Magnetoresistance Measurements on both Sides of Metal-Insulator Transition in Granular Aluminum.

The magnetoresistance of granular aluminum was measured for a series of samples on both sides of the metal -insulator transition. The temperature range was from 4.2K to 0.42K. The magnetic field ranged up to 9 tesla. It was found that both the disappearance of superconductivity and a maximum of the fractional negative magnetoresistance occur at the metal-insulator transition. The conductivity of the samples near at the critical region becomes smaller than Mott's minimum conductivity as T goes down to low temperature. On the metallic side, the temperature dependence of the conductivity changes from T(' 1/2) to T(' 3/4) as the metal-insulator transition is approached. In the presence of a high magnetic field the conductivity of the insulating samples which are close to the metal -insulator transition also shows T('1.2-1.3) dependence down to the lowest temperature measured. A simple scaling argument enables us to estimate the absolute value of the localization length of the insulating samples.