Controlling materials to create and tune topological phases of matter could
potentially be used to explore new phases of topological quantum matter and to
create novel devices where the carriers are topologically protected. It has
been demonstrated that a trivial insulator can be converted into a topological
state by modulating the spin-orbit interaction or the crystal lattice. However,
there are limited methods to controllably and efficiently tune the crystal
lattice and at the same time perform electronic measurements at cryogenic
temperatures. Here, we use large controllable strain to demonstrate the
topological phase transition from a weak topological insulator phase to a
strong topological insulator phase in high-quality HfTe5 samples. After
applying high strain to HfTe5 and converting it into a strong topological
insulator, we found that the sample's resistivity increased by more than two
orders of magnitude (24,000%) and that the electronic transport is dominated by
the topological surface states at cryogenic temperatures. Our findings show
that HfTe5 is an ideal material for engineering topological properties, and it
could be generalized to study topological phase transitions in van der Waals
materials and heterostructures. These results can pave the way to create novel
devices with applications ranging from spintronics to fault-tolerant
topologically protected quantum computers