  We investigate the vortex lattice and vortex bound states in CsFe$_2$As$_2$
single crystals by scanning tunneling microscopy/spectroscopy (STM/STS) under
various magnetic fields. A possible structural transition or crossover of
vortex lattice is observed with the increase of magnetic field, i.e., the
vortex lattice changes from a distorted hexagonal lattice to a distorted
tetragonal one at the magnetic field near 0.5 T. It is found that a mixture of
stripelike hexagonal and square vortex lattices emerges in the crossover
region. The vortex bound state is also observed in the vortex center. The
tunneling spectra crossing a vortex show that the bound-state peak position
holds near zero bias with STM tip moving away from the vortex core center. The
Fermi energy estimated from the vortex bound state energy is very small. Our
investigations provide experimental information to both the vortex lattice and
the vortex bound states in this iron-based superconductor.
