  We consider certain constant-coefficient differential operators on
$\mathbb{R}^d$ with positive-definite symbols. Each such operator $\Lambda$
with symbol $P$ defines a semigroup $e^{-t\Lambda}$ , $t>0$ , admitting a
convolution kernel $H^t_P$ for which the large-time behavior of $H_P^t(0)$
cannot be deduced by basic scaling arguments. The simplest example has symbol
$P(\xi)=(\eta+\zeta^2)^2+\eta^4$ , $\xi=(\eta,\zeta)\in \mathbb{R}^2$ . We
devise a method to establish large-time asymptotics of $H^t_P(0)$ for several
classes of examples of this type and we show that these asymptotics are
preserved by perturbations by certain higher-order differential operators. For
the $P$ just given, it turns out that $H^t_P(0)\sim c_Pt^{-5/8}$ as
$t\to\infty$ . We show how such results are relevant to understand the
convolution powers of certain complex functions on $\mathbb Z^d$ . Our work
represents a first basic step towards a good understanding of the semigroups
associated with these operators. Obtaining meaningful off-diagonal upper bounds
for $H_P^t$ remains an interesting challenge.
