Unconventional photoinduced charge density wave dynamics in 2H-NbSe2

We investigated temperature (T)-dependent ultrafast near-infrared (NIR) transient reflectivity dynamics in coexisting superconducting (SC) and charge density wave (CDW) phases of layered 2H-NbSe2 using NIR and visible excitations. With visible pump-photon excitation (400 nm) we find a slow high-energy quasiparticle relaxation channel, which is present in all phases. In the CDW phase, we observe a distinctive transient response component, irrespective of the pump-photon energy. The component is marked by the absence of coherent amplitude mode oscillations and a relatively slow, picosecond rise time, which is different than in most of the typical CDW materials. In the SC phase, another tiny component emerges that is associated with optical suppression of the SC phase. The transient reflectivity relaxation in the CDW phase is dominated by phonon diffusive processes with an estimated low-T heat diffusion constant anisotropy of -30. Strong excitation of the CDW phase reveals a weakly nonthermal CDW order parameter (OP) suppression. Unlike CDW systems with a larger gap, where the optical OP suppression involves only a small fraction of phonon degrees of freedom, the OP suppression in 2H-NbSe2 is characterized by the excitation of a large number of phonon degrees of freedom and significantly slower dynamics.