Title: Vortex creep at very low temperatures in single crystals of the extreme type-II superconductor Rh$_9$In$_4$S$_4$,
Abstract: We image vortex creep at very low temperatures using Scanning Tunneling
Microscopy (STM) in the superconductor Rh$_9$In$_4$S$_4$ ($T_c$=2.25 K). We
measure the superconducting gap of Rh$_9$In$_4$S$_4$, finding $\Delta\approx
0.33$meV and image a hexagonal vortex lattice up to close to H$_{c2}$,
observing slow vortex creep at temperatures as low as 150 mK. We estimate
thermal and quantum barriers for vortex motion and show that thermal
fluctuations likely cause vortex creep, in spite of being at temperatures
$T/T_c<0.1$. We study creeping vortex lattices by making images during long
times and show that the vortex lattice remains hexagonal during creep with
vortices moving along one of the high symmetry axis of the vortex lattice.
Furthermore, the creep velocity changes with the scanning window suggesting
that creep depends on the local arrangements of pinning centers. Vortices
fluctuate on small scale erratic paths, indicating that the vortex lattice
makes jumps trying different arrangements during its travel along the main
direction for creep. The images provide a visual account of how vortex lattice
motion maintains hexagonal order, while showing dynamic properties
characteristic of a glass.