Thickness-induced metal to insulator transition in Ru nanosheets probed by photoemission spectroscopy: Effects of disorder and Coulomb interaction.

We investigated the electronic structures of mono- and few-layered Ru nanosheets (N layers (L) with N = 1, ~6, and ~9) on Si substrate by ultra-violet and x-ray photoemission spectroscopies. The spectral density of states (DOS) near EF of ~6 L and 1 L is suppressed as it approaches EF in contrast to that of ~9 L, which is consistent with the Ru 3 d core-level shift indicating the reduction of the metallic conductivity. A power law g(ε) ∝ |ε - εF|α well reproduces the observed spectral DOS of ~6 L and 1 L. The evolution of the power factor α suggests that the transition from the metallic state of ~9 L to the 2-dimensional insulating state with the soft Coulomb gap of 1 L through the disordered 3-dimensional metallic state of ~6 L.

Structural analysis of lysine-4 methylated histone H3 proteins using synchrotron radiation circular dichroism spectroscopy.

We report structural alterations of histone H3 proteins induced by lysine-4 (K4) monomethylation, dimethylation, and trimethylation identified by using synchrotron radiation circular dichroism spectroscopy. Compared with unmethylated H3, monomethylation and dimethylation induced increases in α-helix structures and decreases in ß-strand structures. In contrast, trimethylation decreased α-helix content but increased ß-strand content. The structural differences among K4-unmethylated/methylated H3 may allow epigenetic enzymes to discriminate the substrates both chemically and sterically.

Circular dichroism spectroscopic study on structural alterations of histones induced by post-translational modifications in DNA damage responses: lysine-9 methylation of H3.

We report the global structural alterations in histone H3 proteins induced by lysine-9 mono-, di- and trimethylation, which are part of the critical post-translational modifications for DNA damage responses, identified using synchrotron radiation circular dichroism (CD) spectroscopy. Compared with unmodified H3, mono- and dimethylation increases the number of α-helices and decreases the numbers of ß-strands, while trimethylation decreases the α-helix content and increases the ß-strand content. Comparison of the secondary-structure contents of these histone H3 proteins suggests that the methylation-induced structural alterations occur at residues not only close to but also distant from the methylated sites. Such global structural alterations may regulate the interactions of methylated histones with other molecules, such as histone-binding proteins in DNA damage repair processes.

Development of laser-based scanning µ-ARPES system with ultimate energy and momentum resolutions.

We have developed a laser-based scanning angle-resolved photoemission spectroscopy system (µ-ARPES) equipped with a high precision 6-axis control system, realizing not only high-resolution photoemission spectroscopy in energy and momentum, but also spatial resolution of a µm scale. This enables our µ-ARPES system to probe fine details of intrinsic electronic states near the Fermi level such as the superconducting gaps and lifetime broadening.

Electron Number-Based Phase Diagram of Pr_{1-x}LaCe_{x}CuO_{4-δ} and Possible Absence of Disparity between Electron- and Hole-Doped Cuprate Phase Diagrams.

We performed annealing and angle resolved photoemission spectroscopy studies on electron-doped cuprate Pr_{1-x}LaCe_{x}CuO_{4-δ} (PLCCO). It is found that the optimal annealing condition is dependent on the Ce content x. The electron number (n) is estimated from the experimentally obtained Fermi surface volume for x=0.10, 0.15 and 0.18 samples. It clearly shows a significant and annealing dependent deviation from the nominal x. In addition, we observe that the pseudo-gap at hot spots is also closely correlated with n; the pseudogap gradually closes as n increases. We established a new phase diagram of PLCCO as a function of n. Different from the x-based one, the new phase diagram shows similar antiferromagnetic and superconducting phases to those of hole doped ones. Our results raise a possibility for absence of disparity between the phase diagrams of electron- and hole-doped cuprates.

DNA damage response induces structural alterations in histone H3-H4.

