  First-order phase transitions in solids are notoriously challenging to study.
The combination of change in unit cell shape, long range of elastic distortion,
and flow of latent heat leads to large energy barriers resulting in domain
structure, hysteresis, and cracking. The situation is still worse near a triple
point where more than two phases are involved. The famous metal-insulator
transition (MIT) in vanadium dioxide, a popular candidate for ultrafast optical
and electrical switching applications, is a case in point. Even though VO2 is
one of the simplest strongly correlated materials, experimental difficulties
posed by the first-order nature of the MIT as well as the involvement of at
least two competing insulating phases have led to persistent controversy about
its nature. Here, we show that studying single-crystal VO2 nanobeams in a
purpose-built nanomechanical strain apparatus allows investigation of this
prototypical phase transition with unprecedented control and precision. Our
results include the striking finding that the triple point of the metallic and
two insulating phases is at the transition temperature, T_tr = T_c, which we
determine to be 65.0 +- 0.1 C. The findings have profound implications for the
mechanism of the MIT in VO2, but in addition they demonstrate the importance of
such an approach for mastering phase transitions in many other strongly
correlated materials, such as manganites and iron-based superconductors.
