Ultrafast Dynamics of the Charge Density Wave State in Layered CuxTiSe2

TL;DR

This study investigates the ultrafast dynamics of charge density waves in CuxTiSe2, revealing a quantum phase transition at x=0.04 from commensurate to incommensurate CDW, with implications for understanding competing orders and superconductivity.

Contribution

It provides the first detailed transient optical reflectivity analysis of the quantum phase transition in CuxTiSe2, highlighting the decoupling of driving mechanisms and loss of lattice coherence.

Findings

Quantum phase transition at x=0.04 from commensurate to incommensurate CDW

Decoupling of excitonic and phonon mechanisms at the transition

Loss of phonon coherence indicating reduced lattice order

Abstract

We report on a transient optical reflectivity study of the charge density wave (CDW) in CuxTiSe2 single crystals. Our measurements reveal that the system undergoes a quantum phase transition at x=0.04 from a strong commensurate CDW (x<0.04) to a soft incommensurate CDW (x>0.04). We find that the cooperative driving mechanisms for the commensurate CDW, the excitonic insulator mechanism and the soft L$_1$$^-$ phonon mode, decouple at x=0.04 with the observed fluctuations in the folded Se-4p band dominating the transition. We also demonstrate a loss of coherence in the A$_{1g}$ phonon with increased copper intercalation of the parent lattice, indicating a loss of long-range lattice order. These findings provide compelling evidence that TiSe2 undergoes a quantum phase transition upon copper intercalation from a state of commensurate charge order to a state with a different symmetry in which the new charge order coexists with the superconducting phase.