  Quantum spin Hall (QSH) insulators, a new class of quantum matters, can
support topologically protected helical edge modes inside bulk insulating gap,
which can lead to dissipationless transport. A major obstacle to reach wide
application of QSH is the lack of suitable QSH compounds, which should be
easily fabricated and has large size of bulk gap. Here we predict that single
layer ZrTe$_5$ and HfTe$_5$ are the most promising candidates to reach the
large gap QSH insulators with bulk direct (indirect) band gap as large as 0.4
eV (0.1 eV), and robust against external strains. The 3D crystals of these two
materials are good layered compounds with very weak inter-layer bonding and are
located near the phase boundary between weak and strong topological insulators,
which pave a new way to future experimental studies on both QSH effect and
topological phase transitions.
