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  • Articles: DFG German National Licenses  (139)
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  • Articles  (139)
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  • 11
    Electronic Resource
    Electronic Resource
    Integrable and nonintegrable classical spin clusters (1987)
    Springer
    The European physical journal 65 (1987), S. 363-374 
    Details
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The nonlinear dynamics is investigated for a system ofN classical spins. This represents a Hamiltonian system withN degrees of freedom. According to the Liouville theorem, the complete integrability of such a system requires the existence ofN independent integrals of the motion which are mutually in involution. As a basis for the investigation of regular and chaotic spin motions, we have examined in detail the problem of integrability of a two-spin system. It represents the simplest autonomous spin system for which the integrability problem is nontrivial. We have shown that a pair of spins coupled by an anisotropic exchange interaction represents a completely integrable system for any values of the coupling constants. The second integral of the motion (in addition to the Hamiltonian), which ensures the complete integrability, turns out to be quadratic in the spin variables. If, in addition to the exchange anisotropy also singlesite anisotropy terms are included in the two-spin Hamiltonian, a second integral of the motion quadratic in the spin variables exists and thus guarantees integrability, only if the model constants satisfy a certain condition. Our numerical calculations strongly suggest that the violation of this condition implies not only the nonexistence of a quadratic integral, but the nonexistence of a second independent integral of motion in general. Finally, as an example of a completely integrableN-spin system we present the Kittel-Shore model of uniformly interacting spins, for which we have constructed theN independent integrals in involution as well as the action-angle variables explicitly.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01303725
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  • 12
    Electronic Resource
    Electronic Resource
    Integrable and nonintegrable classical spin clusters (1988)
    Springer
    The European physical journal 70 (1988), S. 251-268 
    Details
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract This study investigates the nonlinear dynamics of a pair of exchange-coupled spins with biaxial exchange and single-site anisotropy. It represents a Hamiltonian system with 2 degrees of freedom for which we have already established the (nontrivial) integrability criteria and constructed the integrals of the motion provided they exist. Here we present a comparative study of the phase-space trajectories for two specific models with the same symmetry properties, one of which (the XY model with exchange anisotropy) is integrable, and the other (the XY model with single-site anisotropy) nonintegrable. In the integrable model, the integrals of the motion (analytic invariants) can be reconstructed numerically by means of time averages of dynamical variables over all trajectories. In the nonintegrable model, such time averages over trajectories define nonanalytic invariants, where the nonanalyticities are associated with the presence of chaotic trajectories. A prominent feature in the nonintegrable model is the occurrence of very long time scales caused by the presence of low-flux cantori, which form “sticky” coats on the boundary between chaotic regions and regular islands or “leaky” walls between different chaotic regions. These cantori dominate the convergence properties of time averages and presumably determine the long-time asymptotic properties of dynamic correlation functions. Finally, we present a special class of integrable systems containing arbitrarily many spins coupled by general biaxial exchange anisotropy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01318307
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  • 13
    Electronic Resource
    Electronic Resource
    Anomale Röntgenstreuung schwerer Atome im Bereich derK undL-Kanten nach der wellenmechanischen Dispersionstheorie. I (1953)
    Springer
    The European physical journal 136 (1953), S. 491-510 
    Details
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01338941
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  • 14
    Electronic Resource
    Electronic Resource
    Effect of a temperature dependent effective quasiparticle mass on the surface impedance of YBa 2 Cu 3 O 7-x (2000)
    Springer
    The European physical journal 14 (2000), S. 605-610 
    Details
    ISSN: 1434-6036
    Keywords: PACS. 74.25.Nf Response to electromagnetic fields (nuclear magnetic resonance, surface impedance, etc.) - 74.20.De Phenomenological theories (two-fluid, Ginzburg-Landau, etc.) - 74.76.Bz High- films
