Inelastic neutron scattering on iron-based superconductor BaFe2(As,P)2

Inelastic neutron scattering has been performed on powder sample of an iron-based superconductor BaFe₂(As_0.65P_0.35)₂ with superconducting transition temperature (T_c) = 30 K, whose superconducting (SC) order parameter is expected to have line node. In the normal state, constant-E scan of dynamical structure factor, S(Q, E), exhibits a peak structure centered at momentum transfer Q ～ 1.20 Å^?1, corresponding to antiferromagnetic wave vector. Below T_c, the redistribution of the magnetic spectral weight takes place, resulting in the formation of a peak at E ～ 12 meV and a gap below 6 meV. The enhanced magnetic peak structure is ascribed to the spin resonance mode, evidencing sign change in the SC order parameter similar to other iron-based high-T_c superconductors. It suggests that fully-gapped s_± symmetry dominates in this superconductor, which gives rise to high-T_c (=30 K) despite the nodal symmetry.     

Two-dimensional and three-dimensional Fermi surfaces of superconducting BaFe2(As(1-x)P(x))2 and their nesting properties revealed by angle-resolved photoemission spectroscopy

We have studied the three-dimensional shapes of the Fermi surfaces (FSs) of BaFe(2)(As(1-x)P(x))(2) (x=0.38), where superconductivity is induced by isovalent P substitution and by angle-resolved photoemission spectroscopy. Moderately strong electron mass enhancement has been identified for both the electron and hole FSs. Among two observed hole FSs, the nearly two-dimensional one shows good nesting with the outer two-dimensional electron FS, but its orbital character is different from the outer electron FS. The three-dimensional hole FS shows poor nesting with the electron FSs. The present results suggest that the three dimensionality and the difference in the orbital character weaken FS nesting while partial nesting among the outer electron FSs of d(xy) character and/or that within the three-dimensional hole FS becomes dominant, which may lead to the nodal superconductivity.     

Enhanced Fermi-surface nesting in superconducting BaFe2(As(1-x)P(x))2 revealed by the de Haas-van Alphen effect

The three-dimensional Fermi-surface morphology of superconducting BaFe2(As0.37P0.63)2 with T(c)=9 K is determined using the de Haas-van Alphen effect. The inner electron pocket has a similar area and k(z) interplane warping to the observed hole pocket, revealing that the Fermi surfaces are geometrically well nested in the (π,π) direction. These results are in stark contrast to the fermiology of the nonsuperconducting phosphides (x=1), and therefore suggest an important role for nesting in pnictide superconductivity.     

Angle-resolved photoemission study of no chemisorption on Ag(111) surface

An angle-resolved photoemission study using synchrotron radiation has been carried out to investigate the adsorption of NO on Ag(111). At 150 K, NO is adsorbed molecularly on the Ag(111) surface. The NO molecule is found to have an upright orientation from a study using incidence angle dependent measurements and symmetry selection rules. The upright orientation is also confirmed from the study of the shape resonance of the 5σ level. Photon induced dissociation of the chemisorbed NO was observed and is briefly discussed.     

Angle-resolved photoemission from CO on Fe(110)

We report normal emission, angle-resolved photoemission measurements of the system p (1×2)Co-Fe(110) at several photon energies. The lower bind-energy feature, commonly attributed to emission from both the 1π and 5σ molecular orbitals of CO, shifts from 6.9 to 8.1 eV (relative to the Fermi energy) as the photon energy is increased from 21 to 30 eV. We attribute this dispersion to a large variation in cross section for photoionization of the 1π- and 5σ-derived orbitals and use symmetry selection rules to Identify the 6.9 eV component as the 1π and the 8.1 eV component as the 5σ. This ordering is reversed from the gas phase ordering.     

Atomically-flat,chemically-stable,superconducting epitaxial thin film of iron-based superconductor,cobalt-doped BaFe2As2

The epitaxial growth of Fe-based superconductors such as Co-doped SrFe₂As₂ (SrFe₂As₂:Co) was reported recently, but has still insufficient properties for a device application because they have rough surfaces and are decomposed by reactions with water vapor in an ambient atmosphere. This paper reports that epitaxial films of Co-doped BaFe₂As₂ grown at 700 ^°C show the onset superconducting transition temperature of ～20 K. The transition is sharper than those observed on the SrFe₂As₂:Co films, which would originate from their improved crystallinity. These films also have atomically-flat surfaces with step-and-terrace structures and exhibit chemical stability against exposure to water vapor.     

Angle-resolved photoemission study of NbC(100): Theory and experiment

The electronic band structure of NbC_0.83 has been investigated with angle-resolved photoelectron spectroscopy using HeI, NeI and ArI radiation. Experiments were carried out on the (100) surface with the emission azimuth lying in the (001) and the (0 1) symmetry planes. The experimental data are compared with theoretical spectra for NbC_1.0(100). The results show that most of the structures observed in the experimental spectra can be well explained by the bulk energy band structure of NbC_1.0. The large differences observed when comparing the spectra obtained in the two emission planes are discussed.     

