  One of important challenges in condensed-matter physics is to realize new
quantum states of matter by manipulating the dimensionality of materials, as
represented by the discovery of high-temperature superconductivity in
atomic-layer pnictides and room-temperature quantum Hall effect in graphene.
Transition-metal dichalcogenides (TMDs) provide a fertile platform for
exploring novel quantum phenomena accompanied by the dimensionality change,
since they exhibit a variety of electronic/magnetic states owing to quantum
confinement. Here we report an anomalous metal-insulator transition induced by
3D-2D crossover in monolayer 1T-VSe2 grown on bilayer graphene. We observed a
complete insulating state with a finite energy gap on the entire Fermi surface
in monolayer 1T-VSe2 at low temperatures, in sharp contrast to metallic nature
of bulk. More surprisingly, monolayer 1T-VSe2 exhibits a pseudogap with Fermi
arc at temperatures above the charge-density-wave temperature, showing a close
resemblance to high-temperature cuprates. This similarity suggests a common
underlying physics between two apparently different systems, pointing to the
importance of charge/spin fluctuations to create the novel electronic states,
such as pseudogap and Fermi arc, in these materials.
