Vortex dynamics investigated using low temperature scanning tunneling microscope: collective motion and collective pinning of vortices in NbSe$_{2}$

We report the observation of the collective motion and the collective pinning of vortices of NbSe$_{2}$ under various magnetic fields and from $\sim $6 K to $\sim $2 K using scanning tunneling microscopy (STM). Scanning Tunneling Spectroscopic maps were taken continuously, providing ``movies'' of the vortex motion. Based on the analysis of this data, the speed of the collective motion of vortices was $\sim $1 pm/s. The vortices showed clear signs of weak collective flux pinning. We will quantify the collective transport behavior of vortices based on our STM measurements and compare with the pertinent theories as well as related measurements done by other groups.