  The low-temperature phase diagram of multilayer graphene heterostructures is
largely defined by the exchange-driven instability that lifts the four-fold
isospin degeneracy. Such instability gives rise to the quarter- and half-metal
phases, which are key to our understanding of other emergent phenomena. Recent
theoretical works shed light on a new type of Coulomb-driven instability. It is
proposed that the exchange interaction between trigonal-warping-induced Fermi
pockets could induce charge carriers to condense into one of the Fermi pockets,
giving rise to a net polarization in the momentum space. Here, we report the
observation of spontaneous momentum polarization in Bernal bilayer graphene
using angle-resolved nonlinear transport measurement at the second-harmonic
frequency. With excellent angular precision, we show that the polar axis of the
momentum polarization is tunable with varying carrier density, electric field,
and magnetic field. The dominating influence of the momentum-space instability
reveals a natural connection between broken symmetries, and the isospin
degeneracy lifting in the half- and quarter-metal phases.
