  Twisted double bilayer graphene (TDBG) under a transverse electric field is
predicted to exhibit a topological valley Chern number 2 at charge neutrality,
protected by an emergent valley symmetry, which can manifest in the Hofstadter
spectra. Here, we report the experimental observation of a universal closing of
the charge neutrality gap of TDBG at 1=2 magnetic flux per unit cell, which is
in agreement with theoretical predictions for a valley Chern number 2 gap, and
provides evidence for both the emergent symmetry and valley Chern bands.
Furthermore, our theoretical analysis of the interaction effect and
experimental data shows that the interaction energy is larger than the
flat-band bandwidth in TDBG near 1 degree, but does not alter the emergent
valley symmetry and the single-particle band topology.
