Analysis of collective pinning and depinning of the flux line lattice in pristine 2H-NbSe$_{2}$

Larkin and Ovchinnikov predicted collective pinning of the flux line lattice (FLL) in type II superconductors several decades ago. The collective pinning results from the interplay between strong vortex-vortex interaction and randomly distributed weak pinning centers in a media. The evidence of collective pinning was previously observed at a magnetic field, H, close to H$_{c2}$ in current-driven transport experiments on the macroscopic scale. But there still exists a lack of understanding of collective pinning on the microscopic level. In this talk, we show collective pinning and depinning of the FLL in pristine 2H-NbSe$_{2}$ in a long time series (15 days), measured by a low temperature scanning tunneling microscope. We observed the motion of the FLL within an area of 400 nm $\times $ 400 nm, with an initial magnetic field of 0.5 T. The motion was caused by the very slow decay of magnetic field ($\sim $ 5 nT/s) in a defective superconducting magnet. The average speed of FLL was $\sim $ 2.5 pm/s, lower than previously reported. Using highly time resolved data, we will further discuss the average direction of motion, the strength of pinning centers in pristine 2H-NbSe$_{2}$, flux line mass, and the difference between current-driven and field-driven FLL motions.