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1Author    YuA. Ossipyan, O. V. Zharikov, V. L. Matukhin, V. N. Anashkin, I. A. Safin, A. I. Pogorelcev, G. B. Teitelbaum, Yba, Cu, IodinationRequires cookie*
 Title    Cu NQR Study in a High-J c Superconductor YBa 2 Cu 3 0 6A l x  
 Abstract    NQR-spectra and nuclear relaxation of 63 Cu in ceramic samples of the Y-Ba-Cu-O system with various oxygen content have been compared with those in iodinated YBa 2 Cu 3 O eA . 
  Reference    Z. Naturforsch. 47a, 21—26 (1992); received August 8 1991 
  Published    1992 
  Keywords    NQR-spectra, Relaxation, Superconductivity 
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 TEI-XML for    default:Reihe_A/47/ZNA-1992-47a-0021.pdf 
 Identifier    ZNA-1992-47a-0021 
 Volume    47 
2Author    W. W. Warren, R. E. WalstedtRequires cookie*
 Title    NQR and NMR Studies of Spin Dynamics in High T c Superconducting Cuprates  
 Abstract    Recent NQR and NMR studies of Cu spin dynamics in metallic YBa 2 Cu 3 0 6+JC are reviewed. The Cu nuclear relaxation rates are shown to be inconsistent with weak correlation (band) theory and the Korringa relation, but the Cu spins retain important characteristics of itinerancy. These include pairing of the Cu d-spins in the superconducting state and strong sensitivity of the Cu spin dynamics to changes in carrier concentration associated with variation of the oxygen content. Samples of reduced oxygen content (T c = 60 K) are found to contain a high concentration of Cu + Cu(l) sites associated with "full chain-empty chain" vacancy ordering. These samples also exhibit remarkable precursive suppression of planar Cu(2) nuclear relaxation above T c . 
  Reference    Z. Naturforsch. 45a, 385—392 (1990); received October 25 1989 
  Published    1990 
  Keywords    NMR, NQR, Nuclear relaxation, Spin fluctuations, Superconductivity 
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 TEI-XML for    default:Reihe_A/45/ZNA-1990-45a-0385.pdf 
 Identifier    ZNA-1990-45a-0385 
 Volume    45 
3Author    S. Gärtner, H. Riesemeier, Y. Kitaoka, V. Müller, K. Lüders, E. Kaldis, S. RusieckiRequires cookie*
 Title    Cu-NQR Frequencies and Linewidths in High-T c Superconductors  
 Abstract    The Cu-NQR lines of various high-T c superconductors at the Cu(2) site appear between 15 and 40 MHz. There exists an experimental correlation showing that the NQR frequency increases with decreasing T c . By comparison, it can be concluded from these results that a formal valence state of the Cu ion around 2 + is favourable for superconductivity. All lines exhibit a strong broadening compared to Cu0 2 crystals, which is attributed to lattice modulations within the Cu0 2 layers. This effect also influences the temperature dependence of the linewidth in YBa 2 Cu 3 0 7 _ a . 
  Reference    Z. Naturforsch. 47a, 27—29 (1992); received August 3 1991 
  Published    1992 
  Keywords    NQR, Superconductivity, High-T c compounds, Resonance frequency, Linewidth 
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 TEI-XML for    default:Reihe_A/47/ZNA-1992-47a-0027.pdf 
 Identifier    ZNA-1992-47a-0027 
 Volume    47 
4Author    Robert Schöllhorn, Wolfgang SchrammRequires cookie*
 Title    Topotactic Electron/Ion Transfer Reactions of Niobium Chalcogenides Nb3X4 with Tunnel Structure  
 Abstract    Cathodic reduction of Nb3S4 and Nb3Se4 electrodes in electrolytes containing cations of appropriate size results in the formation of ternary phases A^[Nb3X4p-. The cations A n+ are taken up into empty channels of the rigid lattice framework and exhibit high ionic mobility at room temperature. 
  Reference    Z. Naturforsch. 34b, 697—699 (1979); received February 19 1979 
  Published    1979 
  Keywords    Topotactic Reactions, Intercalation Compounds, Niobium Chalcogenides, Ionic Conductivity, Superconductivity 
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 TEI-XML for    default:Reihe_B/34/ZNB-1979-34b-0697.pdf 
 Identifier    ZNB-1979-34b-0697 
 Volume    34 
5Author    MichaelC. Böhm, Christoph SaalRequires cookie*
 Title    On a Direct Superconducting Pairing Mechanism  
 Abstract    We suggest that superconductivity can be traced back to a first-order interaction between the charge carriers which does not necessarily involve second-order electron-phonon coupling. For small molecular model systems it is demonstrated that the formation of Cooper pairs can lead to an attenuation of the destabilizing influence of the Pauli antisymmetry principle (PAP). We suggest that this attenuation of a fermionic quantum constraint is the driving force for the superconducting transition. Whenever the PAP is activated in single-particle hoppings of electrons, the corresponding moves reduce the electronic derealization; they raise the ensemble energy. The stability of a fermionic system is enhanced with decreasing influence of the PAP. Moves of Cooper pairs are not influenced by any quantum constraint of the intersite type. This behaviour differs from the well-known fermionic constraints. The quantum statistics of Cooper pairs is of a mixed type combining a fermionic on-site and a bosonic intersite behaviour. It coincides with the quantum statistics of so-called hard core bosonic ensembles. In the present work Cooper pair formation has been studied for smaller molecular models. The electronic Hamiltonian employed is of the two-parameter Hubbard-type. The solid state phenomenon superconductivity is correlated with the molecular concepts of "antiaromaticity" and "aromaticity". Arguments are given which support the present interpretation. 
  Reference    Z. Naturforsch. 54a, 718—738 (1999); received May 26 1999 
  Published    1999 
  Keywords    Superconductivity, Possible First-Order Pairing Interaction, Quantum Statistics, Pauli Antisymmetry Principle, Superconductivity vs Antiaromaticity and Aromaticity 
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 TEI-XML for    default:Reihe_A/54/ZNA-1999-54a-0718.pdf 
 Identifier    ZNA-1999-54a-0718 
 Volume    54