Angle-resolved photoemission studies of C (2 × 2) oxygen overlayer on Ni (100)

We report angular-resolved photoemission experiments with 44 eV p-polarized radiation on a C (2 × 2) oxygen overlayer on Ni as a function of polar angle θ. A photoemission calculation using a single step model has been performed. We show that with our present expemental equipment, it is very difficult to discriminate the p_z and p_x,p_y states of oxygen, because of hybridization with Ni s-p levels, except at some high symmetry points for which partially polarized light may give a measurable distinction.     

Angle-resolved photoemission study of the Li-induced structures of the Si(001) surface

We have investigated dispersion relations of surface bands induced by Li adsorption on the Si(001) surface at room temperature by Angle-Resolved PhotoE-mission Spectroscopy (ARPES). The characteristic spectral changes in the Energy Distribution Curves (EDCs) with increasing Li coverage implicate that Li adatoms occupy on-top sites of the dimer-forming silicon atoms resulting in 0.5 and 1.0 monolayers for the completion of the first two Li-induced ordered (2×2): Li and (2×1): Li phase, respectively. The unique upward dispersions of the surface bands for these Li-induced structures are ascribed to the size effect of a Li adatom. We further discuss a mechanism which might account for the metallie peak found at the early stage of Li adsorption.     

Angle-resolved photoemission study of CoSi2 nanofilms grown on Si(111) substrates

We have carried out an angle-resolved photoemission study for CoSi₂ nanofilms grown on the Si(111)-7×7 substrates. The surface of CoSi₂(111) nanofilm changes from the bulk-truncated surface to the surface with additional Si-bilayer by annealing at higher temperature above 825 K. The angle-resolved photoemission spectra of the CoSi₂ nanofilm annealed at 853 K show the spectral features originated from the surface resonance state on the CoSi₂ surface terminated by Si-bilayer. From the detailed photoemission study, we discuss the surface electronic structure in CoSi₂(111) nanofilms grown on Si(111) substrates.     

Angle-resolved photoemission study of Ag nanofilms grown on fcc transition-metal (111) substrates

A comparative study of the electronic structures of Ag nanofilms on the pseudomorphic metastable fcc Fe(111) and bulk-like fcc Co(111) substrates has been carried out to investigate their quantized electronic structures. The photoemission spectra of both Ag nanofilms exhibit the fine structures derived from the quantized sates of Ag sp valence electron. The nanofilm-thickness dependences of the binding energy of the quantized states are reproduced by the calculated results based on the phase accumulation model. From the angle-resolved photoemission measurements, the effective masses of the quantized electronic states along the direction parallel to the nanofilm surface were directly determined. We discuss the electronic hybridization effect between quantized states in Ag nanofilm and 3d-derived electronic states in transition-metal substrates.     

Angle-resolved photoemission study of Pd(111) and Co/Pd(111)

Electronic structure of epitaxial Co films on Pd(111) has been investigated as a function of the Co overlayer thickness, by using angle-resolved photoemission spectroscopy (ARPES). It is found that experimental band structures for Pd(111) and work functions for Co/Pd(111) agree reasonably well with the results of band structure calculations. Compared to Pd(111), valence band ARPES spectra of Co/Pd(111) support the existence of the induced magnetic polarization of Pd atoms near the interface, suggesting large hybridization between Co 3d and Pd 4d states near the interface.     

Angle-resolved photoemission from CO chemisorbed on Rh(110)

He(II)-excited UPS spectra of CO chemisorbed on Rh(110) have been acquired in symmetry-equivalent directions relative to the substrate. In these spectra a marked anisotropy was noted in the emission from the 4σ level. Comparison of the experimental spectra with calculations of the differential photoionization cross-sections suggests that the CO molecule is chemisorbed in an end-on configuration with the carbon end nearer to the surface.     

Angle-resolved photoemission spectroscopy study of the (101) surface of the Kondo insulator CeNiSn

The electronic structure of CeNiSn, which is considered a possible topological Kondo insulator, has been investigated by employing synchrotron radiation excited angle-resolved photoemission spectroscopy (ARPES). We have found that the easy cleavage plane in CeNiSn is (101), for which we have investigated the Fermi surface (FS) and band structures. The measured FS and ARPES for the (101) plane are described well by the calculated FS and band structures, obtained from the DFT calculations. The measured ARPES bands and photon energy map show that the metallic states crossing the Fermi level have the 3D nature, casting a negative suspicion for the existence of the topological surface states of the 2D character in CeNiSn. The Ce 4f Kondo resonance peak is observed in Ce 4d → 4f resonant photoemission spectroscopy, suggesting the importance of the Ce 4f electrons in determining the temperature-dependent topological electronic structure of CeNiSn.     

Study of the structure of a superconducting state of Co-doped BaFe2As2 multiband compounds

The terahertz and infrared spectra of the complex dynamic conductivity, as well as the temperature dependences of the density of a superconducting condensate and the electronic specific heat of superconducting Ba(Fe_{1 ? x} Co _x )₂As₂ compounds, have been analyzed within a Bardeen-Cooper-Schrieffer-like model of a multiband superconductor with strong coupling. It has been shown that the superconducting state of these compounds is determined by three (one electronic and two hole) weakly interacting condensates. The order parameters of the condensates are: Δ₁ ≈ 15 cm^?1, Δ₂ ≈ 21 cm^?1, and Δ₃ ≈ 30–35 cm^?1. The results significantly refine the existing notions on the structure of the superconducting state of Co-doped BaFe₂As₂ multiband compounds.