’beyond graphene’ compounds, including films of transition-metal dichalcogenides (TMDs), using the recently proposed strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation (meta-GGA) to the density functional theory. The SCAN meta-GGA results are compared with those based on the local density approximation (LDA) as well as the generalized gradient approximation (GGA). As expected, the GGA yields expanded lattices and softened bonds in relation to the LDA, but the SCAN meta-GGA systematically improves the agreement with experiment. Our study suggests the efficacy of the SCAN functional for accurate modeling of electronic structures of layered materials in high-throughput calculations more generally.

Angle-Resolved Spectroscopy of Positronium Emission from a Cu(110) Surface

The affinity A_{Ps} of positronium (Ps) atoms for a metal is the negative of the maximum kinetic energy with which Ps is emitted into vacuum when thermalized positrons in a metal encounter the surface. When this quantity is measured by ground state Ps time of flight (TOF), the precision is severely limited by the short triplet state lifetime of 142 ns. By quickly converting the emitted Ps atoms into long-lived Rydberg states, we are able to dramatically increase the TOF to allow precision measurements of A_{Ps}. From our measurements made on a Cu(110) sample at T=128  K, we find A_{Ps}(128  K)=(-2.476±0.010_{stat}±0.013_{syst})  eV, compared with the result A_{Ps}(128  K)=(-2.545±0.010_{num}±0.010_{syst})  eV found using highly accurate generalized gradient approximations for both electrons and positrons within density functional theory. Such precision opens up opportunities in the quest for an improved density functional.

Understanding the magnetic interaction between intrinsic defects and impurity ions in room-temperature ferromagnetic Mg1-xFexO thin films

Understanding the nature and characteristics of the intrinsic defects and impurities in the dielectric barrier separating the ferromagnetic electrodes in a magnetic tunneling junction is of great importance for understanding the often observed 'barrier-breakdown' therein. In this connection, we present herein systematic experimental (SQUID and synchrotron-radiation-based x-ray absorption spectroscopy) and computational studies on the electronic and magnetic properties of Mg1-xFexO thin films. Our studies reveal: (i) defect aggregates comprised of basic and trimer units (Fe impurity coupled to 1 or 2 Mg vacancies) and (ii) existence of two competing magnetic orders, defect- and dopant-induced, with spin densities aligning anti-parallel if the trimer is present in the oxide matrix. These findings open up new avenues for designing tunneling barriers with high endurance and tunneling effect upon tuning the concentration/distribution of the two magnetic orders.

Experimental Observation of Redox-Induced Fe-N Switching Behavior as a Determinant Role for Oxygen Reduction Activity

The commercialization of electrochemical energy conversion and storage devices relies largely upon the development of highly active catalysts based on abundant and inexpensive materials. Despite recent achievements in this respect, further progress is hindered by the poor understanding of the nature of active sites and reaction mechanisms. Herein, by characterizing representative iron-based catalysts under reactive conditions, we identify three Fe-N4-like catalytic centers with distinctly different Fe-N switching behaviors (Fe moving toward or away from the N4-plane) during the oxygen reduction reaction (ORR), and show that their ORR activities are essentially governed by the dynamic structure associated with the Fe(2+/3+) redox transition, rather than the static structure of the bare sites. Our findings reveal the structural origin of the enhanced catalytic activity of pyrolyzed Fe-based catalysts compared to nonpyrolyzed Fe-macrocycle compounds. More generally, the fundamental insights into the dynamic nature of transition-metal compounds during electron-transfer reactions will potentially guide rational design of these materials for broad applications.

Extracting the cation distributions in NiFe2-xAlxO4 solid solutions using magnetic Compton scattering

We discuss the ground state electronic structure and magnetization properties of a series of NiFe2-x Al x O4 solid solutions (x  =  0.0, 0.4, 0.8, 1.2, 1.6, and 2.0) using magnetic Compton scattering measurements, together with parallel first-principles computations. In this way, we systematically extract the complicated cation distributions in this ferrite system as a function of x. The relationship between the electronic ground state, magnetism, and cation distributions is explained in terms of a model, validated by our first-principles computations, wherein the magnetic properties of the three cation distributions are assumed to be different. A good accord is found between the computed and measured magnetic Compton profiles.

