  Charge density wave (CDW) is a collective quantum phenomenon in metals and
features a wave-like modulation of the conduction electron density. A
microscopic understanding and experimental control of this many-body electronic
state in atomically thin materials remain hot topics in condensed matter
physics. Here we report an interface and/or Zr intercalation induced
semiconductor-metal phase transition, as well as a concomitant (2 $\times$ 2)
CDW order in 1T-ZrX$_2$ (X = Se, Te) thin films prepared on graphitized
SiC(0001) substrates. Also observed has been a sizable CDW energy gap up to 22
meV opened at the Fermi level. Fourier-transformed scanning tunneling
microscopy reveals a rather simple Fermi surface, consisting only of Zr
4d-derived conduction band at the corners of the Brillouin zone. Our finding
that such a simple electronic structure is compatible with the CDW phase proves
intriguing and challenges several prevailing scenarios for the formation of CDW
in transition metal dichalcogenides.
