  Liquid-liquid transition is an intriguing phenomenon in which a liquid
transforms into another liquid via the first-order transition. For molecular
liquids, however, it always takes place in a supercooled liquid state
metastable against crystallization, which has led to a number of serious
debates concerning its origin: liquid-liquid transition vs. unusual
nano-crystal formation. Thus, there have so far been no single example free
from such debates. Here we show the first firm experimental evidence that the
transition is truly liquid-liquid transition and not nano-crystallization for a
molecular liquid, triphenyl phosphite. We kinetically isolate the reverse
liquid-liquid transition from glass transition and crystallization with an
extremely high heating rate of flash differential scanning calorimetry, and
prove the reversibility and first-order nature of liquid-liquid transition. Our
finding not only deepens our physical understanding of liquid-liquid transition
but also will initiate a new phase of its research from both fundamental and
applications viewpoints.