Visualizing the mixed bonding properties of liquid boron with high-resolution x-ray Compton scattering

Bonding characteristics of liquid boron at 2500 K are studied by using high-resolution Compton scattering. An excellent agreement is found between the measurements and the corresponding Car-Parrinello molecular dynamics simulations. Covalent bond pairs are clearly shown to dominate in liquid boron along with the coexistence of diffuse pairs. Our study reveals the complex bonding pattern of liquid boron and gives insight into the unusual properties of this high-temperature liquid.

Proposed parameter-free model for interpreting the measured positron annihilation spectra of materials using a generalized gradient approximation

Positron annihilation spectroscopy is often used to analyze the local electronic structure of materials of technological interest. Reliable theoretical tools are crucial to interpret the measured spectra. Here, we propose a parameter-free gradient correction scheme for a local-density approximation obtained from high-quality quantum Monte Carlo data. The results of our calculations compare favorably with positron affinity and lifetime measurements, opening new avenues for highly precise and advanced positron characterization of materials.

Extracting the redox orbitals in Li battery materials with high-resolution x-ray compton scattering spectroscopy

We present an incisive spectroscopic technique for directly probing redox orbitals based on bulk electron momentum density measurements via high-resolution x-ray Compton scattering. Application of our method to spinel Li_{x}Mn_{2}O_{4}, a lithium ion battery cathode material, is discussed. The orbital involved in the lithium insertion and extraction process is shown to mainly be the oxygen 2p orbital. Moreover, the manganese 3d states are shown to experience spatial delocalization involving 0.16±0.05 electrons per Mn site during the battery operation. Our analysis provides a clear understanding of the fundamental redox process involved in the working of a lithium ion battery.

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.

Redox orbitals in LixMn2O4(0 < x < 2) studied by X-ray Compton scattering

LixMn2O4 is attracting much interest as a positive electrode material for Li-ion rechargeable batteries. Redox orbitals of LixMn2O4 under the charge or discharge process are not fully understood yet. Some band calculations have pointed out that intercalated Li 2s electrons occupy Mn sites or down-spin Mn 3d bands [1,2]. On the other hand molecular orbital calculation has reported the Li 2s electrons occupy O sites [3]. To clarify the redox orbital is important to understand the electrochemical reaction in the electrodes. In this study we have investigated the redox orbitals in LixMn2O4 by X-ray Compton scattering. Compton profiles were measured at BL08W of SPring-8, Japan. The energy of incident X-rays were 115keV and the scattering angle was 165 degrees. Energy spectra of Compton scattered X-rays were measured using a two-dimensional position sensitive detector. The measurements were performed under room temperature and vacuum conditions. Samples are polycrystalline of LixMn2O4 (x=0.5, 1.1, 1.2, 1.8 and 2.0). In order to clarify the redox orbitals of LixMn2O4, we obtained difference Compton profiles which represent the incremental electronic states on Li intercalation. Comparing the results with KKR-CPA and DFT calculations, we found that the O 2p bands play an important role for the redox process in LixMn2O4 with 0<x<2.

Engineering a Robust Photovoltaic Device with Quantum Dots and Bacteriorhodopsin

We present a route toward a radical improvement in solar cell efficiency using resonant energy transfer and sensitization of semiconductor metal oxides with a light-harvesting quantum dot (QD)/bacteriorhodopsin (bR) layer designed by protein engineering. The specific aims of our approach are (1) controlled engineering of highly ordered bR/QD complexes; (2) replacement of the liquid electrolyte by a thin layer of gold; (3) highly oriented deposition of bR/QD complexes on a gold layer; and (4) use of the Forster resonance energy transfer coupling between bR and QDs to achieve an efficient absorbing layer for dye-sensitized solar cells. This proposed approach is based on the unique optical characteristics of QDs, on the photovoltaic properties of bR, and on state-of-the-art nanobioengineering technologies. It permits spatial and optical coupling together with control of hybrid material components on the bionanoscale. This method paves the way to the development of the solid-state photovoltaic device with the efficiency increased to practical levels.

