  An exciton is an electron-hole pair bound by attractive Coulomb interaction.
Short-lived excitons have been detected by a variety of experimental probes in
numerous contexts. An excitonic insulator, a collective state of such excitons,
has been more elusive. Here, thanks to Nernst measurements in pulsed magnetic
fields, we show that in graphite there is a critical temperature (T = 9.2 K)
and a critical magnetic field (B = 47 T) for Bose-Einstein condensation of
excitons. At this critical field, hole and electron Landau sub-bands
simultaneously cross the Fermi level and allow exciton formation. By
quantifying the effective mass and the spatial separation of the excitons in
the basal plane, we show that the degeneracy temperature of the excitonic fluid
corresponds to this critical temperature. This identification would explain why
the field-induced transition observed in graphite is not a universal feature of
three-dimensional electron systems pushed beyond the quantum limit.
