Non-Thermal Phonon Dynamics and Excitonic Condensed States Probed by Surface-Sensitive Electron Diffraction

Background Introduction

The interaction between excitons and phonons determines the energy flow in photo-excited materials and controls the emergence of related phases. With the advancement of materials science, electron or X-ray pulse techniques for probing three-dimensional structural dynamics can reveal the intensity of electron-phonon interactions, the decay channels of strongly coupled modes, and the evolution of three-dimensional order. However, the inherent anisotropy of two-dimensional materials and functional heterostructures, and their need for access to remote plane phonon polarization, have spurred the demand for new technologies.

Research Source

This paper is authored by Felix Kurtz, Tim N. Dauwe, Sergey V. Yalunin, Gero Storeck, Jan Gerrit Horstmann, Hannes Böckmann, and Claus Ropers, from the Max Planck Institute for Multidisciplinary Sciences and the University of Göttingen, and was published in the journal “Nature Materials” in March 2024.

Research Details

Through the introduction of ultrafast low-energy electron diffuse scattering (ULEEDS), this paper resolves non-equilibrium phonon dynamics in 1T-TiSe2 and quantifies the contribution of excitons to the structural order parameters.

a) Research Process

The research is divided into several steps: 1. Surface Photo Pump Measurements: Using a low-energy electron diffuse scattering instrument to trace surface structural dynamics, particularly charge density wave (CDW) phase transitions. 2. Photo-Excited Phonon Population Dynamics: Using momentum-resolved reflection geometry electron backscatter images to reveal the time evolution of photo-induced lattice dynamics. 3. Data Analysis and Theoretical Calculations: Utilizing ultrafast time-momentum resolved photon spectroscopy techniques and theoretical models to probe and analyze hard-to-observe non-equilibrium states.

Surface photo-pumping measurements capture photo-induced phonon features by detecting surface structural responses, which is crucial for understanding the equilibrium and non-equilibrium states within layered materials. Experiments were conducted on 1T-TiSe2 single crystal samples cooled to 30K using ultrafast low-energy electron beams for measurements.

b) Main Results

Phonon Dynamics: 1. Time-Dependent Distribution: By comparing intensity distributions after pump-probe delays, the lattice dynamics induced by photo-excitation were revealed. Preliminary observations found: - Partial suppression of main lattice peaks with increased diffuse scattering intensity at Brillouin zone boundaries. - Over time, the main lattice peak intensity further declined, while the intensity in the adjacent area increased.

1. Background Signals and Scattering: Background signals were produced by inelastic scattering between incident electrons and specific phonons. Using a single phonon scattering model, it was found that background intensity is proportional to the phonon density of states, suggesting the generation of slow phonons at the Brillouin zone center.

2. Mode Contribution: Model analysis established through other experiments found that low-frequency ZA (low-frequency acoustic) modes contribute the most to the scattering background.

Charge Density Wave Dynamics: 1. Changes in Structural Order Parameters: By quantifying the time evolution of superstructure peaks, changes in structural order parameters were studied. The results indicated that structural order parameters were suppressed by thermal cooling and partially restored to a quasi-thermalized state. 2. Excitonic and Peierls Mechanism: Combining data from previous experiments and current findings, it was determined that excitonic condensation contributes approximately 30% to the overall lattice distortion, with the remainder controlled by the Peierls mechanism.

Overall, experiments showed that the structural fingerprint of excitonic condensed states is particularly sensitive to photo-doping, while also demonstrating the potential to separate thermalized and non-thermal dynamics.

c) Conclusions and Research Significance

This paper achieved high momentum-resolved observations of non-equilibrium phonon dynamics through ULEEDS, demonstrating the stepwise generation of ZA phonons and the quantum thermalization mechanism in layered materials. The research results indicate that ULEEDS can more detailedly resolve surface-related equilibrium states, providing a method for deepening the understanding of phonon scattering and exciton-phonon interactions within two-dimensional materials.

The novel findings in this study have significant academic value in the field of physics and provide theoretical foundations and technical support for the development of new electronic and photonic devices based on two-dimensional materials.

d) Research Highlights

1. Revealing the Process of ZA Phonon Generation: It was found that the generation of ZA phonons involves a stepwise process over different time scales, reflecting complex phonon interaction mechanisms.
2. Quantitative Study of Excitons and Structural Distortion: Precisely quantifying the contribution of excitonic condensed states to overall lattice distortion, providing an in-depth understanding of the microscopic mechanisms of electron interactions in TiSe2.
3. Application of New Technology: For the first time, ULEEDS technology was applied to two-dimensional materials, demonstrating its significant potential in probing surface state non-equilibrium dynamics.

Summary

Through innovative experimental techniques, this paper unveils the complex non-equilibrium phonon dynamics and exciton-phonon interactions in 1T-TiSe2 materials, providing valuable experimental data and theoretical models for further study of the thermodynamic behavior of two-dimensional materials. This not only brings new perspectives to material physics research but also provides a solid scientific basis for the design of future electronic and photonic devices.