  Advances in attosecond core-level spectroscopies have successfully unlocked
the fastest dynamics involving high-energy electrons. Yet, these techniques are
not conventionally regarded as an appropriate probe for low-energy
quasiparticle interactions that govern the ground state of quantum materials,
nor for studying long-range order because of their limited sensitivity to local
charge environments. Here, by employing a unique cryogenic attosecond beamline,
we identified clear core-level signatures of long-range charge-density-wave
(CDW) formation in a quasi-2D excitonic insulator candidate, even though
equilibrium photoemission and absorption measurements of the same core levels
showed no spectroscopic singularity at the phase transition. Leveraging the
high time resolution and intrinsic sensitivity to short-range charge
excitations in attosecond core-level absorption, we observed compelling
time-domain evidence for excitonic correlations in the normal-state of the
material, whose presence has been subjected to a long-standing debate in
equilibrium experiments because of interfering phonon fluctuations in a similar
part of the phase space. Our findings support the scenario that short-range
excitonic fluctuations prelude long-range order formation in the ground state,
providing important insights in the mechanism of exciton condensation in a
quasi-low-dimensional system. These results further demonstrate the importance
of a simultaneous access to long- and short-range order with underlying
dynamical processes spanning a multitude of time- and energy-scales, making
attosecond spectroscopy an indispensable tool for both understanding the
equilibrium phase diagram and for discovering novel, nonequilibrium states in
strongly correlated materials.
