Angle-resolved Core-level Photoemission Research Articles

Effect of Anisotropic Hybridization in YbAlB_{4} Probed by Linear Dichroism in Core-Level Hard X-Ray Photoemission Spectroscopy.

We have probed the crystalline electric-field ground states of pure |J=7/2,J_{z}=±5/2⟩ as well as the anisotropic c-f hybridization in both valence fluctuating systems α- and β-YbAlB_{4} by linear polarization dependence of angle-resolved core level photoemission spectroscopy. Interestingly, the small but distinct difference between α- and β-YbAlB_{4} was found in the polar angle dependence of linear dichroism, indicating the difference in the anisotropy of c-f hybridization, which may be a key to understanding a heavy Fermi liquid state in α-YbAlB_{4} and a quantum critical state in β-YbAlB_{4}.

Optical Process of Linear Dichroism in Angle-Resolved Core-Level Photoemission Reflecting Strongly Correlated Anisotropic Orbital Symmetry

We revisit the formulations and simulations of angular distributions in polarization-dependent core-level photoemission spectra of strongly correlated electron systems, in order to explain the recently discovered linear dichroism (LD) in the core-level photoemission of 4f-based rare-earth compounds. Owing to the selection rules for the optical process of core-level excitations, the LD originating from the anisotropic outer localized charge distributions determined by the ground-state orbital symmetry can be observed. Our simulations show that core d-level excitations are essential for the LD in localized ions having a cubic symmetry, which is absent in the p-orbital excitations.

Linear Dichroism in Angle-Resolved Core-Level Photoemission Spectra Reflecting 4fGround-State Symmetry of Strongly Correlated Cubic Pr Compounds

We report experimentally observed linear dichroism in angle-resolved core-level photoemission spectra of PrIr2Zn20 and PrB6 in cubic symmetry. The different anisotropic 4f charge distributions between the compounds due to the crystalline-electric-field splitting are responsible for the difference in the linear dichroism, which has been verified by spectral simulations with the full multiplet theory for a single-site Pr3+ ion in cubic symmetry. The observed linear dichroism and polarization-dependent spectra in two different photoelectron directions for PrIr2Zn20 are reproduced by theoretical analysis for the Gamma_3 ground state, whereas those of the Pr 3d and 4d core levels indicate the Gamma_5 ground state for PrB6.

The surface stability of Cr2O3(0 0 0 1)

The surface of chromia (Cr2O3) has a surface electronic structure distinct from the bulk and a packing density distinct from the bulk. More than a demarcation between the solid and the vacuum, the surface differs from the bulk of chromia, not just because of a partial occupancy of chromium sites, but also because of an increased number of unoccupied surface oxygen sites (vacancy sites), evident in angle-resolved core level photoemission. In spite of the structural differences that exist at the surface, there is, as yet, no evidence that these complications affect the surface Debye temperature beyond the most simple of assumptions regarding the lower coordination of the surface. Using low-energy electron diffraction (LEED), the effective surface Debye temperature (∼490 K) is found to be lower than the bulk (∼645 K) Debye temperature of Cr2O3(0 0 0 1). This surface effective Debye temperature, indicative of vibrations along the surface normal, uncorrected for anharmonic effects, has a value reduced from the effective bulk Debye temperature yet close to the value √2 expected from a simple mean field argument.

Photoemission Linear Dichroism Reveals Cubic Wave Functions

Researchers have succeeded in identifying the Γ8 ground state of cubic YbB12 by means of state-of-the-art angle-resolved core-level photoemission linear dichroism in the hard X-ray region.

Anisotropic Electric Conductivity of DelafossitePdCoO2Studied by Angle-Resolved Photoemission Spectroscopy

An explicit connection between the electronic structure and the anisotropic high conductivity of delafossite-type PdCoO2 has been established by angle-resolved photoemission spectroscopy (ARPES) and core-level x-ray photoemission spectroscopy. The ARPES spectra show that a large hexagonal electronlike Fermi surface (FS) consists of very dispersive Pd 4d states. The carrier velocity and lifetime are determined from the ARPES data, and the conductivity is calculated by a solution of the Boltzmann equation, which demonstrates that the high anisotropic conductivity originates from the high carrier velocity, the large two-dimensional FS, and the long lifetime of the carriers.

Surface and bulk components in angle-resolved core-level photoemission spectroscopy of graphite

Angle-resolved x-ray photoelectron spectroscopy of the $\text{C}\text{ }1s$ line of highly oriented pyrolytic graphite reveals two components separated by $194\ifmmode\pm\else\textpm\fi{}6\text{ }\text{meV}$, with markedly different asymmetry parameters and natural linewidth. Analysis of the intensity of these peaks as a function of angle demonstrates that the higher binding-energy peak is associated with the outermost graphite layer(s). The conflicting core-level photoemission data from graphite in the literature are addressed in light of these measurements.

