Single-particle and Collective Mode Couplings Associated with 1-and 2-directional Electronic Ordering in Metallic Rte 3

The coupling of phonons with collective modes and single-particle gap excitations associated with one (1d) and two-directional (2d) electronically-driven charge-density wave (CDW) ordering in metallic RTe3 is investigated as a function of rare-earth ion chemical pressure (R=Tb, Dy, Ho) using femtosecond pump-probe spectroscopy. From the T -dependence of the CDW gap ∆CDW and the amplitude mode (AM) we find that while the transition to a 1d-CDW ordered state at Tc1 initially proceeds in an exemplary mean-field (MF) -like fashion, below Tc1, ∆CDW is depressed and departs from the MF behavior. The effect is apparently triggered by resonant mode-mixing of the amplitude mode (AM) with a totally symmetric phonon at 1.75 THz. At low temperatures, when the state evolves into a 2d-CDW ordered state at Tc2 in the DyTe3 and HoTe3, additional much weaker mode mixing is evident but no soft mode is observed.