  Twisted bilayers of two-dimensional materials have emerged as a highly
tunable platform for studying broken symmetry phases. While most interest has
been focused on emergent states in systems whose constituent monolayers do not
feature broken symmetry states, assembling monolayers that exhibit ordered
states into twisted bilayers can also give rise to interesting phenomena. Here,
we use large-scale first-principles density-functional theory calculations to
study the atomic structure of twisted bilayer $\mathrm{{N}b{S}e_2}$ whose
constituent monolayers feature a charge density wave. We find that different
charge density wave states coexist in the ground state of the twisted bilayer:
monolayer-like $3\times 3$ triangular and hexagonal charge density waves are
observed in low-energy stacking regions, while stripe charge density waves are
found in the domain walls surrounding the low-energy stacking regions. These
predictions, which can be tested by scanning tunneling microscopy experiments,
highlight the potential to create complex charge density wave ground states in
twisted bilayer systems and can serve as a starting point for understanding
superconductivity occurring at low temperatures.