Doping evolution of the electronic structure in the single-layer cuprateBi2Sr2−xLaxCuO6+δ: Comparison with other single-layer cuprates

We have performed angle-resolved photoemission and core-level x-ray photoemission studies of the single-layer cuprate ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}\mathrm{Cu}{\mathrm{O}}_{6+\ensuremath{\delta}}$ (Bi2201) and revealed the doping evolution of the electronic structure from the lightly doped to optimally doped regions. We have observed the formation of the dispersive quasiparticle band, evolution of the Fermi ``arc'' into the Fermi surface, and the shift of the chemical potential with hole doping as in other cuprates. The doping evolution in Bi2201 is similar to that in ${\mathrm{Ca}}_{2\ensuremath{-}x}{\mathrm{Na}}_{x}\mathrm{Cu}{\mathrm{O}}_{2}{\mathrm{Cl}}_{2}$ (Na-CCOC), where a rapid chemical potential shift toward the lower Hubbard band of the parent insulator has been observed, but is quite different from that in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ (LSCO), where the chemical potential does not shift, yet the dispersive band and the Fermi arc and/or surface are formed around the Fermi level already in the lightly doped region. The (underlying) Fermi surface shape and band dispersions are quantitatively analyzed using tight-binding fit, and the deduced next-nearest-neighbor hopping integral ${t}^{\ensuremath{'}}$ also confirms the similarity to Na-CCOC and the difference from LSCO.

Momentum-dependent low energy losses in angle-resolved core level photoemission spectra

Angle-resolved photoemission spectra of the Sb 4d and Bi 5d core levels show spectral features split-off in energy by Δ E∼170–320 meV from the main peaks. The appearance of a second component in the Bi 5d spectrum was previously assigned to a surface related binding energy shift. However, use of synchrotron radiation ranging from 30 to 150 eV and angular scans along different symmetry directions of the (1 1 1) surface Brillouin zone show that the split-off peaks disperse as a function of the electron momentum k . Additionally, the peak width and intensity of the split-off peak oscillates between final state symmetry points. Since the d 5/2 and d 3/2 components show the same features symmetry arguments rule out crystal-field effects as the source of the observed peak splitting. The data can be explained by a k-dependent loss function consisting of several characteristic direct and indirect low energy interband transitions excited in various parts of the bulk Brillouin zone.

A comparison of surface segregation for two semi-Heusler alloys: TiCoSb and NiMnSb

Very different types of surface segregation are found for very similar Heusler alloy materials. We observed significant manganese and antimony segregation to the surfaces and near surface regions of the semi-Heusler alloys NiMnSb and TiCoSb respectively. The Mn and Sb surface enrichment was characterized by angle resolved core level photoemission. Indications of surface disorder from low energy electron diffraction provide complimentary evidence of segregation.

Angular dependence of magnetic dichroism and spin polarization in angle-resolved core-level photoemission

Photoelectron diffraction theory is applied to the investigation of the dependence of magnetic dichroism and spin polarization on the emission direction of photoelectrons in angle-resolved core-level photoemission. It is shown that photoelectron diffraction effects may cause significant modulations in the angular dependence of magnetic dichroism in angle-resolved core-level photoemission. Furthermore, it is pointed out that both magnetic dichroism and spin polarization should show the same angular dependence on the electron emission direction. Results of photoelectron diffraction theory are discussed in detail for p-type core-level photoemission in comparison with those of the single-atom approach. The applied theory gives a good explanation of the experimental data of Fe 3p and Co 2p photoelectrons excited in thin Fe and Co films, respectively.

Multiple-scattering theoretical approach to magnetic dichroism and spin polarization in angle-resolved core-level photoemission

Magnetic dichroism in spin- and angle-resolved core-level photoemission is investigated within the relativistic one-step model of photoemission based on multiple-scattering theory. Photoelectron scattering in emission from $3p$ levels of low-index Fe and Ni surfaces is found to affect intensities, dichroism, and photoelectron spin polarizations considerably, in particular in off-normal emission. In general, the calculated spectra agree well with their experimental counterparts in spectral shape, size of the dichroic signal, and spin polarization. Magnetic circular dichroism in a chiral setup is analyzed theoretically using appropriately defined asymmetries. Further, core-level energy positions and photoemission are compared with those obtained within an atomic model.

Surfaces of the perovskite manganitesLa1−xCaxMnO3

Angle-resolved core-level photoemission has been utilized to study the surfaces of crystalline thin films of manganite ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ ($x=0.1$ and 0.35). It is found that both the surface termination and Ca surface concentration depend on the bulk Ca concentration. For ${\mathrm{La}}_{0.65}{\mathrm{Ca}}_{0.35}{\mathrm{MnO}}_{3},$ the terminal layer is predominately Mn-O, while for ${\mathrm{La}}_{0.9}{\mathrm{Ca}}_{0.1}{\mathrm{MnO}}_{3},$ the majority of the surface is La/Ca-O terminated. Both surfaces exhibit a large enhancement in the Ca concentration compared to the bulk with the ${\mathrm{La}}_{0.9}{\mathrm{Ca}}_{0.1}{\mathrm{MnO}}_{3}$ sample, having a larger surface Ca concentration than for the ${\mathrm{La}}_{0.65}{\mathrm{Ca}}_{0.35}{\mathrm{MnO}}_{3}$ material.

