In recent years, transition metal dichalcogenides (TMDs) have garnered great
interest as topological materials -- monolayers of centrosymmetric
β-phase TMDs have been identified as 2D topological insulators (TIs), and
bulk crystals of noncentrosymmetric γ-phase MoTe2 and WTe2 have
been identified as type-II Weyl semimetals. However, ARPES and STM probes of
these TMDs have revealed huge, "arc-like" surface states that overwhelm, and
are sometimes mistaken for, the much smaller topological surface Fermi arcs of
bulk type-II Weyl points. In this letter, we use first-principles calculations
and (nested) Wilson loops to analyze the bulk and surface electronic structure
of both β- and γ-MoTe2, finding that β-MoTe2
(γ-MoTe2 gapped with symmetry-preserving distortion) is an
inversion-symmetry-indicated Z4-nontrivial (noncentrosymmetric,non-symmetry-indicated) higher-order TI (HOTI) driven by double band
inversion. Both structural phases of MoTe2 exhibit the same surface features
as WTe2, revealing that the large Fermi arcs are in fact not topologically
trivial, but are rather the characteristic split and gapped fourfold surface
states of a HOTI. We also show that, when the effects of SOC are neglected,
β-MoTe2 is a nodal-line semimetal with Z2-nontrivial
monopole nodal lines (MNLSM). This finding confirms that MNLSMs driven by
double band inversion are the weak-SOC limit of HOTIs, implying that MNLSMs are
higher-order topological semimetals with flat-band-like hinge states, which
we find to originate from the corner modes of 2D "fragile" TIs.Comment: Final version, 5 pg main text + 18 pg supplement, 4 + 6 figures,
abstract abridged for arXiv posting - see paper for full abstrac