  Quantum spin Hall (QSH) insulators have gapless topological edge states
inside the bulk band gap, which can serve as dissipationless spin current
channels protected by the time-reversal symmetry. The major challenge currently
is to find suitable materials for this topological state. Here, we predict a
new large-gap QSH insulator with bulk direct band gap of $\sim$0.18 eV, in
single-layer Bi$_{4}$Br$_{4}$, which could be exfoliated from its
three-dimensional bulk material due to the weakly-bonded layered structure. The
band gap of single-layer Bi$_{4}$Br$_{4}$ is tunable via strain engineering,
and the QSH phase is robust against external strain. In particular, because
this material consists of special one-dimensional molecular chain as its basic
building block, the single layer Bi$_{4}$Br$_{4}$ could be easily torn to
ribbons with clean and atomically sharp edges, which are much desired for the
observation and application of topological edge states. Our work thus provides
a new promising material for experimental studies and practical applications of
QSH effect.
