  Using small angle neutron scattering, we have investigated higher-order peaks
in the skyrmion-lattice phase of Cu$_2$OSeO$_3$, in which two different
skyrmion lattices, SkX1 and SkX2, are known to form. For each skyrmion-lattice
phase, we observed two sets of symmetrically inequivalent peaks at the
higher-order-reflection positions with the indices $(110)$ and $(200)$. Under
the condition where the SkX1 and SkX2 coexist, we confirmed the absence of the
scattering at $\mathbf{Q}$ positions combining reflections from the two phases,
indicating a significantly weak double-scattering component. Detailed analysis
of the peak profile, as well as the temperature and magnetic-field dependence
of the peak intensity, also supports the intrinsic higher-order modulation
rather than the parasitic double scattering. The two higher-order modulations
show contrasting magnetic-field dependence; the former $(110)$ increases as the
field is increased, whereas the latter $(200)$ decreases. This indicates that,
in Cu$_2$OSeO$_3$, skyrmions are weakly distorted, and the distortion is
field-dependent in a way that the dominant higher-order modulation switches
from $(110)$ to $(200)$ under field. Monte Carlo simulations under sweeping
external magnetic field qualitatively reproduce the observed magnetic-field
dependence, and suggests that the higher-order modulations correspond to the
superlattices of weak swirlings appearing in the middle of the original
triangular-latticed skyrmions.

Strongly Correlated Electrons