Electronic excitations on silver surfaces
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The surface electronic structure and optical response of Ag has been studied using angle- and energy-resolved photoyield (AERPY) and angle-resolved photoemission spectroscopy (ARPES) using low energy photons (2.7--18 eV). ARPES data for Ag(100) exhibit an unexpected dispersing feature which cannot be assigned to a direct transition peak in the $\ensuremath{\Gamma}\mathrm{X}$ direction of the bulk Brillouin zone. The origin of this feature is found in the surface mediated indirect transitions related to the direct transition in the $\ensuremath{\Gamma}\mathrm{L}$ direction. From the AERPY experiments, the adlayer standing-wave-like bulk plasmon mode is clearly observed in Ag(100) and Ag(111) thin films. The silver multipole plasmon is observed both on Ag adlayers and bulk single crystal surfaces at 3.7 eV. No signature of the multipole plasmon is observed around 6.7 eV, in disagreement with the prediction of the $s\ensuremath{-}d$ polarization model.Keywords:
Brillouin zone
Photon energy
We have studied the electronic structure of EuFe2As2-xPx using high resolution angle-resolved photoemission spectroscopy. Upon substituting As with the isovalent P, which leads to a chemical pressure and to superconductivity, we observe a non-rigid-band like change of the electronic structure along the center of the Brillouin zone (BZ): an orbital and kz dependent increase or decrease in the size of the hole pockets near the Gamma - Z line. On the other hand, the diameter of the Fermi surface cylinders at the BZ corner forming electron pockets, hardly changes. This is in stark contrast to p and n-type doped iron pnictides where, on the basis of ARPES experiments, a more rigid-band like behavior has been proposed. These findings indicate that there are different ways in which the nesting conditions can be reduced causing the destabilization of the antiferromagnetic order and the appearance of the superconducting dome.
Inverse photoemission spectroscopy
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We have measured the collective electronic excitations of the Al(111) surface by means of angle-resolved high-resolution electron-energy-loss spectroscopy. Loss spectra reveal both the monopole and the multipole surface plasmons. The measured dispersion of the monopole surface plasmon is negative, as predicted by calculations of the dynamic response of the electron at the surface of a free-electron-like sample. The aluminum multipole surface plasmon, which was not detected in previous electron-energy-loss investigations, is clearly observed in present spectra.
Localized surface plasmon
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This thesis describes the studies on the electronic structures of several novel quantum materials by using the Angle-Resolved Photoemission Spectroscopy (ARPES) technique: Topological Dirac semimetals represent a new state of quantum matter that offers a platform for realizing many exotic physical phenomena. Our ARPES experiment systematically revealed the electronic structures of MSiS (M=Zr, Hf) family and confirmed the Dirac line-nodes states in both compounds. By comparing the spin-orbit coupling (SOC) effect, we demonstrated that the degeneracy of bands at certain positions of Brillouin zone (BZ) are protected by non-symmorphic symmetry of the crystal. Two dimensional (2D) semiconductors with high carrier mobility and moderate band gap are particularly attractive for their potential broad applications in electronic devices. By using both ARPES and scanning tunneling microscopy (STM) techniques, we comprehensively revealed the electronic structure of newly discovered air-stable oxide semiconductor Bi2O2Se. Surface patterns (consisting of 50% Se vacancies) were found on the cleaved sample surface. Remarkably, we found no evidence of undesired in-gap states even in the presence of these surface defects. Spatially resolved ARPES technique has been recently developed, which features its ability to focus the incident beam into submicron region. By using the technique, we studied the electronic structure of square graphene system. The sides of the graphene are found to be aligned with underlying Cu direction, indicating the important role of substrate during the growth of graphene. In addition, large domain twisted bilayer graphene (tBLG) was artificially constructed. The van Hove singularity in its band structure caused by interlayer interaction was directly revealed by our ARPES measurement.
Brillouin zone
Bilayer graphene
Surface States
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The surface electronic structure and optical response of Ag has been studied using angle- and energy-resolved photoyield (AERPY) and angle-resolved photoemission spectroscopy (ARPES) using low energy photons (2.7--18 eV). ARPES data for Ag(100) exhibit an unexpected dispersing feature which cannot be assigned to a direct transition peak in the $\ensuremath{\Gamma}\mathrm{X}$ direction of the bulk Brillouin zone. The origin of this feature is found in the surface mediated indirect transitions related to the direct transition in the $\ensuremath{\Gamma}\mathrm{L}$ direction. From the AERPY experiments, the adlayer standing-wave-like bulk plasmon mode is clearly observed in Ag(100) and Ag(111) thin films. The silver multipole plasmon is observed both on Ag adlayers and bulk single crystal surfaces at 3.7 eV. No signature of the multipole plasmon is observed around 6.7 eV, in disagreement with the prediction of the $s\ensuremath{-}d$ polarization model.