Energy transfer performance of mechanical nanoresonators coupled with electromagnetic fields

: We study the energy transfer performance in electrically and magnetically coupled mechanical nanoresonators. Using the resonant scattering theory, we show that magnetically coupled resonators can achieve the same energy transfer performance as for their electrically coupled counterparts or even outperform them within the scale of interest. Magnetic and electric coupling are compared in the nanotube radio, a realistic example of a nano-scale mechanical resonator. The energy transfer performance is also discussed for a newly proposed bio-nanoresonator composed of magnetosomes coated with a net of protein fibers.

Muonium emission into vacuum from mesoporous thin films at cryogenic temperatures

We report on muonium (Mu) emission into vacuum following μ(+) implantation in mesoporous thin SiO(2) films. We obtain a yield of Mu into vacuum of (38±4)% at 250 K and (20±4)% at 100 K for 5 keV μ(+) implantation energy. From the implantation energy dependence of the Mu vacuum yield we determine the Mu diffusion constants in these films: D(Mu)(250 K)=(1.6±0.1)×10(-4)  cm(2)/s and D(Mu)(100 K)=(4.2±0.5)×10(-5) cm(2)/s. Describing the diffusion process as quantum mechanical tunneling from pore to pore, we reproduce the measured temperature dependence ∼T(3/2) of the diffusion constant. We extract a potential barrier of (-0.3±0.1) eV which is consistent with our computed Mu work function in SiO(2) of [-0.3,-0.9] eV. The high Mu vacuum yield, even at low temperatures, represents an important step toward next generation Mu spectroscopy experiments.

Persistence of covalent bonding in liquid silicon probed by inelastic x-ray scattering

Metallic liquid silicon at 1787 K is investigated using x-ray Compton scattering. An excellent agreement is found between the measurements and the corresponding Car-Parrinello molecular dynamics simulations. Our results show persistence of covalent bonding in liquid silicon and provide support for the occurrence of theoretically predicted liquid-liquid phase transition in supercooled liquid states. The population of covalent bond pairs in liquid silicon is estimated to be 17% via a maximally localized Wannier function analysis. Compton scattering is shown to be a sensitive probe of bonding effects in the liquid state.

Efficient magnetic torque transduction in biological environments using tunable nanomechanical resonators

Electromagnetic interactions with biological systems promise new possibilities in medical applications and synthetic biology. Creating a controlled action in biological systems requires an efficient transduction of the electromagnetic energy to thermal or mechanical biosignals. In this paper, we present the design and optimization for a nano-scale magnetic torque transducer based on a tunable nanomechanical resonator. Operating in the resonance regime allows the presented system to efficiently absorb a large amount of energy from the source. In addition, systems tuned on well separated resonance frequencies may operate simultaneously without any interference. We describe the theoretical model of the system and show the possibility of achieving the resonance in biological settings for a system with reasonable dimensions.

Auger-mediated sticking of positrons to surfaces: evidence for a single-step transition from a scattering state to a surface image potential bound state

We present the observation of an efficient mechanism for positron sticking to surfaces termed here Auger-mediated sticking. In this process the energy associated with the positrons transition from an unbound scattering state to a bound image potential state is coupled to a valence electron which can then have sufficient energy to leave the surface. Compelling evidence for this mechanism is found in a narrow secondary electron peak observed at incident positron kinetic energies well below the electron work function.

Role of oxygen electrons in the metal-insulator transition in the magnetoresistive oxide La2-2xSr1+2xMn2O7 probed by compton scattering

We have studied the [100]-[110] anisotropy of the Compton profile in the bilayer manganite. Quantitative agreement is found between theory and experiment with respect to the anisotropy in the two metallic phases (i.e., the low temperature ferromagnetic and the colossal magnetoresistant phase under a magnetic field of 7 T). Robust signatures of the metal-insulator transition are identified in the momentum density for the paramagnetic phase above the Curie temperature. We interpret our results as providing direct evidence for the transition from the metalliclike to the admixed ionic-covalent bonding accompanying the magnetic transition. The number of electrons involved in this phase transition is estimated. Our study demonstrates the sensitivity of the Compton scattering technique for identifying the number and type of electrons involved in the metal-insulator transition.

