  Total Routhian surface calculations have been performed to investigate
rapidly rotating transfermium nuclei, the heaviest nuclei accessible by
detailed spectroscopy experiments. The observed fast alignment in $^{252}$No
and slow alignment in $^{254}$No are well reproduced by the calculations
incorporating high-order deformations. The different rotational behaviors of
$^{252}$No and $^{254}$No can be understood for the first time in terms of
$\beta_6$ deformation that decreases the energies of the $\nu j_{15/2}$
intruder orbitals below the N=152 gap. Our investigations reveal the importance
of high-order deformation in describing not only the multi-quasiparticle states
but also the rotational spectra, both providing probes of the single-particle
structure concerning the expected doubly-magic superheavy nuclei.
