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  • 1
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We investigate theoretically the problem of a superconducting matrix containing paramagnetic rare earth impurities with crystal-field split energy levels. There are two competing mechanisms which change the superconducting transition temperatureT c. One is inelastic charge scattering of conduction electrons from the aspherical part of the 4f charge distribution, which leads to an increase inT c similar to that of optical phonons. The other and often predominant mechanism comes from the exchange interaction, which depressesT c and can be very effective even among non-magnetic levels via off-diagonal matrix elements. Crystalline fields serve to alter the effectiveness of the two kinds of scattering depending upon the symmetry character of the low-lying levels, and in favorable cases one may study separately the effects of the two types of scattering by adding different impurities to a given host. We find that crystal-field levels at energies quite high compared tok B·Tc can still have an important effect onT c. It is shown that the crystalline-field splitting should be directly observable as structure in the tunneling characteristics.
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 248 (1971), S. 154-158 
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The model of linear chains is used to study the lattice softening of A 15-compounds in strong magnetic fields. It is shown that a strong field stabilizes the cubic phase due to the Zeeman energy of the conduction electrons. Experiments are suggested to test our theoretical findings.
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  • 3
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present a formalism for treating the problem of impurities in a lattice of magnetic rare earth ions. Latter are subject to a crystalline field and special attention is paid to non-Kramers ions in a singlet ground state. Our calculations are restricted to the paramagnetic regime. We derive the conditions for magnetic localized modes to occur and discuss the appearance of local magnetic instabilities. It is shown that the impurity effects are especially large if the system is close to a magnetic phase transition. Furthermore we compute the influence of impurities on the magnetic transition temperature. For the case of vacancies or nonmagnetic impurities the dependence of the Curie temperature on impurity concentration is derived. It is demonstrated that small amounts of impurities can often completely suppress magnetic ordering.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 21 (1975), S. 369-379 
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A theory of rotationally invariant spin-lattice interactions in rare earth systems is presented. It is shown that rotational invariance to leading order is ensured only if rotational interactions of first and second order in the displacements are included simultaneously in the spin-lattice Hamiltonian. The rotational second-order interactions yield effects which are as large as those of the linear rotational interaction. It is pointed out that a corresponding statement should hold also for pure strain interactions. The phonon Green's function is calculated for the paramagnetic phase of rare earth systems. It is found that in an applied magnetic field the rotational interactions cause measureable changes of the phonon dispersion and the sound velocity even for cubic symmetry. These effects turn out to be of the same order of magnitude as the conventional field-dependent strain effects and are qualitatively different from the latter. The results of our theory are illustrated by the example of SmSb, and quantitative predictions for the transverse sound velocities are given.
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  • 5
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We study the mutual influence of magnetic and elastic properties of a Rare Earth system as an external magnetic field is applied. It is demonstrated that a magnetic phase transition should show up in the temperature dependence of certain sound velocities. Furthermore the magnetic field should induce a hybridization between the magnetic excitons and phonons. Although the theory is valid for any crystal field level scheme we use for an explicit example a singletΓ 1 groundstate with aΓ 4 triplet first excited state.
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  • 6
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A new variational approach is introduced for the calculation of correlation energies of molecules. It is based on the local character of the correlated electron motion. Based on the approach we calculate correlation energies for simple systems. These include various Hubbard models as well as a model of the H6-ring for which exact results are available. Our finding is that the proposed approach appears to be simpler and more economical for numerical work than conventional CI-methods. More than 90% of the correlation energy is obtained in all cases considered.
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  • 7
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A theory is developed for the Faraday effect and Cotton-Mouton effect of acoustic phonons in paramagnetic Rare Earth systems. The effects occur if degenerate transverse acoustic phonons are propagating in the direction of an applied magnetic field and perpendicular to it. The theory is formulated for cubic crystals but a generalization to other crystal symmetries is straightforward. The theoretical findings are applied to TmTe and it is demonstrated that both effects are large enough to be observable.
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  • 8
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Description / Table of Contents: Résumé Nous avons étudié théoriquement la conductance par effet tunnel dans la limite d'un voltage nul. Ce phénomène est intéressant pour mettre en évidence expérimentalement les transitions du premier ordre dans les supraconducteurs à haut champ. Nous calculons et évaluons numériquement cette conductance pour une valeur quelconque du libre parcours moyen de la diffusion spin-orbite. Dans nos calculs nous distinguons le cas des films minces placés dans un champ parallèle et le cas des matériaux dont la valeur deℵ est élevée placés dans un champ perpendiculaire. Dans le premier cas les propriétés anomales apparaissent pour une valeur assez élevée de ce libre parcours moyen. Nous avons enfin déterminé la valeur minimale de ce libre parcours moyen compatible avec l'existence d'une transition du premier ordre, cette valeur est beaucoup plus petite qu'on le croit d'ordinaire.
    Abstract: Zusammenfassung Wir untersuchen die Tunnelleitfähigkeit bei verschwindender Spannung für Supraleiter in hohen Magnetfeldern und zeigen, daß diese eine vielversprechende experimentelle Methode sein sollte, um Phasenübergänge erster Ordnung in solchen Systemen zu messen. Formeln für die Tunnelleitfähigkeit bei verschwindender Spannung werden für beliebige Werte der Spin-Bahn Wechselwirkung berechnet und numerisch ausgewertet. Wir unterscheiden dabei zwischen dünnen Schichten im parallelen Magnetfeld und Materialien mit großemℵ-Wert in senkrechtem Feld. Im Falle dünner Schichten treten Anomalien auf, falls die freie Weglänge für spin-flip Prozesse genügend groß ist. Schließlich bestimmen wir die kleinste freie Weglänge für spin-flip-Prozesse, die noch mit einem Phasenübergang erster Ordnung verträglich ist. Es zeigt sich, daß diese freie Weglänge viel kleiner ist, als bis jetzt allgemein angenommen wurde.
    Notes: Abstract The zero bias tunneling conductance is investigated theoretically and shown to be a promising experimental tool for detecting first order phase transitions in high field superconductors. We calculate and evaluate numerically the zero bias tunneling conductance for arbitrary spin orbital mean free path. In doing the calculations we distinguish between thin films in a parallel field and highℵ-value materials in a perpendicular field. In the former case anomalous properties appear for sufficient long spin orbital mean free path. Finally we determine the minimum spin orbital mean free path which is compatible with a first order phase transition and show that it is much smaller than commonly believed.
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  • 9
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract When dealing with electronic correlations, the latter are often treated without considering changes in the one-particle density. This is a good approximation in systems in which electronic correlations are sufficiently weak, but it is not justified any longer when they become strong. The treatment of electronic correlations within the local ansatz is generalized by the inclusion of one-particle operators. Nonorthogonalized local operators are used for their description. The generalized scheme allows also for an improvement of self-consistent field calculations by including the effects of a larger basis set via one-particle operators.
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  • 10
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract It is shown that thed-electron surface density of states of a transition metal is strongly influenced not only by “off-diagonal” surface effects (e.g. reduction of nearest neighbors at the surface) but also by “diagonal” surface effects. Latter result from differences in the effective atomic potential for differentd-orbitals. They lead generally to large resonant contributions to the surface density of states. It is demonstrated that the “diagonal” effects must be included in any realistic calculation of the surface density of states.
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