Drexel

Ultrafast dynamics in normal and CDW phase of NbSe2

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Published on Thursday, 25 June 2020 14:10

In this recent work we have performed broadband ultrafast optical pump-probe experiments in which we have varied laser fluence and temperature to explore the electron-phonon dynamics in NbSe₂ single crystals. 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 T_CDW with subsequent hardening below T_CDW. From first principle 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 τ₃~100 ps 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 optical densities. At highest fluences we observe electronic melting of the CDW and disappearance of the mode hardening below T_CDW. For more details see the manuscript.