  Revealing the energy and spatial characteristics of impurity induced states
in superconductors is essential for understanding their mechanism and
fabricating new quantum state by manipulating impurities. Here by using
high-resolution scanning tunneling microscopy/spectroscopy, we investigated the
spatial distribution and magnetic field response of the impurity states in
(Li1-xFex)OHFeSe. We detected two pairs of strong in-gap states on the
"dumbbell" shaped defects. They display clear damped oscillations with
different phase shifts and a direct phase-energy correlation. These features
have long been predicted for classical Yu-Shiba-Rusinov (YSR) state, which are
demonstrated here with unprecedented resolution for the first time. Moreover,
upon applying magnetic field, all the in-gap state peaks remarkably split into
two rather than shift, and the splitting strength is field orientation
dependent. Via detailed numerical model calculations, we found such anisotropic
splitting behavior can be naturally induced by a high-spin impurity coupled to
anisotropic environment, highlighting how magnetic anisotropy affects the
behavior of YSR states.
