Magnetic Behavior of Type-II Superconductors with Small Ginzburg-Landau Parameters

Abstract

Type-II superconductors with $\kappa \approx 1$ exhibit a first-order phase transition at $H_{c1\mathrm{}}$, which is due to an attractive interaction between flux lines. This behavior shows up as a discontinuous increase of the flux density from zero to a certain value $B_0$. We have measured $B_0$ as a function of temperature $T$, Ginzburg-Landau parameter $\kappa$, and impurity parameter $\alpha =0.882\mathrm{}\frac{{\xi }_0}{l}$, and have determined the critical values of these parameters below which a first-order transition occurs at $H_{c1\mathrm{}}$. We further investigated the dependence of the parameters ${\kappa }_1$ and ${\kappa }_2$ on $l$ and $T$, and we show that only the condition ${\kappa }_1\mathrm{}\left(T\right)=\frac{1}{\sqrt{2}}$ determines the transition from type-I to type-II behavior. Comparison of our experimental results with recent theoretical treatments shows only qualitative agreement.