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Ultrafast Dynamics of Charge-Density Waves - Kai Rossnagel

Université de Kiel - ALLEMAGNE

LPS Amphi moyen

Pumping correlated materials out of equilibrium and stroboscopically probing them by femtosecond time-resolved spectroscopy and diffraction is a fertile research field with many discoveries waiting to be made. The direct dynamical information can in principle be used to determine the couplings between the various electron and lattice degrees of freedom and, in particular, to identify the dominant degrees of freedom. Temporal discrimination may thus provide novel insights into the nature of complex phases and, possibly, the origin of phase transitions. Here, we explore this idea for canonical charge-density-wave (CDW) systems : the blue bronze Rb0.3MoO3 and the transition-metal dichalcogenides 1T-TiSe2, 1T-TaS2, 1T-TaSe2, and 2H-TaSe2. First, we will illustrate the inherent difficulty in identifying the driving force of CDW formation by a quantitative analysis of comprehensive band structure data as obtained by conventional synchrotron-based angle-resolved photoemission spectroscopy (ARPES). We will then show how femtosecond time-resolved ARPES can be used to directly resolve and compare the momentum-dependent energy gap and spectral weight dynamics on the first few 100 femtoseconds after impulsive near-infrared excitation. Surprisingly, different gap melting time scales and mechanisms are found for the different materials, suggesting different natures of the CDWs (e.g., excitonic versus Peierls insulator) with dominant electronic or dominant lattice degrees of freedom, respectively. These results provide a novel view on CDW formation, and also a motivation for future combined time-resolved photoelectron spectroscopy and diffraction experiments at x-ray free-electron lasers.


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