Unusual anisotropy of negative Hall states in (TMTSF)2PF6

We report an unusual orientation dependence of the negative quantum Hall effect (QHE) phases of the organic conductor (TMTSF) 2 PF 6 . Rotations within the b-c crystallographic plane produce a strong enhancement of the negative QHE states as the sample is tilted away from the normal. For rotations within the a-c plane, an opposite but somewhat less pronounced effect is observed. However, no such orientation dependence is found in the positive QHE states. These results establish a fundamental difference between the negative and positive QHE states in (TMTSF) 2 PF 6 . We speculate on the possibility of charge ordering associated with the negative phases.