Light, pressure, and stress induced phase transitions of deuterated Cu(DCNQI)2 radical ion salts

The extremely sharp metal-insulator phase transition at Tc = 60 K of 6-fold deuterated Cu(DCNQI)2 could be shifted to lower temperature T by illumination with light and by mechanical tension, both acting contrarily to the application of pressure. By proper selection of T, using pressure cycles, the samples could be switched to a persistent insulating state. Applying light (of different wavelengths) we could reestablish the conductivity. Correlating the temperature dependence of the crystal; length (interferometric, resolution Δ1/1 < 10−5) with that of the conductivity clearly demonstrates by the smooth change of the crystal length the model of connectivity limited conductivity between the noninverted still metallic microdomains.