  Band topology of materials describes the extent Bloch wavefunctions are
twisted in momentum space. Such descriptions rely on a set of topological
invariants, generally referred to as topological charges, which form a
characteristic class in the mathematical structure of fiber bundles associated
with the Bloch wavefunctions. For example, the celebrated Chern number and its
variants belong to the Chern class, characterizing topological charges for
complex Bloch wavefunctions. Nevertheless, under the space-time inversion
symmetry, Bloch wavefunctions can be purely real in the entire momentum space;
consequently, their topological classification does not fall into the Chern
class, but requires another characteristic class known as the Stiefel-Whitney
class. Here, in a three-dimensional acoustic crystal, we demonstrate a
topological nodal-line semimetal that is characterized by a doublet of
topological charges, the first and second Stiefel-Whitney numbers,
simultaneously. Such a doubly charged nodal line gives rise to a doubled
bulk-boundary correspondence: while the first Stiefel-Whitney number induces
ordinary drumhead states of the nodal line, the second Stiefel-Whitney number
supports hinge Fermi arc states at odd inversion-related pairs of hinges. These
results establish the Stiefel-Whitney topological charges as intrinsic
topological invariants for topological materials, with their unique
bulk-boundary correspondence beyond the conventional framework of topological
band theory.
