Acta Physica Academiae Scientiarum Hungaricae 39. (1975)

1975 / 3. szám - I. Kirschner - K. Sajó - J. Bánkuti: Analytical Expressions for the Dependence of Superconducting Parameters on Alloy Concentration

Acta Physica Academiae Scientiarum Hungaricae, Tomas 39 (3), pp. 167—170 (1975) ANALYTICAL EXPRESSIONS FOR THE DEPENDENCE OF SUPERCONDUCTING PARAMETERS ON ALLOY CONCENTRATION By I. KlRSCHNER, K. Sajó and J. BÁNKUTI LABORATORY FOR LOW TEMPERATURE PHYSICS, ROLAND EÖTVÖS UNIVERSITY, BUDAPEST (Received 26. VIII. 1975) Analytical expressions are given which describe the dependence of the critical temper­ature, of the critical magnetic fields and of the GL parameter on the concentration of the alloy­ing material, characterizing the phase transition within the superconducting state in good agree­ment with the experiments. In one of our previous papers [1] it was discovered that in superconduct­ing alloys of small GL parameter x a specific phase transition of new type occurs on the effect of increasing alloy concentration. This means that having reached a certain critical concentration Cc the previous type I superconducti­vity (Meissner’s state) goes over to type II (mixed state). Our experiments have been carried out with In-Bi specimens containing 0-5 at. % Bi [2]. The evaluation of the experimental results shows that this phase transi­tion occurs at any temperature-dependent x(T) and at any temperature always whenever the value x = 1/|/2=ис has been reached. Therefore this is considered to be the critical value of GL parameter. The occurring phase transition can be characterized with the following fundamental features: 1. The order parameter » describing the superconductivity becomes inhomogeneous as superconductive and normal domains appear. (In the pres­ence of an external current it becomes anisotropic as well). The vector pot­ential has a similar behaviour too. 2. The superconductivity, being previously a surface phenomenon, be­comes a volumetric one. (The transport current and the external magnetic field penetrate into the sample). 3. Instead of one critical magnetic field Hc describing the pure super­conductivity the critical fields Hcl and Hc2 together determine the magnetic penetration. 4. A sudden change occurs in the value of the thermodynamic entropy matrix [3]. 5. A quick change occurs in the macroscopic conductivity matrix. (This registers the superconductive-normal lattice structure). Acta Physica Academiae Scientiarum Hungaricae 39, 1975

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