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<p>We investigate the nonequilibrium electronic and lattice dynamics of the charge-density-wave (CDW) compound 1<i>T</i>-TiSe<sub>2</sub> using ultrafast optical spectroscopy over a wide range of temperatures and pump fluences. We reveal a close relationship between the observed ultrafast dynamical processes and two characteristic temperatures: <i>T</i><sub>CDW</sub> (∼202 K) and <i>T</i>* (∼165 K). Two coherent phonon modes are identified: a high-frequency <i>A</i><sub>1<i>g</i></sub> mode (<i>ω</i><sub>1</sub>) and a lower-frequency <i>A</i><sub>1<i>g</i></sub>-CDW amplitude mode (<i>ω</i><sub>2</sub>). While both modes soften with increasing temperature, in contrast to thermal behavior we observe a pronounced fluence-induced hardening of the CDW amplitude mode on sub-picosecond timescales. This anomalous frequency upshift provides direct evidence for a nonthermal reconstruction of the lattice potential, driven by transient screening of electron-phonon renormalization by the photoexcited carrier plasma. Concomitantly, the excited-state buildup time exhibits an abrupt increase above a well-defined critical fluence below the CDW transition temperature, signaling a qualitative change in carrier relaxation dynamics. The coincidence between phonon hardening and the fluence threshold indicates that ultrafast electronic screening reshapes the effective lattice potential underlying the CDW order, promoting a nonequilibrium metallic-like response without thermal melting. Our results establish ultrafast phonon hardening as a sensitive probe of lattice potential reconstruction and highlight the fragile balance between excitonic correlations and lattice dynamics in photoexcited 1<i>T</i>-TiSe<sub>2</sub>.</p>

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Ultrafast phonon hardening and nonthermal lattice potential reconstruction in the charge-density-wave material 1T-TiSe2

  • 期刊论文

  • 发表日期：

    2026年4月28日

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  • Xue-Qing Ye,
  • Hao Liu,
  • Qi-Yi Wu,
  • Chen Zhang,
  • Xiao-Fang Tang,
  • Bo Chen,
  • Chuan-Cun Shu,
  • Hai-Yun Liu,
  • Yu-Xia Duan,
  • Peter M. Oppeneer,
  • Jian-Qiao Meng

摘要

We investigate the nonequilibrium electronic and lattice dynamics of the charge-density-wave (CDW) compound 1T-TiSe2 using ultrafast optical spectroscopy over a wide range of temperatures and pump fluences. We reveal a close relationship between the observed ultrafast dynamical processes and two characteristic temperatures: TCDW (∼202 K) and T* (∼165 K). Two coherent phonon modes are identified: a high-frequency A1g mode (ω1) and a lower-frequency A1g-CDW amplitude mode (ω2). While both modes soften with increasing temperature, in contrast to thermal behavior we observe a pronounced fluence-induced hardening of the CDW amplitude mode on sub-picosecond timescales. This anomalous frequency upshift provides direct evidence for a nonthermal reconstruction of the lattice potential, driven by transient screening of electron-phonon renormalization by the photoexcited carrier plasma. Concomitantly, the excited-state buildup time exhibits an abrupt increase above a well-defined critical fluence below the CDW transition temperature, signaling a qualitative change in carrier relaxation dynamics. The coincidence between phonon hardening and the fluence threshold indicates that ultrafast electronic screening reshapes the effective lattice potential underlying the CDW order, promoting a nonequilibrium metallic-like response without thermal melting. Our results establish ultrafast phonon hardening as a sensitive probe of lattice potential reconstruction and highlight the fragile balance between excitonic correlations and lattice dynamics in photoexcited 1T-TiSe2.