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  • Articles: DFG German National Licenses  (48)
  • 1
    Electronic Resource
    Electronic Resource
    X-ray absorption spectroscopy studies of high-Tc superconductors (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 63 (1988), S. 4190-4192 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: X-ray absorption spectroscopy measurements on rare-earth (R) and Ba sites in the new superconducting oxides and in binary rare-earth oxides are discussed. All of the near-edge spectra appear to manifest a continuum resonance feature whose energy is quantitatively well correlated with the R–O and Ba–O bond lengths. Evidence supporting a second such feature related to the R/Ba interatomic distance is also presented. Finally, simultaneous consideration of both of these features appears consistent with the inverse square-law expectation for the interatomic distance dependence of the energies of these spectra features.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.340221
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  • 2
    Electronic Resource
    Electronic Resource
    Reentrant Kondo-mixed valent-Kondo regime behavior with T in the 3d CeT2Si2 series (1987)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 61 (1987), S. 3183-3185 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In intermetallic compounds of Ce involving late 3d, 4d, and 5d row transition metals (T), the Ce valence state increases with decreasing T-d-band electron count. Neifeld, et al. [Phys. Rev. B 32, 6928 (1985)] have recently shown (using extensive Ce-L3-edge measurements) that this trend is reversed for the Ce-3d row compounds earlier than Co. This somewhat striking behavior is nowhere more apparent than in the ThCr2Si2 crystal structure series CeT2Si2 with T=Cu, Ni, Co, Fe, Mn, and (MnxCr1−x). In this paper, we will discuss the following: (1) the extension of this crystal structure to its stability limit in the T=(Mn,Cr) substituted system; (2), how this extension permits restoration of the Ce-valence state to the Kondo local-moment regime; (3) low-temperature electrical resistivity measurements which support first the Kondo to mixed valent followed by mixed valent to Kondo regime passage with decreasing 3d electron count in this series; and (4) finally the apparent compatibility of the Ce-Kondo effect with strong 3d antiferromagnetism for some of these materials.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.338921
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  • 3
    Electronic Resource
    Electronic Resource
    Magnetism and valence mixing in Eu systems with contrasting band structures (1987)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 61 (1987), S. 3374-3376 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Eu compounds at or near the strongly magnetic (J=7/2) Eu2+ valence should enter a mixed valent Eu2+δ (||δ||〈1) nonmagnetic state at sufficiently high pressures. We have performed high pressure (P≤250 kbars) and low temperature (T≥4.2 or 77 K) resistivity measurements on semiconducting EuO and metallic Eu(AuxPd1−x)2Si2 (0≤x≤1). For EuO our results show a nonlinear gap variation above 130 kbar with a hybridization gap and ferromagnetism (Tc ≥200 K) being maintained out to 250 kbars. In the Eu(Au,Pd)2Si2 system the cooperative valence transition can be traced to higher pressures with increasing Au content. In the pure Au the magnetic order persists out to 250 kbars. A common theme in both systems is the substantial pressure-induced enhancement of the magnetic ordering temperature precursive to the eventual large Eu valence change.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.338776
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  • 4
    Electronic Resource
    Electronic Resource
    Unstable valence rare earths ion implanted into transition metals: Valence variation studies (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 63 (1988), S. 3692-3694 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The technique of ion implantation is applied to study the problem of valence instabilities of Ce and Eu in transition-metal hosts. L3 x-ray absorption spectroscopy measurements are used to probe the rare-earth valence state in these materials. Typical extreme valence variations are illustrated with Y, Ti, and Pd hosts. The important question of identifying extrinsic oxidation of the rare-earth implant is addressed in detail. The signatures of rare earth oxidation in the x-ray spectra of both bulk compound and implanted materials are discussed using specific examples.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.340688
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  • 5
    Electronic Resource
    Electronic Resource
    Resistivity effects in the Ce(Pd,Rh)3B1 and Ce(Rh,Ru)3B2 system (1987)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 61 (1987), S. 3180-3182 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report Ce-L3-valence state and temperature-dependent resistivity results on two pseudoternary alloy series: CeT3Bn where n=1 with T=(Pd, Rh) and where n=2 with T=(Rh,Ru). Our results indicate that both series pass continuously from a Kondo local-moment regime to a mixed valent regime to a saturated valence regime with decreasing 4d electron concentration on the T sublattice. We correlate the temperature-dependent resistivity results with the Ce-L3-valence variation for both series. For the n=1 series, Kondo-type behavior appears in the local-moment regime, while for the n=2 series, ferromagnetism appears in the local-moment regime. We also note that in the mixed valence regime, for both series, the absolute value of the resistance drops rapidly, indicating a loss of magnetic scattering.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.338920
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  • 6
