  Metallic glasses have so far attracted considerable attention for their
applications as bulk materials. However, new physics and applications often
emerge by dimensional reduction from three dimension (3D) to two dimension
(2D). Here, we study, by molecular dynamics simulations, how the
liquid-to-glass transition of a binary Cu50Zr50 MG is affected by spatial
dimensionality. We find clear evidence that crystal-like structural ordering
controls both dynamic heterogeneity and slow dynamics, and thus plays a crucial
role in the formation of the 2DMG. Although the 2DMG reproduces the dynamical
behaviors of its 3D counterpart by considering Mermin-Wagner-type fluctuations
specific to 2D, this atomic-scale structural mechanism is essentially different
from that for the 3DMG in which icosahedral clusters incompatible with
crystallographic symmetry play a key role in glassy behaviors. Our finding
provides a new structural mechanism for the formation of 2DMGs, which cannot be
inferred from the knowledge of 3DMGs. The results suggest a structural basis
for the glass transition in 2DMG and provide possible explanations for some
previous experimental observations in ultrathin film MGs.
