  In graphite, a moderate magnetic field confines electrons and holes into
their lowest Landau levels. In the extreme quantum limit, two insulating states
with a dome-like field dependence of the their critical temperatures are
induced by the magnetic field. Here, we study the evolution of the first dome
(below 60 T) under hydrostatic pressure up to 1.7 GPa. With increasing
pressure, the field-temperature phase boundary shifts towards higher magnetic
fields, yet the maximum critical temperature remains unchanged. According to
our fermiology data, pressure amplifies the density and the effective mass of
hole-like and electron-like carriers. Thanks to this information, we verify the
persistent relevance of the BCS relation between the critical temperature and
the density of states in the weak-coupling boundary of the dome. In contrast,
the strong-coupling summit of the dome does not show any detectable change with
pressure. We argue that this is because the out-of-plane BCS coherence length
approaches the interplane distance that shows little change with pressure.
Thus, the BCS-BEC crossover is tunable by magnetic field and pressure, but with
a locked summit.
