SPIN POLARON IN A MAGNETIC-FIELD

The influence of the homogeneous magnetic field on a single mobile hol e in a magnetic insulator, as represented by the two-dimensional t-J m odel, is investigated by considering the coupling of the field to the orbital current. The energy of the J=0 system is analyzed via the high -temperature expansion and the small system diagonalization. The susce ptibility is shown to be diamagnetic and diverging at low temperatures T. In contrast, in the antiferromagnetic J>0 case small systems gener ically reveal a tendency towards a paramagnetic response in larger fie lds 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 stren gth. At low B the perturbation and small-system results are consistent with a pronounced diamagnetic susceptibility at T-->0, but indicate a suppressed contribution at intermediate T similar to J.