On operator-valued free convolution powers

We give an explicit realization of the $η$-convolution power of an $A$-valued distribution, as defined earlier by Anshelevich, Belinschi, Fevrier and Nica. If $η:A\to A$ is completely positive and $η\geq\operatorname{id}$, we give a short proof of positivity of the $η$-convolution power of a positive distribution. Conversely, if $η\not\geq\operatorname{id}$, and $s$ is large enough, we construct an $s$-tuple whose $A$-valued distribution is positive, but has non-positive $η$-convolution power.

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