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: The temperature dependent surface impedance Z s ( T ) of single-crystalline YBa 2 Cu 3 O 7-x (YBCO) was analyzed within the two-fluid model in terms of the fraction of paired charge carriers, f s ( T ), and of the quasiparticle scattering time . The usual approach was extended by considering a temperature dependent effective quasiparticle mass m * ( T ), which results from a strong electron-phonon interaction. This effect must not be neglected in the description of high-temperature superconductors due to the large ratio of T c to the Debye temperature T D . The temperature dependence of the penetration depth, , of high-quality YBCO crystals and films could be described with an electron-phonon coupling constant , and using f s ( T ) = 1- ( T / T c ) 2.8 as an approximation of the BCS theory. Different trial phonon spectra were encountered in terms of their ability to reproduce the -data. The scattering time was described by the Bloch-Grüneisen formalism with T D =460 K. Assuming an Einstein spectrum with , a residual resistivity and a fraction of unpaired quasiparticles at T = 0 K yielded a surprisingly good agreement of the model with Z s ( T )-data measured at 87 GHz with a high-quality epitaxial YBCO film between T =4 K and T c . While an exact reproduction of the surface impedance asks for a rigorous theoretical computation, our analysis demonstrates that strong electron-phonon coupling is relevant for discussing the unconventional transport properties of YBCO.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100510051068
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  • 15
    Electronic Resource
    Electronic Resource
    Observation of variable range hopping at “natural” phonon frequencies (1978)
    Springer
    The European physical journal 31 (1978), S. 233-235 
    Details
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Hill's analysis of hopping conductivity data has been applied to ion-bombarded amorphous-silicon samples. The apparent hopping conductivity parameters derived from a standardT −0.25 plot undergo changes by several orders of magnitude when plotted with the exact scaling of the abscissa. A typical example is characterized by a temperature dependence ofdc conductivity according toσ=σ 0 exp (—(T 0/T) m ), withm=0.45,T 0 =6.4⋅104K andσ 0=6.6⋅101 Ω−1 cm−1. Fromσ 0 a phonon frequency of about 3−15⋅1012 s−1 is derived.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01352347
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  • 16
    Electronic Resource
    Electronic Resource
    Nomenclature of steroidal sapogenins (1962)
    Springer
    Cellular and molecular life sciences 18 (1962), S. 404-405 
    Details
    ISSN: 1420-9071
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Zusammenfassung Es werden Einzelheiten für den Gebrauch von Spirostan und Furostan, wenn nötig durch die Präfixe 5α, 5β, 25R und 25S modifiziert, für die Benennung der Steroid-Sapogenine bekanntgegeben. Diese Vorschläge sollen als formale Grundlage zur Nomenklatur gelten.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02151485
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  • 17
    Electronic Resource
    Electronic Resource
    A mnemonic for configurations of steroidal sapogenins (1962)
    Springer
    Cellular and molecular life sciences 18 (1962), S. 253-254 
    Details
    ISSN: 1420-9071
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Zusammenfassung Die Beweggründe für den Gebrauch derD- undL-Konvention in der Nomenklatur der Steroid-Sapogenine sowie ihre Anwendung auf die Konfigurationsanalyse und Formelaufstellung werden diskutiert.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02148214
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  • 18
    Electronic Resource
    Electronic Resource
    Attenuation operators and complex wave velocities for scattering in random media (1996)
    Springer
    Pure and applied geophysics 148 (1996), S. 269-285 
    Details
    ISSN: 1420-9136
    Keywords: Wave scattering ; attenuation ; random media ; reflectivity method
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The concept of attenuation operators and complex velocities is applied to scattering attenuation in two and three dimensions, using the minimum-phase assumption for the attenuation operator. Acoustic 2D finite-difference computations of synthetic seismograms show, that the attenuation operator describes well the decay and lowpass filtering of the averaged wave form, which follows from averaging travel-time-corrected wave forms along the wave front. In the case of exponential random media, analytical forms of the attenuation operators and complex velocities are available. The complex velocities are incorporated into the reflectivity method. As an application, synthetic seismograms are presented for theS n wave, attenuated by lithospheric velocity and density fluctuations. The limitations of attenuation operators and complex velocities for scattering are also discussed. With these quantities it is not possible to model phenomena related to the scattered waves themselves, such as amplitude and travel-time fluctuations along the wave front, codas and precursors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00882063
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  • 19
    Electronic Resource
    Electronic Resource
    Freshwater sediments: Sinks and sources of bromine (1993)
    Springer
    Naturwissenschaften 80 (1993), S. 558-560 
    Details
    ISSN: 1432-1904
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology , Natural Sciences in General
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01149270
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  • 20
    Electronic Resource
    Electronic Resource
    Die Belastung der Elbe mit Schwermetallen Erste Ergebnisse von Sedimentuntersuchungen (1994)
    Springer
    Naturwissenschaften 81 (1994), S. 401-405 
    Details
    ISSN: 1432-1904
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology , Natural Sciences in General
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01132693
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