  Four-dimensional ultrafast transmission electron microscopy (4D-UTEM)
measurements reveal a rich variety of structural dynamic phenomena at a phase
transition in the charge-density-wave (CDW) 1T-TaSe2. Through the
photoexcitation, remarkable changes on both the CDW intensity and orientation
are clearly observed associated with the transformation from a commensurate (C)
into an incommensurate (IC) phase in a time-scale of about 3 ps. Moreover, the
transient states show up a notable "structurally isosbestic point" at a wave
vector of qiso where the C and IC phases yield their diffracting efficiencies
in an equally ratio. This fact demonstrates that the crystal planes parallel to
qiso adopts visibly common structural features in these two CDW phases. The
second-order characters observed in this nonequilibrium phase transition have
been also analyzed based on the time-resolved structural data.
