Possibility of Flat-Band Ferromagnetism in Hole-Doped Pyrochlore Oxides Sn_{2}Nb_{2}O_{7} and Sn_{2}Ta_{2}O_{7}.

Quantum mechanics tells us that the hopping integral between local orbitals makes the energy band dispersive. In a lattice with geometric frustration, however, dispersionless flat bands may appear due to quantum interference. Several models possessing flat bands have been proposed theoretically, and many attracting magnetic and electronic properties are predicted. However, despite many attempts to realize these models experimentally, compounds that are appropriately described by this model have not been found so far. Here we show that pyrochlore oxides Sn_{2}Nb_{2}O_{7} and Sn_{2}Ta_{2}O_{7} are such examples, by performing first-principles band calculation and several tight-binding analyses. Moreover, spin-polarized band calculation shows that the hole-doped systems Sn_{2}Nb_{2}O_{6}N and Sn_{2}Ta_{2}O_{6}N have complete spin polarization, and their magnetic moments are mostly carried by Sn-s and N-p orbitals, which are usually nonmagnetic. These compounds are not only candidates for ferromagnets without a magnetic element, but also will provide an experimental platform for a flat-band model which shows a wide range of physical properties.

Rotatable high-resolution ARPES system for tunable linear-polarization geometry.

A rotatable high-resolution angle-resolved photoemission spectroscopy (ARPES) system has been developed to utilize tunable linear-polarization geometries on the linear undulator beamline (BL-1) at Hiroshima Synchrotron Radiation Center. By rotating the whole ARPES measurement system, the photoelectron detection plane can be continuously changed from parallel to normal against the electric field vector of linearly polarized undulator radiation. This polarization tunability enables us to identify the symmetry of the initial electronic states with respect to the mirror planes, and to selectively observe the electronic states based on the dipole selection rule in the photoemission process. Specifications of the rotatable high-resolution ARPES system are described, as well as its capabilities with some representative experimental results.

High-energy anomaly in the band dispersion of the ruthenate superconductor.

We reveal a "high-energy anomaly" (HEA) in the band dispersion of the unconventional ruthenate superconductor Sr2RuO4, by means of high-resolution angle-resolved photoemission spectroscopy (ARPES) with tunable energy and polarization of incident photons. This observation provides another class of correlated materials exhibiting this anomaly beyond high-T(c) cuprates. We demonstrate that two distinct types of band renormalization associated with and without the HEA occur as a natural consequence of the energetics in the bandwidth and the energy scale of the HEA. Our results are well reproduced by a simple analytical form of the self-energy based on the Fermi-liquid theory, indicating that the HEA exists at a characteristic energy scale of the multielectron excitations. We propose that the HEA universally emerges if the systems have such a characteristic energy scale inside of the bandwidth.

Highly precise and compact ultrahigh vacuum rotary feedthrough.

The precision and rigidity of compact ultrahigh vacuum (UHV) rotary feedthroughs were substantially improved by preparing and installing an optimal crossed roller bearing with mounting holes. Since there are mounting holes on both the outer and inner races, the bearing can be mounted directly to rotary and stationary stages without any fixing plates and housing. As a result, it is possible to increase the thickness of the bearing or the size of the rolling elements in the bearing without increasing the distance between the rotating and fixing International Conflat flanges of the UHV rotary feedthrough. Larger rolling elements enhance the rigidity of the UHV rotary feedthrough. Moreover, owing to the structure having integrated inner and outer races and mounting holes, the performance is almost entirely unaffected by the installation of the bearing, allowing for a precise optical encoder to be installed in the compact UHV rotary feedthrough. Using position feedback via a worm gear system driven by a stepper motor and a precise rotary encoder, the actual angle of the compact UHV rotary feedthrough can be controlled with extremely high precision.

Low-energy (<10 meV) feature in the nodal electron self-energy and strong temperature dependence of the Fermi velocity in Bi{2}Sr{2}CaCu{2}O{8+δ}.

Using low photon energy angle-resolved photoemission, we study the low-energy dispersion along the nodal (π,π) direction in Bi{2}Sr{2}CaCu{2}O{8+δ} as a function of temperature. Less than 10 meV below the Fermi energy, the high-resolution data reveal a novel "kinklike" feature in the electron self-energy that is distinct from the larger well-known kink roughly 70 meV below E{F}. This new kink is strongest below the superconducting critical temperature and weakens substantially at higher temperatures. A corollary of this finding is that the Fermi velocity v{F}, as measured in this low-energy range, varies rapidly with temperature-increasing by almost 30% from 70 to 110 K. The behavior of v{F}(T) appears to shift as a function of doping, suggesting a departure from simple "universality" in the nodal Fermi velocity of cuprates.

Interplay among Coulomb interaction, spin-orbit interaction, and multiple electron-boson interactions in Sr2RuO4.

Using polarization- and hν-dependent angle-resolved photoemission spectroscopy, we uncovered the fine details of a quasiparticle's dynamics of a typical multiband superconductor, Sr2RuO4. We found strong hybridization between the in-plane and out-of-plane quasiparticles via the Coulomb and spin-orbit interactions. This effect enhances the quasiparticle mass due to the inflow of out-of-plane quasiparticles into the two-dimensional Fermi surface sheet, where the quasiparticles are further subjected to the multiple electron-boson interactions. We suggest that the spin-triplet p-wave superconductivity of Sr2RuO4 is phonon mediated.

Isotopic fingerprint of electron-phonon coupling in high-Tc cuprates.

