The Effect of Preparation Method on Superconducting Critical Temperature and Other Properties in Granular Aluminum

Four series of 3D granular aluminum sample were made under different deposition conditions. Measurements were done in the temperature range 4.2 K to 0.4 K and in magnetic fields up to 9 T. It was found that the enhancement of the superconducting transition temperature T_{rm C} depends on the method of preparation. As the metal-insulator transition is approached T _{rm C} is gradually depressed to 1.3 K for three Al-Al_2O _3 series but dropped abruptly to 0.5 K for an Al-Ge series in which the grain size was four or five times larger than in the Al-Al_2O _3 series. Complete superconductivity disappears in all four series at a resistivity (at 4.2 K) of about 0.2 Omega-cm beyond which only quasi-reentrant or insulating behavior can be observed.