Ultrafast dynamics in the high-symmetry and in the charge density wave phase of 2H-NbSe2

A. Anikin, R. D. Schaller, G. P. Wiederrecht, E. R. Margine, I. I. Mazin, G. Karapetrov

Research output: Contribution to journalArticlepeer-review

Abstract

We investigate carrier and collective mode dynamics in 2H-NbSe2 using time-resolved optical pump-probe spectroscopy and compare the results with first-principles calculations. Broadband ultrafast reflectivity studies of 2H-NbSe2 in a wide temperature interval covering the normal, charge density wave (CDW) and superconducting phase were performed. Spectral features observed in the transient reflectivity experiment were associated with specific optical transitions obtained from band structure calculations. Displacive excitation of coherent phonons showed CDW-associated coherent oscillations of the soft phonon mode across the whole spectral range. Temperature evolution of this coherent phonon mode in the low-excitation linear regime shows softening of the mode down to the CDW transition temperature TCDW with subsequent hardening below TCDW. The global fit of the broadband probe data reveals four different relaxation times associated with characteristic electron-electron, electron-phonon, and phonon-phonon relaxation processes. From first-principles calculations of electron-phonon coupling we associate the few picosecond electron-phonon relaxation time τ2 with a specific group of phonons with frequencies around 20 meV. On the other hand, the anomalously long relaxation time of τ3∼100ps is associated with anharmonicity-driven phonon-phonon scattering. All relaxation processes result from anomalies near the second order CDW phase transition that are reflected in the temperature dependencies of the characteristic relaxation times and amplitudes of coherent oscillations. At the highest fluences we observe electronic melting of the CDW and disappearance of the mode hardening below TCDW.

Original languageEnglish
Article number205139
JournalPhysical Review B
Volume102
Issue number20
DOIs
Publication statusPublished - Nov 30 2020

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics

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