Separating Rank Logic from Polynomial Time

In the search for a logic capturing polynomial time most promising candidates are Choiceless Polynomial Time (CPT) and rank logic. Rank extends fixed-point with counting by operator over prime fields. We show that isomorphism problem CFI graphs ℤ 2 i cannot be defined in logic, even if base graph is totally ordered. However, CPT can define this problem. thereby separate from particular time.