Critical femtosecond relaxation dynamics of collective and single-particle excitations through the phase transitions in single crystals of eta-Mo4O11

摘要

We present a systematic study of the single-particle and collective excitations by femtosecond transient reflectivity measurements in single crystals eta-Mo4O11, investigating the dynamics as a function of temperature with two different pump photon energies (3.1 and 1.55 eV). A remarkable slowing down of the relaxation dynamics is observed at the first charge density wave (CDW) transition at T-CDW1 = 105 K associated with hidden one-dimensional Fermi surface (FS) nesting. In contrast, the appearance of the second transition at T-CDW2 associated with further CDW ordering is barely perceptible. The coherent response can be described well by the displacive coherent excitation model of Zeiger et al. [Phys. Rev. B 45, 768 (1992)] assuming a coupling of phonons to the photoexcited quasiparticles. The coupling of the collective modes to the electronic order parameter is found to be weak. The exponential relaxation is discussed in terms of single-particle relaxation and an overdamped collective mode.