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|Title:||SUPERCONDUCTIVITY IN MAGNETIC AND PROXIMITY EFFECT SYSTEMS|
|Authors:||Stephan, Hugo Walter|
|Abstract:||<p>Some aspects of the theory of superconductors containing paramagnetic impurities in the model of Shiba and Rusinov (SR) are examined. The critical magnetic field deviation function is shown to be very sensitive to the SR scattering parameter ∊₀ with the behavior of D(t) correlated with the zero frequency density of states and improved agreement is found with experiment Zn-Mn as compared to the predictions of the theory of Abrikosov and Gor'Kov (AG). Optical absorption and thermal conductivity experiments involving a variety of transition metal alloys are reanalyzed including up to three scattering phase shifts with no significant improvement found over the agreement obtained with only a single phase shift. The electromagnetic coherence length with SR impurities is also considered. Model calculations for superconducting spin-glasses show that systems such as GdᵪCc₁₋ᵪRu₂, which exhibit significant deviations from the AG prediction for the reduction of the critical temperature with impurity concentration are also expected to exhibit significant deviations from AG behavior for the temperature variation of properties such as the thermodynamic critical field and the electromagnetic penetration depth. The model of Lee for reentrant ferromagnetic superconductors is shown to be only in qualitative agreement with the temperature dependence of the free energy difference and thermal conductivity determined experimentally for ErRh₄B₄. This model is not in agreement with the measured peak in ultrasonic attenuation at higher temperature. A variety of properties of proximity effect junctions are considered within the McMillan model. The free energy difference is shown to be given by the standard Bardeen-Stephen result applied to both sides of t he junction. The temperature dependence of the free energy difference differs significantly from that of a BCS superconductor with the deviation function becoming much more negative than the BCS predictiton. Both the free energy difference and the specific heat jump are suppressed by increasing normal film thickness or the addition of paramagnetic impurities and this behavior is relatively insensitive to the details of scattering as described by the SR scattering parameter ∊₀. The optical absorption of the normal side of a proximity effect junction is found to have a small structure associated with the two peaked structure of the density of states predicted by the McMillan model, with the effect of SR model impurities qualitatively the same as in the intrinsic case. The electromagnetic penetration depth at low temperature is found to be quite sensitive to the addition of paramagnetic impurities as well as to the SR scattering parameter ∊₀. Finally, the temperature dependence of the zero bias tunneling conductance of a proximity effect induced superconducting spin-glass is calculated and shown to be in reasonable agreement with experiments involving Ag-Mn.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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