Metal-to-Insulator Transition in an Anderson Insulator With Kondo Impurities

We report a voltage-controlled critical behavior observed in a GaAs epitaxial structure containing a dense array of ErAs nanoparticles. When fabricated with metal electrodes, the structure displays a voltage- and temperature-dependent metal-to-insulator transition and strong hysteresis in the current versus voltage and versus temperature characteristics, with critical temperatures as high as 77 K. Furthermore, we observed a diverging rms deviation of the electrical conductance with respect to the critical bias voltage, which further supports the existence of a phase transition. The insulating phase is governed by Efros-Shklovskii variable range hopping, and the conductance reduces beyond instrument limits as the temperature drops toward zero; supporting it is an Anderson insulator. The metallic phase displays a conductance minimum at a critical temperature behaving similarly to that of single quantum dot due to Kondo resonance, and then the conductance increases as the temperature continues to drop. Furthermore, the metallic phase displays a colossal magnetoresistance under a weak magnetic field at 77 K while the insulating phase does not. And the metal-to-insulator phase transition can be induced by the magnetic field showing a critical discontinuity, similar to that versus temperature and voltage. We propose that the metal-to-insulator phase transition can be explained by the concept of an Anderson insulator with Kondo impurities.