Spin polaron in a magnetic field

The influence of the homogeneous magnetic field on a single mobile hole in a magnetic insulator, as represented by the two-dimensional t-J model, is investigated by considering the coupling of the field to the orbital current. The energy of the J=0 system is analysed via the high-temperature expansion and the small system diagonalization. The susceptibility is shown to be diamagnetic and diverging at low temperatures T. In contrast, in the antiferrmagnetic J>0 case small systems generically reveal a tendency towards a paramagnetic response in larger fields at low T. By employing at T=0 the cumulant expansion we study the ground state in arbitrary B, showing a behavior very sensitive to the character of the quasiparticle dispersion and the magnetic-field strength. At low B the perturbation and small-systems results are consistent with a pronounced diamagnetic susceptibility at T->0, but indicate on a suppressed contribution at intermediate T~J.