Magic Angles and the Ground States in (TMTSF)₂PF₆

Resistance measurements in the “normal” state of (TMTSF)₂PF₆ under pressure for different magnetic field orientations show striking features below 5 K. We observe metallic behavior (dR/dT>0) when the field is aligned along the intermolecular tunneling directions (Lebed magic angles), and nonmetallic behavior (dR/dT<0) for other orientations. The crossover is associated with a reduction in dimensionality when the field applied perpendicular to the tunneling direction exceeds a critical temperature dependent value. At higher temperatures there are further dimensionality crossovers associated with the competition of thermal, electronic, and magnetic energies.