Synchrotron-radiation circular-dichroism spectroscopy was used to reveal that the DNA damage response induces a decrement of α-helix and an increment of ß-strand contents of histone H3-H4 extracted from X-ray-irradiated human HeLa cells. The trend of the structural alteration was qualitatively opposite to that of our previously reported results for histone H2A-H2B. These results strongly suggest that histones share roles in DNA damage responses, particularly in DNA repair processes and chromatin remodeling, via a specific structural alteration of each histone.

Conformation of membrane-bound proteins revealed by vacuum-ultraviolet circular-dichroism and linear-dichroism spectroscopy.

Knowledge of the conformations of a water-soluble protein bound to a membrane is important for understanding the membrane-interaction mechanisms and the membrane-mediated functions of the protein. In this study we applied vacuum-ultraviolet circular-dichroism (VUVCD) and linear-dichroism (LD) spectroscopy to analyze the conformations of α-lactalbumin (LA), thioredoxin (Trx), and ß-lactoglobulin (LG) bound to phosphatidylglycerol liposomes. The VUVCD analysis coupled with a neural-network analysis showed that these three proteins have characteristic helix-rich conformations involving several helical segments, of which two amphiphilic or hydrophobic segments take part in interactions with the liposome. The LD analysis predicted the average orientations of these helix segments on the liposome: two amphiphilic helices parallel to the liposome surface for LA, two hydrophobic helices perpendicular to the liposome surface for Trx, and a hydrophobic helix perpendicular to and an amphiphilic helix parallel to the liposome surface for LG. This sequence-level information about the secondary structures and orientations was used to formulate interaction models of the three proteins at the membrane surface. This study demonstrates the validity of a combination of VUVCD and LD spectroscopy in conformational analyses of membrane-binding proteins, which are difficult targets for X-ray crystallography and nuclear magnetic resonance spectroscopy.

Characteristic two-dimensional Fermi surface topology of high-Tc iron-based superconductors.

Unconventional Cooper pairing originating from spin or orbital fluctuations has been proposed for iron-based superconductors. Such pairing may be enhanced by quasi-nesting of two-dimensional electron and hole-like Fermi surfaces (FS), which is considered an important ingredient for superconductivity at high critical temperatures (high-Tc). However, the dimensionality of the FS varies for hole and electron-doped systems, so the precise importance of this feature for high-Tc materials remains unclear. Here we demonstrate a phase of electron-doped CaFe2As2 (La and P co-doped CaFe2As2) with Tc = 45 K, which is the highest Tc found for the AEFe2As2 bulk superconductors (122-type; AE = Alkaline Earth), possesses only cylindrical hole- and electron-like FSs. This result indicates that FS topology consisting only of two-dimensional sheets is characteristic of both hole- and electron-doped 122-type high-Tc superconductors.

Characterization of intermolecular structure of ß(2)-microglobulin core fragments in amyloid fibrils by vacuum-ultraviolet circular dichroism spectroscopy and circular dichroism theory.

Intermolecular structures are important factors for understanding the conformational properties of amyloid fibrils. In this study, vacuum-ultraviolet circular dichroism (VUVCD) spectroscopy and circular dichroism (CD) theory were used for characterizing the intermolecular structures of ß2-microglobulin (ß2m) core fragments in the amyloid fibrils. The VUVCD spectra of ß2m20-41, ß2m21-31, and ß2m21-29 fragments in the amyloid fibrils exhibited characteristic features, but they were affected not only by the backbone conformations but also by the aromatic side-chain conformations. To estimate the contributions of aromatic side-chains to the spectra, the theoretical spectra were calculated from the simulated structures of ß2m21-29 amyloid fibrils with various types of ß-sheet stacking (parallel or antiparallel) using CD theory. We found that the experimental spectrum of ß2m21-29 fibrils is largely affected by aromatic-backbone couplings, which are induced by the interaction between transitions within the aromatic and backbone chromophores, and these couplings are sensitive to the type of stacking among the ß-sheets of the fibrils. Further theoretical analyses of simulated structures incorporating mutated aromatic residues suggested that the ß2m21-29 fibrils are composed of amyloid accumulations in which the parallel ß-sheets stack in an antiparallel manner and that the characteristic Phe-Tyr interactions among the ß-sheet stacks affect the aromatic-backbone coupling. These findings indicate that the coupling components, which depend on the characteristic intermolecular structures, induce the spectral differences among three fragments in the amyloid fibrils. These advanced spectral analyses using CD theory provide a useful method for characterizing the intermolecular structures of protein and peptide fragment complexes.

