Through relativistic photoemission calculations for the Au(111) surface state at the Fermi level, we study the photon energy dependence of circular dichroism. The dichromatic signal (D S ) pattern changes 23 times with photon energies between 7 and 100 eV, and we have found 13 different patterns in the k map at the Fermi level for the D S from the Au(111) surface state with normal incidence light. We show that the photon energy dependence of D S is very complex even in the simplest case. The sign change in the circular dichroism as a function of photon energy is related to the relative phases of the complex expansion coefficients of different outgoing partial waves in a time-reversed low-energy electron diffraction state. With off-normal incidence, the z component of the incoming photon field is dominant, and the fine structure seen in the D S in the normal incidence case is lost very rapidly, moving from a normal to an off-normal incidence. We also report that the Rashba split surface state of Au(111) has a significant component of d-type orbital due to relativistic effects and the computational setup used.

We used polarization-dependent angle-resolved photoemission spectroscopy (ARPES) to study the high-energy anomaly (HEA) in the dispersion of Nd2−xCexCuO4, (x = 0.123). We have found that at particular photon energies the anomalous, waterfalllike dispersion gives way to a broad, continuous band. This suggests that the HEA is a matrix element effect: it arises due to a suppression of the intensity of the broadened quasi-particle band in a narrow momentum range. We confirm this interpretation experimentally, by showing that the HEA appears when the matrix element is suppressed deliberately by changing the light polarization. Calculations of the matrix element using atomic wave functions and simulation of the ARPES intensity with one-step model calculations provide further proof for this scenario. The possibility to detect the full quasi-particle dispersion further allows us to extract the high-energy self-energy function near the center and at the edge of the Brillouin zone. One of the unique assets of angle-resolved photoemission spectroscopy (ARPES) is the ability to determine the spectral function A(ω, k) in energy and momentum space. The finite width and deviation of the dispersion from that calculated in an independent particle model are interpreted in the majority of cases in terms of manybody effects [1]. In the cuprate high-T c superconductors, various kinks in the dispersion have been discovered which were analyzed in terms of a coupling of the charge carriers to bosonic excitations possibly mediating high-T c superconductivity in these materials. Besides the kinks in the low binding energy (E B ) region (E B ≤ 0.1 eV) at E B = E H 0.3 eV the band appears to bend sharply and seems to proceed almost vertically towards the valence bands. This phenomenon has been termed "waterfall" or high-energy anomaly (HEA) [2]. The HEA has been observed in undoped cuprates [3] as well as in their hole-doped [2, 4-13], and electron-doped derivatives [4,[13][14][15]. In the latter two systems E H shows a d-wave momentum dependence being larger along the nodal direction and smaller near the antinodal point opposite to the momentum dependence of the d-wave superconducting gap [6,12,14]. The values of E H exhibit a difference of ≈ 0.4 eV between hole doped and electron doped cuprates [4]. This difference was interpreted in terms of a shift of the chemical potential [14]. The experimental studies were accompanied by numerous theoretical papers [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31].For the phenomenon of the HEA, a number of explanations have been suggested including Mott-Hubbard models with a transition from the coherent quasi-particle dispersion to the incoherent lower Hubbard band [5,18,21,26,32], a disintegration of the low-energy branch into a holon and spinon band due to a spin charge separation [2], a coupling to spin fluctuations [9,17,19,29,33], a coupling to phonons [7], string excitations of spinpolarons [32], a bifurcation of the quasi-particle band due to an excitation of a bosonic mode of ...

Angle-resolved photoemission spectroscopy (ARPES) is typically used to study only the occupied electronic band structure of a material. Here we use laser-based ARPES to observe a feature in bismuth-based superconductors that, in contrast, is related to the unoccupied states. Specifically, we observe a dispersive suppression of intensity cutting across the valence band, which, when compared with relativistic one-step calculations, can be traced to two final-state gaps in the bands 6 eV above the Fermi level. This finding opens up possibilities to bring the ultra-high momentum resolution of existing laser-ARPES instruments to the unoccupied electron states. For cases where the finalstate gap is not the object of study, we find that its effects can be made to vanish under certain experimental conditions.Angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental probe that has been used extensively to image the occupied electronic states of materials in an energy-and momentum-resolved manner [1,2]. Since it is based on Einstein's photoelectric effect, it cannot directly probe a material's unoccupied electronic states, but it has nevertheless provided signatures of gaps in the unoccupied states [3][4][5][6]. According to the one-step model [7], some electrons, after absorbing photons, may have energies which lie between two unoccupied bands. We call the space between the unoccupied bands a finalstate gap, although this gap may be confined to a limited momentum range, and disperse within that range. The photoemission intensity of these electrons is suppressed, but not suppressed completely, due to the finite widths of the final states. These finite widths represent the small chance that electrons interacting with the medium, primarily through electron-hole pair creation and plasmonic interaction, will have energy within the final-state gap. Typically, this final-state effect in ARPES is not used to measure unoccupied states, which are instead mapped by inverse photoemission[8] or very-low-energy electron diffraction [5,[9][10][11][12].Here we show that laser-based ARPES [13][14][15], under certain conditions, can be used to map final-state gaps in the electronic states of a material. This method provides the following advantages with respect to standard synchrotron-based ARPES: (a) improved momentum resolution and greater bulk sensitivity, due to the lower photon energy range available in laser-ARPES (6-7 eV)[13], * alanzara@lbl.gov and (b) access to unoccupied electron states closer to the Fermi level.Data are shown for cuprate superconductors Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) and Bi 2 Sr 2−x La x CuO 6 (La-Bi2201) along the Γ-Y direction of the Brillouin zone, using ∼6 eV laser ARPES. In these measurements, a final-state gap can be seen as a line of suppressed intensity that disperses in momentum.When the final-state gap crosses the photoemitted valence band, it creates a distortion 100-140 meV below the Fermi level, depending on the photon energy. A second distortion is seen at 20-50 meV, indicating...

We have obtained angle-resolved photoemission (ARPES) spectra from single crystals of the topological insulator material Bi2Te3 using a tunable laser spectrometer. The spectra were collected for eleven different photon energies ranging from 5.57 to 6.70 eV for incident light polarized linearly along two different in-plane directions. Parallel first-principles, fully relativistic computations of photo-intensities were carried out using the experimental geometry within the framework of the one-step model of photoemission. A reasonable overall accord between theory and experiment is used to gain insight into how properties of the initial and final state band structures as well as those of the topological surface states and their spin-textures are reflected in the laser-ARPES spectra. Our analysis reveals that laser-ARPES is sensitive to both the initial state kz dispersion and the presence of delicate gaps in the final state electronic spectrum.