Spin-orbital magnetic response of relativistic fermions with band hybridization

Spins of relativistic fermions are related to their orbital degrees of freedom. In order to quantify the relativistic effect in systems composed of both relativistic and nonrelativistic fermions, we focus on the spin-orbital (SO) crossed susceptibility, which becomes finite in the presence of spin-orbit coupling. The SO crossed susceptibility is defined as the response function of their spin polarization to the "orbital" magnetic field, namely the effect of magnetic field on the orbital motion of particles as the vector potential. Once relativistic and nonrelativistic fermions are hybridized, their SO crossed susceptibility gets modified at the Fermi energy around the band hybridization point, leading to spin polarization of nonrelativistic fermions as well. These effects are enhanced under a dynamical magnetic field that violates thermal equilibrium, arising from the interband process permitted by the band hybridization. Its experimental realization is discussed for Dirac electrons in solids with slight breaking of crystalline symmetry or doping, and also for quark matter including dilute heavy quarks strongly hybridized with light quarks, arising in a relativistic heavy-ion collision process.