Transition from Type-I to Type-II Superconducting Behaviour with Temperature observed by $\mu$SR and SANS

We investigate the superconducting behaviour of Bi doped Pb. Pure lead shows type-I behaviour entering an intermediate state in a magnetic field. High dopings of Bi ($>$3%) lead to type-II behaviour showing a mixed state, where the magnetic field penetrates the superconductor in the form of a flux lattice. At intermediate doping, the sample shows both type-I and type-II behaviour depending on the temperature. This arises because the Ginzburg-Landau parameter $\kappa$ passes through its critical value of $1\mathrm{/}\sqrt{2}$ with temperature.