Unconventional photo-induced charge-density-wave dynamics ‌in 2H-NbSe2 compared to the conventional in 1T-VSe2

R. Venturini,1 A. Sarkar,1 P. Sutar,1 D. Vengust,1 D. Grabnar,1 Z. Jagličić,2,3
Y. Vaskivskyi,1 E. Goreshnik,4 D. Mihailovic,1,5 T. Mertelj1,5

1Department of Complex Matter, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

2Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jamova cesta 2, Ljubljana, Slovenia

3Institute of Mathematics, Physics and Mechanics, Jadranska 19, Ljubljana, Slovenia

4Dept. of Inorganic Chemistry and Technology, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

5Center of Excellence on Nanoscience and Nanotechnology Nanocenter (CENN Nanocenter), Jamova 39, 1000 Ljubljana, Slovenia

We investigated temperature dependent ultrafast near-infrared 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. 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, such as 1T-VSe2.

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 non-thermal 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.

Fig. 1. Temperature dependent transient reflectivity in 2H-NbSe2 and 1T-VSe2.