Experimental evidence of hidden topological surface states in PbBi4Te7.

A topological surface state that is protected physically under the Bi2Te3-like five-layer block has been revealed on the Pb-based topological insulator (TI) PbBi4Te7 by bulk sensitive angle-resolved photoelectron spectroscopy (ARPES). Furthermore, conservation of the spin polarization of the hidden topological surface states is directly confirmed by bulk-sensitive spin ARPES observation. This finding paves the way to realize the real spintronics devices by TIs that are operable in the real environment.

Doping dependence of low-energy quasiparticle excitations in superconducting Bi2212.

: The doping-dependent evolution of the d-wave superconducting state is studied from the perspective of the angle-resolved photoemission spectra of a high-Tc cuprate, Bi2Sr2CaCu2 O8+δ (Bi2212). The anisotropic evolution of the energy gap for Bogoliubov quasiparticles is parametrized by critical temperature and superfluid density. The renormalization of nodal quasiparticles is evaluated in terms of mass enhancement spectra. These quantities shed light on the strong coupling nature of electron pairing and the impact of forward elastic or inelastic scatterings. We suggest that the quasiparticle excitations in the superconducting cuprates are profoundly affected by doping-dependent screening.

'True' bosonic coupling strength in strongly correlated superconductors.

Clarifying the coupling between electrons and bosonic excitations (phonons or magnetic fluctuations) that mediate the formation of Cooper pairs is pivotal to understand superconductivity. Such coupling effects are contained in the electron self-energy, which is experimentally accessible via angle-resolved photoemission spectroscopy (ARPES). However, in unconventional superconductors, identifying the nature of the electron-boson coupling remains elusive partly because of the significant band renormalization due to electron correlation. Until now, to quantify the electron-boson coupling, the self-energy is most often determined by assuming a phenomenological 'bare' band. Here, we demonstrate that the conventional procedure underestimates the electron-boson coupling depending on the electron-electron coupling, even if the self-energy appears to be self-consistent via the Kramers-Kronig relation. Our refined method explains well the electron-boson and electron-electron coupling strength in ruthenate superconductor Sr2RuO4, calling for a critical revision of the bosonic coupling strength from ARPES self-energy in strongly correlated electron systems.

Perpendicular magnetic anisotropy with enhanced orbital moments of Fe adatoms on a topological surface of Bi2Se3.

We have found a perpendicular magnetic anisotropy of iron adatoms on a surface of the prototypical three-dimensional topological insulator Bi2Se3 by using x-ray magnetic circular dichroism measurements. The orbital magnetic moment of Fe is strongly enhanced at lower coverage, where angle-resolved photoemission spectroscopy shows coexistence of non-trivial topological states at the surface.

Vacuum-ultraviolet electronic circular dichroism study of methyl α-D-glucopyranoside in aqueous solution by time-dependent density functional theory.

