  Taking the ground state rotational band in $^{24}$Mg as an example, we
investigate the impurity effect of $\Lambda$ hyperon on collective excitations
of atomic nuclei in the framework of non-relativistic energy density functional
theory. To this end, we take into account correlations related to the
restoration of broken symmetries and fluctuations of collective variables by
solving the eigenvalue problem of a five-dimensional collective Hamiltonian for
quadrupole vibrational and rotational degrees of freedom. The parameters of the
collective Hamiltonian are determined with constrained mean-field calculations
for triaxial shapes using the SGII Skyrme force. We compare the low-spin
spectrum for $^{24}$Mg with the spectrum for the same nucleus inside
$^{25}_{\Lambda}$Mg. It is found that the $\Lambda$ hyperon stretches the
ground state band and reduces the $B(E2:2^+_1 \rightarrow 0^+_1)$ value by
$\sim 9%$, mainly by softening the potential energy surface towards the
spherical shape, even though the shrinkage effect on the average proton radius
is only $\sim0.5%$.
