  In a solid material strong interactions between the electrons can lead to
surprising properties. A prime example is the Mott insulator, where the
suppression of conductivity is a result of interactions and not the consequence
of a filled Bloch band. The proximity to the Mott insulating phase in fermionic
systems is the origin for many intriguing phenomena in condensed matter
physics, most notably high-temperature superconductivity. Therefore it is
highly desirable to use the novel experimental tools developed in atomic
physics to access this regime. Indeed, the Hubbard model, which encompasses the
essential physics of the Mott insulator, also applies to quantum gases trapped
in an optical lattice. However, the Mott insulating regime has so far been
reached only with a gas of bosons, lacking the rich and peculiar nature of
fermions. Here we report on the formation of a Mott insulator of a repulsively
interacting two-component Fermi gas in an optical lattice. It is signalled by
three features: a drastic suppression of doubly occupied lattice sites, a
strong reduction of the compressibility inferred from the response of double
occupancy to atom number increase, and the appearance of a gapped mode in the
excitation spectrum. Direct control of the interaction strength allows us to
compare the Mott insulating and the non-interacting regime without changing
tunnel-coupling or confinement. Our results pave the way for further studies of
the Mott insulator, including spin ordering and ultimately the question of
d-wave superfluidity.