The vacuum-ultraviolet (VUV) electronic circular dichroism (ECD) spectrum of methyl α-D-glucopyranoside (methyl α-D-Glc) was measured down to 163 nm in aqueous solution using a synchrotron-radiation VUV-ECD spectrophotometer. The spectrum exhibited two characteristic ECD peaks around 170 nm, which depend on the trans (T) and gauche (G) configurations of the hydroxymethyl group at C-5. To elucidate the influences of the T and G configurations on the spectrum, the ECD spectra of three rotamers (α-GT, α-GG, and α-TG) of methyl α-D-Glc were calculated using time-dependent density functional theory (TDDFT) combined with molecular dynamics simulation. A linear combination of the ECD spectra of these three rotamers, which differ markedly from each other, produced a methyl α-D-Glc spectrum similar to that observed experimentally. The spectrum was assignable to the n-σ* transitions of the ring oxygen and methoxy oxygen with minor contributions from the hydroxyl oxygen. The differences in α-GT, α-GG, and α-TG spectra were attributed to fluctuations of the configurations of the hydroxymethyl group at C-5 and the hydroxyl group at C-4, which strongly affected the orientations of intramolecular hydrogen bonds around the ring oxygen. These findings demonstrate that combining VUV-ECD and TDDFT is useful for structural characterization of saccharides in aqueous solution.

Chiral orbital-angular momentum in the surface states of Bi2Se3.

We performed angle-resolved photoemission (ARPES) experiments with circularly polarized light and first-principles density functional calculation with spin-orbit coupling to study surface states of a topological insulator Bi2Se3. We observed circular dichroism (CD) as large as 30% in the ARPES data with upper and lower Dirac cones showing opposite signs in CD. The observed CD is attributed to the existence of local orbital-angular momentum (OAM). First-principles calculation shows that OAM in the surface states is significant and is locked to the electron momentum in the opposite direction to the spin, forming chiral OAM states. Our finding opens a new possibility for strong light-induced spin-polarized current in surface states. We also provide a proof for local OAM origin of the CD in ARPES.

Core-to-Rydberg band shift and broadening of hydrogen bonded ammonia clusters studied with nitrogen K-edge excitation spectroscopy.

Nitrogen 1s (N 1s) core-to-Rydberg excitation spectra of hydrogen-bonded clusters of ammonia (AM) have been studied in the small cluster regime of beam conditions with time-of-flight (TOF) fragment-mass spectroscopy. By monitoring partial-ion-yield spectra of cluster-origin products, "cluster" specific excitation spectra could be recorded. Comparison of the "cluster" band with "monomer" band revealed that the first resonance bands of clusters corresponding to N 1s → 3sa(1)/3pe of AM monomer are considerably broadened. The changes of the experimental core-to-Rydberg transitions ΔFWHM (N 1s → 3sa(1)/3pe) = ~0.20/~0.50 eV compare well with the x ray absorption spectra of the clusters generated by using density functional theory (DFT) calculation. The broadening of the core-to-Rydberg bands in small clusters is interpreted as being primarily due to the splitting of non-equivalent core-hole N 1s states caused by both electrostatic core-hole and hydrogen-bonding (H(3)N···H-NH(2)) interactions upon dimerization. Under Cs dimer configuration, core-electron binding energy of H-N (H-donor) is significantly decreased by the intermolecular core-hole interaction and causes notable redshifts of core-excitation energies, whereas that of lone-pair nitrogen (H-acceptor) is slightly increased and results in appreciable blueshifts in the core-excitation bands. The result of the hydrogen-bonding interaction strongly appears in the n-σ* orbital correlation, destabilizing H-N donor Rydberg states in the direction opposite to the core-hole interaction, when excited N atom with H-N donor configuration strongly possesses the Rydberg component of anti-bonding σ* (N-H) character. Contributions of other cyclic H-bonded clusters (AM)(n) with n ≥ 3 to the spectral changes of the N 1s → 3sa(1)/3pe bands are also examined.

Secondary-structure analysis of alcohol-denatured proteins by vacuum-ultraviolet circular dichroism spectroscopy.

