  Two new Raman modes below 100 cm^-1 are observed in twisted bilayer graphene
grown by chemical vapor deposition. The two modes are observed in a small range
of twisting angle at which the intensity of the G Raman peak is strongly
enhanced, indicating that these low energy modes and the G Raman mode share the
same resonance enhancement mechanism, as a function of twisting angle. The 94
cm^-1 mode (measured with a 532 nm laser excitation) is assigned to the
fundamental layer breathing vibration (ZO (prime) mode) mediated by the twisted
bilayer graphene lattice, which lacks long-range translational symmetry. The
dependence of this modes frequency and linewidth on the rotational angle can be
explained by the double resonance Raman process which is different from the
previously-identified Raman processes activated by twisted bilayer graphene
superlattice. The dependence also reveals the strong impact of electronic-band
overlaps of the two graphene layers. Another new mode at 52 cm^-1, not observed
previously in the bilayer graphene system, is tentatively attributed to a
torsion mode in which the bottom and top graphene layers rotate out-of-phase in
the plane.