    Electronic Resource
    Electronic Resource
    The near-edge x-ray absorption spectroscopy of RT2Si2 and RT2Ge2 compounds (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 63 (1988), S. 3503-3505 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: X-ray absorption near-edge spectroscopy (XANES) measurements on 1:2:2 compounds of the form RT2X2 with R=Ce and Eu, T=a transition metal, and X=Si or Ge are presented. XANES measurements on each of the three sublattices are used to elucidate elements of the electronic structure of these compounds. In particular, the evidence for the strongly bonded T-X planes, the coupling of the Ce valence-state stability to the T-X planes, and the response of the X-p orbitals to varying transition-metal components are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.340723
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  • 7
    Electronic Resource
    Electronic Resource
    Superconducting and magnetic properties of RBa2Cu3O7−x compounds (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 63 (1988), S. 4187-4189 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Superconducting properties of RBa2Cu3O7−x compounds (R=Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, and Yb) have been studied by dc electrical resistivity and ac magnetic susceptibility techniques. Except for R=La, which is superconducting below 50 K, RBa2Cu3O7−x compounds exhibit sharp resistive and inductive transitions in the 90-K range. The nearly constant value of Tc for magnetic R ions indicates a very weak interaction between R ions, as anticipated from the known crystal structure of these materials. The effects of annealing in oxygen and argon, and air quenching, on the superconducting properties are also discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.340245
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  • 8
    Electronic Resource
    Electronic Resource
    Correlation between electronic states of O, Cu, and Ba in several high-Tc superconductors (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 63 (1988), S. 4193-4195 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: X-ray photoemission measurements of Y-, Sm-, Eu-, and Yb-based high-temperature superconductors show a clear correlation between Cu 2p satellite features, which previous work has shown are associated with materials which exhibit superconductivity, and a high-binding-energy peak in the O 1s spectrum. In samples with varying oxygen content the intensity of the Cu 2p satellites changes, reflecting the Cu valence which varies between 1+ and 2+. The high-binding-energy O peak appears to track the Cu valence. This high-binding-energy peak may be indicative of the presence of O 2p holes, suggested in some models as necessary for superconductivity. The Ba 3d peak position also changes systematically with the Cu valence.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.341147
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  • 9
    Electronic Resource
    Electronic Resource
    Systematics of thin films formed by excimer laser ablation: Results on SmBa2Cu3O7 (1988)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 53 (1988), S. 703-704 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Excimer laser ablation was used to prepare thin films from a bulk target of SmBa2Cu3O7. The systematic variation in film thickness, stoichiometry, and microstructure, as a function of laser fluence and angle from the target surface normal, was determined. We show that films which are stoichiometric over wide solid angles can be rapidly prepared from this material by use of high laser fluence. Films prepared from various Bi-Sr-Ca-Cu-O targets at high laser fluence also reproduce bulk stoichiometry. In contrast, films prepared by this technique from a YBa2Cu3O7 target are deficient in Ba and Cu for laser fluences required to produce compositional homogeneity over wide areas.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.100646
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  • 10
    Electronic Resource
    Electronic Resource
    Nature of the pulsed laser process for the deposition of high Tc superconducting thin films (1988)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 53 (1988), S. 1431-1433 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The pulsed laser thin-film deposition process can enable preparation of thin films of complex composition with good control over the film stoichiometry. The film compositions are similar to that of the target pellet and as a consequence this technique appears to be an ideal method for preparing high Tc thin films on a variety of substrates.The factors which contribute to this beneficial phenomenon have been explored by a laser ionization mass spectrometry (LIMS) and a post ablation ionization (PAI) neutral velocity analysis technique in order to determine the mass and velocities of the laser ejected material. In addition, x-ray absorption measurements on films deposited onto substrates at room temperature were performed in order to identify the presence of short-range crystalline order in the films. Both of these studies rule out the ejection of stoichiometric clusters of material from the pellet during the laser ablation/deposition process. Instead, binary and ternary suboxides are emitted from the target pellet. These suboxides most likely have unit sticking coefficient to the substrate which could contribute to the preservation of the film stoichiometry. The velocity distribution of several neutral species (e.g., BaO) indicates that particles have energies of several eV. Thus the effective temperatures of the emitted species are ∼15×103 K, and these energetic particles may facilitate growth of the crystalline films at low substrate temperatures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.100458
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