Electron glass in ultrathin granular Al films at low temperatures

Quench-condensed granular Al films of sheet resistance similar to 10 k Omega/square display hysteresis and ultraslow, nonexponential relaxation in the resistance when the temperature is varied below 300 mK. The hysteresis is nonlinear and the relaxation time does not obey the Arrhenius form. Furthermore, large resistance fluctuations having a 1/f-type power spectrum are observed at low temperatures, with a low-frequency cutoff which shifts to lower frequencies with decreasing temperature. These observations combine to provide a coherent picture that there exists a new glassy electron state in ultrathin granular A1 films, with a growing correlation length at low temperatures.