To elucidate the structural characteristics of alcohol-denatured proteins, we measured the vacuum-ultraviolet circular dichroism (VUVCD) spectra of six proteins-myoglobin, human serum albumin, α-lactalbumin, thioredoxin, ß-lactoglobulin, and α-chymotrypsinogen A-down to 170 nm in trifluoroethanol solutions (TFE: 0-50%) and down to 175 nm in methanol solutions (MeOH: 0-70%) at pH 2.0 and 25°C, using a synchrotron-radiation VUVCD spectrophotometer. The contents of α-helices, ß-strands, turns, poly-L-proline type II helices (PPIIs), and unordered structures of these proteins were estimated using the SELCON3 program, including the numbers of α-helix and ß-strand segments. Furthermore, the positions of α-helices and ß-strands on amino acid sequences were predicted by combining these secondary-structure data with a neural-network method. All alcohol-denatured proteins showed higher α-helix contents (up to ~ 90%) compared with the native states, and they consisted of several long helical segments. The helix-forming ability was higher in TFE than in MeOH, whereas small amounts of ß-strands without sheets were formed in the MeOH solution. The produced α-helices were transformed dominantly from the ß-strands and unordered structures, and slightly from the turns. The content and mean length of α-helix segments decreased as the number of disulfide bonds in the proteins increased, suggesting that disulfide bonds suppress helix formation by alcohols. These results demonstrate that alcohol-denatured proteins constitute an ensemble of many long α-helices, a few ß-strands and PPIIs, turns, and unordered structures, depending on the types of proteins and alcohols involved.

Te concentration dependent photoemission and inverse-photoemission study of FeSe1-xTex.

We have characterized the electronic structure of FeSe1-x Te x for various x values using soft x-ray photoemission spectroscopy (SXPES), high-resolution photoemission spectroscopy (HRPES) and inverse photoemission spectroscopy (IPES). The SXPES valence band spectral shape shows that the 2 eV feature in FeSe, which was ascribed to the lower Hubbard band in previous theoretical studies, becomes less prominent with increasing x. HRPES exhibits systematic x dependence of the structure near the Fermi level (EF): its splitting near EF and filling of the pseudogap in FeSe. IPES shows two features, near EF and approximately 6 eV above EF; the former may be related to the Fe 3d states hybridized with chalcogenide p states, while the latter may consist of plane-wave-like and Se d components. In the incident electron energy dependence of IPES, the density of states near EF for FeSe and FeTe has the Fano lineshape characteristic of resonant behavior. These compounds exhibit different resonance profiles, which may reflect the differences in their electronic structures. By combining the PES and IPES data the on-site Coulomb energy was estimated at 3.5 eV for FeSe.

End station for nanoscale magnetic materials study: combination of scanning tunneling microscopy and soft X-ray magnetic circular dichroism spectroscopy.

We have constructed an end station for nanoscale magnetic materials study at the soft X-ray beamline HiSOR BL-14 at Hiroshima Synchrotron Radiation Center. An ultrahigh-vacuum scanning tunneling microscope (STM) was installed for an in situ characterization of nanoscale magnetic materials in combination with soft X-ray magnetic circular dichroism (XMCD) spectroscopy experiment. The STM was connected to the XMCD experimental station via damper bellows to isolate it from environmental vibrations, thus achieving efficient spatial resolution for observing Si(111) surface at atomic resolution. We performed an in situ experiment with STM and XMCD spectroscopy on Co nanoclusters on an Au(111) surface and explored its practical application to investigate magnetic properties for well-characterized nanoscale magnetic materials.

Efficient spin resolved spectroscopy observation machine at Hiroshima Synchrotron Radiation Center.

Highly efficient spin- and angle-resolved photoelectron spectrometer named ESPRESSO (Efficient SPin REsolved SpectroScopy Observation) machine has been developed at the beamline BL-9B in Hiroshima Synchrotron Radiation Center. Combination of high-resolution hemispherical electron analyzer and the high-efficient spin detector based on very low energy electron diffraction by the ferromagnetic target makes the high-energy resolution and angular resolution compatible with spin- and angle-resolved photoemission (SARPES) measurement. 7.5 meV in energy and ±0.18° in angular resolution have been achieved with spin resolution. The ESPRESSO machine, combination of quick energy-band dispersion measurement and Fermi surface mapping by two-dimensional electron detector for the spin integrated ARPES and the high-efficient spin analysis by the efficient spin detector realizes the comprehensive investigation of spin electronic structure of materials.