Angle-resolved photoemission spectroscopy with low-energy tunable photons along the nodal direction of oxygen isotope substituted Bi(2)Sr(2)CaCu(2)O(8+delta) reveals a distinct oxygen isotope shift near the electron-boson coupling "kink" in the electronic dispersion. The magnitude (a few meV) and direction of the kink shift are as expected due to the measured isotopic shift of phonon frequency, and are also in agreement with theoretical expectations. This demonstrates the participation of the phonons as dominant players, as well as pinpointing the most relevant of the phonon branches.

Interplay of electron-lattice interactions and superconductivity in superconductivity in Bi2Sr2CaCu2O8+delta.

Formation of electron pairs is essential to superconductivity. For conventional superconductors, tunnelling spectroscopy has established that pairing is mediated by bosonic modes (phonons); a peak in the second derivative of tunnel current d2I/dV2 corresponds to each phonon mode. For high-transition-temperature (high-T(c)) superconductivity, however, no boson mediating electron pairing has been identified. One explanation could be that electron pair formation and related electron-boson interactions are heterogeneous at the atomic scale and therefore challenging to characterize. However, with the latest advances in d2I/dV2 spectroscopy using scanning tunnelling microscopy, it has become possible to study bosonic modes directly at the atomic scale. Here we report d2I/dV2 imaging studies of the high-T(c) superconductor Bi2Sr2CaCu2O8+delta. We find intense disorder of electron-boson interaction energies at the nanometre scale, along with the expected modulations in d2I/dV2 (refs 9, 10). Changing the density of holes has minimal effects on both the average mode energies and the modulations, indicating that the bosonic modes are unrelated to electronic or magnetic structure. Instead, the modes appear to be local lattice vibrations, as substitution of 18O for 16O throughout the material reduces the average mode energy by approximately 6 per cent--the expected effect of this isotope substitution on lattice vibration frequencies. Significantly, the mode energies are always spatially anticorrelated with the superconducting pairing-gap energies, suggesting an interplay between these lattice vibration modes and the superconductivity.

Laser based angle-resolved photoemission, the sudden approximation, and quasiparticle-like spectral peaks in Bi2Sr2CaCu2O(8+delta).

A new low photon energy regime of angle-resolved photoemission spectroscopy is accessed with lasers and used to study the high T(C) superconductor Bi2Sr2CaCu2O(8+delta). The low energy increases bulk sensitivity, reduces background, and improves resolution. With this we observe spectral peaks which are sharp on the scale of their binding energy--the clearest evidence yet for quasiparticles in the normal state. Crucial aspects of the data such as the dispersion, superconducting gaps, and the bosonic coupling kink are found to be robust to a possible breakdown of the sudden approximation.

Angle-resolved photoemission spectroscopy study of adsorption process and electronic structure of silver on ZnO(1010).

The adsorption process and valence band structure of Ag on ZnO(1010) have been investigated by angle-resolved photoelectron spectroscopy utilizing synchrotron radiation. The coverage-dependent measurements of the Ag 4d band structure reveal that the Ag bands with a dispersing feature are formed even at low coverages and that the basic structure of the bands is essentially the same throughout the submonolayer region. These results indicate that the Ag atoms aggregate to form islands with an atomically ordered structure from the low coverages. Upon annealing the Ag-covered surface at 900 K, the Ag 4d band undergoes only a minor change, suggesting that the ordered structure within the Ag islands is persistent against mild annealing. From the dispersive feature of the Ag 4d states, we propose that the atomic structure has locally rectangular symmetry with a good lattice matching with the ZnO(1010) surface.

Kink in the dispersion of layered strontium ruthenates.

We present detailed energy dispersions near the Fermi level along the high symmetry line GammaX on the monolayer and bilayer strontium ruthenates Sr2RuO4 and Sr3Ru2O7, determined by high-resolution angle-resolved photoemission spectroscopy. A kink in the dispersion is clearly shown for the both ruthenates. The energy position of the kink and the slope in the low-energy part near the Fermi level are almost identical between them, whereas the dispersion in the high-energy part varies, like the behavior of the kink for the cuprate superconductors.

Ta 5d band symmetry of 1T-TaS1.2Se0.8 in the commensurate charge-density-wave phase.

We present a detailed angle-resolved photoemission study on the layered transition-metal dichalcogenide 1T-TaS1.2Se0.8 in the commensurate charge-density-wave (CDW) phase. A drastic reduction in the spectral weight along the high symmetry line GammaM, particularly around the point M, is observed when s-polarized light was used. This implies that the initial state must be symmetric with respect to a mirror plane perpendicular to the line GammaK, which is consistent with conventional band calculations in the absence of the CDW. We conclude that there is only a limited amount of modification of the electronic structure of 1T-TaS1.2Se0.8 in the commensurate CDW phase due to the CDW-related potential.

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.

Fusarium mycotoxins (fumonisins, nivalenol, and zearalenone) and aflatoxins in corn from Southeast Asia.

Corn samples collected from the Philippines, Thailand, and Indonesia were surveyed for the natural occurrence of Fusarium mycotoxins (fumonisins, trichothecenes, and zearalenone) and aflatoxins. Fumonisins B1 and B2 were found in over 50% of corn samples in individual countries, and their co-occurrences with aflatoxins at the incidence of 48% were noted. In addition to these mycotoxins, a trichothecene, nivalenol, and an estrogen, zearalenone, both mycotoxins of Fusarium species, were detected in these Southeast Asian samples. This is the first report on the simultaneous occurrence of two carcinogenic mycotoxins, fumonisins and aflatoxins, together with Fusarium mycotoxins (nivalenol and zearlenone) in corn from Asian tropics.