Dynamics of Collective Modes in an unconventional Charge Density Wave system BaNi$_{2}$As$_{2}$

TL;DR

This study investigates the collective dynamics of an unconventional charge density wave in BaNi₂As₂ using time-resolved optical spectroscopy, revealing multiple amplitude modes and their evolution across phase transitions, indicating a complex CDW behavior.

Contribution

It provides the first detailed analysis of CDW collective modes in BaNi₂As₂, showing their evolution and resilience, and suggests a link between CDW order and structural phase transitions.

Findings

Existence of several CDW amplitude modes.

Smooth evolution of modes through phase transition.

Unconventional resilience of CDW to perturbation.

Abstract

BaNi$_{2}$As$_{2}$ is a non-magnetic analogue of BaFe$_{2}$As$_{2}$, 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 exceptional resilience of CDW against perturbation, implying an unconventional origin of the underlying electronic instability.