61 research outputs found
Fermi surfaces and quasi-particle band dispersions of the iron pnictides superconductor KFe2As2 observed by angle-resolved photoemission spectroscopy
We have performed an angle-resolved photoemission study of the iron pnictide
superconductor KFe2As2 with Tc 4 K. Most of the observed Fermi surfaces show
almost two-dimensional shapes, while one of the quasi-particle bands near the
Fermi level has a strong dispersion along the kz direction, consistent with the
result of a band-structure calculation. However, hole Fermi surfaces \alpha and
\zeta are smaller than those predicted by the calculation while other Fermi
surfaces are larger. These observations are consistent with the result of a de
Haas-van Alphen study and a theoretical prediction on inter-band scattering,
possibly indicating many body effects on the electronic structure.Comment: 4 pages, 5 figures. Proceeding of the 9th International Conference on
Spectroscopies in Novel Superconductors (SNS2010
Slater to Mott crossover in the metal to insulator transition of Nd2Ir2O7
We present an angle-resolved photoemission study of the electronic structure
of the three-dimensional pyrochlore iridate Nd2Ir2O7 through its magnetic
metal-insulator transition. Our data reveal that metallic Nd2Ir2O7 has a
quadratic band, touching the Fermi level at the Gamma point, similarly to that
of Pr2Ir2O7. The Fermi node state is, therefore, a common feature of the
metallic phase of the pyrochlore iridates. Upon cooling below the transition
temperature, this compound exhibits a gap opening with an energy shift of
quasiparticle peaks like a band gap insulator. The quasiparticle peaks are
strongly suppressed, however, with further decrease of temperature, and
eventually vanish at the lowest temperature, leaving a non-dispersive flat band
lacking long-lived electrons. We thereby identify a remarkable crossover from
Slater to Mott insulators with decreasing temperature. These observations
explain the puzzling absence of Weyl points in this material, despite its
proximity to the zero temperature metal-insulator transition
Normal State Spin Dynamics of Five-band Model for Iron-pnictides
Normal state spin dynamics of the recently discovered iron-pnictide
superconductors is discussed by calculating spin structure factor S(q, omega)
in an itinerant five-band model within RPA approximation. Due to the
characteristic Fermi surface structure of iron-pnictide, column like response
is found at (pi, 0) in extended Brillouin zone in the undoped case, which is
consistent with the recent neutron scattering experiment. This indicates that
the localized spin model is not necessary to explain the spin dynamics of this
system. Furthermore, we show that the temperature dependence of inelastic
neutron scattering intensity can be well reproduced in the itinerant model. We
also study NMR 1/T_1T in the same footing calculation and show that the
itinerant model can capture the magnetic property of iron-pnictide
superconductors.Comment: 4 page
d- and f-orbital correlations in the REFeAsO compounds
We estimate theoretically the strength of the local Coulomb interaction for
the Fe 3d and Ce 4f shells in the REFeAsO compunds. In LaFeAsO and CeFeAsO we
obtain values of the local Coulomb interaction parameter U for both Fe and Ce
which are larger than those of elemental Fe and Ce metals. The Fe 3d bandwidth
of REFeAsO is found to increase slightly as one moves along the RE-series.
Using a combined local density approximation and dynamical mean-field theory
(LDA+DMFT) approach, we study the behaviour of the localized 4f states along
the rare-earth oxyarsenides REFeAsO series (RE=Ce,Pr,Nd). In CeFeAsO the
occupied Ce 4f band is located just below the Fe 3d band leading possibly to a
Kondo screening of the 4f local moment under applied pressure, while the
unscreened local moment behaviour is expected for the Pr and Nd compounds.Comment: 7 pages, 2 figures, 1 tabl
Effect of Polypropylene fibres on the Workability parameters of Extrudable Cementitious Material
Additive manufacturing in construction industry has been introduced as an aspiration for a more sustainable built environment and currently evolving with high demand amongst researches. This study is an investigation of the influence of polypropylene (PP) fibre addition on the workability parameters of a new extrudable concrete mixture. As the quality of final printed structure prominently depends on the fresh state properties of concrete, this investigation mainly focused on the rheological properties such as workability (flow), setting time, extrudability and buildability. These parameters were systematically investigated through a small scale experimental process with time after mixing. The selected control mix with Ground granulated blast furnace slag (GGBS) and Silica Fume (SF) was used in this analysis. The Control cementitious specimens without fibre inclusion and with fibre addition in different volume fraction of binder, ranging from 0.5% to 3% were printed. The results showed that the fibre addition of 0, 0.5 and 1.0% have the better flowability and extrudability compared to 1.5, 2 and 3%. Also, reduction in the print quality was assessed visually with increasing fibre percentage. However, results indicated that the initial setting time is comparatively low for those mixes with higher fibre inclusion which is required for better bond strength between layers. Moreover, higher fibre content caused better buildability and shape retention in the extruded samples
Electronic Structure Calculation by First Principles for Strongly Correlated Electron Systems
Recent trends of ab initio studies and progress in methodologies for
electronic structure calculations of strongly correlated electron systems are
discussed. The interest for developing efficient methods is motivated by recent
discoveries and characterizations of strongly correlated electron materials and
by requirements for understanding mechanisms of intriguing phenomena beyond a
single-particle picture. A three-stage scheme is developed as renormalized
multi-scale solvers (RMS) utilizing the hierarchical electronic structure in
the energy space. It provides us with an ab initio downfolding of the global
band structure into low-energy effective models followed by low-energy solvers
for the models. The RMS method is illustrated with examples of several
materials. In particular, we overview cases such as dynamics of semiconductors,
transition metals and its compounds including iron-based superconductors and
perovskite oxides, as well as organic conductors of kappa-ET type.Comment: 44 pages including 38 figures, to appear in J. Phys. Soc. Jpn. as an
invited review pape
Determination of Superconducting Gap of SmFeAsFxO1-x Superconductors by Andreev Reflection Spectroscopy
The superconducting gap in FeAs-based superconductor SmFeAs(O1-xFx) (x = 0.15
and 0.30) and the temperature dependence of the sample with x = 0.15 have been
measured by Andreev reflection spectroscopy. The intrinsic superconducting gap
is independent of contacts while many other "gap-like" features vary
appreciably for different contacts. The determined gap value of 2D = 13.34
+/-0.47 meV for SmFeAs(O0.85F0.15) gives 2D/kBTC = 3.68, close to the BCS
prediction of 3.53. The superconducting gap decreases with temperature and
vanishes at TC, in a manner similar to the BCS behavior but dramatically
different from that of the nodal pseudogap behavior in cuprate superconductors.Comment: 13 pages, 9 figures, Special Issue of Physica C on Superconducting
Pnictide
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