Importance of local band effects for ferromagnetism in hole-doped La2CuO4 cuprate superconductors

Band calculations for supercells of La((2-x))Ba(x)CuO(4) show that the rigid band model for doping is less adequate than what is commonly assumed. In particular, weak ferromagnetism can appear locally around clusters of high Ba concentration. The clustering is important at large dilution, and averaged models for magnetism, such as the virtual crystal approximation, are unable to stabilize magnetic moments. These results give support to the idea that weak ferromagnetism can be the cause of the destruction of superconductivity at high hole doping.

Physisorption of positronium on quartz surfaces

Whether positronium (Ps) can be physisorbed on a material surface is of great fundamental interest, since it can lead to new insight regarding quantum sticking and is a necessary first step to try to obtain a Ps2 molecule on a material host. Experiments in the past have produced evidence for physisorbed Ps on a quartz surface, but firm theoretical support for such a conclusion was lacking. We present a first-principles density-functional calculation of the key parameters determining the interaction potential between Ps and an alpha-quartz surface. We show that there is indeed a bound state with an energy of 0.14 eV, a value which agrees very well with the experimental estimate of approximately 0.15 eV. Further, a brief energy analysis invoking the Langmuir-Hinshelwood mechanism for the reaction of physisorbed atoms shows that the formation and desorption of a Ps2 molecule in that picture is consistent with the above results.

Anomalous electronic correlations in the ground state momentum density of Al(97)Li(3)

We report high resolution Compton scattering measurements on an Al(97)Li(3) disordered alloy single crystal for momentum transfer along the [100], [110], and [111] symmetry directions. The results are interpreted via corresponding Korringa-Kohn-Rostoker coherent potential approximation first-principles computations. By comparing spectra for Al(97)Li(3) and Al, we show that the momentum density in the alloy differs significantly from the predictions of the conventional Fermi-liquid picture and that the ground state of Al is modified anomalously by the addition of Li.

Study of colloidal quantum-dot surfaces using an innovative thin-film positron 2D-ACAR method

Nanosized inorganic particles are of great interest because their electronic properties can be easily tailored, providing a tremendous potential for applications in optoelectronic devices, light-emitting diodes, solar cells and hydrogen storage. Confinement of electrons and holes to dimensions comparable to their wavelength leads to quantum-well states with modified wavefunctions and density of states. Surface phenomena are crucial in determining nanoparticle properties in view of their large surface-to-volume ratio. Despite a wealth of information, many fundamental questions about the nature of the surface and its relationship with the electronic structure remain unsolved. Ab initio calculations on CdSe nanocrystals suggest that passivating the ligands does not produce the ideal wurtzite structure and that Se atoms relax outwards irrespective of passivation. Here we show that implanted positrons are trapped at the surface of CdSe nanocrystals. They annihilate mostly with the Se electrons, monitor changes in composition and structure of the surface while hardly sensing the ligand molecules, and we thus unambiguously confirm the predicted strong surface relaxation.

Temperature-dependent orbital degree of freedom of a bilayer manganite by magnetic compton scattering

We have measured temperature-dependent magnetic Compton profiles (MCPs) from a single crystal of La1.2Sr1.8Mn2O7. The MCPs, which involved the scattering of circularly polarized x rays, are in general related to the momentum density of all the unpaired spins in the system. Nevertheless, we show that when the x-ray scattering vector lies along the [110] direction, the number of magnetic electrons of a specific symmetry, i.e., d electrons of x(2)-y(2) symmetry, yield a distinct signature in the MCP, allowing us to monitor substantial changes in the occupancy of the dx(2)(-y(2)) states over the investigated temperature range of 5-200 K. This study indicates that magnetic Compton scattering can provide a powerful window on the properties of specific magnetic electrons in complex materials.

Compton Spectroscopy and Chemical Bonding

In this article we show with the help of two examples how Compton spectroscopy may be used to study the effect of chemical bonding in materials as diverse as a molecular crystal and a high temperature superconductor. Compton spectroscopy has a long history as an investigative method in condensed matter physics and in fact the realisation that the Compton profile is sensitive to the effects of chemical bonding dates back at least fifty years. In the seventies, through the efforts of Weyrich [1] and others [2,3], practical applications of this realisation were first achieved. We argue that such studies are more and more relevant thanks to the availability of synchrotron radiation and efficient computational tools.