Article

Angular dependent resonant photoemission processes at the 2p thresholdsin nickel metal

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Abstract

Angle-resolved valence-band resonant photoemission of nickel metal has been measured close to the 2p core-level thresholds with synchrotron radiation. The well-known 6-eV correlation satellite has an intensity enhancement of about two orders of magnitude at resonance. The angular dependence of the photoemission intensity has been studied as function of photon energy and provides unambiguous evidence for interference effects all the way up to the resonance maximum. The observation of different angular asymmetries, {\beta}, for the valence band and the satellite is discussed in connection to the origin of the resonant photoemission process and the character of the satellite.

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... For x = 0.05, the double-peak structure has essentially vanished in comparison to at x = 0.16 due to the superposition of the strong fcc contribution. (iv) The 6 eV feature [19,31] above the main 2p 3/2 peak is prominent in Ni metal x = 0.0 that is associated with electron correlation effects and narrow-band phenomena [32]. The intensity of the 6 eV feature is very low in the Ni 1−x C x films in comparison to Ni metal even at x = 0.05 due to more delocalized bands. ...
... Moreover, the 6 eV feature in the Ni XAS spectra that signifies electron correlation effects and narrow-band phenomena in metallic Ni [19,31] is washed out in the Ni 1−x C x samples due to the Ni 3d-C 2p orbital overlap that changes the properties of Ni already at very low carbon content. Thus, the spectral profiles of the Ni 1−x C x samples exhibit carbide signatures and exclude metallic nickel. ...
... For x = 0.05, the double-peak structure has essentially vanished in comparison to at x = 0.16 due to the superposition of the strong fcc contribution. (iv) The 6 eV feature [19,31] above the main 2p 3/2 peak is prominent in Ni metal x = 0.0 that is associated with electron correlation effects and narrow-band phenomena [32]. The intensity of the 6 eV feature is very low in the Ni 1−x C x films in comparison to Ni metal even at x = 0.05 due to more delocalized bands. ...
... Moreover, the 6 eV feature in the Ni XAS spectra that signifies electron correlation effects and narrow-band phenomena in metallic Ni [19,31] is washed out in the Ni 1−x C x samples due to the Ni 3d-C 2p orbital overlap that changes the properties of Ni already at very low carbon content. Thus, the spectral profiles of the Ni 1−x C x samples exhibit carbide signatures and exclude metallic nickel. ...
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The crystal structure and chemical bonding of magnetron-sputtering deposited nickel carbide Ni 1−x C x (0.05 x 0.62) thin films have been investigated by high-resolution x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy, Raman spectroscopy, and soft x-ray absorption spectroscopy. By using x-ray as well as electron diffraction, we found carbon-containing hcp-Ni (hcp-NiC y phase), instead of the expected rhombohedral-Ni 3 C. At low carbon content (4.9 at%), the thin film consists of hcp-NiC y nanocrystallites mixed with a smaller amount of fcc-NiC x . The average grain size is about 10–20 nm. With the increase of carbon content to 16.3 at%, the film contains single-phase hcp-NiC y nanocrystallites with expanded lattice parameters. With a further increase of carbon content to 38 at%, and 62 at%, the films transform to x-ray amorphous materials with hcp-NiC y and fcc-NiC x nanodomain structures in an amorphous carbon-rich matrix. Raman spectra of carbon indicate dominant sp 2 hybridization, consistent with photoelectron spectra that show a decreasing amount of C–Ni phase with increasing carbon content. The Ni 3d–C 2p hybridization in the hexagonal structure gives rise to the salient double-peak structure in Ni 2p soft x-ray absorption spectra at 16.3 at% that changes with carbon content. We also show that the resistivity is not only governed by the amount of carbon, but increases by more than a factor of two when the samples transform from crystalline to amorphous.
... Such spectroscopies probe respectively the radiative and non radiative autoionization decay of a core hole, and the signal can be strongly enhanced with respect to the non resonant mode. The element and orbital selectivity of core level resonant spectroscopies allows to access higher order multipoles which are left unexplored by MCD in X-ray absorption (XAS) [1][2][3][4][5][6], to distinguish and enhance specific electronic excitations and satellites [7,8], collective magnetic excitations [9], ultrafast and charge transfer dynamics [10][11][12] and to detect quadrupolar transitions towards localized empty states [13,14]. In particular, RPES has recently been applied to several correlated materials [15][16][17][18][19] and full two dimensional angular scans of resonantly emitted electrons in moderately correlated materials have also been carried out [20,21]. ...
... We again observe a region of deconstructive interference ( Fig.4(b)) and then a strong enhancement for the third photon energy (Fig.4(c)), which is different for the two spin channels. The massive enhancement of the signal observed here does not imply strong interference effects: an analysis of the different contributions in the amplitude reveals that, in our case, the enhancement is given essentially by the resonant excitation alone, as was also found in other cases [8]. ...
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