Hysteresis, fluctuations, and correlations in the electron glass

Quench-condensed ultrathin granular Al films, with normal-state sheet resistance close to 10 kΩ/€, are superconducting below 2.5 K in zero magnetic field. Above a critical field, they are only weakly insulating at mK temperatures. However, in this field-driven normal state, the films demonstrate a glassy electronic behavior such as strong hysteresis and ultraslow, non-exponential relaxation in film resistance when the temperature is varied below 300 mK. The hysteresis is nonlinear and can be suppressed by a dc bias voltage. The relaxation time does not obey the Arrhenius form, indicating the existence of a broad distribution of low energy barriers. Furthermore, large resistance fluctuations, having a 1/f-type power spectrum with a low-frequency cut-off, are observed at low temperatures. With decreasing temperature, the amplitude of the fluctuation increases and the cut-off frequency decreases. We argue that the fluctuation power spectrum reveals a growing correlation length with decreasing temperature in the electron glass.