What constitutes substantial evidence of incompetence in a criminal case?

In this context, "substantial evidence" has been defined as evidence that is sufficient to raise a reasonable doubt concerning the defendant's competence to stand trial. (Welch, supra, 20 Cal.4th at p. 738.) "Substantial evidence of incompetence exists when a qualified mental health expert who has examined the defendant states under oath, and ' " 'with particularity,' " ' a professional opinion that because of mental illness, the defendant is incapable of understanding the purpose or the nature of the criminal proceedings against him, or of cooperating with counsel. [Citations.] [] The defendant's demeanor and irrational behavior may also, in proper circumstances, constitute substantial evidence of incompetence." (People v. Mai, supra, 57 Cal.4th at pp. 1032-1033.)