154 research outputs found
Electronic band gap reduction and intense luminescence in Co and Mn ion-implanted SiO
Cobalt and manganese ions are implanted into SiO over a wide range of
concentrations. For low concentrations, the Co atoms occupy interstitial
locations, coordinated with oxygen, while metallic Co clusters form at higher
implantation concentrations. For all concentrations studied here, Mn ions
remain in interstitial locations and do not cluster. Using resonant x-ray
emission spectroscopy and Anderson impurity model calculations, we determine
the strength of the covalent interaction between the interstitial ions and the
SiO valence band, finding it comparable to Mn and Co monoxides. Further, we
find an increasing reduction in the SiO electronic band gap for increasing
implantation concentration, due primarily to the introduction of Mn- and
Co-derived conduction band states. We also observe a strong increase in a band
of x-ray stimulated luminescence at 2.75 eV after implantation, attributed to
oxygen deficient centers formed during implantation.Comment: 8 pages, 6 figure
Effect of 3d-doping on the electronic structure of BaFe2As2
The electronic structure of BaFe2As2 doped with Co, Ni, and Cu has been
studied by a variety of experimental and theoretical methods, but a clear
picture of the dopant 3d states has not yet emerged. Herein we provide
experimental evidence of the distribution of Co, Ni, and Cu 3d states in the
valence band. We conclude that the Co and Ni 3d states provide additional free
carriers to the Fermi level, while the Cu 3d states are found at the bottom of
the valence band in a localized 3d10 shell. These findings help shed light on
why superconductivity can occur in BaFe2As2 doped with Co and Ni but not Cu.Comment: 18 pages, 8 figure
XPS and DFT study of pulsed Bi-implantation of bulk and thin-films of ZnO - the role of oxygen imperfections
An atomic and electronic structure of the bulk and thin-film morphologies of
ZnO were modified using pulsed Bi-ion implantation (1x1017 cm-2 fluence, 70 min
exposure under Bi-ion beam, EBi+ = 30 keV, pulsed ion-current density of not
more than 0.8 mA/cm2 with a repetition rate of 12.5 Hz). The final samples were
qualified by X-ray photoelectron core-level and valence band mapping
spectroscopy applying ASTM materials science standard. The spectroscopy data
obtained was discussed on the basis of DFT-models for Bi-embedding into ZnO
host-matrices. It was established that in the case of direct Bi-impurities
insertion into the employed ZnO-host for both studied morphologies neither the
only "pure" Bi2O3-like phase nor the only "pure" Bi-metal will be preferable to
appear as a secondary phase. An unfavorability of the large cluster
agglomeration of Bi-impurities in ZnO-hosts has been shown and an oxygen 2s
electronic states pleomorphizm was surely established.Comment: 20 pages, 8 figures, 2 tables, accepted to Appl. Surf. Sc
ELECTRONIC STRUCTURE OF FeSi
The full set of high-energy spectroscopy measurements including X-ray
photoelectron valence band spectra and soft X-ray emission valence band spectra
of both components of FeSi (Fe K_beta_5, Fe L_alpha, Si K_beta_1,3 and Si
L_2,3) are performed and compared with the results of ab-initio band structure
calculations using the linearized muffin-tin orbital method and linearized
augmented plane wave method.Comment: 11 pages + 3 PostScript figures, RevTex3.0, to be published in
J.Phys.:Cond.Matte
Pleomorphic structural imperfections caused by pulsed Bi-implantation in the bulk and thin-film morphologies of TiO2
The results of combined experimental and theoretical study of substitutional
and clustering effects in Bi-doped TiO2 hosts (bulk and thin-film morphologies)
are presented. Bi-doping of the bulk and thin-film titanium dioxide was made
with help of pulsed ion-implantation (E(Bi+) = 30 keV, D = 1 * 1017 cm-2)
without posterior tempering. The X-ray photoelectron spectroscopy (XPS)
qualification (core-levels and valence bands) and Density-Functional Theory
(DFT) calculations were employed in order to study the electronic structure of
Bi-ion implanted TiO2 samples. According to XPS data obtained and DFT
calculations, the Bi -> Ti cation substitution occurs in Bi-implanted bulk
TiO2, whereas in the thin-film morphology of TiO2:Bi the Bi-atoms have
metal-like clusters segregation tendency. Based on the combined XPS and DFT
considerations the possible reasons and mechanism for the observed effects are
discussed. It is believed that established peculiarities of bismuth embedding
into employed TiO2 hosts are mostly the sequence of pleomorphic origin for the
formed "bismuth-oxygen" chemical bonding.Comment: 19 pages, 6 figures, accepted in Appl. Surf. Sc
Correlation effects in Ni 3d states of LaNiPO
The electronic structure of the new superconducting material LaNiPO
experimentally probed by soft X-ray spectroscopy and theoretically calculated
by the combination of local density approximation with Dynamical Mean-Field
Theory (LDA+DMFT) are compared herein. We have measured the Ni L2,3 X-ray
emission (XES) and absorption (XAS) spectra which probe the occupied and
unoccupied the Ni 3d states, respectively. In LaNiPO, the Ni 3d states are
strongly renormalized by dynamical correlations and shifted about 1.5 eV lower
in the valence band than the corresponding Fe 3d states in LaFeAsO. We further
obtain a lower Hubbard band at -9 eV below the Fermi level in LaNiPO which
bears striking resemblance to the lower Hubbard band in the correlated oxide
NiO, while no such band is observed in LaFeAsO. These results are also
supported by the intensity ratio between the transition metal L2 and L3 bands
measured experimentally to be higher in LaNiPO than in LaFeAsO, indicating the
presence of the stronger electron correlations in the Ni 3d states in LaNiPO in
comparison with the Fe 3d states in LaFeAsO. These findings are in accordance
with resonantly excited transition metal L3 X-ray emission spectra which probe
occupied metal 3d-states and show the appearance of the lower Hubbard band in
LaNiPO and NiO and its absence in LaFeAsO.Comment: 6 pages, 5 figure
Structural defects induced by Fe-ion implantation in TiO2
X-ray photoelectron spectroscopy (XPS) and resonant x-ray emission
spectroscopy (RXES) measurements of pellet and thin film forms of TiO with
implanted Fe ions are presented and discussed. The findings indicate that
Fe-implantation in a TiO pellet sample induces heterovalent cation
substitution (Fe Ti) beneath the surface region. But
in thin film samples, the clustering of Fe atoms is primarily detected. In
addition to this, significant amounts of secondary phases of Fe are
detected on the surface of all doped samples due to oxygen exposure. These
experimental findings are compared with density functional theory (DFT)
calculations of formation energies for different configurations of structural
defects in the implanted TiO:Fe system. According to our calculations, the
clustering of Fe-atoms in TiO:Fe thin films can be attributed to the
formation of combined substitutional and interstitial defects. Further, the
differences due to Fe doping in pellet and thin film samples can ultimately be
attributed to different surface to volume ratios.Comment: 7+ pages, 3 Figure, to appear in J. Appl. Phy
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