  Layered tellurides have been extensively studied as a platform for
investigating the Fermi surface (FS) nesting-driven charge density wave (CDW)
states. EuTe4, one of quasi-two-dimensional (quasi-2D) binary rare-earth
tetratellurides CDW compounds, with unconventional hysteretic transition, is
currently receiving much attention. Here, the CDW modulation vector, momentum
and temperature dependence of CDW gaps in EuTe4 are investigated using
angle-resolved photoemission spectroscopy. Our results reveal that (i) a FS
nesting vector q ~ 0.67 b* drives the formation of CDW state, (ii) a large
anisotropic CDW gap is fully open in the whole FS, and maintains a considerable
size even at 300 K, leading to appearance of semiconductor properties, (iii) an
abnormal non-monotonic increase of CDW gap in magnitude as a function of
temperature, (iv) an extra, larger gap opens at a higher binding energy due to
the interaction between the different orbits of the main bands.
