  The state of matter above the critical point is terra incognita, and is
loosely discussed as a physically homogeneous flowing state where no
differences can be made between a liquid and a gas and where properties undergo
no marked or distinct changes with pressure and temperature. In particular, the
structure of supercritical state is currently viewed to be the same everywhere
on the phase diagram, and to change only gradually and in a featureless way
while moving along any temperature and pressure path above the critical point.
Here, we demonstrate that this is not the case, but that there is a
well-defined structural crossover instead. Evidenced by the qualitative changes
of distribution functions of interatomic distances and angles, the crossover
demarcates liquid-like and gas-like configurations and the presence of
medium-range structural correlations. Importantly, the discovered structural
crossover is closely related to both dynamic and thermodynamic crossovers
operating in the supercritical state, providing new unexpected fundamental
interlinks between the supercritical structure, dynamics and thermodynamics.
