  Motivated by the recent proposal of the spinon Fermi surface spin liquids for
several candidate materials such as YbMgGaO4, we explore the experimental
consequences of the external magnetic fields on this exotic state.
Specifically, we focus on the weak field regime where the spin liquid state is
well preserved and the spinon remain to be a good description of the magnetic
excitations. From the spin-1/2 nature of the spinon excitation, we predict the
unique features of the spinon continuum when the weak magnetic field is applied
to the system. Due to the small energy scale of the exchange interactions
between the local moments in the spin liquid candidate like YbMgGaO4, our
proposal for the spectral weight shifts and spectral crossing in the magnetic
fields can be immediately tested by inelastic neutron scattering experiments.
Several other experimental aspects about the spinon Fermi surface and the
spinon excitations are discussed and proposed. Our work provides an
experimental scheme to examine the fractionalized spinon excitation and the
candidate spin liquid states in YbMgGaO4, the 6H-B phase of Ba3NiSb2O9 and
other relevant materials.
