  Let $H:=-\Delta+V$ be a nonnegative Schr\"odinger operator on $L^2({\bf
R}^N)$, where $N\ge 2$ and $V$ is a radially symmetric function decaying
quadratically at the space infinity. In this paper we consider the
Schr\"odinger heat semigroup $e^{-tH}$, and make a complete table of the decay
rates of the operator norms of $e^{-tH}$ in the Lorentz spaces as $t\to\infty$.
