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Ultrafast dynamics in 1T−TiS2 single crystals: From quasi-flat-band-mediated carrier recombination to thermally triggered coherent acoustic phonons

Shijie Ma1, Cunzhe Lou2, Jingjing Yang3, Ze Yan4,5, Peng Suo1,2, Yidan Zhang1, Kaiwen Sun1, Xian Lin1, Yanfeng Guo4,5 et al.

Hongzhi Zhou3,*, Jie Li2,†, and Guohong Ma1,2,‡

  • *Contact author: zhouhongzhi@ucas.ac.cn
  • †Contact author: lij@shu.edu.cn
  • ‡Contact author: ghma@staff.shu.edu.cn
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Phys. Rev. Research 8, 033176 – Published 14 August, 2026

DOI: https://doi.org/10.1103/87mq-t3yf

Abstract

Carriers in flat bands (FBs) possess high effective masses and are predicted to exhibit unconventional transport properties. The study of the photocarrier dynamics in FBs is of fundamental importance for understanding many-body interactions and the emergence of exotic quantum phenomena. 1T−TiS2, a layered transition-metal dichalcogenide featuring a quasiflat band near the Fermi level, provides an ideal platform for investigating nonequilibrium carrier dynamics in electronic flat-band systems. Here, we employed transient absorption microscopy to uncover a picosecond, quasi-flat-band-mediated bimolecular recombination of photoexcited electrons and holes confined within the flat-band states in the 80–295 K experimental window. In addition, the launch of coherent acoustic phonons in TiS2 crystals at 45 GHz is demonstrated, from which sound velocity and Young's modulus are determined to be 4.39 kms−1 and 62 GPa, agreeing well with the prediction from the first-principles calculation. The present study provides microscopic insight into ultrafast energy relaxation and the associated phonon generation in layered materials.

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Phys. Rev. Research 8, 033176– Published 14 August, 2026
  • Received 12 February 2026
  • Accepted 14 July 2026
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