Epitaxial Fe100−xCox/Ag(001) alloy films: Structure and element-specific magnetic moments from magnetic linear dichroism

The structure and element-specific local magnetic moments in Fe100−xCox/Ag(001) (16<x<70) epitaxial ultrathin alloy films have been determined by photoelectron diffraction (PED) and related magnetic linear dichroism in angle-resolved core-level photoemission, using a laboratory x-ray source. The PED peaks of the Fe and Co 2p core levels for any of the alloy compositions are at emission angles as expected for a bcc lattice. The dichroism increases for the Fe 2p3/2 core level with increasing x, whereas that of the Co 2p3/2 dichroism is virtually independent on the alloy composition. The dichroism of both the Fe and the Co core levels scales well with the calculated dependence of the local magnetic moment on the alloy composition rather than with the average saturation magnetization.

Photoelectron diffraction: A source for magnetic dichroism in angle-resolved photoemission from ferromagnets

Magnetic dichroism has been measured in angle-resolved core-level photoemission from the Fe 2p and 3p levels in epitaxially grown ultrathin films of Fe(001) and in an amorphous metallic glass with composition Fe{sub 78}B{sub 13}Si{sub 9}. Unpolarized Al K{alpha} and Mg K{alpha} radiation was used for excitation, leading to diffraction patterns that are dominated by forward scattering along low-index crystallographic directions. The Fe(001) data for both total intensity and magnetic dichroism are quantitatively compared to theoretical calculations at both a two-atom single-scattering level and a multiatom multiple-scattering level. Strong effects on the magnetic dichroism due to photoelectron diffraction are found, and the combined angle and energy dependence of the dichroism shows a characteristic {open_quotes}checkered{close_quotes} pattern that should be generally observable in all single crystals. Comparing dichroism data obtained for single-crystal Fe films with those obtained from the amorphous glass and from two-atom and multiatom diffraction theory further permits estimating the relative contributions of free-atom-like dichroism and of photoelectron diffraction, with the free-atom dichroism (that is dominant along low-index directions) being smaller by about a factor of 2 to 4 than the maximum diffraction dichroism (that dominates away from low-index directions). Such photoelectron-diffraction-produced magnetic dichroism thus should provide a useful tool formore » studying magnetic order near single-crystal surfaces. The deviation of the zero in the dichroic asymmetry from the low-index directions is also found via photoelectron diffraction theory to be very sensitive to the s-to-d partial-wave phase difference, and the experimental data permit estimating this quantity and the solid-state effects on it. {copyright} {ital 1998} {ital The American Physical Society}« less

Core level studies of sodium tungsten bronze, Na 0.64WO 3(110) 3×1

We present angle-resolved core level photoemission studies of Na 0.64WO 3(110)3×1. The spectra are interpreted in terms of a recent model which shows two facetted reconstructions present simultaneously at the surface. W 4f and Na 2p core level peak positions and intensities are interpreted on the basis of the different local atomic co-ordination spheres within the model.

Progress in understanding VUV photoemission in low-dimensionality systems

High-resolution angle-resolved photoemission and core-level photoemission measurements were performed on single crystal samples of the quasi-one-dimensional material (TaSe 4) 2I and the cuprate superconductor Bi 2Sr 2CaCu 2O 8+x. Resistivity measurements were performed on both materials. X-ray diffraction, transmission electron microscopy (TEM), and AC susceptibility measurements were performed on the cuprate samples. The data indicate that photoemission studies provide key information in understanding the physics issues of both materials.

Progress in understanding VUV photoemission in low-dimensionality systems

High-resolution angle-resolved photoemission and core-level photoemission measurements were performed on single crystal samples of the quasi-one-dimensional material (TaSe 4 ) 2 I and the cuprate superconductor Bi 2 Sr 2 CaCu 2 O 8+x . Resistivity measurements were performed on both materials. X-ray diffraction, transmission electron microscopy (TEM), and AC susceptibility measurements were performed on the cuprate samples. The data indicate that photoemission studies provide key information in understanding the physics issues of both materials.

Magneto-dichroic effects in energy- and angle-resolved photoemission: contrast mechanisms for the elementally sensitive imaging of magnetic domains

A number of magneto-dichroic effects has recently been reported in energy- and angle-resolved core-level photoemission with both circularly and linearly polarized light. These various dichroisms can be employed to obtain magnetic sensitivity in photoelectron microscopy. A combination of linear (MLDAD) and circular magnetic dichroism in angle-resolved photoemission (MCDAD) may provide complementary information about the domain structure.

Crystalline symmetry effects in X-ray magnetic circular dichroism in angle-resolved core-level photoemission

The dependence of magnetic circular dichroism (MCD) in core-level photoemission on the experimental geometry and direction of electron emission has been studied experimentally using photoemission from the 2p and 3p levels of iron. The MCD has been measured both along emission directions of high symmetry, and along general emission directions within a single mirror plane. The observed angular variations cannot be accounted for by model calculations based on absorption, or based on angle-resolved emission into photoelectron states of a single, oriented atom. They are consistent with a model where the photoelectron states of the crystalline symmetry appropriate to couple to the detector are used.