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Next: Tinkham's Formula Fits Up: Experimental Results: Angular Previous: Type I Superconductivity

Type II/1 Superconductivity and C4KHg


The possibility of type I superconductivity in C4KHg was discussed above. In this case a first-order transition is observed at Hc rather than a second-order one at Hc2. There is also the possibility of a first-order transition at the lower critical field, Hc1, even in type II superconductors.[11] There are materials which, as the applied field is swept up, exhibit first a first-order transition at Hc1 and then a second-order transition at Hc2, as explained below. These superconductors are called ``type II/1'' to distinguish them from the more commonly encountered ``type II/2'' variety which have second-order transitions both at Hc1 and Hc2.[252] According to Auer and Ullmaier, who studied type II/1 superconductivity in TaN and NbN films, ``type II/1 superconductivity is a general phenomenon for all low-kappa type-II superconductors''.[11] If this statement is correct, the numbers in Table gif suggest that type II/1 superconductivity should occur for a sizable range of angles in C4KHg, especially close to Tc .

Type II/1 transitions occur in materials which have attractive interactions among vortices for large intervortex separations. (Of course the intervortex interactions must be repulsive at small separations, or else the vortices will coalesce into macroscopic normal regions, and the material will be type I rather than type II/1.) When this is the case, there will be an intervortex separation d0 which minimizes the interaction energy. If the vortex-interaction energy terms dominate the condensation energy in the total free energy, then once one vortex has been nucleated, the sample can reduce its energy by admitting more flux until the average vortex separation is d0. The result is that in type II/1 superconductors there is a discontinuity in the magnetization at Hc1, which corresponds to a first-order transition.[11]

Since type II/1 behavior impacts upon the beginning of the field sweep and not the region near Hc2, it cannot affect the measurement of the upper critical field directly. The influence of type II/1 behavior on the measurement of Hc2(theta) must come through its effect on the shape of the field sweeps, just like the indirect influence of the demagnetization. The shape of the transition could be significantly changed by a first-order transition, and this could bias the data analysis procedure enough to influence Hc2(theta). There is no definite evidence for type II/1 superconductivity in C4KHg, but its presence seems likely according to the Auer-Ullmaier analysis.[11] The presence or absence of a first-order transition could undoubtedly be resolved with magnetization measurements if a 1 K SQUID magnetometer were available.

next up previous contents
Next: Tinkham's Formula Fits Up: Experimental Results: Angular Previous: Type I Superconductivity (Alison Chaiken)
Wed Oct 11 22:59:57 PDT 1995