Dynamics of collective modes in an unconventional charge density wave system BaNi2As2

BaNi 2As 2 is a non-magnetic analogue of BaFe2 As2 , the parent compound of a prototype
pnictide high-temperature superconductor, displaying superconductivity already at ambient
pressure. Recent diffraction studies demonstrated the existence of two types of periodic
lattice distortions above and below the triclinic phase transition, suggesting the existence of
an unconventional charge-density-wave (CDW) order. The suppression of CDW order upon
doping results in a sixfold increase in the superconducting transition temperature and
enhanced nematic fluctuations, suggesting CDW is competing with superconductivity. Here,
we apply time-resolved optical spectroscopy to investigate collective dynamics in BaNi 2 As 2.
We demonstrate the existence of several CDW amplitude modes. Their smooth evolution
through the structural phase transition implies the commensurate CDW order in the triclinic
phase evolves from the high-temperature unidirectional incommensurate CDW, and may
indeed trigger the structural phase transition. Excitation density dependence reveals excep-
tional resilience of CDW against perturbation, implying an unconventional origin of the
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underlying electronic instability.