[Paper Review] Ultrafast decoupling of atomic sublattices in a charge-density-wave material

Jun Li, Junjie Li+13 more|arXiv (Cornell University)|Mar 24, 2019

Electronic and Structural Properties of OxidesMaterials Science8 references3 citations

TL;DR

Using ultrafast MeV electron diffraction, this study reveals that photoexcitation in the charge-density-wave material 1T-TaSeTe induces a transient state where the Ta and Se/Te sublattices decouple, evidenced by simultaneous enhancement and suppression of Bragg peak intensities. This decoupling, driven by nonmetal-anion dynamics, unveils a new pathway for laser-induced manipulation of lattice order parameters and access to hidden quantum phases.

ABSTRACT

Atomic rearrangements within crystals lie at the foundation of electron-phonon-coupled phenomena such as metal-insulator transition and superconductivity. Advanced laser-pump-probe studies have recently focused on various charge-density-wave (CDW) materials to sharpen our understanding of the charge-lattice entanglement, where non-thermal melting of the CDW state is evident from the enhanced Bragg diffraction peak intensities - attributed to the dominance of the metal-atom dynamics over the nonmetal-anion one. Here using ultrafast MeV electron diffraction on the prototypical CDW material 1T-TaSeTe, we observe an unusual coexistence of systematically enhanced and suppressed Bragg peak intensities upon the CDW suppression, indicating a dominance of nonmetal-anion dynamics during photoexcitation. By tracking these atomic trajectories quantitatively through the ultrafast process, we identify a transient state that manifests itself as an unexpected decoupling of the Ta and Se/Te sublattices. These findings unambiguously unveil a new kind of laser manipulations of lattice order parameters, which has potentials in creating new quantum states and discerning hidden phases such as intra-unit-cell orders.

Motivation & Objective

• To investigate the ultrafast dynamics of atomic sublattices in charge-density-wave (CDW) materials following photoexcitation.
• To resolve the dominant atomic motions responsible for CDW suppression in 1T-TaSeTe.
• To identify transient lattice states that may reveal hidden quantum orders.
• To explore laser-induced manipulation of lattice order parameters beyond conventional electron-phonon coupling.

Proposed method

• Ultrafast MeV electron diffraction (UED) was employed to probe atomic lattice dynamics on sub-100 fs timescales.
• A laser pump pulse initiated photoexcitation, followed by a delayed electron probe to capture time-resolved diffraction patterns.
• Time-resolved Bragg peak intensities were analyzed to track atomic displacements and sublattice decoupling.
• Quantitative atomic trajectory reconstruction was performed from the diffraction data to identify distinct dynamics of Ta and Se/Te sublattices.
• The analysis focused on changes in Bragg peak intensities to infer lattice distortions and transient structural states.
• Comparison of peak intensity trends (enhanced vs. suppressed) revealed the dominance of nonmetal-anion motion over metal-atom dynamics.

Experimental results

Research questions

• RQ1 What atomic motions dominate during the ultrafast suppression of the charge-density-wave order in 1T-TaSeTe?
• RQ2 How do the Ta and Se/Te sublattices respond differently to photoexcitation on femtosecond timescales?
• RQ3 Can transient decoupling of sublattices be observed and quantified in time-resolved diffraction experiments?
• RQ4 What structural signatures indicate the formation of a transient state with broken sublattice coherence?
• RQ5 What implications does this decoupling have for creating or detecting hidden quantum phases?

Key findings

• Simultaneous enhancement and suppression of Bragg peak intensities were observed, indicating distinct and opposing dynamics between Ta and Se/Te sublattices.
• The nonmetal-anion (Se/Te) sublattice exhibited dominant motion during CDW suppression, contrary to the conventional assumption of metal-atom dominance.
• A transient state emerged where the Ta and Se/Te sublattices decoupled, evidenced by non-coherent atomic displacements.
• Quantitative reconstruction of atomic trajectories confirmed the decoupling effect, with Se/Te atoms moving independently of Ta atoms.
• The observed decoupling reveals a new mechanism for laser control of lattice order parameters in quantum materials.
• This transient decoupling state may provide access to hidden intra-unit-cell orders and new quantum phases.