Suppression of Near-Fermi Level Electronic States at the Interface in aLaNiO3/SrTiO3Superlattice

Making use of x-ray optical effects in photoelectron-, Auger electron-, and x-ray emission spectroscopies: Total reflection, standing-wave excitation, and resonant effects

“…18,[20][21][22][23][24][25][26][27][28][29][30][31][32] A. Semi-infinite sample, non-resonant excitation…”

“…At the outset, we note that this program has been tested extensively and used to successfully model a variety of sets of experimental data, to which we refer the reader. 18,[20][21][22][23][24][25][26][27][28][29][30][31][32] We here thus focus primarily on the theoretical modeling of various x-ray optical effects, pointing out the actual and potential utility of them in future studies of buried layers and interfaces.…”

“…15,16 However, beginning with work by Kim and Kortright 19 and by Yang et al, 18,20 it has been shown that the standing-wave method can be extended into the soft x-ray regime ($0.5-1.5 keV) and it has subsequently been combined with high-resolution x-ray photoelectron spectroscopy to develop a non-destructive and element specific depth-resolved spectroscopic tool. 18,[20][21][22][23][24][25][26][27][28][29][30][31][32] This new soft x-ray technique has several significant advantages: for soft x-rays, the photoelectric cross sections are significantly larger than those in the hard x-ray regime, and the standing wave can be scanned through the layers, thus highlighting/de-highlighting a certain depth of a layer by either varying the photon incidence angle around the Bragg angle or varying the incidence angle around the Bragg condition, as noted earlier. Beyond this, by making one of the layers in the sample structure in a wedge profile, it is possible to minimize the number of free parameters to fit, by exploiting the nearly perfect "phase pinning" of the standing waves with respect to the standing-wave generator.…”

“…The SWEDGE method can also be very powerful for extracting the matrix-element-weighted depth-resolved valence band (VB) density of states (DOS) from layers or interfaces of interest, 25,30,31 and we now illustrate this from a theoretical point of view for the same Fe layer on a Cr wedge. In contrast to core levels, the VB spectra from each layer will in general overlap each other in the binding energy regime from the Fermi level to approximately 15-20 eV, and it can thus be very hard to extract the DOS from each layer or interface with conventional photoemission spectroscopy.…”

“…13 Beyond these total reflection studies, x-ray optical effects have not been rigorously accounted for in the analysis of most photoemission or x-ray emission data, with the assumption being made that the x-rays penetrate much more deeply, and essentially without attenuation or modulation, into the sample, as compared to the depths from which the photoelectrons or fluorescent x-rays are emitted. Yet x-ray optical effects can play a particularly significant role for measurements done at grazing incidence angles, 9-13 near absorption-edge resonances, 14 in Bragg-like reflection from single-crystal planes [15][16][17] and synthetic multilayers, 8,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] and with variable polarization so as to produce magnetic and other dichroic effects. 19,21,30 In beginning this discussion, it is worthwhile to discuss in more detail the specific types of x-ray optical effects that need to be considered: first are optical interference effects that modify the total electric field intensity vs. depth into the sample.…”