Exciton-polaron transition dynamics in the correlated Van der Waals material NiPS3

Long Cheng1, Kaibo Zheng2,3, Xuan Luo4, Peter Uhd Jepsen1, Binbin Zhou1

1Department of Electrical and Photonics Engineering, Technical University of Denmark, 2800 Kongens Lyngby, Denmark
2Department of Chemistry, Technical University of Denmark, 2800 Kongens Lyngby, Denmark
3Department of Chemical Physics and NanoLund, Lund University, Box 124, Lund 22100, Sweden
4Key Laboratory of Materials Physics, Institute of Solid State Physics, HFIPS, Chinese Academy of Sciences, Hefei, 230031, China

NiPS3, a typical van der Waals transition metal phosphorous trisulfide material, has attracted significant interest due to its strongly correlated electrons and unconventional phenomena. These characteristics make it a promising platform for investigating the unique interaction among multiple degrees of freedom in condensed systems. Recent discoveries of spin-orbit-entangled excitons arising from Zhang-Rice states and exciton-driven transient antiferromagnetic metallic states have revealed the microscopic properties and collective behavior of electrons coupling with spin, orbital, and magnon, enhancing the optical manipulation of coherent many-body interactions [1-2].
Moreover, as a negative charge transfer insulator, localized electrons in NiPS3 dominate intricate interactions with their surrounding environment. Interactions between electrons and ionic vibrations (lattice distortion) yield exotic composite quasiparticles. Utilizing an optical-pump air-plasma-based terahertz probe (OPTP) system with a frequency range covering optical phonon modes (shown in Figure 1a) and temporal resolution of a few tens of femtoseconds, we aim to monitor in real-time the formation and dissociation of these quasiparticles.
As shown in Figure 1b, after photon excitation into a non-equilibrium state, the THz response of NiPS3 exhibits extraordinary behavior. After the initial sharp rise due to charge transfer transition, we observe a rapid decline on a remarkable timescale of ~110 fs, one order of magnitude faster than previous reports. Subsequently, the THz response becomes negative and enters a ~30 ps relaxation process. Combining the specific response of the materials to auxiliary evidence, we attribute the initial ~110 fs drops to conventional exciton formation. Notably, the negative responses and subsequent slow relaxation are linked to exciton-phonon interactions, paralleling conventional excitonic behavior.

Figure 1: (a) THz waveforms transmitted through free-space (cyan) and NiPS3 sample (red). The inset shows the corresponding spectra. (b) The transient THz wave reflection curves at 75K (black), 188 K (cyan), and 300 K (red) are shown as dotted points. The curves of 75 K and 188 K are vertically shifted for clarity. The solid curves correspond to the exponential fitting for the fast and slow dynamics. The inset illustrates the temperature dependence of the slow process time constants.

In this study, we explore well-known excitonic features and uncover a new aspect: the coexistence of self-trapped excitonic-polaron dynamics in NiPS3. This novel perspective opens new avenues for exploring complex many-body behavior in condensed systems, a crucial step towards understanding the optoelectronic properties of materials and unlocking their potential in various applications.

[1] S. Kang, K. Kim, B. H. Kim, et al., “Coherent many-body exciton in van der waals antiferromagnet NiPS3,” Nature 583(7818), 785–789 (2020)
[2] C. A. Belvin, E. Baldini, I. O. Ozel, et al., “Exciton-driven antiferromagnetic metal in a correlated van der waals insulator,” Nat. Comm. 12(1), 4837 (2021)

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