Schrödinger Semigroup is compact if potential goes to infinity

Question

Several papers (e.g. this one: arXiv:0810.3275v1 [math.SP] 17 Oct 2008 ) claim that if $H=-\Delta+V$ and $V(x)\to\infty$ if $|x|\to\infty$, then the semigroup $e^{-tH}$ is eventually compact.

Does anyone know any references for that? I looked in Reed Simon but couldn't find anything.

Regards, and thanks in advance,

Frank