  A Zeeman field affects the metallic heavy fermion ground state in two ways:
(i) it splits the spin-degerenate conduction sea, leaving spin up and spin down
Fermi surfaces with different band curvature; (ii) it competes with the Kondo
effect and thus suppresses the mass enhancement. Taking these two effects into
account, we compute the quasiparticle effective mass as a function of applied
field strength within hybridization mean field theory. We also derive an
expression for the optical conductivity, which is relevant to infrared
spectroscopy measurements.
