Dynamic competition between phason and amplitudon observed by ultrafast multimodal scanning tunneling microscopy

TL;DR

This study introduces a multimodal ultrafast STM-based technique to observe and analyze the dynamic competition between phason and amplitudon excitations in an unconventional charge density wave insulator, revealing new insights into collective mode interactions.

Contribution

The paper presents a novel multimodal ultrafast microscopy platform and uncovers the dynamic interplay and splitting of phason modes, advancing understanding of collective excitations in quantum materials.

Findings

Observation of a massive phason at 0.22 THz with temperature-dependent behavior.

Discovery of a daughter phason mode at 0.11 THz arising from mode splitting.

Evidence that the daughter phason suppresses the amplitudon, indicating competitive interactions.

Abstract

The intertwining between two ordered states that arise from the same interactions is reflected in the dynamics of their coupled collective excitations. While the equilibrium phase diagram resulting from such intertwined orders has been extensively studied, the dynamic competition between non-equilibrium modes is a largely unexplored territory. Here, we introduce a multimodal STM-based pump-probe technique, that combines ultrafast tunneling microscopy (USTM), ultrafast point-contact spectroscopy (UPC), and optical pump-probe reflectance (OPPR) on femtosecond timescale, all within a single instrument. Using this platform, we investigate the collective excitations of the unconventional charge density wave insulator (TaSe4)2I. Our UPC measurements reveal charge oscillations at 0.22 THz, with a temperature dependence that matches the theoretically predicted behavior of the long-sought massive phason gaining mass through the Anderson-Higgs mechanism. Unexpectedly, the data also reveals a second mode at 0.11 THz exhibiting similar temperature and polarization dependence with comparable mode intensity. These features, along with the robust 1/2 frequency ratio locking suggest that the 0.11 THz phason is a 'daughter mode' that arises from the splitting of the 0.22 THz massive phason into two massless phasons via parametric amplification, analogous to the decay of a neutral pion into two photons. Strikingly, comparison with OPPR data reveals that the daughter phason competes with and suppresses the amplitudon at proximate frequency. Our studies reveal an unexplored mechanism for the generation and extinction of collective excitations in quantum materials and pave the way for a microscopic understanding of ultrafast phenomena.