Brillouin zone
Photon energy
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We have investigated atomic and electronic structures of a clean Pd(111) surface using low energy electron diffraction (LEED) and angle-resolved photoemission spectroscopy (ARPES). A typical clean LEED pattern with a 3-fold symmetry has been observed, corresponding to that for an fcc (111) surface. ARPES measurements have been performed along the TEX> symmetry lines, from which the experimental band structure of Pd(111) has been determined. The experimental band structure and work function of Pd(111) surface are found to agree well with the calculated band structure of bulk Pd and the calculated work function of Pd(111), respectively. However, the peak positions in the experimental band structure are located closer to the Fermi level than in the theoretical band structure by 0.1~0.8 eV, depending on the -points in the Brillouin zone. In additin, the experimental band widths are narrower than the theoretical band widths by about 0.5eV. The effects of the localized surface Pd 4d states and the Coulomb interaction between Pd 4d bulk electrons have been discussed as possible origins of such discrepancies between experiment and theory.
Brillouin zone
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We measure the electronic structure of FeSe from within individual orthorhombic domains. Enabled by an angle-resolved photoemission spectroscopy beamline with a highly focused beam spot (nano-ARPES), we identify clear stripelike orthorhombic domains in FeSe with a length scale of approximately 1--5 $\ensuremath{\mu}\mathrm{m}$. Our photoemission measurements of the Fermi surface and band structure within individual domains reveal a single electron pocket at the Brillouin zone corner. This result provides clear evidence for a one-electron-pocket electronic structure of FeSe, observed without the application of uniaxial strain, and calls for further theoretical insight into this unusual Fermi surface topology. Our results also showcase the potential of nano-ARPES for the study of correlated materials with local domain structures.
Brillouin zone
Orthorhombic crystal system
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We measure the electronic structure of FeSe from within individual orthorhombic domains. Enabled by
an angle-resolved photoemission spectroscopy beamline with a highly focused beamspot (nano-ARPES), we
identify clear stripe-like orthorhombic domains in FeSe with a length scale of approximately 1-5 m. Our
photoemission measurements of the Fermi surface and band structure within individual domains reveal a single
electron pocket at the Brillouin zone corner. This result provides clear evidence for a one-electron pocket
electronic structure of FeSe, observed without the application of uniaxial strain, and calls for further theoretical
insight into this unusual Fermi surface topology. Our results also showcase the potential of nano-ARPES for the
study of correlated materials with local domain structures.
Brillouin zone
Orthorhombic crystal system
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We show the three-dimensional electronic structure of the Kondo lattice CeIn3 using soft x-ray angle resolved photoemission spectroscopy in the paramagnetic state. For the first time, we have directly observed the three-dimensional topology of the Fermi surface of CeIn3 by photoemission. The Fermi surface has a complicated hole pocket centred at the {\Gamma}-Z line and an elliptical electron pocket centred at the R point of the Brillouin zone. Polarization and photon-energy dependent photoemission results both indicate the nearly localized nature of the 4f electrons in CeIn3, consistent with the theoretical prediction by means of the combination of density functional theory and single-site dynamical meanfield theory. Those results illustrate that the f electrons of CeIn3, which is the parent material of CeMIn5 compounds, are closer to the localized description than the layered CeMIn5 compounds.
Brillouin zone
Inverse photoemission spectroscopy
Lattice (music)
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We report comprehensive angle-resolved photoemission investigations on the electronic structures and nematicity of the parent compounds of the iron-based superconductors including CeFeAsO, BaFe2As2, NaFeAs, FeSe and undoped FeSe/SrTiO3 films with 1, 2 and 20 layers. While the electronic structure near the Brillouin zone center Γ varies dramatically among different materials, the electronic structure near the Brillouin zone corners (M points), as well as their temperature dependence, are rather similar. The electronic structure near the zone corners is dominated by the electronic nematicity that gives rise to a band splitting of the dxz and dyz bands below the nematic transition temperature. A clear relation is observed between the band splitting magnitude and the onset temperature of nematicity. Our results may shed light on the origin of nematicity, its effect on the electronic structures, and its relation with superconductivity in the iron-based superconductors.
Brillouin zone
Iron-based superconductor
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We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the undoped and electron-doped iron pnictides BaFe 2- x Co x As 2 (Ba122) ( x = 0, 0.14) and studied the Fermi surfaces (FSs) and band dispersions near the Fermi level. The FS sheets we observed are consistent with the shrinkage of the hole-like pockets around the Brillouin Zone (BZ) center and the expansion of the electron pockets around the BZ corner in the electron-doped compound as compared to the undoped parent compound. Band dispersions and FSs around the BZ center strongly depend on the photon energy, indicating a three-dimensional (3D) electronic structure. This observation suggests that antiferromagnetism and superconductivity in the pnictides have to be described in terms of an orbital-dependent 3D electronic structure, where FS nesting is not necessarily strong.
Brillouin zone
Inverse photoemission spectroscopy
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