Ultrafast dynamics in (TaSe$_4$)$_2$I triggered by valence and core-level excitation

TL;DR

This study investigates the ultrafast out-of-equilibrium dynamics of (TaSe$_4$)$_2$I using optical and free-electron laser spectroscopies, revealing how different excitations influence charge density wave phases and their symmetry.

Contribution

It demonstrates the energy-dependent relaxation dynamics of the metallic sub-system and the selective excitation of the CDW amplitude mode, highlighting the importance of valence band excitation for coupling to ordered states.

Findings

Core-level excitation mimics valence band excitation in the insulating sub-system.

Metallic sub-system relaxation depends on photo-excitation energy.

CDW amplitude mode is excited only at low-photon-energy.

Abstract

In this work, we study the out-of-equilibrium dynamics of the paradigmatic quasi-one-dimensional material (TaSe$_4$)$_2$I, that exhibits a transition into an incommensurate CDW phase when cooled just below room temperature, namely at T$_{\rm{CDW}} $= 263 K. We make use of both optical laser and free-electron laser (FEL) based time-resolved spectroscopies in order to study the effect of a selective excitation on the normal-state and on the CDW phases, by probing the near-infrared/visible optical properties both along and perpendicularly to the direction of the CDW, where the system is metallic and insulating, respectively. Excitation of the core-levels by ultrashort X-ray FEL pulses at 47 eV and 119 eV induces reflectivity transients resembling those recorded when only exciting the valence band of the compound - by near-infrared pulses at 1.55 eV - in the case of the insulating sub-system. Conversely, the metallic sub-system displays a relaxation dynamics which depends on the energy of photo-excitation. Moreover, excitation of the CDW amplitude mode is recorded only for excitation at low-photon-energy. This fact suggests that the coupling of light to ordered states of matter can predominantly be achieved when directly injecting delocalized carriers in the valence band, rather than localized excitations in the core levels. On a complementary side, table-top experiments allow us to prove the quasi-unidirectional nature of the CDW phase in (TaSe$_4$)$_2$I, whose fingerprints are detected along its $c$-axis only. Our results provide new insights on the symmetry of the ordered phase of (TaSe$_4$)$_2$I perturbed by a selective excitation, and suggest a novel approach based on complementary table-top and FEL spectroscopies for the study of complex materials.